Fundamentals of Colorand Appearance

This book is intended to provide a foundationfor understanding color and appearance to anaudience of people who are involved in colorevaluation and color management. It also presentsthe tools available to monitor and control colorreproduction in a variety of applications.

Most people consider color subjectively. The perceptionand knowledge of color is often limited to a child’s“Crayola®’’ days or to the bits and pieces of informa-tion that are picked up in everyday life. Very little istaught about color in schools. Yet, industry considerscolor a very important characteristic for evaluatingproducts.

Many books, papers, and journals have been writtenabout color. They cover theories and principles forevaluation, formulation, batch correction, weightingfunctions, optimum pigment loading, and measure-ment techniques. Unfortunately, most of the colorliterature available today is written by scientists toexplain the physics, science, and mathematicssupporting the instrumentation used to measure colorand the notations used to communicate it.

Through years of experience, we have assembled thismaterial to provide a logical and practical approach tosolving color and appearance issues related toindustrial applications. We hope that this informationwill be valuable to you.

Some of the industries and applications for color are:

• Apparel: ink, leathergoods, plastics,and textile applications

• Appliances: paint, plastics, laminates,and ink applications

• Automotive: ink, paint, plastics, laminates,and textile applications

• Building Products: ink, paint, paper, plastics,and textile applications

• Food & Food Packaging: ink, dyes, paper, and plastics applications

• Furniture: ink, paint, plastics, laminates,and textile applications

• Health & Beauty: cosmetics, ink, pharmaceuticals,plastics, and textile applications

• Manufacturing: ink, paint, paper, plastics, andtextile applications

• Printing: ink, paper, and plastics applications

• Pharmaceuticals: dyes, pigments, ink, paper,and plastics applications

• Retail: color coordination of in-process materialsand finished goods for all industries andapplications listed above

About this Book

FundamentalsFundamentals of Color and Appearance

Table of Contents

Section 1: Introduction to Color and AppearanceColor and Appearance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.1Color Perception . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.1Understanding Appearance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.1

Light Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.2Artificial Light Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.2Electromagnetic Spectrum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.2Visible Spectrum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.2Additive Color Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.2Subtractive Color Mixtures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.3

Object Interaction with Light Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.4Light Distribution from Objects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.4Reflected Light . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.4Absorbed Light . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.4Geometric Attributes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.4Gloss . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.6

Human Observer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.7Eye Physiology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.7Color Deficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.7Factors That Affect Color Vision . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.9

Color Communication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.9Munsell Color Order System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.9Color Naming Conventions and Standards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.11Definition of a Color Order System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.11

Section 2: Quantifying ColorQuantifying Sources, Objects and Observer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.1The CIE System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.1CIE Chromaticity Diagram (1931) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.1

Quantifying Light Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.3Color Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.3Planckian Curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.3Standard Illuminants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.3Color Rendering Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.3Spectral Power Distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.4Color Evaluation Illuminants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.4

Quantifying Objects Using Spectral Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.7Spectral Reflectance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.7Fluorescent Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.7Spectral Transmittance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.7

Quantifying Observers — Observer Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.81931 Standard Observer (2 Degree Observer) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.81964 Observer (10 Degree Observer) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.8

Putting the Numbers Together — Tristimulus Values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.8Sources and Illuminants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.10Spectral Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.10Observer Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.10Calculating Tristimulus Values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.10Refining CIE, XYZ Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.10

Color Spaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.10Chromaticity (xyY) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.10CIE 1976 Uniform Color Space . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.12CIELab . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.12CIELCh . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.12Lab . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.12CMC (l:c) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.14FMC-2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.14

Section 3: Instrumentation

Effective Color EvaluationAdvantages of Color Measurement Instrumentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.1

Colorimeters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.1

Spectrophotometers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.2Light Source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.2UV Filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.245/0 Illumination Optics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.2Integrating Sphere Optics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.3Diffraction Grating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.3Detector Array . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.3Microprocessor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.5

Sphere-Based Spectrophotometer Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.5Spectrophotometer Performance Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.5

Innovations in Spectrophotometers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.6Goniospectrophotometers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.6

Glossmeter Instruments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.8Glossmeter Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.8ASTM D523 Test Method for Specular Gloss . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.8

Section 4: Visual Quality Control

Visual Color Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4.1ASTM D1729-89 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4.1Establishing Your Visual Color Evaluation Program . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4.1

Light Source Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4.1Natural Daylight Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4.2Artificial Daylight Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4.2Fluorescent Light Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4.3Incandescent Light Sources (Illuminant A) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4.3Horizon Daylight . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4.3

Viewing Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4.3Viewing Geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4.3Neutral Surrounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4.4

Metamerism Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4.4

Color Standards and Sample Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4.5The Ideal Color Standard . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4.5Color Standard Storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4.5Sample Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4.5Visual Tolerancing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4.6Evaluating Color Vision . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4.6Color Vision and Color Discrimination Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4.7Communicating Color Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4.7Visual Color Evaluation Documentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4.8Visual Color Evaluation Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4.8

Section 5: Instrumental Quality ControlConsistency in Color Communication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.1Establishing Your Instrumental Quality Control Program . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.1

Color Standards and Sample Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.1Color Standard Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.1Physical Standards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.1Numerical Standards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.2Sample Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.2

Illuminant and Observer Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.3Daylight Illuminants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.3Horizon Daylight . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.3Incandescent Illuminants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.4Fluorescent Illuminants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.4Observer Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.4

Color Difference in Color QC 5.4Deltas in Color Difference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.4Total Color Difference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.4Nonsymmetrical Color Perception in Color QC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.5

Tolerancing (Creating a Tolerance) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.6Tolerances Based on Individual Deltas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.7Manual Tolerancing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.8Total Color Difference Tolerances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.8

Pass/Fail Analysis (Applying a Tolerance to Color Data) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.9Special Pass/Fail Situations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.9

Indices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.9Yellowness Indices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.9Whiteness Indices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.10Strength Indices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.11Metamerism Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.12Orange Juice (OJ) Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.12TAPPI Brightness and Opacity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.12AATCC Gray Scales . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.13XYZ Ratios . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.13

Limitations of Color Measurement Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.13Correlation with Visual Response . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.13Interinstrument Agreement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.14

Instrumental Color QC — A Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.14Proactive Color Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.14Cost of Poor Color Quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.14Phases of a Color Quality Program . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.14

Instrumental Color Evaluation Documentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.15Instrumental Color Evaluation Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.15

Appendixes:Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .G.1References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .R.1

1.1

Color and AppearanceThe phenomenon of color results from the physicalinteraction of light energy with an object and thesubjective experience of an individual observer. Whenphysical interaction and subjective experience arecombined, color becomes a psychophysical response.

Color PerceptionThree factors can influence the perception of color:

1. a light source

2. an object being viewed

3. an observer (person) viewing the object

The combination of these three elements is calledthe observer situation.

A light source is an object that emits radiant energy(light). The human eye is sensitive to this energy.Emission is characterized by the relative amountof energy emitted at each wavelength in the visiblespectrum. The light source that illuminates theobject affects color perception, since individualsources contain varying quantities of each of thevisible wavelengths of light.

The spectral reflectance (or transmittance) of anobject characterizes the color makeup or color“fingerprint” of that object. The spectral reflectionor transmission curve of the object represents itgraphically and provides a way to quantify samplecolor numerically. Since samples vary in color, sodoes that graph depicting the energy beingabsorbed or reflected.

Example: A red object looks red primarily because itreflects red wavelengths more than green and blue.Green and blue wavelengths are selectively absorbed.

The human visual system (observer) affects theperception of a color. Human color vision deficienciesaffect color perception.

Understanding AppearanceColor is one aspect of a broader subject knownas appearance. Appearance consists of two majorcategories.

Chromatic Attributes are characteristics that relate tocolor such as hue, value, and chroma.

Geometric Attributes are characteristics associatedwith light distribution from an object including gloss,haze, texture, shape, viewing angle, and surround.

Both chromatic and geometric attributes affectvisual perception.

object

observerlight source

Observer Situation

Section 1: Introductionto Color and Appearance

1

Introduction toColor and Appearance

1.2

Light EnergyKnown as visible light, white light is part of an evenlarger classification of energy known as the electro-magnetic spectrum. In 1666, Isaac Newton passed abeam of sunlight through a glass prism that refractedit into a visible spectrum. This experiment demon-strated that white light can be split into many colorsranging from violet to red.

Example: A rainbow is a naturally occurring example ofwhite light (daylight) separated into a full spectrum of color.Water droplets in the atmosphere act as little prisms thatseparate the light into its component colors.

As an energy form, light can be characterized in termsof frequency and time (cycles per second). If we holdtime constant (e.g., 1 second), we see higher frequencyis related to a shorter repeat cycle. The cycle is calledwavelength and is symbolized by the Greek letter λ(lambda). Violet light is refracted the most because ofits lower and, therefore, shorter wavelengths.

Artificial Light SourcesArtificial light sources are produced to approximateradiant energy across the full visible spectrum. Anincandescent lamp, daylight lamp, or a fluorescenttube appear white to the eye. White light, that is de-scribed by the components separated by the prism,is the primary focus of color and appearance technology.

Electromagnetic SpectrumLight, or any radiant energy form, is measured asdistance traveled versus time. That distance is commonlydescribed in terms of wavelength and the amountof energy at a given wavelength. The electromagneticspectrum consists of visible light and other formsof electromagnetic energy. X-rays and ultravioletenergy are located at lower wavelengths whileinfrared, microwave, TV, radio and electricity arelocated at the higher wavelengths.

Visible light wavelengths are expressed in nanome-ters — millionths of a millimeter. Comparing theseunits to inches, there are 25,400,000 nanometers tothe inch. The relative insensitivity of the human eyelimits it to the visible part of the electromagneticspectrum — a narrow section of wavelengths rangingfrom approximately 380 to 760 nanometers.

Although humans are only visually sensitive to visiblelight energy, the presence of other forms of electro-magnetic energy impacts our daily lives. We feel the

warming effects of infrared energy produced froman incandescent lamp or radiated from the sun.Ultraviolet energy can cause a “reemission” in thevisible spectrum. Optical whiteners and brightenerstake advantage of this phenomenon. Energies atshorter wavelengths include gamma and x-rays thathave many valuable applications in nuclear andinternal medicine.

Visible SpectrumThe colors that we recognize are found at thefollowing wavelengths:

• red between 630 and 700 nm• orange between 590 and 630 nm• yellow between 560 and 590 nm• green between 480 and 560 nm• blue between 480 and 360 nm

The commonly taught acronym that helps peopleremember the colors in the visible spectrum is ROYG BIV. The acronym helps us remember Red, Orange,Yellow, Green, and Blue, as well as Indigo and Violet.Indigo and violet are included in the blue region ofthe visible spectrum, but can be seen separately.

Human vision does not respond equally to eachwavelength in the visible spectrum. Although varyingwith different wavelengths, sensitivity peaks at about550 nm (green) for daytime vision. At twilight, theeye’s sensitivity shifts to shorter wavelengths at about510 nm (lower levels of illumination).

Additive Color ProcessBy mixing different wavelengths of energy (or colors)to create white light, we observe the additive colorprocess. The additive primaries (red, green and blue)are defined by the three visually dominant sectionsof visible light. In theory, we can mix these additiveprimaries in desired proportions to produce any color.By mixing all three primaries in relatively equalamounts, we can produce white light.

A color television, computer monitor, or stage lightingall demonstrate the additive color process. Threebeams of light projected through red, green and bluefilters illustrate how these additive mixtures react.

By superimposing two primary colors, we produce asecondary color. Example, when red and green lightsare superimposed, we obtain yellow; green and blueproduce cyan; blue and red produce magenta. Com-bining two secondary colors will produce white light.

1.3

additive color process

green

red

yellow cyan

magenta

white

blue

Light Energy

10-7

10-6

10-5

10-4

10-3

10-2

1.0-1

1.0

102

103

104

105

107

108

109

1010

1011

1012

1013

1014

10

wavelength

106

gamma rays

cosmic rays

x-rays

infrared

microwaves

radar

television

radio

ultrasonic

ultraviolet

visible light

electromagneticspectrum

400 nm 500 nm 600 nm 700 nm

visible light

cyan

magentayellow

green blue

red

black

subtractive color process

white light

colors. By combining all three primary colors,the resulting image appears black (since all lightis absorbed).

Subtractive color mixing is used in the printing industry.We refer to printing as a four-color process: threeprimaries (cyan [C], magenta [M], and yellow [Y])and black [K]. (Black ink is used to achieve a deeperblack than could be made by mixing the three prima-ries.) Taken together, these four inks are called the“process” colors. Color prints or transparencies can beseparated into millions of dots using an optical color sepa-rator. Four separate printing plates are produced foreach of the process colors. When a printing press (e.g.,offset press) “reassembles” the dots of process inks onpaper substrate, a full range of color can be achieved.

Subtractive Color MixturesWhile additive mixtures are achieved by blending coloredlights, subtractive mixtures are produced by combiningcolor pigments or dyes. Any material containing pigmentsor dyes will either reflect, absorb or transmit light energy.This results in color being produced according to thesubtractive color theory. Furthermore, objects absorbingcertain wavelengths will reflect opposing wavelengthsof energy. For example, a yellow object absorbs blueenergy and reflects yellow energy.

A cyan colorant subtracts the red component of whitelight, magenta subtracts green light, and yellowsubtracts blue light. Blending two subtractive primariescreates the secondary mixtures of red, green and blue.Cyan, magenta and yellow are the subtractive primary

1.4

Diffuse transmission occurs when light is dispersedwhile leaving an object’s surface in all directions. Atlow levels this is seen as haze.

Regular transmission occurs when light passesthrough an object predominantly undisturbed(without diffusion) and is altered only in its color.

Reflected LightAs white light strikes the object, a small portion of itis reflected from the first surface at the same surfaceangle as it originated. The angle of incidence of alight source striking a surface equals the angle ofreflection from the surface. This highlight or reflec-tion from the first surface remains as white lightbecause it is a reflection of light energy at all wave-lengths. We see it as shininess or gloss.

Absorbed LightAbsorption plays a major role in delivering thatportion of light that finally reaches the eye. Lightis selectively absorbed by an object based on the pres-ence of physical substances such as dyes and pigments(colorants). A red object absorbs blue light and otherwavelengths and primarily reflects the red light that isseen by an observer. If all of the light is absorbed, theobject appears black and is said to be opaque.

Geometric AttributesObjects can be identified according to how theymodify incident light.

When white light strikes the surface of an opaque(non-metallic) object, such as a red object, severalimportant interactions occur that result in the objectbeing perceived as red. Most of the light enters thered object and certain wavelengths (e.g., blue andgreen) are selectively absorbed by the pigments. Redwavelengths scattered into the air enter the eye. Theyare interpreted by the brain as red.

Metallic finishes, such as gold, brass or silver, havea unique color in their specular reflectance. Thespecular reflectance from a metallic surface is thecolor of the metal.

Small flakes of aluminum or mica are suspended ina pigmented paint coating in metallic or metal-flakefinishes. They exhibit a change in lightness wheneither viewing angle or angle of illumination ischanged.

We have used the terms color pigments and dyes.It is important to identify the differences betweenthese two terms. Colorant is a generic term used todenote a color pigment or dye. Although there areexceptions, pigments are finely ground insolubleparticles, usually available as a powder. Pigments aredispersed or suspended in a liquid vehicle, such as abase to make paint. By contrast, dyes are soluble andare usually mixed with water or solvent (e.g., presscake or paste to make ink). These media allowcolorants to be absorbed better by the product to becolored such as textile or paper.

Object Interactionwith Light EnergyThe object is the second element of the observersituation. Basic physics states that all objects modifyand distribute the light waves that interact with them.The geometric attributes of an object (shape, texture,opacity, and so forth) affect how the light waves aremodified and distributed.

Light Distribution from ObjectsWhen light strikes an object, one or more of the fourprimary types of light distribution occur.

Specular reflection (gloss) occurs when a smallfraction of light (generally between one and ten percent)reflected at the first surface of the object is un-changed. It will appear as a white highlight to anobserver. This is also known as the mirror-likereflection from a smooth surface.

On a rough or matte surface, light is diffuselyreflected (scattered) in all directions that are visibleto an observer. Non-metallic color surfaces exhibitthis diffuse reflection. In comparison, smoothsurfaces exhibit more directional reflection.

Scattering occurs when light energy encountersparticles and is slowed down, redirected, or refracted.This can happen at the surface of or inside an object.A refractive index identifies how much light is sloweddown compared to air, which is assigned an index ofone. Scattering is responsible for the opacity orhiding power of a colorant. Sunlight scattering bymolecules in the earth’s atmosphere is responsiblefor the blue sky and red sunrise.

1.5

Primary Types of Light Distribution from Objects

regular transmission

Geometric Attributes

specular reflection diffuse reflection diffuse transmission

transparent objecttranslucent object

specularreflection

incident

glossy object

small flakesof aluminumor mica

diffusereflection

opaque object

diffusereflection

incidentspecularreflection

redpigments

metallic object

matte object

1.6

These effects can be measured using a goniospectro-photometer. This specialized instrument measuresthe object at several viewing angles. When viewednear the specular reflection or facing the observer,they appear lighter — this is known as the face color.As the angle increases away from the specular reflec-tion, the lightness of the color decreases — this isreferred to as the flop color. The apparent differencebetween the two objects is referred to as the travel.

When a metallic finish is viewed at an angle near thespecular, the object’s lightness is at its maximum andcolor saturation at its minimum. This appearanceattribute is known as luster. This effect is desirable inautomotive finishes. The more rapidly the luster de-creases with the angle of view, the more apparent thecurvature and contours of the car surface appear.These effects are also seen in other materials such assatin and taffeta fabrics that demonstrate a change inluster as the viewing angle changes.

Pearlescent finishes use small particles of mica thatact as interference filters to provide a change in luster,and hue or chroma as illumination or angle of viewchanges. Pearlescent finishes are popular for packag-ing, cosmetics, and auto finishing since they cause achange in luster or apparent color on curved surfaces.

Many factors contribute to an apparent colordifference in metallic and pearlescent finishes.Two objects that appear to match under one set ofangular conditions of illumination and viewing, butnot all, are goniochromatic and demonstrate goniochro-matism. Specialized methods for visual evaluationand multiangle instrumentation (goniospectropho-tometers) have been designed to measure this effect.For visual evaluation, GretagMacbeth has developedthe SkyLight® viewing booth, in support of the effortsof the ASTM E12 committee on the Measurementof Metallic and Pearlescent Colors. The SkyLightprovides a D65 filtered tungsten light source, fifteenchoices of viewing angles, and a calibrated gray scaleto determine the magnitude of any apparent colordifference. For instrumental evaluation, Macbethoffers the Auto-Eye® goniospectrophotometer.

Translucent objects are less than 100% opaque.Examples of translucent materials include printinginks, plastics, liquids, sheer textiles, filters and coloredglass products. The translucency of a product makesvisual evaluation and instrumental measurements farmore complex. Thickness must be controlled. Theperception of a product is dependent on its substrateor backing. Transmitted and reflected light must bothbe considered in these evaluations.

Light that is scattered forward and backward resultsin what is perceived as haze. In transparent prod-ucts, haze refers to the small quantity of light thatis scattered while light passes through an objectto make it look less than clear. Transparent prod-ucts include clear liquids, glass, and plastic films.Both reflected and transmitted light must beconsidered when evaluating these products forcolor and appearance. Visual or instrumentalcolor and appearance assessments require anunderstanding of the materials used in order toprovide repeatable reporting of these attributes.

GlossGloss is the property of a surface that involvesspecular reflection and is responsible for lustrous ormirror-like appearance. Next to color, gloss is thesecond most important attribute that can be mea-sured objectively. In general, the higher the gloss, thedarker the object will appear, and the lower the gloss,the lighter the object will appear.

Gloss is one of the easiest measurements to under-stand, but it is also one of the most misapplied. Gloss isa visual sensation that a human observer experienceswhen light is distributed from a surface. It is a geomet-ric attribute much like haze or transparency. Gloss isperceived differently from color and is not limited tothe three dimensions of color perception.

Some factors that affect gloss include shape, texture,angle of view, curvature of the surface, and objectdirectionality. The human eye accounts for all theseparameters simultaneously and combines them intoa single subjective gloss appraisal. This explains whyseveral observers can reach different conclusionsabout gloss. No single objective measurement of glosswill provide a perfect correlation to human assessment.

1.7

Human ObserverThe human observer is the final prerequisite forhuman color perception. The human eye and brainfunction both as receiver and analyzer. An under-standing of how the eye functions provides someawareness of how we perceive color and appearance.

Eye PhysiologyThe eye has an outer protective covering called thecornea. It is a transparent membrane that bends lightrays as they enter the eye. The light rays travelthrough an opening in the iris called the pupil. Themovement of the iris causes the pupil to change insize. The size of the pupil controls the amount oflight that enters the back of the eye. The light nextpasses through the lens, which changes in thicknessto bring the image of an object into focus on thelight sensitive retina.

The retina is the innermost layer of the eye and con-tains two types of visual receptors, rods and cones.Approximately one hundred and twenty million rodsprovide humans with night or scotopic vision. Thereare approximately five to seven million cones in theretina layer of the eye that provide for our photopicor color vision. There are three types of cones. Bluecones are sensitive to short wavelengths. Green conesare sensitive to medium wavelengths and red cones tolonger wavelengths. Light adaptation or photopicvision, a function of our cones, occurs when we move

from a dark environment to a bright one. Althoughpainful, our light adaptation occurs in milliseconds.Dark adaptation, however, known as our scotopicvision, is a function of our rods and can take as longas twenty minutes. This slower adaptation to darknessexplains why driving at dusk is the most dangeroustime to drive.

Rods and cones in the back of the eye transformimages into chemical energies that stimulate millionsof nerve endings. These nerve impulses are transportedto the brain by the optic nerve, where the signals areinterpreted.

The color vision we are born with is our inheritanceand remains with us with only slight modificationsthroughout life. There are various levels of colordeficient vision that are linked to the X chromosome.Since males have a single X chromosome (that is in-herited from their mothers), and females have two(one inherited from each parent), color deficient visionoccurs in 1 of 12 males and in only 1 of 250 females.

Color DeficiencyColor deficient vision is caused by a partial or com-plete absence of one or more of the three types ofcones. The most common form of color deficientvision is a partial green defective. These individualsare Deuteranomalous and called Deutans if they havea total absence of green receptors. The second mostcommon color deficiency involves partial red vision.Protoanomalous or Protans have all red receptorsmissing. A Tritanomalous has partial blue vision,and individuals with the total absence of bluereceptors are called Tritans. A Monochromathas only one type of cone.

The term color blind is often misused in labelinganyone with any degree of color defective vision.For an individual to be considered totally colorblind, he or she must lack all three receptors.This condition occurs only once in 40,000. Thisindividual is an Achromat. People with normal colorvision are referred to as Trichromats. In the twomost common forms of color deficiencies, (Deutansand Protans), reds and greens are confused. Tritans,on the other hand, see differences between red andgreen, but have difficulties with green and blue.For these reasons, green traffic lights are nevera pure green, but a bluish-green to accommodateall common color deficiencies.

Cross Section of the Human Eye

fovea

vitreous humor optic nerve

lens

iris

cornea

aqueoushumor ciliary

muscle

retina

retina

ciliarymuscle

1.8

Factors That Affect Color Vision

simultaneous contrast

chameleon effect

complementary afterimage

before after

1.9

Factors That Affect Color VisionMany circumstances affect human color vision.

An important factor often overlooked in performingcolor evaluation is the presence of a surround. Thesurround color can influence color judgment signifi-cantly and can cause what is known as simultaneouscontrast.

Colors of medium value and chroma will appear tochange in the direction of lighter, brighter or darker,less saturated colors surrounding them. This is knownas the chameleon effect. A chameleon changes itsskin color to blend into its environment, just as somecolors evaluated next to others will appear to shift inthe direction of the adjoining colors.

Staring at colors for prolonged periods reducessensitivity to them, resulting in a reverse or comple-mentary afterimage that appears until color balanceis restored.

The term adaptation describes how the eye automati-cally changes its sensitivity to a wide range of viewingconditions. Familiar colors or objects remain largelyunchanged with variations in lighting or viewing condi-tions. This phenomenon is called color constancy.For example, imagine the clothes you are wearing.The blue suit you put on at home (incandescent light)still looks blue when you go outdoors (daylight) andinto the office (fluorescent light). These sources arequite different in energy composition. It is quiteremarkable that when we view colors under thesedifferent sources, color is still perceived as being inthe same color family.

Color CommunicationA major problem often arises when we communicatecolor to others. For example, imagine someonewearing rose-colored clothing. You probably have apretty vivid image of the color. The problem with thisstatement, however, is that roses are not always red.In fact, they can be red, yellow, pink, white, orvirtually any color imaginable.

A human observer with normal color vision candistinguish seven to ten million different colors.Specifying or communicating color choices is onlypossible when we can establish an orderly relation-ship among colors.

Munsell® Color Order SystemColor order systems provide a way for us to communi-cate color meaningfully. The first system to do thissuccessfully was the Munsell Color Order System.It is probably the most recognized system, and wasdeveloped by Professor Albert H. Munsell in 1905.

Albert Munsell was a commercial artist and teacherin need of a precise way to describe color. He establisheda method to specify and show the relationships amongcolors using three attributes: hue, value, and chroma.

In 1918, Munsell founded the Munsell Color Companyto provide physical color standards in the form ofpainted paper swatches. This operation, now a partof GretagMacbeth, is a principal supplier of suchstandards to colorists in business, science, education,and industry.

Hue is that attribute of a color by which we distin-guish red from green, blue from yellow, and so forth.There is a natural order of hues: red, yellow, green,blue, purple. One can mix paints of adjacent colorsin this series to obtain a continuous variation fromone color to the other. For example, red and yellowmay be mixed in any proportion to obtain all thehues from red through orange to yellow. The samemay be said of yellow and green, green and blue, blueand purple, and purple and red. This series returnsto the starting point, so all colors can be arranged ina circle. Munsell called red, yellow, green, blue, andpurple the principal hues and placed them at equalintervals around a circle. He inserted five intermedi-ate hues: yellow-red, green-yellow, blue-green, pur-ple-blue and red-purple, to make a total of ten hues.For simplicity, he used the initials for each color assymbols to designate the ten hue sectors: R, YR, Y,GY, G, BG, B, PB, P and RP.

Munsell arbitrarily divided the hue circle into 100steps, of equal visual change in hue, with the zeropoint at the beginning of the red sector. Hue may beidentified by a number ranging from 0 to 100 (shownin the outer circle). This may be useful for statisticalrecords, cataloging and computer programming.However, the meaning is more obvious when the hueis identified by the hue sector and a step, based on ascale of ten, within that sector. For example, the huein the middle of the red sector is called five red, andis written 5R. (The zero step is not used, so there is a10R hue, but no 0 YR.) This method of identifyinghue is shown on the inner circle.

1.10

Munsell Color Order System

Munsell color space

chroma

value

strong chromaweak chroma

1.11

products in the graphic arts industry. The ASTMrecognizes N 7/ for viewing surrounds used in criti-cal color matching applications.

The Munsell color order system itself is applicableto all possible colors. The American Society for Test-ing and Materials (ASTM) describes the StandardTest Method for specifying color by the MunsellSystem in D1535. Unknown colors can be identifiedand communicated in Munsell notation by visualcomparison to the Munsell chips available in theMunsell Book of Color or through computer programs.

Color NamingConventions and StandardsThe Munsell color order system remains the mostrecognized method for identifying and specifyingcolor. It is described in unabridged dictionaries andencyclopedias as well as in specialized publications onart, design, color photography, television, printing,paint, textiles and plastics.

The Inter-Society Color Council (ISCC) and theNational Bureau of Standards (NBS) developed asimplified language of color based on the MunsellSystem. The Munsell space was divided into 267regions. Boundaries were defined based on hue, valueand chroma. Each region was named using an ISCC-NBS hue and a standard set of adjectives (pale, dark,light, moderate, brilliant, strong, deep and vivid).

The Munsell color order system has been widelyused in many fields of color science, most notably asa model of uniformity for colorimetric spaces andhas, itself, been the subject of many scientific studies.

Definition of a Color Order SystemA distinction must be made between color ordersystems and color collections. Any group of colorscan be a collection. For a collection of colors to be acolor order system, several requirements must be met:

• The collection must represent all colors in a three-dimensional color space.

• Colors must have a logical visual progressionorrelationship among one another.

• Any color introduced into the system must fitinto the system in a logical sequence, and a colornotation can be derived based on its relationshipto other colors.

Value indicates the lightness of a color. The scaleof value ranges from 0 for pure black to 10 for purewhite. Black, white and the grays between them arecalled neutral colors. They have no hue. Colors thathave a hue are called chromatic colors. The valuescale applies to chromatic as well as neutral colors.

Chroma is defined as the degree of departureof a color from the neutral color of the same value.Colors of low chroma are sometimes called weak,while those of high chroma are said to be highlysaturated, strong or vivid. Imagine mixing a littlevivid red paint with a gray paint of the same value.If you started with gray and gradually added thered until you achieved the original vivid red color,you would develop a series of gradually changingcolors that increase in chroma. The scaling of chro-ma is intended to be visually uniform and is verynearly so. The units are arbitrary. The scale startsat zero, for neutral colors, but there is no arbitraryend to the scale. As new pigments have becomeavailable, Munsell color chips of higher chroma havebeen made for many hues and values. The chromascale for normal reflecting materials extends beyond20 in some cases. Fluorescent materials may havechromas as high as 30.

Munsell hue, value and chroma can be variedindependently so all colors can be arranged accord-ing to the three attributes in a three-dimensionalspace. The neutral colors are placed along a verticalline, called the neutral axis, with black at the bottom,white at the top, and all grays in between. The dif-ferent hues are displayed at various angles aroundthe neutral axis. The chroma scale is perpendicularto the axis, increasing in an outward direction. Thisthree-dimensional arrangement of colors is calledMunsell color space.

In the Munsell system, color is identified by its Hue(H), Value (V) and Chroma (C). These attributes arewritten in a form H V/C, also referred to as Munsellnotation. For a vivid red having a hue of 5R, a valueof 6 and a chroma of 14, the complete notation is 5R6/14. When a finer division is needed for any attribute,decimals are used. For example, 5.3R 6.1/14.4.

The notation for a neutral color is written: N V/.(The chroma of a neutral color is zero, but is customary toomit the zero in the notation.) The notationN 1/ denotes a black, a very dark neutral, while N 9/denotes a white, a very light neutral. The notation fora middle gray is N 5/. ANSI specifies N 8/ for interi-ors of booths and surround colors for evaluating

1.12

• A slight variation of an existing color can bedescribed easily by partial steps between colors,and a new notation can be derived.

• The system should provide values that allow easycommunication of a color even when a sample is notprovided.

• The medium used to represent the system must beconsistent and reproducible. Colors represented fromone book or fan deck to another should be stable.Paint on paper is usually used to represent thesesystems because of its consistency and long shelf life(4 to 6 years).

Ostwald System (1931) — This color system wasbased on the use of a disk colorimeter. It providesa scale having constant white, black or hue. Eventhough Ostwald’s system was based on the system-atic mixing of colorants, it is truly not a color ordersystem since it is arranged on colorant behavior andnot on color space. Additionally, partial stepsbetween colors are not available.

The Natural Color System (Swedish 1970) —This system is based on four principal pure hues:red, yellow, green, and blue. These principal huesare arranged at 90 degrees from each other byperceptual steps of saturation (chroma) andblackness. One criticism of this system is thatit does not provide equal perceptual steps of hue.

OSA Uniform Color Scales System (1977) —This system provides 558 colors spaced accordingto redness, greenness, yellowness, blueness, andlightness. It allows for the addition of any color andprovides uniform steps or spaces from the centerof the system. The problem experienced with thissystem is that samples at the outer edge are notequally spaced since the system originates fromthe center.

Chroma Cosmos 5000 (Japan CRI 1979) —This system provides 5000 samples based on theMunsell System. They are arranged by constantchroma and not by hue. This system has goodcorrelation with human visual perception but doesnot take into consideration several human factors.Regarding lightness, chroma and hue, mostobservers have the greatest acceptance to variationsin lightness (least perceptible), a lesser acceptanceto variations in chroma, the least acceptanceto variations in hue.

2.1

Quantifying Sources,Objects and ObserversVisual color perception (with its subjective observer)has evolved to a scientific method that is capable ofusing instrumentation to measure color objectively.This method is based on the ability to describe thetypical observer response numerically.

The human visual system is now understood wellenough to simulate it with mathematical models.While no mathematical model can simulate the fullcomplexity of human visual perception, the use of asingle model enables one to get consistent andrepeatable results.

Standard mathematical models quantify source,object and observer as a function of wavelength.

• Sources are quantified as illuminants.

• Objects are quantified by spectral data.

• Observers are quantified by the observer functions.

The CIE SystemThe International Commission on Illumination orCIE (abbreviated from the French CommissionInternationale de l’Éclairage) is devoted to stan-dardization in illumination and related areas thatinclude color and appearance.

The CIE color system was developed on the premisethat color is the combination and interaction of lightenergy, an object and an observer. In 1931, the CIEdefined numbers which could be used to represent acolor viewed under a standard light source by thestandard observer.

The CIE’s accomplishments included:

• The development of a standard observer whichdescribed how an “average” human sees color.

• The definition of standard illuminants(specification for light sources for color comparison).

• The calculation of tristimulus values whichrepresent how the human visual system responds toa given color.

• The transformation of tristimulus values intomore understandable chromaticity coordinates(xyY color space).

• The CIE chromaticity diagram.

CIE Chromaticity Diagram (1931)The CIE Chromaticity Diagram contains some veryimportant information which will aid the discussionon quantifying light sources, so we will introduceit here. We will come back to it again later in thissection when we discuss color spaces in general.

When the full range of perceptible colors is graphed,the result is a characteristic horseshoe shape. Colorsnear the edge of the graph are highly saturated. Thecenter of the graph is determined by the chromatic-ity coordinates (hue and chroma) of the illuminant.Colors closer to this center are less saturated andnearly neutral (black, gray or white).

The curved line that defines the horseshoe shaperepresents pure spectral colors, as those projectedby a prism. Blue wavelengths plot at the extreme left.Green wavelengths plot along the top. Red wavelengthsplot along the extreme right. Highly saturated pur-ples and magentas (which are actually mixtures ofspectral blue and red) plot along the straight linethat connects the ends of the horseshoe curve.This line represents the limits of our visual sensitivity.

Section 2:Quantifying Color 2

Quantifying Color

2.2

chromaticity diagram showing isothermic lines

.2 .3 .4 .5 .6

.2

.3

.4

.5

y

x

CIE 1931 Chromaticity Diagram

wavelength in microns source A

source B

source C

source D

source E

CIE standard illuminants

A

B

C

D65

D75

F2

TL84

Incandescent

Noon Daylight

Average Daylight

Average North Sky Daylight

North Sky Daylight

Cool White Fluorescent

Narrow Band Fluorescent

2856K

4874K

6770K

6500K

7500K

4150K

4100K

Illuminant Description Color Temperature

chromaticity diagram

2.3

The CIE diagram demonstrates one of its remark-able properties: If you mix two colors with differentchromaticity coordinate points, any of the mixturesthat can be achieved can be found on a straight lineconnecting the two points. This property assistsanalysis of additive color mixing. For example, whenthe chromaticity coordinates of three primary colorguns of a television or monitor are plotted on theCIE diagram, straight lines connecting the coordi-nates form a triangle. Any color within the triangleor color gamut can be produced. Those lying out-side of it cannot.

Quantifying Light Sources

Color TemperatureAt the center of almost every CIE diagram is a curve.This curve originates in the dark red region of thediagram, proceeds through the white region, andends in blue. This curve represents the black bodycurve or Planckian Locus, named in honor of MaxPlanck, (Karl Ernst Ludwig Planck, a German physi-cist who lived from 1858-1947). In 1900, Planckderived an equation that relates the spectral charac-teristics of light emitted from a glowing body toincreases in temperature of that same body. An ironbar placed in a furnace appears dull red as it beginsto heat. The bar proceeds through red-orange, white,and finally blue-white as temperature rises. In the sameway, a filament in an incandescent lamp changes coloras varying voltages are applied. Planck’s Law can beused to designate the relative color temperature of alight source and can be expressed as absolute temper-ature (Kelvin). The Kelvin scale (a thermodynamictemperature scale) has the same unit size as those inthe Celsius scale, except they start at absolute zero(minus 273.16 degrees Celsius).

Planckian CurveTechnically, a color temperature designation canapply to an incandescent lamp only, and for thosesources that adhere to the Planckian Curve. However,in illumination engineering, the terms ApparentColor Temperature and Correlated Color Tempera-ture are often used to specify a degree of the whitenessof fluorescent-, high intensity discharge and daylightlamps. Even daylight does not exactly match the blackbody curve. It should be understood that color tem-perature alone is one of the weakest specificationsfor a light source. Consider a household incandescentsource and a warm white fluorescent source. Bothhave the same correlated color temperature at3000K, however they render colors very differently.

If a section of the 1931 CIE diagram containing theblack body curve is enlarged, there can be an infinitenumber of chromaticity coordinates that could repre-sent any correlated or apparent color temperature.For this reason the American National Standards In-stitute (ANSI) has specified a range of chromaticitiesacceptable for a specific color temperature. Becauseof the inconsistencies associated with using chroma-ticity coordinates, these are a very weak specificationfor any light source when used alone.

Standard IlluminantsIn addition to the black body curve located at thecenter of most CIE diagrams, there are also alpha-numeric designations: A, B, C, and D65. Theserepresent standard illuminants that have beenidentified by the CIE and other standardizationcommittees including ANSI. Known as CIE StandardIlluminants, they are mathematical reference modelsused for performing visual or instrumental calcula-tions. The physical simulation of an illuminant iscalled a light source. Some illuminants (A, B, D55,D65 and D75) can be represented by actual lightsources. Others (such as C) cannot. Therefore,all light sources can be illuminants, but not allilluminants can be light sources.

Color Rendering IndexThe Color Rendering Index (CRI) expresses thedegree to which a range of colors appears familiaror natural under a particular light source. TheCRI system is based on how a light source affectsour color judgment of eight special pastel colorsand nine supplemental special colors. The lightsource to be evaluated is compared to a referencesource at a specified color temperature. Themaximum CRI rating is 100. Lamps that haveratings of 90 or above are considered goodfor color evaluation.

Lamps with color temperatures below 5000K arecompared to a tungsten filament lamp, which isarbitrarily given a CRI of 100. For light sources above5000K, the reference source is a phase of daylight thatmatches its correlated color temperature.

The Color Rendering Index for a light source isbased on an arbitrary reference source. This does notmean that the reference source has good color ren-dering properties. Color Rendering Index ratings arethe average performance for a light source comparedto reference colors. Better responses for some colorscan be concealed in the overall average with poorerperformance in other colors. Two lamps with the

2.4

same correlated color temperature and Color Ren-dering Index may differ remarkably in their abilityto render one or more colors. A Color RenderingIndex is only an indicator of the color renderingability for a light source. It is useful only in specify-ing a source when its limitations and deficienciesare understood.

Spectral Power DistributionDue to the great variations in light sources in usetoday, understanding the critical difference in theenergy content or wavelength balance for each sourceis essential to understanding overall performance.A spectroradiometer measures the relative energy ofa light source across the visible spectrum as a functionof wavelength. Results can be usually represented aswatts per nanometer. A light source can be charac-terized by its Spectral Power Distribution, or SPDcurve. These curves and their related numerical dataare convenient ways to identify and characterize lightsources and illuminants. The spectral power distribu-tion of a light source is one of the most valuable toolsused to determine how well a light source renders ordistorts color.

Daylight Spectral Power Distribution —The spectral quality of natural daylight constantlychanges from hour to hour, day to day, season toseason, and place to place.

Natural daylight is not available at night or in interiorrooms. For these reasons, companies have developedtechnologies that simulate daylight phases.

Differences in the spectral composition of the threedaylight phases are demonstrated in their SPD curves.D75 has slightly more blue energy than D65. D65 hasmore blue energy than D50 and D50 has more redcontent than either D65 or D75. As a rule, all phasesof daylight have very similar curves, and most colormatches found to be acceptable under one of theseilluminants will be acceptable under the other two.

Color Evaluation IlluminantsThe CIE specified certain light sources to use for colorevaluation. The chart to the right lists the commonlyselected illuminants used to perform visual colorevaluation or color instrumental measurements.Each illuminant has its own unique spectral powerdistribution curve and identifying numerical data.When looking at SPD curves, the important part isthe visual spectrum from 380 nm to 760 nm.

Daylight sources are preferred sources for colorevaluation. D65 has the entire spectrum in close toequal amounts. The equal energy distribution is onefactor that makes daylight the preferred source.Other light sources do not exhibit similar charact-eristics. The abbreviations D50, D65, and D75 areused to designate the phases of daylight recognizedby the CIE.

Daylight can be simulated in several ways. The bestmethod for simulating daylight is a filtered tungstenhalogen source. If you decide to use a daylight fluo-rescent source, choose one that is specially designedfor color evaluation. The least desirable choice fordaylight simulation is commercial daylight fluorescentfixtures. These units are generally optimizing for lampefficiency, not color rendering properties.

According to the Illumination EngineeringSociety of North America (IES), “The only wayto effectively reproduce or simulate daylightis by filtering a continuous spectrum tungstenhalogen source.” This patented technology waspioneered by GretagMacbeth and can be foundin all of our SpectraLight® products. It remainsthe most accurate reproduction of daylightin use throughout the world today.

If you opt to use daylight fluorescent simulation,keep in mind that commercial daylight fluorescentlamps utilize only three phosphors, resulting in spikesand depressions in the SPD. These effects enhancesome colors, but gray out others. GretagMacbethDaylight fluorescent lamps have a patented sevenphosphor coating that eliminates the color exaggera-tion caused by most commercially available daylightfluorescent sources. GretagMacbeth’s patentedcoating also doubles usable lamp life.

Another issue associated with commercial fluores-cent lamps is that an additional green phosphor isused to increase lamp efficiency (lumen-per-watt ratio).Human sensitivity to color peaks at 550 nm (green).The addition of green phosphor heightens humanvisual perception and the perceived efficiency of thelamp follows. Commercial fluorescent lamps aremanufactured to achieve maximum efficiency ratherthan maximum color rendering capabilities. Onecannot have good color rendering and energyefficiency — a tradeoff must be made.

2.5

Color Rendering Index

Cool White

Cool White Dx

Warm White

Warm White Dx

Daylight

GretagMacbeth D50 Fluor

GretagMacbeth D65 Fluor

GretagMacbeth D75 Fluor

TL84

Ultralume 30

Horizon

Tungsten

Mercury Vapor

Metal Halide

Xenon

High Pressure Sodium

Low Pressure Sodium

0.373 0.385

0.376 0.368

0.436 0.406

0.440 0.403

0.316 0.345

0.340 0.360

0.313 0.324

0.299 0.310

0.375 0.380

0.440 0.406

0.492 0.416

0.424 0.399

0.373 0.415

0.396 0.390

0.324 0.324

0.519 0.418

0.569 0.412

4250

4050

3020

2940

6250

5150

6520

7550

4100

3000

2300

3190

4430

3720

5920

2100

1740

62

89

52

73

74

93

94

95

85

85

100

100

32

60

94

21

-.44

Lamp Designation CIE Chromaticity Coordinates x y

Correlated ColorTemperature - Kelvin

Color RenderingIndex

5000

K

5000K

7500

K

7500K

6500

K

6500K

Daylight SPD Curves1.5

0400 700wavelength (nm)

400 700

norm

alize

d sp

ectra

l pow

er(W

/%NM

/100

0000

LM

)

600

0

SpectraLight D65

400 700

norm

alize

d sp

ectra

l pow

er(W

/%NM

/100

0000

LM

)

600

0

CIE D65

GretagMacbeth D65 fluorescent

400 700

norm

alize

d sp

ectra

l pow

er(W

/%NM

/100

0000

LM

)

600

0

wavelength (nm)

wavelength (nm)

wavelength (nm)

commercial daylight D65

400 700

norm

alize

d sp

ectra

l pow

er(W

/%NM

/100

0000

LM

)

600

0wavelength (nm)

Quantifying Light Sources

2.6

Illuminant A, also known as incandescent or tung-sten halogen, has a curve that provides very littleblue and green energy, and large amounts of yellow,orange, and red energy. For this reason, the incan-descent lamps found in homes or used as accentlighting in retail stores enhance yellows, oranges andreds and suppress or gray out blues.

Fluorescent lamps such as Cool White or TL84have some dramatic differences in the shape oftheir curves. In fluorescent lamp technology, a tubeof glass or envelope is capped at each end with acoiled wire filament called an electrode. The tube isfilled with argon gas and a small amount of mercuryvapor. The inside of the envelope is coated withphosphor. The lamp ballast provides a high voltageelectric arc to start the lamp. The voltage is thenreduced to maintain a plasma arc between the elec-trodes. The combination of this plasma arc and themercury vapor gas produces a short wave ultravioletenergy that is absorbed by the phosphor coating,causing it to fluoresce. All fluorescent lamps producea continuous spectrum characterized by the chemicalcomposition of the phosphor coating. The mercurylines or spikes at specific wavelengths are caused bythe mercury gas.

Distortions hinder the ability of fluorescent lampsto render all colors equally. For example: Cool WhiteFluorescent strengthens oranges, yellows, blues, andgreens and suppresses or grays out most reds. As aresult of the phosphors found in a Cool White lamp,energy is emitted in wide patterns across the visiblespectrum. Cool White Fluorescent is known as wideband fluorescent. In order to improve viewing con-ditions, newer triphosphor lamp technology utilizesrare earth phosphors that were introduced as TL84and Ultralume. These produce three narrow spikesof blue, green, and red-orange energy to create anillusion of white light. Known as narrow band fluo-rescent lamps, they depend on the human brain toaverage the visual response across the spectrum.Narrow band lamps tend to make a space look morecolorful as the three peak phosphors compress allcolors into blue, green, and red-orange, in orderto increase the contrast among colors. wavelength (nm)

400 700

norm

alize

d sp

ectra

l pow

er(W

/%NM

/100

0000

LM

)

600

0

Cool White Fluorescent

wavelength (nm)

Illuminant A

400 700

norm

alize

d sp

ectra

l pow

er(W

/%NM

/100

0000

LM

)

600

0wavelength (nm)

SPD Curves for Common Illuminants

400 700

norm

alize

d sp

ectra

l pow

er(W

/%NM

/100

0000

LM

)

600

0

TL84

2.7

Quantifying ObjectsUsing Spectral DataIt is important to understand the interaction thatoccurs between objects and light waves in order tobe able to relate them to measurement data. Theperceived color of an object is directly related to itsspectral characteristics. Spectral characteristics arespecified by reflectance (or transmittance) as a func-tion of wavelength. Spectral characteristics of amaterial, for the purpose of color measurement, area property of the individual piece being measuredand are independent of other variables (such as thelight source and observer). Spectral data aremeasured with a spectrophotometer.

Spectral ReflectanceAs white light strikes the red object, a small portionis reflected from the first surface at the same surfaceangle as it originated. The angle of incidence of alight source striking a surface equals the angle ofreflection from the surface. This highlight or reflec-tion from the first surface remains as white lightbecause it is a reflection of light energy at all wave-lengths. We see it as shininess or gloss. Every colorhas its own unique characteristic of absorbing andreflecting white light energies. By plotting thesereflectance and absorption properties against thevisible spectrum, reflectance curves are generated.

A spectral curve can be used to identify a specificcolor. A white surface reflects all light energy acrossthe visible spectrum. Its reflectance curve is a straightline at approximately 90 to 100% reflectance. Blackabsorbs almost all light energy, so its reflectance curveis flat and approaches 0% reflectance. A mid-rangegray, made by combining equal portions of whiteand black, is represented by a straight line at 50%reflectance.

For opaque materials, spectral reflectance is mea-sured because reflected light is what the eye perceives.Reflectance is expressed as a percentage, rangingfrom 100% (where material reflects all light strikingit) to 0% (where it absorbs all light striking it). Forexample, a bright red tile may reflect 75% in the redregion of the visible spectrum, and reflect only 10%in the blue and green regions.

Fluorescent MaterialsFluorescent materials absorb energy at a short wave-length and emit the energy at a longer wavelength.

Emitted energy and reflected energy of the samewavelength are perceived (by both spectrophotometerand human eye) as if all the energy was simply reflect-ed. Therefore, the object seems to reflect more lightof that color than that with which it was illuminated.To the eye, a fluorescent color is very bright and vivid,and a fluorescent white is “whiter than white.” To thespectrophotometer, a fluorescent object can have“reflectance” values greater than 100%. For this rea-son, fluorescent materials require special consider-ation to attain meaningful results.

Spectral TransmittanceFor transparent materials, spectral transmittancedetermines the color, because the eye perceiveslight that passes through the material. Transmit-tance is expressed as a percentage, ranging from100% (where material transmits all light strikingit) to 0% (where it absorbs all light striking it). Forexample, a deep red filter may transmit 80% in thered region of the visible spectrum and transmitonly 1% in the blue and green regions.

redobject

spectral reflectance curves

green

red

gray

blue

refle

ctan

ce %

black

white

spectral reflectance curve400 nm 500 nm 600 nm 700 nm

125

100

75

25

refle

ctan

ce %

50

0

400 nm 500 nm 600 nm 700 nm

125

100

75

25

50

0

white fluorescent

Quantifying Objects

2.8

Quantifying Observers —Observer FunctionsIn addition to defining standard illuminants, theCIE conducted experiments to quantify the standardobserver. The development of the standard observeris the basis for all instrumental color measurement.

1931 Standard Observer (2 Degree Observer)In 1927, a test fixture was designed to allow an ob-server to “dial” in the correct amount of red, greenand blue light to match a given color. Imagine anobserver seated in front of this fixture. The observerwould look at a white screen through an aperturehaving a 2 degree field of view (much like looking atone’s thumbnail at arm’s length). The observer thenwould be asked to match a test light visually on oneside of the screen by adjusting the intensity of red,green and blue lights. The quantities of the threeprimary light sources required to match the testsource were named tristimulus values. The test wascontinued until the observers matched colors repre-senting the entire visual spectrum.

A 2 degree field of view was used in the experiments.This meant the observer would be able to use onlythe region of the retina known as the fovea. The 2degree field of view was selected since the fovea hasthe highest concentration of cones, and it is thecones that are responsible for color sensation.

Two separate experiments were conducted by phys-icists John Guild and David Wright. Guild used 7observers and Wright used 10. The data from thetwo independent observer groups were combinedmathematically.

The experimental results proved that not all colorscan be matched using the set of primaries. In somecases, light from one of the primaries had to be addedto the test color to deliver a match. Adding light tothe test color was considered the equivalent of sub-tracting it from the other two primaries, resulting inthe test color being described by a combination ofpositive and negative tristimulus values.

In order for the data to be easily used in ongoingstandardization work, the CIE believed it would benecessary to eliminate all negative numbers. Toaccomplish this, they mathematically transformedthe standard tristimulus curves so that all red, greenand blue responses were positive. This change meant

that the new primary set of red, green and blueoutput could not be produced by an existing lamp.Despite this, the 1931 CIE standard observer curveswere adopted as “standard” response curves for theaverage observer.

The three standard observer curves relate directlyback to how the human eye functions. They representthe three responses of the human eye. We knowthese as our color matching function. The three colormatching functions are given symbols x-bar, y-bar,and z-bar. These functions correspond to the spectralsensitivity of the human eye with the highest sensi-tivity in the green range (electromagnetic spectralenergy at 550 nm).

This standard observer function and the relatedmathematical data remain the worldwide basis forcolor measurement and computation.

1964 Observer (10 Degree Observer)It was later determined that color values calculatedusing the 2 degree observer do not always correlatewell with visual assessment, since most visual assess-ments are done with a field of view greater than 2degrees. Subtle differences exist when a wider areaof view is used, particularly in the blue-green regionof the spectrum.

In 1964, the CIE defined a supplemental observer toprovide better correlation with commercial colormatching. The supplemental observer is based oncolor matching experiments which were conductedusing a 10 degree field of view.

Repeatability of the standard observer was found tobe more accurate using a larger field of view. Today,the 10 degree observer is most widely used in colorformulation and color quality control.

Putting the Numbers Together —Tristimulus ValuesHaving defined standard illuminants and standardobservers, the CIE was then able to define tristimu-lus values. Tristimulus values are numbers thatrepresent how the human visual system respondsto a given color. These are calculated from thenumerical data used to describe each of thecomponents responsible for color perception.

2.9

standard observer responses

2.00

1.50

1.00

.50

Rela

tive

Resp

onse

CIE 1931

CIE 1976

Quantifying Observers

2˚ and 10˚ observers

eye

fovea

10˚2˚

CIE standard observer experiment

whi

te s

cree

n

mas

king

scr

een

red

green

black partition

observer

testlamp

blue

400 nm 500 nm 600 nm 700 nm

2.10

Sources and IlluminantsA light source is defined as a physical origin of light,such as a tungsten lamp. An illuminant is a numeri-cal representation of a source. The set of numbersused in this numerical representation describes howmuch light of each wavelength the source contains.Illuminants have been created to represent mostcommonly available sources.

Spectral DataSpectral data (reflectance or transmittance) specifiesthe spectral reflectance or transmittance characteris-tics of an object. This is measured on a spectropho-tometer and shows how much light (as a percentageof incident light) of each wavelength is reflected ortransmitted by the object.

Observer FunctionsThe response of the average normal human eye ateach wavelength has been measured through exten-sive experimentation by the CIE. Since there arethree color sensor types, there are three observerfunctions that comprise what is known as the stan-dard observer.

Calculating Tristimulus ValuesSource, object and observer are integrated mathemat-ically to calculate a response value called a tristimulusvalue. Since the standard observer contains three re-sponse functions, three response values are calculatedto describe (in mathematical terms) how the humanvisual system responds to a single color.

The observer situation and its three elements cannow be represented numerically. By multiplying theobject color (as reflectance or transmittance values)by the mathematical data representing the lightsource and standard observer functions (x-bar, y-bar,and z-bar), we arrive at the tristimulus values. Theseare also referred to as cap X, cap Y, and cap Z. Mostobservers have difficulty in identifying a color fromits tristimulus values.

Refining CIE, XYZ SystemsThe CIE, XYZ System was designed to providea method to determine whether two colors havingthe same tristimulus values match (using a specificilluminant and observer). If the calculated XYZvalues for two colors were the same, the pair wouldmatch when viewed under the light used in theoriginal calculation. However, these same two objectsmay not match when viewed under a different light

source. Thus, XYZ values do not allow you to detectmetamerism.

In addition, CIE X, Y and Z values are not practicalfor describing object color (and were never intendedto be used this way). An example of this can be shownif one is given a set of tristimulus values X = 18.34,Y = 11.19, Z = 6.68 (based on Illuminant D65/10˚Observer) and asked to describe the color theyrepresent. Unless you were very experienced usingtristimulus values, you would be unable to determinethat these values represent a dark, yellowish red.Color scientists have developed equations that trans-late tristimulus values into numbers that can be moreeasily understood by the common observer and canbe plotted in color space.

Color SpacesA color space is an arrangement of colors in someorderly fashion. Some color spaces are physicalarrangements of colors based on visual assessment(for example, the Munsell system). For colormeasurement, the most useful color spaces are onesthat are calculated based on tristimulus values.

Many advances have been made to refine the bound-aries of color space and describe color difference.Color spaces evolved out of the necessity to providea uniform method to spread colors relative to theirvisual differences and to describe color in numericalterms that make sense to us. Opponent-type scaleswere developed from the theoretical mechanism thatthe eye and brain actually use to perceive color.

Chromaticity (xyY)Chromaticity coordinates (hue and chroma) are asimple transformation of tristimulus values that canbe represented on a two-dimensional graph. Thethird dimension is tristimulus Y (luminosity), whichrepresents how bright the color is. Using the chroma-ticity coordinate system, all possible colors can bedesignated by their x and y coordinates on the 1931CIE chromaticity diagram. Chromaticity coordinatesare calculated from tristimulus values as follows:

The primary disadvantage of this color space is thatequal spatial distances on the graph do not representequal visual color differences. In other words, it is anonuniform color space.

x XX Y Z

=+ +( )

, y YX Y Z

=+ +( )

2.11

Color Spaces

380-420440

460

480

490

520 530 540 550 560570 580

590600

610 620 630 640 650

700-780 nm680

∞

6000

10000

20000

40003000 2000

1000

C

BE

A

D65

1500

.5

Blue

YellowishGreen Yellow

Green

YellowOrangeYellow

Yellowish

Pink

Pink

PurplishPink

ReddishOrange

GreenishBlue

Bluish Green

Green

OrangeGreenish Yellow

White

ReddishPurple

PurplishRed

Red

Purple

Purplish Blue

470

x

u´

v´v´

CIE 1976 U.C.S. chromaticity diagram

.6

.4

.3

.2

.1

.0

.6.5.4.3.2.1 u´

.6.5.4.3.2.1.0

.6

.5

.4

.3

.2

.1

.0

480

470

500

560

540

580

600

650

620

450

.8.4.2.0 .6

.6

.4

.2

.0

.8

CIE 1931 x, y chromaticity diagram

y

520

2.12

CIE 1976 Uniform Color SpaceUnlike the CIE color space from 1931 that was a visualcolor space, in 1976, the CIE defined a uniformcolor space. Relative to the 1931 CIE ChromaticityDiagram, you can see that the shape of the 1976 CIEdiagram has been changed. The hues from yellow tobluish green have been compressed, while the huesfrom blue to red have been expanded. In the 1976CIE diagram, equal spatial distances on the graphrepresent equal visual color differences.

CIELabCIELab color space supports the accepted theoryof color perception based on three separate colorreceptors, RGB (Red, Green, and Blue), in the eye.When reflected light reaches these receptors, theyare excited. This results in three sets of signals beingsent to the brain: light or dark, red or green, andyellow or blue. They are opposing in that one receivesa red signal or a green, but not both. Does one eversee a red green or green red? The answer is no, buta yellow green or a blue red can be sensed.

There is a relationship between lightness and Munsellvalue. This relationship was established so that thelightness scale would be based on uniform steps, asMunsell had already established in his color scale.

The opponent type color scales are derivedmathematically from the CIE values. As definedby the CIE in 1976, CIELab color space and colordifference formula were modified for use wherea more perceptually uniform color space thanCIE, XYZ was required.

L* is a measure of the lightness of an object andranges from 0 (black) to 100 (white).

a* is a measure of redness (positive a*) or greenness(negative a*).

b* is a measure of yellowness (positive b*) orblueness (negative b*).

These coordinates (a* and b*) approach zero for neutralcolors (white, grays, and blacks). The higher the valuesfor a* and b* are, the more saturated a color is.

Now, for example, a color that has an:X value of 18.34,Y value of 11.19 and aZ value of 6.68

can be described in terms of uniform CIELab colorspace with an:

L* value of 39.90,a* value of 48.04 and ab* value of 17.18.

This is meaningful since a positive a* and b* valuesignify the yellowness and redness of the colorimmediately. The L* signifies that the color is dark(using the 0 to 100 scale for relative darkness andlightness of a color).

CIELChIn addition to the use of L, a, b, colors can alsobe defined by the parameters of lightness, chromaand hue (LCh). This method is based on the CIELabcolor space, but describes the location of a colorin space by use of polar coordinates rather thanrectangular coordinates.

L* is a measure of the lightness of an object, rangingfrom 0 (black) to 100 (white).

C* is a measure of chroma (saturation) andrepresents distance from the neutral axis.

h is a measure of hue and is represented as anangle ranging from 0 degrees to 360 degrees.Angles that range from 0 degrees to 90 degreesare reds, oranges, and yellows. 90 degrees to 180degrees are yellows, yellow-greens, and greens.180 degrees to 270 degrees are greens, cyans(blue-greens) and blues. From 270 degrees to360 degrees are blues, purples, magentas, andreturn to reds. An h value that is equal to 360degrees is reported as 0 degree.

Hunter LabThis color space was developed by Richard S. Hunterin 1942 for use with three-filter colorimeters. The L,a, and b notations represent the same color charac-teristics as in the CIELab system. For any given color,CIELab values and Hunter Lab are similar. However,the CIELab equations are the current CIE recom-mendation, and are in more common use. The Labcolor space is generally used only by manufacturerswho need to correlate with historical data stored inHunter Lab values.

2.13

Color Spaces

whiteL*

green-a*

blue-b*

yellow+b*

black

red+a*

CIELab

90˚yellow

+b*

180˚green

-a*

hue

h˚

C*

270˚blue-b*

0˚red+a*

CIELCh

Pale Gray (nearly wht.)Medium GrayBrilliant RedBrilliant YellowGreenDeep Blue

83.7059.6043.7083.3056.8029.30

-0.500.00

37.101.90

-30.008.0

Example L* b*a*

0.500.50

18.7077.0015.40

-17.90

Pale Gray (nearly wht.)Medium GrayBrilliant RedBrilliant YellowGreenDeep Blue

83.7059.6043.7083.3056.8029.30

Example L* hC*

315.0˚270.0˚26.8˚88.6˚

152.7˚294.1˚

0.710.50

41.5577.0233.7219.61

3.1

Effective Color EvaluationVisual color evaluation will always be subjective. It relieson individual eye-brain limitations, includingmemory loss, color vision defects and eye fatigue.Differences in viewing conditions, light sources, andother factors need to be understood and addressedto ensure good visual evaluation. Finally, color differencesare difficult to quantify and communicate. Forexample, what do we mean when we say somethingis too light or too red?

While the final judge will always be the human ob-server, color measurement instrumentation is avail-able that can assist the observer in making colordecisions. Instruments are tools that improve theeffectiveness and efficiency of color control.

Example: While a carpenter could build a house withouta tape measure, the process would be laborious and waste-ful. The carpenter would have to approximate the lengthof boards and other materials used in construction andthen cut them until the proper lengths were achieved. Thetape measure allows the carpenter to accomplish the taskfaster and with less waste by providing a method to makerepeatable measurements consistently and accurately.

Advantages of ColorMeasurement InstrumentationColor instrumentation is more sensitive than even thebest trained human eye. Instruments can detect colordifferences between a standard and a trial far belowthe minimum perceptible level. A wide variety of colormeasurement instrumentation is available today, in-cluding tristimulus colorimeters and spectrophotometers.

Color measurement instrumentation does not replacethe colorist. Instead, it provides the colorist with aresource to achieve consistent acceptable color fasterand with less rework. Effective color evaluation requiresan interaction between the subjective human observerand an objective instrument. The ultimate goal is toarrive at numerical assessments that agree with what theeye sees and to use these assessments to communi-cate the acceptability of color quality.

ColorimetersThe term colorimeter is used to describe any colormeasurement instrument. A true colorimeter usesfilters (glass or plastic) in combination with a lightsource and detector to emulate the three colorresponse functions of the eye. A colorimeter pro-vides color data only, is based on a single observer(2 or 10 degrees), and typically one illuminant(C or D65). Because colorimeters only providedata for a single illuminant, these instrumentsare “blind” to metamerism.

Although colorimeters have improved in recent years,there are inherent problems associated with filter-based technology and tungsten halogen light sources.

These filter-based instruments are subject to repeat-ability and interinstrument agreement problems. Fil-ters can change or degrade over time, resulting incolor data that constantly vary.

Colorimeters use a tungsten halogen light source thatinherently has very little blue or green energy. Thiscan cause problems when collecting data on dark objects.

Tungsten halogen light sources must be warmed upto stabilize lamp performance, making it impossibleto take immediate measurements. Measurementstaken when the instrument is first turned on versusafter it has warmed up have shown discrepancies indata collected.

Tungsten halogen light sources generate heat. Someinstrument designs require that the tungsten lamp be oncontinuously in order to eliminate warm-up effects.Unfortunately this can cause sample heating thatcould result in thermochromic effects. Thermochromiccolors can change significantly while they are in themeasurement port of these instruments.

Tungsten halogen lamps change over the life of thelamp. Unless the optics and electronics of the instru-ment are capable of correcting for lamp aging, driftin the instrumental color data will occur.

Section 3:Instrumentation

3

Instrumentation

3.2

SpectrophotometersA spectrophotometer compares the amount oflight that is shined onto an object with the amountof light that is reflected back from that object. Theratio of these two measurements determines the per-centage of light the object reflects back. This ratio iscalculated at each wavelength in the visible spectrum.The major components of a spectrophotometer area light source, UV filter, optics, diffraction grating,detector array, and microprocessor.

Light SourceGretagMacbeth spectrophotometers use a pulsedxenon light source. The pulsed xenon flash illumi-nates the object uniformly by sending a powerfulone hundred thousand watt burst of light into theilluminating optics (sphere or 45/0 assembly). Sincethe light has high intensity, but short duration, theenergy does not appreciably change any thermo-chromic properties of the sample.

Pulsed xenon is more powerful than a tungstenhalogen lamp for improved measurement reproduc-ibility through noncontact measurement. This prop-erty is particularly useful in measuring dark objects.

Another benefit of pulsed xenon is that it provideslight almost exactly the same color as normal day-light, an important factor for correlation to visualevaluation. This is very important when measuringoptically brightened or fluorescent objects.

Lastly, pulsed xenon can provide in excess of one millionflashes. This translates into five to seven measurement-years without requiring a lamp change.

UV FilterSome spectrophotometers have a filter for removingor partially removing ultraviolet (UV) radiation fromthe light source. If an object has fluorescent agents,UV radiation can change the apparent reflectance ofan object (“apparent” to the spectrophotometer).

To determine if an object is fluorescent, take two read-ings: one with the UV component included, and onewith the UV component excluded. If the object hasfluorescent agents, the two readings will be significantlydifferent. The degree of fluorescence depends on theabsolute amount of excitation energy present in the lightsource. For many fluorescent materials, the excitationwavelength is in the ultraviolet region.

Different instruments have different ways of handlingUV. Some instruments utilize removable UV filters.Readings with filter in place and with filter removedcan be compared to determine emission wavelength,and degree of fluorescence relative to other fluorescentobjects. Be careful when comparing results betweentwo instruments of this type because the percentagereflectance measurement depends on the individuallamp in the instrument. Other instruments have anadjustable UV filter. UV content of the source maybe adjusted to achieve a known quantity of UV,or a known reading on a standard fluorescentmaterial. Absolute degree of fluorescence maythen be quantified.

45/0 Illumination OpticsTo illuminate an object, spectrophotometers haveone of two types of illumination optics: sphere or45/0 (“forty-five zero”). Illumination optics are selected toinclude or exclude the specular component. Thecolors of roughened surfaces of some textiles (flannel),paints (sand paints), and plastics (textured polysty-rene) depend on the angle at which they are viewed.

Using 45/0 viewing delivers a good approximationof the typical visual evaluation technique. 45/0includes the effect or appearance of gloss and texturein the measurement. This appearance measurementsimulates how the human eye would see a sample.

The 0/45 design is based on the set-up of a visualevaluation where the incident light is overhead at0 degrees and the viewing angle is 45 degrees(or the optics are reversed as in 45/0). It has beenshown through experimentation that the same resultscan be achieved with 45/0 as with 0/45. An object canbe evaluated with the light source directly overheadand viewed at 45 degrees, (preferred), or the sameevaluation can be obtained by illuminating an objectat 45 degrees and viewing directly overhead.

An advantage of 45/0 instrument design is that it canprovide annular (circular) illumination of the object.

rela

tive

outp

ut

1.5

1.0

.50

0

300 nm 500 nm 600 nm 700 nm

tungstenhalogen

pulsedxenon

2.0

Pulsed Xenon vs Tungsten Halogen

3.3

This eliminates the large variations that can occurwhen measuring directional objects such as corduroy,brushed aluminum and textured plastics. Both 0/45and 45/0 viewing allow for a product to be evaluatedonly for appearance. The diffuse reflectance or scat-tered light is measured, while the specular or glosscomponent is excluded from the measurement.

Annular illumination optics light the object at45 degrees in a complete circle. To visualize this,imagine a cone whose apex angle is 45 degreesto the cone’s axis. The object is placed at the tipof the cone. The sides of the cone represent lightstriking the object. The object is “viewed” by thespectrophotometer along the cone axis, throughthe base of the cone.

If an object has a specular component of reflection,it will be excluded by 45/0 illumination automatical-ly. Why? Light that is reflected specularly will bereflected at the same angle as it strikes the object.With 45/0 illumination optics, light that is reflectedaway at 45 degrees will not be “viewed” by thespectrophotometer.

Integrating Sphere OpticsD8 geometry uses an integrating sphere with diffuseillumination and an 8 degree viewing angle. Theintegrating sphere is a hollow ball that is coated witha highly reflective white substance. The object ispresented to the instrument by placing it in front ofthe viewing port opening located at the front of theinstrument. The light source illuminates the spherethrough another port and is reflected from the spherewalls to illuminate the object from all directions. Thisdiffuse illumination minimizes the effect of objecttexture and sample directionality. The light reflectedfrom the object exits through a third viewing portat 8 degrees from an axis that is perpendicularto the object.

Light reflects away from a surface at an angle that isequal to its incident illumination. Sphere geometryhas the unique ability to include or exclude the spec-ular component from the measurement. To excludethe specular component, a small piece of the spherelocated at 8 degrees opposite the viewing angleis removed; any specular reflection will simplybe a reflection of a very dark hole, and thereforewill not be included in the reading.

The specular port is located at 8 degrees oppositethe viewing port. When this port is closed, thespecular component of the light source (relativeto the sample) is included in the measurement.This is known as Specular Component Included(SCI) and is a true color measurement. SCImeasures the total reflectance, which equalsthe diffuse plus the specular reflectance.

Specular Component Included measurement:

• Includes all angles of illumination, ignoringsurface characteristics.

• Is independent of object surface characteristics(gloss or texture).

• Is taken with the object positioned flush with thesample viewport.

• Measures “true color.”

• Is used widely for color matching.

The specular exclusion port can also be openedwhen taking measurements. When this port is open,the specular reflection from the object is eliminatedfrom the measurement. This measurement excludesthe specular component and correlates well withvisual evaluation of the object’s surface. This isknown as Specular Component Excluded (SCE).SCE measures the diffuse reflectance only and issimilar to the 45/0 design.

Specular Component Excluded measurement:

• Is similar to 0/45, depending on gloss level.

• Characterizes the effects of an object’s surface.

• Measures color and appearance.

• Can be compared to SCI measurements forestimation of gloss.

The comparison of SCI and SCE measurements ofthe same object estimates the appearance effects ofgloss. A difference of approximately 4% between thetwo measurements is equivalent to approximately 100gloss units; 2% would be 50 gloss units, and so forth. Thismethod only estimates gloss. The use of a glossmeter isstill the best way to characterize an object’s gloss.

Diffraction GratingLight that reflects from an object into the “viewing”optics (measuring optics) is a mixture of variouswavelengths. Before spectral reflectance can be mea-sured, this light must be divided into its individualspectral components. Mixed light that is shined ontoa diffraction grating is refracted as a spectrum (as ifit were shined through a prism). This is a very pre-cise way to divide white light into its component col-ors. Another term often used for diffraction gratingis monochromator.

Detector ArrayThe detector array is a row of photodetectors. Themonochromator projects the spectrum onto this ar-ray. Each photodetector senses the quantity of lightenergy at each measurement point in the spectrum,and then records it as an electronic signal.

3.4

Illumination Optics

light source

specularexclusion port

8˚

specimen

integrating sphere

0/45 illumination

light source

SCI

specimen atreflectance port

specimen atreflectance port

specular exclusionport open

SCE

45/0 illumination

sphere optics

diffraction grating

detectors

diffractiongrating

detectordiffraction

grating

light source

light source

8˚8˚

8˚8˚

light source

3.5

MicroprocessorThe microprocessor is a tiny computer inside thespectrophotometer. This computer takes the energyreadings from the detector array and calculates whatpercent reflectance each amount of energy represents.

A spectrophotometer calculates the ratio of light re-flected or transmitted from an object, wavelength bywavelength across the entire visible spectrum relativeto the values of a white ceramic calibration tile withknown absolute reflectance values. Data collected areknown as spectral data. The spectrophotometer cancalculate color values based on any combination ofobserver and illuminant using these spectral data.It is the spectral data that give a spectrophotometerversatility in all its applications, including color for-mulation, batch correction, shade sorting, trend analysis,pass/fail analysis, and strength calculations.

Many current spectrophotometer designs use apulsed xenon light source in conjunction with a dif-fraction grating or monochromator, detector array,and a microprocessor.

Sphere-BasedSpectrophotometer OperationIn a sphere-based spectrophotometer, the light reflectedfrom the object leaves the viewport and is analyzed.Reflected or transmitted light contains various wave-lengths of energy, based on the light source and theselective absorption properties of the object itself.The diffraction grating (or monochromator) functionslike a prism and separates light into its componentcolors. The spectrum is projected onto a detector ar-ray that distinguishes quantities of energy acrossthe entire visible spectrum. The detectors measure 31or 16 data points across the spectrum (at intervals of10 or 20 nanometers respectively). The microprocessor,a small computer in the spectrophotometer, calculatesthe percent reflectance at each interval.

The sphere design allows for some unique measure-ment techniques for the evaluation of objects. Theseinclude transmittance measurements, UV measurementsand adjustable area of view.

Transmittance measurements can be made by placingan object between the integrating sphere and thediffraction grating. This makes it possible to mea-sure liquids and translucent or transparent materialsin much the same way as the eye perceives the colorof light that is transmitted through them.

Transmittance is expressed as a percentage, rangingfrom 100% (where material transmits all light strikingit) to 0% (where it absorbs all light striking it). For

example, a deep red filter may transmit 80% in thered region of the visible spectrum, but only transmit1% in the blue and green regions.

If the light source used has ultraviolet energy, ablocking filter can be used to eliminate ultravioletenergy from entering the sphere (UV excluded) andthereby exclude it from the measurement. When thisblocking filter allows the ultraviolet energy into thesphere (UV included), the ultraviolet energy is partof the measurement. The effects of optical brighten-ers, whitening agents, fluorescent dyes, or pigmentscan be seen by comparing UV excluded and UV in-cluded measurements of the same object.

Some instruments can vary the area of object mea-sured. Small Area View (SAV) can be used for smallsolid objects or curved surfaces. Large Area View (LAV)is preferred when measuring textured surfaces suchas textiles, foods or multicolor objects. Several in-struments are equipped with an intermediate Medi-um Area View (MAV), and Very Small Area View(VSAV). SAV and VSAV measurements are best usedwhere measurement of a small area or highly curvedsurface is desired. Measurement repeatability shouldbe checked when the sample is not a solid color orwhen measuring highly curved surfaces.

Some instruments are able to make adjustingmeasurements to compensate for instrument drift.Many sphere-based spectrophotometers in usetoday are pseudo-dual beam instruments. Theseinstruments use a beam switch to measure the objectand an independent reference measurement of thewall of the sphere. A true dual-beam instrumenthas a separate reference light source that providesa simultaneous measurement of the object and thesphere wall. True dual-beam technology increasesinstrument accuracy and prevents drift. TheGretagMacbeth Color-Eye® 7000 Series offerstrue dual-beam spectrophotometry.

Spectrophotometer Performance IssuesColor spectrophotometer performance dependson several factors, including the initial instrumentcalibration. The instrument comes with a calibratedwhite ceramic standard for reflectance measure-ments. This is the permanent reference material thatis used for calibration according to a manufacturer’sspecifications. Never mix calibration tiles amonginstruments, or use any other tiles for calibratingthe instrument.

The performance of the white calibration standard iscritical to overall instrument performance. In mostcases, the white ceramic material used to make thestandard is stable and reproducible. Operators shouldexercise care in keeping the tile clean and free fromsurface defects such as scratches and chips.

3.6

For transmittance measurements an air blank is usedfor calibration, and in many cases the white calibra-tion tile itself can be applied to “complete” thesphere at the viewport.

During calibration, the instrument measures the energyratio of this calibration tile (that is, the reflected ener-gy versus incident energy) at each wavelength and as-signs the known reflectance values stored in memoryto these ratios.

Calibration is a necessary part of good measurementprocedure. Calibration is recommended:

• At least every eight hours.

• Every time the instrument power is turned on or interrupted.

• When you select a different measurement mode (reflection or transmission).

• When changing from SCI to SCE.

Calibrating more frequently will do no harm. Checkthe instrument operation manual to determine howoften you should calibrate your spectrophotometer.

It is important to consider instrument performancewhen selecting a spectrophotometer. Repeatabilityand interinstrument agreement are two issues thatare typically considered when purchasing an instrument.

Repeatability refers to the instrument’s ability to pro-vide the same measurement data each time it readsthe same object.

Interinstrument agreement refers to how well themeasurement data from the instrument agree withdata from other instruments of the same type andsetup. The chart below represents repeatability andinterinstrument agreement specifications for someGretagMacbeth spectrophotometers.

Innovations inSpectrophotometers

GoniospectrophotometersThe appearance of metallic and pearlescent materialsis difficult to measure consistently because it varieswith the angle of illumination and viewing. No singlegeometric conditions can characterize color variationsof these surfaces accurately.

Effect surfaces can be characterized with measure-ments at several angles away from the speculardirection (aspecular angles). Measurements at multipleaspecular angles, e.g., 20, 45, 75, and 110 degrees,identify the variations in color representative ofsample appearance.

Goniospectrophotometers are available in both porta-ble and benchtop models. They provide multianglemeasurements of effect surfaces such as metallic andpearlescent finishes, grained plastics and anodizedaluminum. Goniospectrophotometers are widely usedin automotive applications, since many of the newautomobiles feature metallic or pearlescent finishes.They are also a valuable tool for automotive refinishapplications. Since goniospectrophotometers canproperly characterize metallic finishes, they eliminatethe need to repaint an entire car in order to repairone body section.

Some goniospectrophotometers utilize a singlelight source multiangle viewing system. This meth-od of viewing has a single light source and placesthe observer at various angles along the viewingpath. This requires that the light source be on whileseveral different measurement conditions change.Measurement time can be extensive. During themeasurement time, the operator must remainperfectly still or the data will be affected.

Other goniospectrophotometers utilize a multiplelight source multiangle measuring system. TheGretagMacbeth Auto-Eye® 640 series of goniospec-trophotometers are a good example of a multiplelight source, multiangle measuring system. Thepulsed xenon illumination and pressure sensorsensure accurate sample positioning. Measurementtime is extremely short. This “stop motion” measure-ment virtually freezes the sample so movementis not an issue. Good interinstrument agreementis achieved by a three-point triggering system thatensures a uniform measurement plane.

Since goniospectrophotometers are typically usedin automotive manufacturing and automotiverefinish, other features to look for which will ensuremeasurement accuracy include temperature sensorsthat identify when samples are out of the tempera-ture range and an automatic alert for recalibration.

GretagMacbeth SpectrophotometersInstrument Geometry Repeatability Interinstrument

Agreement

Color-Eye 7000A D/8 0.01 0.08

Color-Eye 3100 D/8 0.02 0.12

Color-Eye 2180 D/8 0.04 0.12

ColorChecker® 545 45/0 0.04 0.12

Color-Eye 2145 45/0 0.04 0.12

3.7

Goniospectrophotometer Designs

sample

25˚45˚

75˚

110˚

specularangle

light source

diffusereflection

specularreflection

single light source

multiple light sourcesample

video dataA/D

converter

graphics display

buttons

fiber optic

element detector array

microprocessorand memory

xenon lamp

25˚45˚

75˚

110˚ note: all anglesare relative to thespecular direction

lens

virtual specular direction(direction in which thesystem is looking)

holographic grating

xenon lampxenon lamp

xenon lamp

3.8

Glossmeter InstrumentsSince it is not possible to produce an equivalentgloss stimulus, no standard observer for gloss hasbeen developed. Surface gloss is referenced to howa perfect mirror reflects light. In the real world,most surfaces are less than perfect. Specularreflection associated with gloss can vary from oneobject surface to another. Although not easilyperceived, gloss can vary across the surface of thevery same object.

Gloss measurements should not be confused withpercent reflectance measurements that are per-formed with a reflectometer. Reflectance measure-ments identify how efficiently a surface reflects light.A reflectometric measurement is derived frompercent reflectance and is done by comparing theobject to a white standard (100% reflectance) and ablack standard (0% reflectance).

A variety of test methods can be used to measuregloss objectively. Discussion here is limited to simple,single angle, specular gloss and its measurementusing standard gloss meters.

Glossmeter DesignGloss values are stated in gloss units. Gloss measure-ment is a mathematical comparison of the surfacebeing measured to a defined standard, usually blackpolished glass, with a known gloss value. Measure-ment is done using a glossmeter. The geometryof these instruments is prescribed by national andinternational standards such as ASTM D523. Sinceglossmeters have a green filter located in frontof the light source, they are not sensitive to any colordifferences between objects. Light that is projectedonto the surface of the object at 60 degrees to thenormal (perpendicular to the surface) is the mostimportant angle used to characterize gloss. Reflectedlight at this angle is collected at the angle oppositeto the incidence, based on the principle that theangle of incident light equals the angle of reflectedlight. Light that is collected is analyzed usinga photodetector, and a gloss value is interpolated.With some glossmeters, a second detector is usedat the light source to correct for lamp aging anddifferences in filaments. This detector also indicateswhen a lamp change is required to ensure instru-ment performance and stability.

Gloss scales were originally calibrated to provide avalue of 1,000 gloss units by comparison to a perfectfirst surface mirror. It is possible to have gloss valuesgreater than 100 gloss units. Products, such aspolished aluminum, chrome plated parts andmetallic plastics, can demonstrate gloss values of 200to 300 or higher.

ASTM D523 Test Methodfor Specular GlossLengthy visual evaluations (primarily paint objectevaluations) performed in the late 1930s demon-strated that the 60 degree angle provides the bestoverall estimate of gloss. The 60 degree methodwas adopted by the American Society for Testingand Materials (ASTM) in ASTM D523 (last revisedin 1989). Today, this method is more widely usedthan any other gloss test procedure. ASTM D523Standard Test Method for Specular Gloss coversthe measurement of the specular gloss of non-metallic specimens for gloss geometry of 20,60 and 85 degrees.

A major problem with the use of industrial glossinstrumentation is the misuse of various geometries.Three separate gloss angles were selected to providethe best correlation with visual gloss evaluation.

ASTM standard D523 clearly defines the instrumentgeometry to use:

• The 20 degree geometry is used for high glossfinishes (advantageous for comparing specimenshaving 60 degree gloss values higher than 70gloss units).

• The 60 degree geometry is used for comparingmost specimens and for determining when the 20or 85 degree geometry may be more applicable.

• The 85 degree geometry is used for comparingspecimens for sheen or near grazing shininess. It ismost frequently applied when specimens have 60degree gloss values lower than 10 gloss units.

4.1

Visual Color EvaluationBy definition, visual color evaluation will always besubjective. Standardization is extremely importantwhen attempting to communicate color results withincompanies or to suppliers and customers. Once standardconditions for visual evaluation have been establishedand a standardized language for communicating whatis seen can be agreed upon, visual color evaluation can bea very effective way to perform color quality control.

Even when instrumental color quality control programsare used, methods for visual color evaluation needto be established. Visual color evaluation will:

• Ensure that there is a correlation between whatthe instrument and software calculate versus whatthe eye actually sees.

• Serve as the final judge when any discrepancy arises.

ASTM D1729-89The American Society for Testing and Materials (ASTM)provides guidelines to establish standardized conditionsfor visual evaluation of color difference of opaquematerials in standard D1729-89. ASTM D1729-89uses the term color difference to relate a target colorto a color from a production run. Color differenceis what people consider most important when they arejudging acceptable product appearance.

Establishing Your VisualColor Evaluation ProgramIn establishing your own visual color evaluationprogram, you should account for all the factors thatrelate to the observer situation (light source, objectand observer). The key elements that should beaddressed are:

Light Source Selection — To obtain consistentresults, everyone who judges product color should beviewing these products using the same light source.

Viewing Conditions — Viewing angle, backgroundand surround must also be controlled for accurateand reproducible color comparisons.

Color Standards and Sample Preparation —The importance of good color standards cannot beoverstated. Consistent results can be obtained onlywhen everyone is comparing to the same standard.

Color Vision Testing — While you cannot controlhuman color vision, you can test individuals todetermine their ability to see small color differences.

Metamerism Evaluation — Since many coloredmaterials are used under various light sources,it is important to evaluate them under multiplelight sources to detect for metamerism.

Communicating Color Differences — In order tocommunicate color differences effectively, a commonlanguage is needed.

Light Source SelectionConsider these factors when you select a light sourceto view a color:

• If you need to communicate color and appearancewith others (other manufacturing plants, suppliers,customers, etc.), make certain that you all use thesame light sources.

• If you are following an established procedure orstandard method, choose the specified light source(s).

• If you are free to choose your own light source(s),choose a light source that fits your needs. For choosinglight sources, the most important question to answeris, “Under what lighting conditions will my productor materials be viewed?”

Because so many different light sources are used inindustry, it is generally recommended that you evaluatecolor under two or more light sources. Ideally, youshould select two very different sources, and one ofthe sources should be artificial daylight. Productslike the GretagMacbeth SpectraLight® and TheJudge® are designed to allow you to evaluate colorsusing multiple sources.

Section 4:Visual Quality Control

4

Visual Quality Control

4.2

Common Light Sources

Daylight D75 Horizon Daylight

Cool White Fluorescent Illuminant A (Incandescent)

Natural Daylight SourcesDaylight is the preferred source for visual evaluationbecause it is visually more correct than other lightsources. Other light sources tend to influence ourcolor judgment. North Sky Daylight was originallychosen because it is more consistent year round.This means a person can make color evaluationsusing light that enters a room from a north-facingwindow (south-facing in the southern hemisphere)at any time of year. The choice of North Sky Day-light was validated by industries that are required tograde and sort whites and off-whites, such as textile,paper and graphic arts products. The blue sourcefound in North Sky Daylight makes it easier to seesubtle differences in whites, off-whites and yellowprinting inks.

Artificial Daylight SourcesWith the advent of viewing booths, light sourcesthat simulate daylight are preferred for visual colorevaluation since they are more consistent than theirnatural counterparts. Furthermore, natural daylightis not available at night or in interior rooms. Thechoice of a specific daylight source is based on

recommended industry practices or internationalstandards that pertain to a specific product orapplication. The abbreviations D50, D65, and D75are used to designate the phases of daylight that arerecognized by the CIE.

North Sky Daylight at 7500K (D75) is the lightproduced from a moderately overcast sky as onefaces north in the northern hemisphere (south inthe southern hemisphere).

Average North Sky Daylight at 6500K (D65)conforms with international standards in Europe,the Far East, and South America. It is also used toprovide correlation with instrumental measure-ments. The Detroit Color Council, in conjunctionwith the SAE (Society of Automotive Engineers), hasrecently adopted the use of D65 for visual evaluationof automotive interiors and exteriors.

Noon Sky Daylight at 5000K (D50) is the desiredlight source for performing color quality anduniformity evaluation in the graphic arts industry,as specified in ANSI standard PH 2.32 and ISOstandard 3664.

4.3

Fluorescent Light SourcesFluorescent light sources are commonly found in officebuildings, factories, stores and increasingly in the home.Cool White Fluorescent (CWF) at 4150K is a commonwide band fluorescent light source that simulates USoffice or store lighting. Other available sources includeTL84 at 4100K, a commercial narrow band fluores-cent used in Europe and Ultralume 30 at 3000K, acommercial narrow band fluorescent used in the US.

Incandescent LightSources (Illuminant A)Incandescent and tungsten light sources are com-monly found in homes and used as accent lightingin stores. Illuminant A at 2856K is used to simulatethese environments.

Horizon DaylightHorizon Daylight at 2300K is provided by usinga tungsten halogen lamp operated at half power. Itprovides the light quality that is found in early morn-ing sunrise or late afternoon sunset. If you can getan acceptable color match using Horizon Daylight(when the sky is the reddest) and North Sky Daylight(when the sky is the bluest), you will probably have agood match in any phase of daylight (all day long).

Viewing ConditionsWhen performing visual color evaluation it is criticalto control the conditions associated with viewinggeometry and sample surround.

Viewing GeometryTo obtain consistent results when performing visualcolor evaluation, it is important to control both theviewing angle (direction from which object is viewed)and the incident angle (direction from which lightstrikes the object).

ASTM D1729-89 recommends the use of 0/45 geometry.In this configuration, the object is laid flat on thetable or in the viewing booth and light strikes it fromdirectly above. The viewer observes the object bylooking down at an angle of 45 degrees.

An easy way to ensure that the viewing angleis maintained at 45 degrees is to make sure thatthe distance from the object to the observer is equalto the distance from the surface to the observer’seye. This is shown in the second figure to the right,“maintaining proper viewing geometry”.

Viewing Geometry

0/45 viewing geometry

observer

surface

45˚

light source

90˚

a

a

maintaining properviewing geometry

observer

object

45/0 viewing geometry

observer

surface

45˚

light source

4.4

Equivalent results can be obtained when lightstrikes an object at 45 degrees and is viewed from0 degrees (45/0 geometry). In this situation, lightoriginates from above the observer’s head whilethe object is tilted 45 degrees away from the person.The observer views the object at an angle of 45degrees away from the light source. Using 45/0viewing geometry is subject to error. Commonerrors that occur are:

• The sample is mounted on a stand in the viewingbooth to maintain the 45 degree angle accurately.However, this does not take into account observersof varying heights. A tall observer views the sampleat a different angle from a short observer.

• The observer tilts the object by hand to create the45 degree angle, while maintaining the proper view-ing angle. It is difficult for the observer to ensurethat the object is actually tilted at 45 degrees andthat the sample is being viewed at 45 degrees fromthe light source.

Neutral SurroundsNeutral surrounds should be used for color evalua-tion to eliminate phenomena such as simultaneouscontrast, chameleon effect and complementaryafterimage. Neutrals are gray, white or black. Colordifferences are more easily detected when the back-ground color is the same value (lightness or darkness)as the color being evaluated. Subtle differences inwhite or off-white colors are best perceived on a

white background, while differences in intermediatecolors are best perceived using a neutral gray back-ground. Dark and high gloss samples require a flatblack or black velvet background to detect faint dif-ferences. Viewing booths should use a matte grayneutral surround with a Munsell notation betweenN 5/ and N 7/.

Metamerism EvaluationMetamerism occurs when a sample pair appears tomatch under one set of viewing conditions, but notunder another. The samples are metamers and thematch is called a metameric match. Metamerism isvery common. We have all experienced metamerism,but may not have known the technical term for it.A metameric match can be determined by a visualevaluation of samples in a viewing booth equippedwith multiple light sources.

Metameric Pairs have the same color coordinates(XYZ values) for a given set of viewing conditions,yet have different spectral reflectance (or transmittance)curves. In spectrophotometry, metamerism can beidentified when the reflectance curves of two samplescross over one another three or more times. In thisinstance, they will provide a conditional match, mean-ing that under a specified set of viewing conditionsthey will not match.

Metamerism Evaluation

samples do notmatch underother lightsources, i.e.,fluorescent

=

refle

ctan

ce %

75

50

100

reflectance curves of a metameric pair

fluorescent

400 nm 500 nm 600 nm 700 nm

25

0

daylight

= samples appearto match underone light source,i.e., daylight

4.5

Observer Metamerism can also occur when samplesthat appear to match to a group of observers, do notmatch to an individual observer. In reality, thisindividual could have slightly variant color vision.This situation becomes even more difficult when thisindividual is the customer. Color vision and discrimi-nation tests provide some insight into the differencesbetween observers.

Geometric Metamerism is exhibited when a pair ofsamples match at one angle of illumination, but donot match when the angle of illumination or viewingangle is changed. This situation often occurs withmaterials that are directional such as velvets, suedes,broadlooms, and plastics. Consistent sample prepara-tion is critical for proper evaluation of these materials.

Color Standardsand Sample PreparationTo make visual color evaluations, a trial (batch) mustbe compared to a target, or standard, as it is commonlycalled. A physical standard is a piece of material thatexhibits an acceptable color to all concerned parties.

The color standard should represent the “ideal” color.Many times a customer submits a master standard toa vendor. The vendor produces a trial for approvalas the working standard. Agreement of acceptabilityis critical in determining color limits or tolerancesfor a product. Since visual assessment is the ultimatejudge of acceptability, it is important that the vendorestablish good communications with the customer.

The Ideal Color StandardPhysical specimens are affected by a variety of factorssuch as light, heat, age and other ambient variables.Therefore, it is important to consider the followingcharacteristics when selecting a standard.

• Stability: How is the standard affected by light,temperature, heat, age, and other ambient variables?

• Reproducibility: Can the standard be reproducedby the manufacturing process, or does it representa one-time-only color?

• Availability: If the standard is damaged or lost,can it be replaced?

• Consistency: How consistent is the color standardfrom run to run?

• Uniformity: Does the standard appear uniform?Does its uniformity impact color measurement?

• Representation: Does the standard accuratelyrepresent the product color? Surface texture?Gloss? Angular dependency attributes (e.g., metallics,pearlescents)?

Color Standard StorageTo maintain your color standards, it is importantthey are stored properly. The following generalguidelines should be followed.

Protect standards from exposure to light, heat andairborne contamination. Paper specimens are commonlystored in airtight black plastic bags in a freezer. Besure the selected storage method will not deterioratethe sample. For example, some highly acidic paperscan react with the standard, and plastic bags thatcontain plasticizers can alter standards.

Create duplicates of the standards (when possible) bydividing a single sample into swatches or producingmultiple standards within the same production run.Store these samples under controlled temperatureand humidity.

Sample PreparationSome of the factors to be considered when evaluatinga specimen (standard or trial) are:

• Directionality• Opacity, translucency or transparency• Photochromism• Thermochromism• Fluorescence• Surface

— uniformity— texture— smoothness— flatness and curvature— thickness— backing

• Surround• Pressure or tension• Size and overall shape

This list is not complete. It is intended to encourageyou to investigate the physical aspects of your materialsso that you can better interpret both visual andinstrumental color and appearance assessments.

Directionality occurs if the appearance or colordata changes when the sample is rotated 90 de-grees. If this occurs, sample orientation is criticalfor proper evaluation. Products that demonstratedirectionality include textiles, extruded plastics,aluminum and many coatings.

4.6

Opacity is determined by holding the samples inquestion up to a bright light source. If light passesthrough these samples, they are not opaque. If asample is not opaque, a substrate should be used toback the sample prior to visual evaluation.

Photochromism is a reversible change of a material’scolor that is observed when it is exposed to light.Materials that demonstrate photochromism includevarious pharmaceuticals, plastics and cosmetics.These products must be evaluated using consistentlevels of illumination or measurement results will vary.

Thermochromism is a change in color that resultsfrom a temperature change. Colorists who work withmolded plastics, ceramics, glass and baked-on coat-ings wait for samples to cool to room temperaturebefore making a visual color evaluation. Colors thatexhibit thermochromatic shifts include many yellows,oranges and reds.

Fluorescence occurs when optical brighteners,whitening agents, and fluorescent dyes or pigmentsare added to products in an attempt to enhancetheir whiteness. Fluorescent agents absorb energyin the ultraviolet region of the spectrum and emitthis energy in the blue region of the visible spec-trum. The effect of such fluorescent agents makeswhite and yellow-white products appear whiter.Fluorescent colorants (dyes and pigments) are addedto produce more brilliant colors. This include materi-als such as Day-Glo*. Optical brighteners, whiteningagents, fluorescent dyes and pigments are found in

many textiles, plastics, and paper products. Fluores-cent agents can be detected visually by using a view-ing booth that is equipped with an ultraviolet sourcesuch as that found in The Judge II or SpectraLightII from GretagMacbeth.

Visual TolerancingWhen customers make color decisions, they are doingso based on the color difference between the stan-dard and the trial submitted. (The observer shouldhave the standard and trial touching when makingthis evaluation.) Rarely does a trial perfectly matchthe standard.

The customer’s ability to observe a color differenceis called perceptibility. The limits of acceptability(maximum acceptable difference) will hopefullybe greater than those of perceptibility (minimumperceptible difference). Color difference acceptabilityis not absolute and is entwined with the psychologyof color perceptions. To further complicate matters,the customer making the color decision may be affect-ed by cost considerations, time of year, or a numberof other factors. Munsell produces specialized colorcharts to help companies establish visual tolerancesfor their colors.

Munsell® Color Tolerance Sets for visual color con-trol specify a target (or centroid) and two limits (toler-ances) for each of the three dimensions of color (i.e.,two hue limits, two value limits, and two chroma lim-its). The chart is arranged with a centroid color in thecenter surrounded by six limits. Apertures (windows)between colors facilitate comparison to color of prod-ucts, packaging or specimens.

Munsell color tolerance sets allow you to determineif a trial falls within an acceptable range (volumeof acceptability). Even when color is controlled byinstrumental measurements, a color tolerance setis a useful aid in visualizing limits of acceptabilityand reaching a clear understanding among buyers,sellers and producers.

Evaluating Color VisionNormal color vision is the prerequisite for individualsmaking color judgments. However, it is not unusualto find individuals making color decisions who havenever had their color vision tested. With the useof color computer systems, there is a tendency todownplay the need for color vision testing. However,the eye still makes the final color decision in mostindustries. Even when using color measurement

Munsell color tolerance set

color tolerance set

Visual Tolerancing

targetcolor

4.7

instrumentation, the intent is to obtain meaningfulnumbers that agree with the human eye-brain decision.

Color Vision and ColorDiscrimination TestsSeveral color vision and color discrimination testsare available to evaluate individual color perception.Standardized lighting at the recommended colortemperature (usually illuminant C) is critical for adminis-tering these tests. Errors in color judgment mayresult if controlled viewing conditions are not used.

Dvorine Book of Pseudoisochromatic Plates(Harcourt Brace Jovanovich, Inc.) are designed so thenormal observer will see two numbers on each plate.Red-green deficient observers see a group of unre-lated dots. These systems are not suitable for quanti-tative color vision testing and are limited in theirability to identify the type of color deficiency.

Farnsworth-Munsell 100 Hue Test (availablefrom GretagMacbeth) offers a simple method fordetermining color vision abnormalities and testingcolor discrimination. The observer is required toarrange 85 samples, divided into four groups, inconsecutive color order. This test makes it possibleto sort observers with normal color vision accordingto their level of chromatic discrimination. Thissystem was not designed to be a color aptitudetest. However, it may be used with other teststo corroborate evidence of color deficiency.

Inter-Society Color Council Color MatchingAptitude Test (discontinued in 1988) requires anobserver to match 48 chips with very small differenc-es in saturation or chroma in four hues. This test wasdesigned to identify individuals with superior colordiscrimination skills.

Japanese Color Aptitude Test (Japanese Color Re-search Institute) extensively evaluates an observer’scolor aptitude. It provides tests in Color Naming,Color Memory, Recognition of the Three ColorAttributes (Hue, Value and Chroma) and Ability toDistinguish Small Color Differences. This test requirestwo days to complete all five sections. It involves a largenumber of samples and cannot be self-administered.

Farnsworth Lantern (available from GretagMacbeth,and also known as the Navy Lantern) was designedto screen the more severe forms of red-green colorvision defects. It is used by all branches of the armedforces of The United States and Canada. Test lightssubtend very small angles to simulate distant signallights. The observer is required to identify randomtwo-light combinations of red, green or white.

HVC Color Vision Skill Test (Lou Graham andAssociates, Greensboro, NC) requires the observerto discriminate very small color differences in fourhues. The color differences are below accepted toler-ances for most industries. The objective is to categorizeobservers based on levels of superiority. This systemwas developed to replace the ISCC Color MatchingAptitude Test.

CommunicatingColor InformationIt’s one thing to be able to see color differences.It’s another thing to be able to communicate them.For example, what does “It needs more punch”mean? An understanding of Munsell’s three attributes— hue, value and chroma — is a critical first stepin being able to communicate color differences.Munsell offers a variety of educational materialsto help teach the Munsell system, including theMunsell Student Color Set that teaches the systemby having students arrange colored chips accordingto the Munsell system.

Hue is generally communicated in terms of redder,yellower, greener or bluer. This is based on how thehue of the color being viewed compares to the standard.It depends on where the hue falls on the Munsellhue circle. For example, a red object might be yel-lower than the standard or it might be bluer. (Itwould have little meaning to describe one red objectas redder than another.) A yellow object could beeither redder or greener than a yellow standard. Agreen object could be either yellower or bluer than agreen standard, and a blue object could be eithergreener or redder than the standard.

Value is almost always communicated using the termslighter and darker.

Chroma is often described as more saturated or lesssaturated. Common terms used to communicatechroma differences are stronger and weaker. Which-ever terms are chosen to describe chroma, it is bestto have everyone use the same terms. Avoid usingterms like dirtier, cleaner, punchier.

Standardizing the language that is used to communi-cate color difference cannot ensure that two viewerswill see color differences the same way. However, it doesminimize the error caused by a miscommunication.

4.8

To add more meaning when describing color differ-ences, use adjectives that indicate the magnitude ofthe difference, such as moderately or slightly. Forexample, a red sample might be slightly yellower,moderately lighter and slightly more saturated thanthe standard.

To make sure different observers are using quantifiableterms like moderately or slightly in the same manner,it is best to create a few sets of samples of differentcolored objects and label them based on whether thedifferences are considered slight or moderate. Ideally,the samples used for this exercise should exhibit colorchange in only one direction (for example, value).This can be done by collecting different trials over aperiod of time.

Visual Color EvaluationDocumentationThe best way to ensure that everyone is followingyour standard visual color evaluation method is toput it in writing. This written method should becommunicated to customers, suppliers and anyoneinvolved in making color decisions at your company.

A sample outline for a visual color evaluation methodis given below to encourage the use of similar methodsfor individual product applications or company-widestandardization and communication.

bluer thanstandard

Communicating Color Information

yellower thanstandard

changes in hue

standard

darker thanstandard

lighter thanstandard

changes in value

standard

stronger thanstandard

weaker thanstandard

changes in chroma

standard

Visual Color Evaluation Method

Viewing Conditions

Lighting Product: GretagMacbeth SpectraLight II with UV

Primary Light Source: D65 Filtered Tungsten

Secondary Light Sources: Cool White Fluorescent,

Illuminant A

Viewing Geometry: 0/45

Surround: Munsell N 7/

Color Standards & Sample Preparation

Sample Size (standard and trial): 3" x 5" (desired)

Sample Orientation: Side-by-side, always touching

Color Standard Storage: Filed in nonacidic envelope

Color Tolerance: Munsell Color Tolerance Set

Color Analysis

Color Vision Testing: Farnsworth-Munsell

100 Hue Test

Color Communication: Based on Munsell

Hue Terms: Redder, Yellower, Greener, Bluer

Value Terms: Lighter, Darker

Chroma Terms: Stronger, Weaker

Modifiers: Slightly, Moderately, Much

5.1

Consistency inColor CommunicationConsistency means doing things the same way eachtime, and this is important for effective color commu-nication. We recommend that you establish a practiceand, if possible, do not change it. Consistencyis important in many aspects of color measurement.

Establishing Your InstrumentalQuality Control ProgramIn establishing your own instrumental color qualitycontrol program, many factors need to be addressed.If possible, you should standardize your choices.However, in some cases you many need to use differ-ent options for different customer requirements orunique samples. Be aware that when you do this, thereadings obtained with different options are notnecessarily comparable.

Color Standard Selection — Determine whethera physical or numerical standard will be used. In theselection of a physical standard, consider the following:

1. What specific size limitations or requirements arethere for the standard? For example, if you measuretrials or batch samples using the small area viewplate, should you consider using a small standard?

2. What specific shape requirements are there forthe standard? Curved? Textured?

3. How thick is the standard material?

4. Is the material opaque, translucent or transparent?

5. If the standard is expendable, will you have toproduce a quantity of acceptable standard material?

6. Can you substitute a more stable material to beused as a standard? For example, could you substitutea ceramic standard for a high gloss paint standard?

Consider the advantages and disadvantages of thephysical or numerical standard before deciding which

one to use. These advantages and disadvantages willbe dependent on, but not limited to, the standards,instruments, procedures, and processes you use.

Sample Preparation — Determine a sample prepara-tion method that is appropriate for your application.

Instrument Selection and Configuration —Determine what type of instrument is best for yourapplication (sphere or 45/0, 10 nm or 20 nm, trans-mission or reflection, etc.). Once you have made yourselection, determine what instrument configurationis appropriate for your application (large or small area of view, specular component included or excluded,UV included or excluded, etc.). For more informationon the options available, see the discussions in SectionFour on spectrophotometers and sphere-basedspectrophotometer operation.

Mathematical Configuration — Determine whichmathematical configuration (illuminant, observer,color difference formula, indices, etc.) is appropriatefor your application and use that configurationexclusively, if possible.

Color Standards andSample Preparation

Color Standard SelectionWhen using an instrument, you can work from aphysical or instrumental standard. In both cases,make certain that the color standard truly representsthe desired product color and can be achieved in themanufacturing process.

Physical StandardsIf a physical standard is used, make certain that thecolor standard meets the guidelines that we recommendin Section Three.

Measure the physical standard using the spectropho-tometer that best meets your needs and record themeasurement in a data log (in duplicate when possible).You should also enter the standard in the colorimetric

Section 5:Instrumental Quality Control

5

Instrumental

Quality Control

5.2

software package of your choice. After you havecompleted your entries of standards, and beforeturning off the computer, backup your database.

Check the standard regularly against the numericaldata to monitor drift in color or appearance. If astandard begins to deteriorate, replace it. Makecertain that you contact vendors to advise them onstandard replacement procedures.

Advantages of Physical StandardsIf the numbers are ever questionable, a trial can alwaysbe compared visually to the physical standard.

A physical standard decreases dependency on abso-lute agreement between color measuring systems,provided the systems have at least the same opticaldesign (geometry and spectral resolution).

Disadvantages of Physical StandardsPhysical standards may change over time due todeterioration of the material, or from constant han-dling (fingerprints, scratches, soil buildup, etc.),or they may be lost.

Depending on the material, it may be difficult toproduce or obtain several pieces (for standards atseparate sites) that match. At the heart of any suc-cessful color measurement program is a consistentmethod of specimen preparation and presentation.Individuals who make color measurements must beaware of how “less than ideal” the specimens are andwhat effect any preparation procedure may have onthe optical properties of the specimens.

Numerical StandardsA numerical standard is a set of reflectance valuesthat were obtained either from a physical standardor by deriving a standard. The reflectance valuesmay be stored on paper or electronically. Technically,standards in the database file are numerical stan-dards. Ideally, a numerical standard is defined byspectral data rather than color coordinates.

Advantages of Spectral DataElectronically stored numerical standards are easilyaccessible (just load them from the disk) and maybe communicated quickly and easily by telephone,fax, e-mail, etc. In addition, numerical standardsnever change over time, as could a physical standard.

Disadvantages of Spectral DataIf the numbers were obtained from a measurement,the numerical standard may not be valid on a system

different from the one on which it was first measured.Reflectance values should be traded only betweensystems with a high level of agreement.

Color coordinates typically require only threenumbers to define the standard (in CIELab: L*, a*and b*), versus 40 spectral data points. However,because the standard is specified for only oneilluminant, it is useless for measuring degreeof metamerism. In addition, the standard canbe used only on the system from which the originalvalues came, because absolute accuracy of all colormeasuring systems is poor.

Sample PreparationMost color measurement problems can be tracedfirst to inconsistent sample preparation and mea-surement technique.

Measuring your standards is an important point andshould be a fundamental part of your color qualitycontrol program when using color instrumentation.Once a working standard has been established, itshould be measured carefully. Color values shouldbe stored in a database for future retrieval and com-parison. This eliminates total dependence on thephysical specimen by establishing a fixed numericaltarget, set when the standard was at its optimum.An added benefit is the capability to monitor changein the working standard so that it can be replacedwhen necessary.

An average of two or more readings may be requiredto ensure repeatability between specimens (trials andstandards) and operators. In most applications, asingle reading will not represent the color accurately.This is a critical step in obtaining meaningful num-bers that agree with the eye.

Directionality is determined visually or by use of aspectrophotometer. In both cases, a sample is said tobe directional if its appearance or color data changeswhen the sample is rotated 90 degrees. If this occurs,sample orientation is critical for proper evaluation.Products that demonstrate directionality include tex-tiles, extruded plastics, aluminum and many coatings.

Opacity can also be determined visually or throughinstrumental measurement. Samples in question areheld up to a bright light source. If light passes throughthis sample, they are not opaque. If a spectrophotometeris to be used, transmission measurement is recommend-ed. If this is not possible, a substrate should be usedto back the samples prior to instrumental measurement.

5.3

Photochromism describes the phenomenon ofreversible change in color demonstrated by somematerials when they are exposed to light. Materialsthat demonstrate photochromism include variouspharmaceuticals, plastics and cosmetics. Theseproducts must be evaluated using consistent levelsof illumination.

Thermochromism is observed when color changesas a result of a change in temperature. Many industriesare aware of this phenomenon. Colorists who workwith molded plastics, ceramics, glass and baked-oncoatings always wait for their samples to cool toroom temperature before taking an instrumentalmeasurement. Colors that exhibit thermochromaticshifts include many yellows, oranges and reds.

Fluorescence occurs when optical brighteners,whitening agents or fluorescent dyes or pigmentsare added to products in an attempt to enhancetheir whiteness.

The effect of such fluorescent agents makes whiteand yellow-white products appear whiter. Fluorescentcolorants (dyes and pigments) are added to producemore brilliant colors. This include materials such asDay-Glo* additives. Optical brighteners, whiteningagents, and fluorescent dyes and pigments are foundin many textiles, plastics, and paper products.A spectrophotometer can be used to determinethe effects of fluorescent agents provided its lightsource contains ultraviolet energy and a filter that caneliminate these energies. Two measurements arerequired, one with the UV included and one withUV excluded. If fluorescent agents are present,the two measurements may be quite different.

Illuminant and Observer SelectionSimilar to visual evaluation, it is important to considerthese factors when you select an illuminant:

• If you need to communicate color and appearancewith others (other manufacturing plants, suppliers,customers, etc.), make certain that you all use thesame illuminants.

• If you are following an established procedure orstandard method, choose the specified illuminant(s).

• If you are free to choose your own illuminant(s),choose an illuminant that fits your needs. For choosingilluminants, the most important question to answeris, “Under what lighting conditions will my productor materials be viewed?”

Daylight IlluminantsLight sources that simulate daylight are preferredfor visual color evaluation because of the extremerange they cover, while their complementary illumi-nants are used for instrumental testing includingcolor matching and metamerism testing.

The two most common illuminants for simulatingdaylight are Illuminant C and Illuminant D65.They are essentially the same, but choose D65if possible (it is more current and can bereproduced as a light source).

The choice of a specific daylight source is basedon recommended industry practices or internationalstandards that pertain to a specific productor application. The abbreviations D50, D65,and D75 are used to designate the phasesof daylight that are recognized by CIE.

North Sky Daylight at 7500K (D75) is the lightproduced from a moderately overcast sky as one facesnorth in the northern hemisphere (south in thesouthern hemisphere). D75 is used for performingvisual evaluation of opaque materials as recom-mended in ASTM (American Society for Testing andMaterials) designation D1729.

Average North Sky Daylight at 6500K (D65) conformswith international standards in Europe, the Orient,and South America. It is also used to provide corre-lation with instrumental measurements. The DetroitColor Council, in conjunction with the SAE (Societyof Automotive Engineers), has recently adopted theuse of D65 for visual evaluation of automotive interi-ors and exteriors.

Noon Sky Daylight at 5000K (D50) is the desiredlight source for performing color quality and uniformityevaluation in the graphic arts industry, as specified inANSI standard PH 2.32 and ISO standard 3664.

Horizon DaylightHorizon Daylight at 2300K is provided by usinga tungsten halogen lamp operated at half power.It provides the light quality that is found in earlymorning sunrise or late afternoon sunset. If youcan get an acceptable color match using HorizonDaylight (when the sky is the reddest) and NorthSky Daylight (when the sky is the bluest), you willprobably have a good match in any phase of daylight(all day long).

5.4

Incandescent IlluminantsChoose Illuminant A if the lighting conditions aretungsten (filament) lighting (light bulbs).

Fluorescent IlluminantsMost indoor fluorescent lighting is represented byIlluminant CWF-2. Other fluorescent illuminants areIlluminant DLF-7, Illuminant NBF-11, Illuminant TL83,Illuminant TL84 and Illuminant U30.

• Illuminant DLF-7 and Illuminant NBF-11represent fluorescent sources common in theUnited States.

• Illuminant TL83 and Illuminant TL84 representfluorescent sources common in Europe and Asia.

• Illuminant U30 represents a fluorescent sourceused in Sears and several other department stores.

If you are using a GretagMacbeth SpectraLight®

Booth or Luminaire, it best to select the illuminantsdesigned to match them. SPL Hor represents theHorizon setting on the SpectraLight booth. SPLD65 and SPL D75 represent the two most commonDaylight settings.

Observer SelectionFor most quality control applications, it does notmatter which observer you choose, 2 or 10 degrees,as long as you continue to use that same observer forall measurements. Most users choose a specific observerfunction because they need to be consistent with otherusers who have already specified an observer function.If you have no restrictions on which observer tochoose, the 10° observer is the best choice. Numericalresults will correlate better with visual results.

Color Difference in Color QCNumerical comparison of trials to the standard isdone by calculating color difference.

Deltas in Color DifferenceThe difference in absolute color coordinates betweena trial and a standard quantifies the color differencebetween the two. These differences are called Deltas.Deltas may be positive or negative, and this affectshow they are interpreted.

Special Case: ∆Η∆Η∆Η∆Η∆Η*Change in hue angle alone (∆h) is not meaningful,because the visual difference this represents depends

also on the chroma of the standard and trial. Forexample, a hue angle change of 5 degrees may notbe visible for a pale color, yet can be unacceptablefor a highly saturated color. ∆h (small h) is rarely, ifever, reported because it is virtually meaningless.

For this reason, change in hue is expressed as thedistance ∆H* (with a capital H) along the chromaarc for the standard. Therefore, for a specific changein angle (let’s say 5 degrees), ∆H* is larger for a pairof colors that are far from the neutral axis, andsmaller for a pair of colors near the neutral axis.

Other common deltas include:

∆L*: negative means the trial is darker, positivemeans the trial is lighter.

∆a*: negative means the trial is greener (or less red),positive means the trial is redder (or less green).

∆b*: negative means the trial is bluer (or less yellow),positive means the trial is yellower (or less blue).

∆C*: negative means the trial is less saturated,positive means the trial is more saturated.

∆H*: negative means the trial is located clockwise incolor space from the standard, positive means the trialis located counter-clockwise in color space from thestandard. The meaning of this must be interpretedwith the location of the standard. For example, if thehue angle (h) of the standard is 90 degrees, positive∆H* means the trial is greener, negative ∆H* meansthe trial is redder (more towards orange).

Total Color DifferenceWhen a standard and trial are plotted in a uniformcolor space, the distance between them representsthe total color difference. Total color difference in auniform color space is always called ∆E (Delta E). ∆Eis always positive, since a “negative distance” wouldhave no meaning.

∆E is a single value that tells the size of differencebetween the standard and trial; it tells nothing aboutthe direction of the color difference. Two trials mayhave the same ∆E versus the standard; one could belighter with the correct hue and chroma, while theother might be darker, off hue and too high chroma.

The general equation for calculating ∆E is:

The values substituted for ∆l, ∆m, and ∆n depend onthe color space that is selected. For example, in CIELab,∆L*, ∆a*, and ∆b* are substituted respectively.

∆ ∆ ∆ ∆E l m n2 2 2

5.5

Color Differences

= standard

rectangular coordinates L*a*b*

= batch

+b*

DE*

Da*

DL*+a*

-a*

-b*

Db*

+a*

+DC*

+DH*

+DL*+DE*

DH - deltahue angle

+b*

= standard

= batch

polar coordinates L*C*h*

neutralgray axis

5.6

of three axes along hue, chroma, and lightness basedon CIELCh. Color values for samples are essentiallythe same as those for LCh. The mathematics of CMCchange the size and shape of the ellipse as it movesthrough color space.

CMC (l:c) refers to the elliptical tolerance. Sampleswith DEcmc less than 1.0 fall within an ellipse, whilethose samples with a DEcmc greater than 1.0 falloutside an ellipse. The equation is designedso that a color difference of 1.0 represents anacceptable color difference.

The lightness-to-chromaticity ratio, (l:c), refersto the weighting factors for lightness and chroma.The CMC color difference equation provides adjust-ment of tolerance ellipses to allow more or lessemphasis on lightness or chroma. Specific ratiosand commercial factors have been identified byAATCC (American Association of Textile Coloristsand Chemists) for use in textile applications. CMCremains the most popular method for elliptical toler-ancing found in today’s color measurement software.

Tolerancing (Creating a Tolerance)Before pass/fail analysis can be performed, it is nec-essary to determine what degree of color differenceis acceptable, and what degree of color difference isnot acceptable. This process of creating a toleranceis called tolerancing.

Once a color standard has been established, accept-ability limits (color tolerances) can be defined. Inmany color applications, separate color tolerancesmay be needed for each individual product color.Key factors to consider are:

• Consistency is critical. Select one color equationand document it accurately.

• Specify exact calculation. Color instrument softwareprovides the user many selections and combinations.This alone is a major source of user confusion.

• Avoid “fudge” factors between different color equa-tions. Use a single equation and specify Delta E inCIELab, FMC-2, CMC, and so forth.

• Numbers should be confirmed by visual accept-ability. The goal of instrumental color analysis is toarrive at values that agree with human assessment.

For practical color quality control, ∆E can be extremelyuseful for pass/fail decision making, because onlyone value needs to be checked. However, no uniformcolor space correlates perfectly with visual response.Therefore, equivalent ∆E values (numbers) do notnecessarily represent equivalent visual differences.Two color difference formulas attempt to account forthis: CMC (l:c) and FMC-2.

CIELab values (L*a*b*) can be used to identify theamount of change in color as a result of change inillumination. For example, a red sample might beevaluated under D65 daylight and then under illu-minant F2 (Cool White Fluorescent). Under illumi-nant F2, the sample will appear slightly lighter, lessred and less yellow.

CIELab values also allow us to calculate color differ-ences between an established product master or stan-dard and a production trial or sample as shown inthe example below.

Nonsymmetrical ColorPerception in Color QCThe phenomenon of nonsymmetrical color percep-tion and tolerancing was first defined by work ofMacAdam in 1942. Using the 1931 CIE Diagram,MacAdam plotted just-noticeable color differencesor color difference thresholds under constant lighting.The smallest differences were perceived in green, thenyellow, followed by red. The greatest acceptable dif-ferences were found in the blue region of color space.MacAdams’ work became the basis for elliptical tol-erancing that is found in color control software today.

Ongoing work to refine color difference formulascontinues. In 1983, CMC (Colour MeasurementCommittee of the Society of Dyers and Colourists)was introduced under British Standard 6923.

CMC is a modification to the CIELab equation thatimproves correlation between visual and instrumentalsample tolerancing. CMC defines ellipses made up

Color Difference NumbersTrial Standard Difference Visual Terms

L* = 40.40 L* = 39.90 DL* = 0.50 Lighter

a* = 48.04 a* = 54.39 Da*=-6.36 Less Red

b* = 13.83 b* = 17.18 Db* = -3.35 Less Yellow

2.14

CMC (l:c)CMC is a color difference equation that wasdeveloped by the Color Measurement Committee ofthe Society of Dyers and Colorists. It was originallyintended for pass/fail tolerancing within the textileindustry. It has gained wide acceptance in a varietyof industries, since its development.

The CMC color difference equation resulted from amodification of the CIELab equation and is basedon the lightness, chroma and hue of a color (L,C,h).This modification provides a numeric value, DEcmc,which describes the color difference between astandard and a sample in a more uniform colorspace. This enables the use of a single numbertolerance for judging the acceptability of a colormatch in which the tolerance is independent of thecolor of the standard. This single number, DEcmc,represents the volume of the acceptance ellipsoidaround the standard. The ellipsoid automaticallyvaries in size/shape depending upon the position ofthe color in color space. The graphic shows how theequations allow the shape, size and orientation ofthe ellipse to change as it moves throughout CIELabcolor space with a DEcmc of 1.0.

The equation enables the user to alter the overall sizeof the ellipse to provide better correlation with visualacceptability of their product. The eye typically allowsfor greater shifts in the lightness direction than ineither the chroma or hue direction. This is the purposeof the 1:c ratio within the CMC equation. A user canindicate the desired ratio of lightness to chroma in theellipsoid. The ratio selected depends upon the productand the requirements for producing a “color match.”Generally, it is recommended that a ratio of 2:1be evaluated and, if necessary, altered higher orlower depending upon performance. A CMC (1:c)ratio of 1:1 is recommended for the judgment ofcolor difference perceptibilty, while a ratio of 2:1is recommended for the judgment of colordifference acceptabilty.

FMC-2FMC-2 is not really a color space, but rather a colordifference equation developed by Friele, MacAdamand Chickering (hence the abbreviation FMC). Itwas developed from test results based on just-noticeable difference.

XYZxy values are simply tristimulus values andchromaticity coordinates and represent the absolutevalues displayed when FMC-2 is selected.

DRG and DYB are the delta values between twocolors. ∆L is an abbreviation for “Delta Light/Dark”and represents change in lightness/darkness. DRGis an abbreviation for “Delta Red/Green” andrepresents change in redness/greenness. DYBis an abbreviation for “Delta Yellow/Blue” andrepresents change in yellowness/blueness.

Delta E (∆∆∆∆∆E) represents total color difference.The FMC-2 equations are designed so that a ∆Eof 1.0 represents a just-noticeable difference,regardless of the color in question.

CMC (l:c)

huechroma

lightnessLC

hPale Gray (nearly wht.)

Medium Gray

Brilliant Red

Brilliant Yellow

Green

62.051

27.148

20.025

62.345

17.933

63.461

27.682

13.632

62.730

24.718

Example X value Z valueY value

74.413

32.353

8.832

12.356

19.734

Color Spaces

FMC-2

5.7

Tolerances

+ deltab*

yellower

- deltab*

bluer

rectangular tolerances

+ deltaa*

redder

+1.0

+0.5

+0.5 +1.0-1.0 -0.5

- deltaa*

greener

+ deltab*

yellower

- deltab*

bluer

+ deltaa*

redder

- deltaa*

greener

elliptical tolerances

-0.5

- 1.0

+1.0

+0.5

+0.5 +1.0-1.0 -0.5-0.5

-1.0

polar tolerances

Chroma

Hue

= Standard

• In many cases, but not all, the final judge is thehuman. Color is accepted or rejected based on itsvisual appearance. This reinforces the importance ofcontrol in visual and instrumental color assessment.

Problems in Setting Color TolerancingThere is great confusion and misunderstandingabout setting color tolerances. The following guide-lines will help eliminate some of the confusion.

• It is better to start with a wide tolerance and tight-en it to acceptable limits, rather than to begin with avery tight tolerance and widen it to fit the data.

• Avoid setting a tolerance at the minimum percep-tible difference. In almost all applications, the limitsof acceptability are greater than limits of perceptibility.

• Tolerances should agree with acceptability criteriathat are established between a customer and supplier.

Tolerances Based on Individual DeltasIndividual tolerance values may be assigned to eachof the deltas in color space. The combined tolerancesdefine a volume of acceptability in color space. Theshape of this volume depends on the color space chosen.

Rectangular Tolerances — In a rectangular-coordinatecolor space (CIELab, Lab), delta-value tolerancesdefine a rectangular volume in color space. Themain advantage is that this volume is easy to imag-ine. The main disadvantage is that colors near thecorners, even though they are within tolerance, arelikely to exhibit an unacceptable visual difference,or if the tolerance boundary cube is made toosmall, you could reject samples numerically thatare visually acceptable.

Polar Tolerances — In a polar-coordinate colorspace (CIELCh), delta-value tolerances definea region shaped like a keystone. Imagine bendinga rectangular wooden beam around until the endsmeet, then cut a small section from the beam. Thesmall section is typical of a volume defined by deltatolerances in a polar coordinate system. The mainadvantage is that this volume corresponds to visualassessment better than a rectangular volume (thoughnot perfectly). The main disadvantage is that thisvolume is difficult to imagine and communicate.

5.8

Manual TolerancingTolerances may be generated manually in any of theforms available (rectangular, polar, or elliptical).Tolerances can be created anew or simply be adjust-ments made to tolerances generated using any of theautomatic techniques.

Rectangular (Manual Tolerancing) — Manual toler-ances for CIELab or Hunter Lab color space definea rectangular volume of acceptance, because theseare rectangular coordinate color spaces. The rectan-gular tolerance volume is specified by positive andnegative tolerance values for ∆L*, ∆a* and ∆b* (or∆L, ∆a and ∆b for Hunter Lab).

The rectangular tolerance volume may not be sym-metrical around the standard (for example, the+∆a* tolerance does not have to be of equal magni-tude as the -∆a* tolerance).

Polar (Manual Tolerancing) — Manual tolerancesfor CIELCh color space define a keystone-shapedvolume of acceptability. The tolerance volume isspecified by positive and negative tolerance valuesfor ∆L*, ∆C* and ∆H*.

A typical tolerance volume is slightly narrower (inthe ∆H* direction) for lower chroma values, andslightly wider for higher chroma values. For numeri-cal delta calculation, both distances are consideredthe same. Numerical delta calculation correlates tothe volume specified only if the tolerance for ± ∆C*is not unreasonably large.

Elliptical (Manual Tolerancing) — An ellipticaltolerance can be specified in any of the availablecolor spaces (CIELab, CIELCh and Lab). CMC andFMC-2 are color difference formulas that apply anelliptical tolerance automatically.

Major/Minor AxesAn ellipsoid (a three-dimensional ellipse) is definedby the lengths of three axes, the longest of which iscalled the major axis. The two others are called minoraxes. The standard is located at the center of theellipsoid.

To define the axes in CIELab (or Hunter Lab), oneaxis is defined by ± ∆L* (or ± ∆L), the second is de-fined by ± ∆a* (or (± ∆a), and the third is defined by± ∆b* (or ± ∆b). To define the axes in CIELCh, oneaxis is defined by ± ∆L*, the second is defined by± ∆C*, and the third is defined by ± ∆H*.

Total Color Difference TolerancesTolerancing based on total color difference (∆E) iseasy to implement because only one value (∆E) needsto have limits.

Setting a tolerance for CIELab ∆∆∆∆∆E* creates a sphericalvolume of acceptability in CIELab space. Theoreti-cally, a ∆E* of 1.0 represents the typical threshold ofacceptability. In practice, the proper value depends onthe material, the color of the standard, and ultimatelywhat is acceptable to the concerned parties.

Because CIELab is not a perfectly uniform color space,∆E* should be used to call attention to suspicioustrials rather than as an absolute pass/fail determinant.If a trial fails the established ∆E* tolerance, inspectthe component Deltas, and perform a visual assessment.

Setting a tolerance for CIELCh ∆∆∆∆∆E creates a sphericalvolume of acceptability in CIELCh space. In fact, theresults are exactly the same as setting a ∆E tolerance inCIELab space. Theoretically, a ∆E of 1.0 representsthe typical threshold of acceptability. In practice, theproper value depends on the material, the color ofthe standard, and ultimately what is acceptable tothe concerned parties.

Because CIELCh is not a perfectly uniform colorspace, ∆E should be used to call attention to suspicioustrials, rather than as an absolute pass/fail determi-nant; if a trial fails the ∆E tolerance, inspect thecomponent deltas, and perform a visual assessment.

Setting a tolerance for CMC (l:c) ∆∆∆∆∆E creates anellipsoidal volume of acceptability in CIELCh space.The ∆Ecmc equation was designed such that theshape and size of the ellipse change depending onthe location of the standard in CIELCh space, andso that the ellipse for ∆E of 1.0 represents a just-noticeable-difference from the standard. Therefore,∆Ecmc can be used more confidently as a pass/faildeterminant. If a trial fails the ∆Ecmc tolerance,a visual assessment is recommended.

FMC-2 ∆∆∆∆∆E color difference equation was developedbased on research on just-noticeable-differenceperception. The complex equation convertsMacAdam ellipses into spheres of the same size.The radius of these spheres is 1.0 MacAdam units.Therefore, if the ∆EFMC-2 between trial and standardis less than 1.0, the difference between trial andstandard should not be visible under normalcircumstances.

5.9

Pass/Fail Analysis (Applying aTolerance to Color Data)The whole purpose for generating tolerances is toallow pass/fail analysis to be performed on measuredsamples. Pass/fail analysis is performed as follows:

• A trial is measured.

• If the trial falls inside the tolerance volume, it passes.

• If the trial falls on or outside the tolerancevolume, it fails.

If a trial fails, we recommend using visual evalua-tion. This will help verify that the tolerances areappropriate, or help identify a process problem.If an unacceptably large number of trials fail,tolerances may need to be adjusted or the processmay no longer be under control.

Special Pass/Fail SituationsA trial fails based on two general criteria:

• If the trial plots outside the volume of acceptabilitydefined by the component delta tolerances (for ex-ample, Delta L*, Delta a*, etc.), or

• If the total color difference (Delta E) between thetrial and the standard is larger than the total colordifference tolerance.

With elliptical tolerancing, the component deltasdefine the lengths of the ellipse axes. Mathematical-ly, the deltas for a trial may fall within the values ofthe tolerance, but fall outside the ellipse defined bythe tolerances.

To understand this, think of the ellipse as a rectan-gle with its corners rounded off; the trial may plotwithin that portion of the rectangle that is cut off byapplying elliptical tolerancing. For this reason, com-ponent deltas for a trial may be indicated as failing,even though the delta is less than the allowable delta(the tolerance value).

When CMC is the selected equation, pass or fail isdetermined on Delta E alone (using the current l:cratio and Delta E tolerance values). When FMC-2is the selected equation, pass or fail is determinedbased on the component delta tolerance values aswell as the Delta E tolerance values.

If a trial fails, what should I do?If a trial fails, but you are not sure why, check thefollowing items:

1. Check the trial Delta E values for all illuminants.If all Delta E values pass,

2. Check the component delta values for all illumi-nants. If all the component deltas seem to pass,

3. Check the color plot for all illuminants; the trialmay be outside the volume of acceptability definedby the current tolerancing options. If the trial appearsto be inside the volume for all illuminants,

4. Check the lightness component delta value; if thelightness delta value is large enough, the trial maystill be outside the volume of acceptability definedby the tolerance options, but still appear as if it fallswithin all three component delta tolerances.

IndicesInterpreting color coordinates can be tedious, depend-ing on exactly which characteristic of the materialyou need to quantify. Indices are single values thatare calculated differently from color coordinates.Most of them are intended to provide a single valuefor assessing the sample, rather than having to inter-pret the meaning of color coordinates.

Yellowness IndicesYellowness is defined by ASTM as “the attributeby which an object color is judged to depart fromcolorless or a preferred white toward yellow.Negative values denote blueness” (ASTM designa-tion E284-93a). The ASTM Yellowness indices areintended to be used for near-white samples that havea dominant (or complementary) wavelength between570 and 580 nm. For samples that do not meet thesecriteria, these index values may be meaningless.

The Yellowness indices report a zero value for mea-surements that match the preferred white. Positivevalues indicate the sample measured deviates fromthe preferred white towards yellow. Negative valuesindicate the sample measured deviates from thepreferred white towards blue.

ASTM D1925 Yellowness Index was developedspecifically for determining the yellowness of homo-geneous, nonfluorescent, nearly colorless transparent

5.10

Values greater than 100 indicate a bluish white,while values less than 100 indicate a yellowish white.

Berger (59) Whiteness Index is specified for illumi-nant C and 2° observer function only. However, theequation is commonly used with other illuminantsand observer functions, and therefore the valueshown will depend on the primary illuminant andthe observer function you have chosen.

The equation for this whiteness index is:

where X, Y and Z are the tristimulus values for thesample, and X0 and Z0 are tristimulus values for theilluminant/observer combination.

Values greater than 33.33 indicate a bluish white,while values less than 33.33 indicate a yellowish white.

Stensby Whiteness Index is defined by the equation:

where L, a and b are Hunter Lab color coordinates.

Values greater than 100 indicate a bluish white,while values less than 100 indicate a yellowish white.

Taube Whiteness Index is specified for the illumi-nant C and 2° observer function only. However, theequation is commonly used with other illuminantsand observer functions, and therefore the valueshown will depend on the primary illuminant andthe observer function you have chosen.

The equation for this whiteness index is:

where Y and Z are the tristimulus values for the sam-ple and Z0 is a tristimulus value for the illuminant/observer combination.

Values greater than 100 indicate a bluish white,while values less than 100 indicate a yellowish white

Hunter (60) Whiteness Index is defined by theequation:

where L and b are Hunter Lab color coordinates.

Values greater than 100 indicate a bluish white,while values less than 100 indicate a yellowish white.

or nearly white translucent or opaque plastics, asviewed under daylight lighting conditions. It canalso be applied to materials other than plastic fittingthis description.

The equation for YI D1925 is:

where Y and Z are the tristimulus values for thesample calculated using CIE Illuminant C.

ASTM E313 Yellowness Index has been appliedsuccessfully to a variety of white or near-white mate-rials, including paints, plastics and textiles. The defi-nition of yellowness for this method is “the attributeby which an object color is judged to depart from apreferred white toward yellow. Negative values de-note blueness.”

The equation for YI E 313 is:

where X, Y and Z are the tristimulus values for thesample calculated using CIE Illuminant C.

Whiteness IndicesWhiteness is defined by ASTM as “the attribute bywhich an object color is judged to approach the pre-ferred white.” A variety of whiteness indices areavailable. Regardless of which one you choose, it isimportant to understand how the numerical valuerelates to visual assessment. It is also important tocommunicate which one you chose to other affectedparties. If other parties have specified which one touse, be sure to use the one specified.

CIE Ganz Whiteness Index is specified by the CIEfor the D65 in combination with either 2° or 10°observer function. However, the equation is com-monly used with other illuminants, and thereforethe value shown will depend on the primaryilluminant you have chosen.

The equation for this whiteness index is:

WICIE

= Y - 800(xn - x) - 1700 (y

n - y)

where Y is the tristimulus value for the sample, xand y are chromaticity coordinates for the sample ascalculated using the illuminant/observer combina-tion, and xn and yn are chromaticity coordinates forthe illuminant/observer combination.

=YI100 x (1.28X – 1.06Z)

Y

=100 x 1 –YI 0.847ZY

= 0.333Y + 125 – 125ZZ0

XX0

WIBerger(59)

= L – 3b + 3aWIStensby

= 400 – 3YZZ0

WITaube

= L – 3bWIHunter

5.11

Strength IndicesIn general, the strength indices are used for deter-mining the difference in strength between the standardand sample. If the standard and sample are preparedfrom different batches of the same colorant, thedifference in strength between the two batchescan be measured.

The strength indices allow you to determine howstrong or weak a trial is in relation to the standard.Strength adjustment of spectral data adjusts the spec-tral data of trials to be equivalent in strength to thestandard; the adjusted spectral data is used for allsubsequent calculations (color coordinates, indices,etc.). This is useful for determining color differenceonly, regardless of strength differences.

Chromatic Strength Index is the ratio of (K/S) forthe trial versus (K/S) for the standard, expressed as apercentage, at a single wavelength. It can be calculat-ed at any user-specified wavelength:

where R is reflectance at the wavelength of maxi-mum absorbance (minimum reflectance), expressedas a decimal fraction (for example, 20%R = 0.20R).

For transmission measurements:

where T is transmittance expressed as a decimalfraction (for example, 20%T = 0.20T).

Percent strength greater than 100% indicates thetrial is stronger than the standard. Percent strengthless than 100% indicates the trial is weaker than thestandard. Percent strength equal to 100% indicatesthe trial is equal in strength to the standard.

Chromatic Strength Index is named chromatic be-cause it is calculated at one wavelength only, usuallyat the wavelength of maximum absorbance (mini-mum reflectance), because this wavelength usuallyaffects the color of the material more than any otherwavelength. The choice of illuminant/observer com-bination has no bearing on this strength index.

Apparent Strength Index is the ratio of the sum ofthe (K/S) values at all visible wavelengths for the trial

versus the sum of (K/S) values at all visible wave-lengths for the standard, expressed as a percent:

where R is reflectance at the wavelength of maxi-mum absorbance (minimum reflectance), expressedas a decimal fraction (for example, 20%R = 0.20R).

For transmission measurements:

where T is transmittance expressed as a decimalfraction (for example, 20%T = 0.20T).

Percent strength greater than 100% indicates thetrial is stronger than the standard. Percent strengthless than 100% indicates the trial is weaker than thestandard. Percent strength equal to 100% indicatesthe trial is equal in strength to the standard.

Apparent Strength Index considers the difference inreflectance at all wavelengths to determine the strengthof the trial. The choice of illuminant/observer combina-tion has no bearing on this strength index.

Integrated Strength Index is the ratio sum of (K/S)values times the sum of the weighting function valuesfor the illuminant/observer combination at all wave-lengths for the trial versus the sum of (K/S) valuestimes the sum of the weighting function values forthe illuminant/observer combination at all wave-lengths for the standard expressed as a percentage:

where R is reflectance at the wavelength of maxi-mum absorbance (minimum reflectance), expressedas a decimal fraction (for example, 20%R = 0.20R).fx, fy and fz are the weighting function values for theilluminant/observer combination.

= 100 x

=

(K / S)trial

(K / S)

(K / S)std

(1–R)2

2R

Strength%

= log10 (1/T)R

= log10 (1/T)R

= 100 x

700

400

(K / S)trial

(K / S)std

Σ700

400Σ

Strength%

= (K / S)( 1– R )2

2R

= 100 x

700

400

(K / S)(fx + fy + fz)trial

(K / S) (fx + fy + fz)std

Σ700

400Σ

Strength%

= (K / S)( 1– R )2

2R

5.12

(2) If the MI value is significantly greater than zero(greater than about 1.0 or 2.0), the trial is likely toexhibit a noticeable color difference from the stan-dard under the source represented by the secondaryilluminant. You may be able to achieve an acceptablecolor difference for both primary and secondaryilluminants, and decrease the MI. If you are con-cerned about color difference with one illuminantonly, your goal is to decrease ∆E for that illuminantalone. If you are concerned about color differencewith two illuminants, your goal is to decrease MIbetween the two illuminants.

Orange Juice (OJ) IndexThis index was developed by the U.S. Departmentof Agriculture for use with the GretagMacbethColor-Eye® 2020+ or GretagMacbeth Color-Eye3000 spectrophotometers equipped with an orangejuice test tube holder accessory.

TAPPI Brightness and OpacityYou can obtain any of the following standards direct-ly from TAPPI (Technical Association of Pulp and PaperIndustries) in the United States:

Telephone (USA): 1-800-332-8686Write: TAPPI, Technology Park/Atlanta,P.O. Box 105113, Atlanta, GA 30310

T 452 TAPPI Brightness (1977) is typically usedonly in the paper industry for evaluating the qualityof untreated pulp, paper, and paperboard. TAPPITest Method T 452 (1977) is not recommended forevaluating the normal color appearance of whites,especially if fluorescent agents are present; use oneof the whiteness indices instead.

T 452 TAPPI Brightness (1987) provides equivalentresults to that described in TAPPI official Test Meth-od T 452. It was originally designated the “officialstandard” in 1987. The result is typically used onlyin the paper industry for evaluating the quality ofuntreated pulp, paper, and paperboard. Test Meth-od T 452 (1987) is not recommended for evaluatingthe normal color appearance of whites, especially iffluorescent agents are present; use one of the white-ness indices instead.

T 525 TAPPI Brightness (1986) provides a measureof the diffuse brightness of pulp (d/0˚). Diffuse re-flectance is measured at an effective wavelength of457 nm. The method is used to evaluate the diffuseblue reflectance factor (diffuse brightness) of pulpby measuring handsheets prepared using TAPPI

For transmission measurements:

where T is transmittance expressed as a decimalfraction (for example, 20%T = 0.20T).

Percent strength greater than 100% indicates thetrial is stronger than the standard. Percent strengthless than 100% indicates the trial is weaker than thestandard. Percent strength equal to 100% indicatesthe trial is equal in strength to the standard.

The Integrated Strength Index is affected by theweighting functions for the illuminant/observercombination. For example, a comparison fora red colorant will have a smaller apparentstrength difference if Illuminant D65 is chosen,than if Illuminant A is chosen.

Metamerism IndexThe Metamerism Index (MI) indicates how likely apair of specimens will exhibit the same color differ-ence under two different light sources (representedby the primary and secondary illuminants). Theequation is:

MI = (∆L*1 −∆L *2 )2 + (∆a *1 −∆a*2)

2 + (∆b *1 −∆b *2 )2

where ∆L*1, ∆a*1, ∆b*1 are the Delta CIELab colorcoordinates between standard and trial calculatedwith the primary illuminant and ∆L*2, ∆a*2, ∆b*2are the Delta CIELab color coordinates betweenstandard and trial calculated with the secondaryilluminant.

If the MI is low, the color difference between thepair is essentially the same for both illuminants. Thismeans the visual difference between the pair will bethe same under both light sources represented. Thecolors may not necessarily match, but the color dif-ference will be the same.

If the MI is high, the color difference between thepair is not the same for both illuminants. Thismeans the visual difference between the pair will bedifferent under the two different light sources repre-sented. The pair may match under one source, butnot under the other.

A practical use for this index is as follows:

(1) If a trial exhibits a very low color difference forthe primary illuminant, it will match the standardunder the source represented by the primary illuminant.

= log10 (1/T)R

5.13

Test Method T 218, “Forming Handsheets forReflectance Tests of Pulp.” TAPPI Test MethodT 525 (1986) is not recommended for evaluatingthe normal color appearance of whites, especiallyif fluorescent agents are present; use one of thewhiteness indices instead.

TAPPI T 425 Opacity is a measure of opacity(sometimes called hiding power or contrast ratio).Basically, it is a measure of how opaque a substance is.It is the ratio of Y tristimulus for the sample mea-sured over a black substrate divided by Y tristimulusfor the sample measured over a white substrate,expressed as a percentage:

This calculation is based on the 1931 standardobserver (2 degree observer) and Illuminant A.A contrast ratio of 100% means the substance istotally opaque (Y is the same no matter what thesubstrate is, black or white). In theory, a perfectlyclear sample would have a contrast ratio of 0%;the lowest practical value is about 1%.

TAPPI T 519 Diffuse Opacity is a measureof printing opacity and should not be confusedwith TAPPI T 425, “Opacity of Paper (15 degrees/diffuse Illuminant A, 89% reflectance backing andpaper backing)” measurement of the opacity. Thiscalculation is based on the 1931 standard observer(2 degree observer) and Illuminant C. The methodindicates the extent to which a single sheet of paperhides (obscures) printed matter on underlying sheetsof similar composition.

AATCC Gray ScalesThe following are special gray scales developed bythe American Association of Textile Chemists andColorists (AATCC).

AATCC Gray Scale for Color Change assesses colorfastness by using a gray scale for determining changesin color and complies with ISO 105-A02-1978 (E). A5-step scale consists of five pairs of non-glossy graycolor chips (or swatches of gray cloth) that illustratethe perceived color differences corresponding tofastness ratings of 5, 4, 3, 2, and 1.

AATCC Gray Scale Staining assesses color fastnessby using a gray scale for determining staining ofadjacent fabrics and complies with ISO 105-A03-1978(E). A 5-step scale consists of five pairs of non-glossygray or white color chips (or swatches of gray cloth)that illustrate the perceived color differences corre-sponding to fastness ratings of 5, 4, 3, 2, and 1.

XYZ RatiosXYZ ratio compare a tristimulus value (X, Y and Z)for the trial versus the corresponding tristimulusvalue for the standard, expressed as a percentage.The ratios are calculated as follows: 100 (Xtrial/Xstd),100 (Ytrial/Ystd), 100 (Ztrial/Zstd). The XYZ ratios can beused to determine relative strength of a trial versusthe standard.

Limitations of ColorMeasurement SystemsColor measurement systems (instrument and softwaretogether) are excellent for color quality control andformulation. However, there are some limitationsthat are important to consider when implementingthese systems.

Correlation with Visual ResponseColor coordinates, differences, indices, and so forththat are calculated with the existing mathematicalmodels correspond fairly well to visual response,under certain conditions. Most of the models weredeveloped in conjunction with visual assessmentexperiments. However, the conditions are usuallyquite limited, and therefore the numerical resultsshould always be interpreted in conjunction withvisual assessment. The numerical results shouldnever be represented as absolute, because theydepend on a large number of variables.

For example, never consider the CIELab valuesprinted on the back of a tile as absolute. Thevalues are there for reference only. They shouldbe used only as absolute targets for the instrumentspecified, and only for the instrument and softwareconfiguration specified. Also, values depend on theilluminant/observer combination.

When developing a standard and tolerances for colorcoordinates or index values, be sure to correlate theresults you get with visual assessment. Continue touse visual assessment until the correlation with nu-merical results is established and well understood.Then, visual assessment can be decreased, sometimesto the point where it is necessary for exceptions only.

Visual assessment should always be used for excep-tions (rejects or returns). If the color measurementsystem rejects a production sample, verify with visualassessment. If a customer rejects a shipment, reassessthe shipment instrumentally and verify with visual

100 x Yoverblack

Yoverwhite

5.14

assessment. Above all, make sure you understandyour customer’s requirements, and how they deter-mine if those requirements are met.

Interinstrument AgreementTheoretically (and ideally), two instruments of thesame design should measure a sample exactly thesame. All measuring devices are subject to variability;spectrophotometers are complex devices with manyinherent variables. Therefore, even two instrumentsof the same design will exhibit slight differences inmeasurement results.

Most modern spectrophotometers of the same exactdesign can measure the same sample to within 0.2CIELab ∆E of each other. The more differences be-tween the instruments being compared, the greaterdifference in absolute values you can expect. Instru-ments do not need to be accurate (meaning “able toprovide absolute measurements of a specimen thatcan be verified by another similar instrument”) be-cause of the way they are used. They are best usedfor color difference measurement, not for absolutecolor measurement.

Instrumental Color QC —A SummaryColor quality can be affected by many dependentand independent variables. As one color formulaconstituent increases, for example, the color willchange in a predictable way (deeper, darker, lighter,and so forth). If independent outside variablessuch as temperature, humidity, ultraviolet exposure,pressure, or other external phenomena occur, colorcan also be affected. For example, temperature cancause thermochromatic changes, while ultravioletexposure could result in color fading.

Proactive Color ControlBefore the final color is determined in the manu-facturing process, it can be checked throughouteach critical step and evaluated for acceptabilityin a defined range or color tolerance. In most casesproactive color control can be conducted withPass/Fail criteria. This process is proactive, becauseit is traceable to a specific part of the color manufac-turing process. The approach is cost effectivebecause it can isolate and fix those variables thatcause the process to go out of control.

Companies that use proactive color control are notfaced with the same excess scrap materials producedat sites that evaluate color reactively.

Cost of Poor Color QualityIn today’s business it is difficult to assign a dollarvalue to acceptable color quality. It is far easier tomeasure the cost of scrap, mismatched goods, pro-duction downtime, and the cost to rework in termsof lost throughput, labor, and material. In general,the more color applications you involve in your pro-cess, the more complicated the color managementprocess becomes. For example, an automobile manu-facturer must manage color for its paint, plastics,textile and ink components. The large community ofsubassembly suppliers to automotive manufacturersneeds accurate color specifications so that the carinterior is acceptable and harmonious to its users.Plastic parts must match the painted parts, and bothmust look right next to fabrics or leather upholstery.

Phases of a Color Quality ProgramThere are several key phases in the development of acolor quality program. No single phase in the follow-ing list is any more important than its neighbor. Everyphase of the process has a beginning (color desired)and end (color achieved).

• Design and color specification.

• Color matching and formulation.

• Visual and instrumental(hardware/software) analysis.

• Quality control of the colormanufacturing process.

• Applications procedures and methods.

• Training and education.

• Retail and consumer preferences.

Reducing variability between the desired color andthe color achieved can be accomplished onlythrough effective communication throughout thesupply chain. This communication must include:

• Accurate specification of thedesired color.

• Accurate color formulation.

• Accurate visual assessment of color.

• Accurate instrumental assessment of color.

5.15

• Accurate quality control of colormanufacturing process.

• Effective training and educational materials.

Satisfaction of all of these requirements delivers thecustomer satisfaction that results in increased salesand ultimately improved share of market for yourproduct.

Instrumental ColorEvaluation DocumentationThe best way to ensure that everyone is followingyour standard visual and instrumental color evalua-tion method is to put it in writing. This writtenmethod should be communicated to customers,suppliers and any persons involved in making colordecisions at your company.

A sample outline for a comprehensive colorevaluation program is given to encourage theuse of similar methods for individual productapplications or company-wide standardizationand communication.

The information provided serves as guidance.Successful color control programs bring togetherall the factors relating to the light source, objectand human observers and apply control, consistencyand communication. Then and only then can colordata be produced that is meaningful and agrees withour visual evaluations and those of our customers.This is the final goal of any color analysis andbrings everything back to the starting point, theeye and brain combination and how color andappearance are perceived.

Instrumental Analysis

Color Measurement

Spectrophotometer: GretagMacbeth Color-Eye 7000A

Sphere-Based (D8)

CIE Observer: 10 degree

CIE Illuminant: D65

Secondary Illuminants: Cool White Fluorescent,

Illuminant A

Color Scale: L*a*b* (CIELab)

Measurement Mode: Reflectance, SCE,

UV included, LAV

Number of Measurements: Average = 3

Color Difference: CIELab Delta E* 1.0 with no

single component greater than 60% of total

Gloss Measurement

Glossmeter: 60 degree

Target Gloss and Tolerance: 45 ± 2 gloss units

Visual Analysis

Viewing Conditions

Lighting Product: GretagMacbeth SpectraLight II with UV

Primary Light Source: D65 Filtered Tungsten

Secondary Light Sources: Cool White Fluorescent,

Illuminant A

Viewing Geometry: 0/45

Surround: Munsell N 7/

Color Standards

Sample Size (standard and trial): 3'' x 5'' (desired)

Sample Orientation: Side-By-Side, Always Touching

Color Standard Storage: Filed in nonacidic envelope

Color Tolerance: Munsell Color Tolerance Set

Color Analysis

Color Vision Testing: Farnsworth-Munsell 100 Hue Test

Color Communication: Based on Munsell

Hue Terms: Redder, Yellower, Greener, Bluer

Value Terms: Lighter, Darker

Chroma Terms: Stronger, Weaker

Modifiers: Slightly, Moderately, Much

Instrumental Color Evaluation Method

G.1

A

AbsorbanceAbsorbance is “light-stopping ability.” The higher thevalue, the more light the sample absorbs. Values typicallyrange from 0 to 3.0. Mathematically, absorbance iscalculated A = log10(1/T) where T is transmittance,expressed as a decimal from 0 to 1 (for example, 0.5Ris 50%R). This is the same equation used to calculatedensity in graphic arts and photography applications.

Achromatic ColorA neutral color (white, gray or black) that has no hue.

Additive Color MixtureMixing of the three primary color lights (red, greenand blue) to obtain colors. For example, combininggreen and red creates yellow, red and blue createsmagenta, and blue and green creates cyan.

AdaptationThe ability of the eye to adjust to different lightsources or light levels. This allows the visual systemto adjust its sensitivity to different lighting conditions.

Angle of IncidenceThe angle at which a beam of light strikes thesurface of an object compared to the perpendicularto the object surface.

Angle of ReflectionThe angle at which a beam of light is reflected fromthe surface of an object compared to the perpendic-ular to the object surface.

Angle of ViewThe angle at which a sample is viewed compared tothe perpendicular to the surface.

Apparent Color TemperatureThe color appearance of a light source related to theabsolute color temperature of a black body radiatorhaving the same color.

AttributeDistinguishing characteristic of a sensation, percep-tion or mode of appearance.

B

Beer’s LawDescribes the mathematical relationship between theabsorption of light energy relative to the concentra-tion of a dye or pigment.

Black Body (Planckian) LocusThe set of points on a chromaticity diagram repre-senting the colors of perfect radiators having variouscolor temperatures.

Black Body RadiatorIn theory, an object that absorbs all energy that comesinto contact with it.

BrightnessBoth the saturation and lightness of product color.

C

ChromaAttribute of color used in the Munsell Color Systemto indicate the degree of departure from a gray ofthe same value. Correlates with dimension of saturation.

ChromaticHaving color (hue); not neutral (black, white or gray).

Chromaticity DiagramIn practical terms, a two-dimensional graph on whicha color may be plotted according to its hue and chroma.The third dimension of this graph is the luminancefactor, or lightness, which is independent of hue orchroma. The location of a point on this graph indicatesroughly what color it is (red, green, blue, purple, andso forth) and how saturated it appears. This informationmust be interpreted with caution, since the coordinatesof neutral colors differ with each illuminant. Thecoordinates can rarely be interpreted with appearanceunless the illuminant is specified. A plot of all colorshas a characteristic horseshoe shape.

CIEThe abbreviation for the French title of the Interna-tional Commission on Illumination, Commission

Glossary of Terms

Glossary

G

G.2

Internationale de l’Éclairage. The Commission isdevoted to standardization in illumination and relatedareas that include color. The CIE operates througha series of committees.

CIE Luminosity Function (Y)A plot of the relative magnitude of the visualresponse as a function of wavelength from about 380to 790 nm, adopted by CIE in 1931.

CIE Standard ObserverThe observer data for a 2 degree field of view, adoptedby the CIE in 1931 to represent the response of theaverage human eye, when adapted to an equal energyspectrum. A supplementary 10 degree observer wasadopted in 1964.

CIE Tristimulus ValuesAmounts (in percent) of the three components (RGB)necessary in a three-color additive mixture requiredfor matching a color.

CIELabA uniform (opponent color scale) color space inwhich colors are located within a three-dimensionalrectangular coordinate system; the three dimensionsare lightness (L*), redness/greenness (a*) andyellowness/blueness (b*). CIELab is part of thecurrent CIE recommendations. A uniform colorspace utilizing an Adams Nickerson cube rootformula, adopted by the CIE in 1976 for usein the measurement of small color differences.Pronounced “see-lab” and also referred to asL*a*b* (pronounced, “el-star”, “ay-star”, “bee-star”).

CIELChA uniform (opponent color scale) color space inwhich colors are located within a three-dimensionalpolar coordinate system; the three dimensions arelightness (L*), chroma (C*), and hue angle (h).CIELCh is part of the current CIE recommendations.To pronounce CIELCh, just say the letters.

CMCAlso CMC (l:c). A color difference formula based onthe CIELCh (opponent color scale) color space, inwhich equivalent total color difference values representequivalent visual differences, regardless of the color.

Color AptitudeThe ability to work with and understand color;includes both inherited skills and work experience.

Color AttributeThree-dimensional characteristic of the appearanceof an object. One dimension usually defines thelightness, the other two together define color.

Color ConstancyRelative independence of perceived object color tochanges in color of the light source.

Color Difference, VisualThe difference between two colors that the humaneye sees. It is usually described in qualitative termssuch as lighter, darker, redder, greener, bluer,yellower, paler, more saturated, and so forth.

Color Difference, NumericalThe difference between color coordinate values fortwo different samples. Numerical color differencequantifies the difference between two colors.

Color Difference EquationsMathematical equations that calculate the magnitudeof difference between two colors. Some equationsconvert CIE coordinates into more uniform differ-ences that more closely simulate visual perception ofcolor difference.

Color Matching FunctionRelative amounts of the three additive primaries (red,green and blue) required to match wavelengths of light.

Color Measurement ScaleA system of specifying numerically the perceivedattributes of color.

Color Rendering IndexMeasure of the amount of color change that objectsexhibit when illuminated by a light source ascompared with the color of those same objects whenilluminated by a reference source of comparablecolor temperature.

Color SpaceIn general, a collection of systematically ordered colors,or a system of ordering colors. A color space canbe defined by a physical collection of samples orby a mathematical system. In the context of instru-mental color measurement, a three-dimensional volumedefined by a set of equations, in which any color may belocated precisely based on instrumental measurement.For example, the three-dimensional volume describedby the CIELab system is called CIELab color space(CIELab is an opponent color scale).

G.3

Color ToleranceAn acceptable color difference between a standard(master) and a trial (batch).

ColorantAny dye or pigment that provides color to a materialor mixture.

Colorant MixtureA mixture of dyes or pigments.

ColorimeterAn instrument designed to measure light reflectedor transmitted by a sample, which can be correlatedwith a psychophysical description of color.

Conditional MatchA set of samples which appear to match under alimited set of conditions such as light source orviewing angle.

Correlated Color TemperatureThe temperature in degrees Kelvin of a point on ablack body locus which most closely resembles thelight source.

D

Delta AbsorbanceThe difference in absorbance values, at eachwavelength, between trials and the standard. Positivedelta absorbance means the trial absorbs more lightthan the standard. Negative delta absorbance meansthe trial absorbs less light than the standard.

Delta E∆E or DE. The generic name for total color differ-ence, and is used to indicate total color differencefor all uniform color spaces. Total color difference(Delta E) is a single number that expresses themagnitude (size, degree, amount) of differencebetween two colors. The value tells nothing aboutthe nature of the color difference.

Delta K/SThe difference in K/S values, at each wavelength,between trials and the standard. Positive deltaK/S means the trial has a higher absorption-to-scattering ratio than the standard. Negative deltaK/S means the trial has a lower absorption-to-scattering ratio than the standard.

Delta ReflectanceThe difference in reflectance values, at eachwavelength, between trials and the standard. Positivedelta reflectance means the trial reflects more lightthan the standard. Negative delta reflectance meansthe trial reflects less light than the standard.

Delta TransmittanceThe difference in transmittance values, at eachwavelength, between trials and the standard.Positive delta transmittance means the trialtransmits more light than the standard. Negativedelta transmittance means the trial transmits lesslight than the standard.

Delta ValueA conversational term for “delta color coordinate”;sometimes, the word “deltas” is used. For example,in CIELab calculations, Delta L*, Delta a* and Deltab* are all called “CIELab delta values” or “CIELabdeltas.” This term is used to facilitate spokencommunication.

Dialog BoxA window that appears temporarily to requestinformation. Many dialog boxes have options youmust choose before Microsoft* Windows* can carryout a command.

Diffuse ReflectionReflection in which light energy is scattered in manydirections by diffusion at or below the surface.

Diffuse TransmissionDiffusion of light energy being transmitted througha sample and subject to the laws of refraction.

Diffused LightNondirectional or scattered light.

EEllipsoidA solid whose plane sections are all ellipses (closedcurves produced when a cone is cut obliquely to itsaxis by a plane).

G.4

FFlairThe change in hue of a sample when the light sourceis changed; the opposite of color constancy.

FluorescenceProcess by which energy, usually UV, is absorbed bycertain chemicals or materials and re-emitted atother, usually longer, wavelengths.

Fluorescent LampA low pressure mercury electric-discharge lamp inwhich a fluorescing coating (phosphor) transformssome ultraviolet energy generated by the dischargeinto visible light.

Fluorescent Whitening Agent (FWA)A fluorescent dye or pigment that absorbs UVenergy and re-emits the energy at a higher wave-length as visible light (violet blue) thereby causinga white appearance.

FMC-2A color difference equation developed by Friele,MacAdam and Chickering. The equation wasderived from the results of an extensive visualassessment experiment. For most colors, a total colordifference (Delta E) value of 1.0 represents a just-noticeable difference.

Foot CandleThe quantity of light at a point on a plane surfaceone foot from and perpendicular to a standard candle.

G

Geometric AttributesThe characteristics associated with light distributionfrom an object including gloss, haze, texture, shape,viewing angle and surround.

Geometric MetamerismThe property exhibited by a pair of samples (usuallyhighly textured) that appear to match at one illuminationand viewing angle, but no longer match when eitherthe angle of illumination or viewing angle is changed.

GlossmeterAn instrument used to measure the amount ofgloss (a term used to describe the relative amountof mirror-like (specular) reflection from the surfaceof a sample). These instruments measure the light

reflected at selected specular angles, such as 20degrees from the perpendicular, 45, 60, 75, and 85degrees. Results obtained are very dependent oninstrument design, calibration technique used,types of samples, and so forth.

GoniochromaticAdjective used to describe a colored material thatexhibits goniochromatism.

GoniochromatismThe phenomenon where the color of a material changesas the angle of illumination or viewing is changed.

GoniospectrophotometerAn instrument used to measure a spectrophotometriccurve at various angles of incidence and reflectance.The angles of incidence and reflectance can bechanged or are offered at fixed intervals (e.g., 15,45, 75, 110 degrees).

HHazeThe scattering of light by a specimen responsible forthe apparent reduction of contrast of objects viewedthrough it or contrast of objects viewed by reflectionat the surface.

HueThe attribute of color used in the Munsell ColorSystem by which we distinguish red from green, bluefrom yellow, and so forth.

I

IlluminantAn illuminant is a mathematical representation ofa theoretical real light source, used for calculatingtristimulus values from a spectrophotometric mea-surement. The numbers represent relative power ofthe theoretical source at each point in the visible spec-trum. The relative power distribution of a real sourcecould be used for calculation, but real sources aredifficult to standardize.

Illuminant AMathematical representation of tungsten halogen(incandescent). Color temperature: 2856K. Usesinclude metamerism testing. Simulates typical homeor store accent lighting.

G.5

Illuminant CMathematical representation of filtered tungstenhalogen (daylight). Color Temperature: 6770K.Uses include metamerism testing. Simulates theCIE average daylight.

Illuminant CWF-2 (F2)Mathematical representation of commercial, wideband fluorescent used in the USA (Cool WhiteFluorescent). Color temperature: 4150K. Usesinclude metamerism testing. Simulates typicaloffice or store lighting in the USA.

Illuminant D50Mathematical representation of noon sky daylight.Color temperature: 5000K. Uses include generalevaluation of color, metamerism testing, andevaluating color uniformity and quality in thegraphic arts industry.

Illuminant D55Mathematical representation of noon sky daylight.Color temperature: 5500K. Uses include metamerismtesting. Simulates the CIE average noon sky daylight.

Illuminant D65Mathematical representation of average north skydaylight. Color temperature: 6500K. Uses includegeneral evaluation of color, metamerism testing,providing visual correlation with spectrophotometricinstrumental readings, and conformance withEuropean and Japanese standards. Simulatesaverage north sky daylight.

Illuminant D75Mathematical representation of north sky daylight.Color temperature: 7500K. Uses include generalevaluation of color, metamerism testing, and visualevaluation of opaque materials as outlined by ASTMD1729. Simulates north sky daylight.

Illuminant DLF-7Mathematical representation of commercial, wideband fluorescent used in the USA (Deluxe). Colortemperature: 6500K. Uses include metamerism testing.

Illuminant NBF-11Mathematical representation of commercial, narrowband fluorescent used in the USA. Color tempera-ture: 4000K. Uses include metamerism testing. USAequivalent to TL84.

Illuminant SPL (D65)Mathematical representation of GretagMacbeth-patented Filtered Tungsten Halogen as found inSpectraLight (Daylight). Color temperature: 6500K.Uses include critical evaluation of color, metamerismtesting, providing visual correlation with spectrophoto-metric instrumental readings, conformance withEuropean and Japanese standards, and agreementwith the current Automotive standard. Simulatesaverage north sky daylight.

Illuminant SPL (D75)Mathematical representation of GretagMacbeth-patented, Filtered Tungsten Halogen as found inSpectraLight (Daylight). Color temperature:7500K. Uses include critical evaluation of color,metamerism testing, and visual evaluation of opaquematerials as outlined by ASTM D1729. Simulatesnorth sky daylight.

Illuminant SPL (HOR)Mathematical representation of Tungsten Halogenas found in SpectraLight (Horizon). Color temperature:2300K. Uses include metamerism testing. Simulatesearly morning sunrise or late afternoon sunset.

Illuminant TL83Mathematical representation of commercial, rareearth phosphor, narrow band fluorescent used inEurope and the Pacific Rim. Color temperature:3000K. Uses include metamerism testing. Simulatestypical office or store lighting in Europe and thePacific Rim.

Illuminant TL84Mathematical representation of commercial, rareearth phosphor, narrow band fluorescent used inEurope and the Pacific Rim. Color temperature: 4100K.Uses include metamerism testing. Simulates typicaloffice or store lighting in Europe and the Pacific Rim.

Illuminant TL85Mathematical representation of commercial, rareearth phosphor, narrow band fluorescent used inEurope and the Pacific Rim. Color temperature: 5000K.Uses include metamerism testing. Simulates typicaloffice or store lighting in Europe and the Pacific Rim.

Illuminant U30Mathematical representation of commercial, rareearth phosphor, narrow band fluorescent. Colortemperature: 3000K. Uses include metamerismtesting. Simulates typical store lighting for Sears.USA equivalent of TL83.

G.6

IncandescentA lamp in which light is produced by a filamentheated by an electric current so that it glows.

Integrating SphereA sphere coated inside with a highly reflective,diffuse material and used to collect for measurementthe light reflected or transmitted by a specimen.

K

K/S (“K over S”)The ratio of the absorption coefficient (K) versusthe scattering coefficient (S) for a reflectance measure-ment. The ratio is derived mathematically from thereflectance measurement as follows:

where R is reflectance expressed as a decimal(for example, 60%R is 0.6R).

Kubelka-MunkPhenomenological turbid-medium theory relatingthe reflectance and transmittance of scattering andabsorbing materials to optical constants. Theoryincludes variables where K represents the absorptioncoefficient and S the scattering coefficient for theconcentrations of colorants. Also known as K over Sdata, this relationship is the basis of virtually allcomputer color matching calculations.

LLab (Hunter Lab)A uniform (opponent color scale) color spacein which colors are located within a three-dimen-sional rectangular coordinate system; the threedimensions are lightness (L), redness/greenness (a),and yellowness/blueness (b).

Lambert’s LawThe flux reflected per unit solid is proportional tothe cosine of the angle measured from the normal(perpendicular) to the surface.

LightElectromagnetic radiation that has a wavelength inthe range from 380 (violet) to about 770 (red)nanometers (nm), and can be perceived by thenormal, unaided human eye.

Light SourceThat element in an instrument or in the visualobserving situation that furnishes radiant energy inthe form of light.

LightnessOne of the three dimensions describing color. Theattribute by which observers distinguish white objectsfrom gray objects and light colored objects from darkcolored objects.

Log AbsorbanceThe base 10 logarithm of absorbance valuesat each wavelength. A spectral plot (log absorbanceversus wavelength) shows that vertical distancesbetween two curves (samples) are virtually the samefor all wavelengths. Therefore, the vertical positionof the curve relates directly to the colorant concen-tration or sample thickness. Also, the shape of thecurve is independent of colorant concentration orsample thickness.

Log K/SThe base 10 logarithm of K/S values at eachwavelength. A spectral plot (log K/S versus wave-length) shows that vertical distances between twocurves (samples) are virtually the same for mostwavelengths. Therefore, the vertical position of thecurve relates directly to the colorant concentration.Also, the shape of the curve is almost independentof colorant concentration.

LusterThe appearance characteristic of a surface that reflectsmore in some directions than it does in other directionsbut not of such gloss as to form clear mirror images.

M

MacAdam EllipsesEllipsoids plotted on the chromaticity diagram thatcorrespond to a just-noticeable difference from thecolor represented by the center of the ellipsoid. Thesize and shape of the ellipsoids depend on theirlocation on the chromaticity diagram.

MacAdam UnitA unit of color difference as calculated by theFMC-2 equation. One (1:0) MacAdam unitcorresponds to a just-noticeable diffference,based on experimental results.

= (K / S)(1–R)2

2R

G.7

MatchGenerally, two colors match if they appear to be thesame (have the same color coordinate values). Theword “match” is often used to indicate a spectralmatch, which means the two colors will appear tobe the same (have the same color coordinates)regardless of illuminant or observer.

Matte FinishA surface which displays no gloss when observed atany angle; a highly diffusely reflecting surface.

Metameric PairA pair of colors which match when viewed in adescribed way, but no longer match if the viewingconditions change.

MetamerismA phenomenon exhibited by a pair of colors whichmatch under one or more sets of real or calculatedconditions and not match when these conditionsare changed.

N

Narrow Band FluorescentGeneric term for fluorescent lamp products such asUltralume and TL84 that produce narrow bands ofvisible light energy as a function of their phosphor blend.

Nonmetameric MatchA pair of colors which appear to be identical to allobservers under all conditions of illumination andviewing; an unconditional match.

Normal Color VisionVision of a normal observer who exhibits nosymptoms of anomalous or defective color response.

OObserver MetamerismA pair of colors which match when viewed by oneobserver, but no longer match when viewed byanother observer.

OpaqueTerm used to describe complete opacity, i.e., thedegree to which a specimen obscures the substratebeneath it; opposite of transparent.

Opponent Color TheoryTheory explaining conceptually how the humanvisual system (eye and brain combination) perceivescolor. To the human visual system, red and green areopposites, and yellow and blue are opposites. To ahuman observer this means that something that isred has no green in it (it may also be blue or yellow),while something that is yellow has no blue in it(it may also be red or green). Something that isneither red nor green is neutral with respect toredness/greenness. Something that is neither yellownor blue is neutral with respect to yellowness/blueness. If a color is neutral with respect to both,it is a “colorless” neutral (such as a black, gray orwhite). This theory is the basis for most uniformcolor spaces (especially CIELab, CIELCh, and Lab).

PPhotochromismA reversible change in color of a specimen due toexposure to light.

Photopic VisionAdjective used to describe vision mediated by thecone receptors in the retina of the eye, which giverise to the sensation of color occurring at high andmedium levels of luminance.

PsychophysicalA term used to describe the area of color sciencewhich deals with the relationship between physicaldescription and the sensory perception resultingfrom them.

RReflectionThe process by which incident light leaves a surfaceor medium from the side on which it is incident.

SSaturationThe attribute of color perception that expresses thedegree of departure from a gray of the same lightness;grays have no saturation.

G.8

International de l’Èclairage (CIE) in 1931. This isbased on the results of a color matching experimentthat used a 2 degree field of view.

Subtractive Color MixtureColorant mixture which must take into account boththe absorption and scattering of two or more of theindividual pigments used in the mixture.

Supplemental Observer(1964, 10 Degree Observer)The 10 degree visual field observer is the supplemen-tary observer adopted by the Commission Interna-tional de l’Èclairage (CIE) in 1964. This is based onthe results of a color matching experiment that useda 10 degree field of view.

SurroundPortion of the visual field immediately surroundingthe central field or pattern of interest.

TThermochromismA reversible change in color of a specimen due tochange in temperature of the specimen. This is typicalof highly saturated (vivid) colors (such as bright reds,yellows, and oranges).

TintA color produced by the mixture of white pigmentor paint with a chromatic pigment or paint.

Total Color Difference (∆ or Delta E)A single number that expresses the magnitude (size,degree, or amount) of a difference between twocolors. The value tells nothing about the natureof the color difference.

TranslucencyAppearance state between complete opacity andcomplete transparency; partially opaque.

TransmissionProcess by which incident light is transmitted througha material or object.

Transmittance (of Light)That fraction of the emitted light of a givenwavelength which is not reflected or absorbed,but which passes through a material or object.

ScatteringThe process by which light passing through granular,fibrous or rough surface matter is redirected over arange of angles.

Scotopic VisionVision mediated by rods alone at very low levels ofillumination; night vision.

SpectralPertaining to the visible spectrum, thus, having to dowith color.

Spectral Power Distribution (SPD)Graphical or numerical representation of radiantenergy per unit interval of wavelength for a givenlight source.

SpectrophotometerA photometric device for the measurement ofspectral transmittance or spectral reflectance.

SpecularHaving the qualities of a speculum or mirror; asmooth reflecting surface.

Specular Component Excluded (SCE)Measurement of reflectance made in such a way thatspecular reflectance is excluded from the measure-ment; diffuse reflectance only.

Specular Component Included (SCI)Measurement of the total reflectance from a surface,including the diffuse and specular reflectance.

Specular GlossRelative luminous fractional reflectance from asurface in the mirror or specular direction.

Specular ReflectionReflectance of a beam of radiant energy at an angleequal but opposite to the incident angle; the mirror-like reflectance.

Standard IlluminantThe relative energy emitted by a real or imaginarylight source that is mathematically defined at eachwavelength across its spectral distribution.

Standard Observer(1931, 2 Degree Observer)The 2 degree visual field observer is the standardobserver recommended by the Commission

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TransparentAdjective to describe a material which transmits lightwithout diffusion or scattering.

Tristimulus ValuesAmounts (in percent) of the three componentsnecessary in a three-color additive mixture requiredfor matching a color; in the CIE System, they aredesignated as X, Y and Z. The illuminant and stan-dard observer color matching functions used mustbe designated; if they are not, the assumption ismade that the values are for the 1931 observer(2 degree field) and Illuminant C.

TurbidityReduction of transparency of a specimen due to thepresence of particulate matter.

U

UltravioletRadiant energy below 380 nm; portion of the elec-tromagnetic spectrum between about 10 and 380 nm.

Uniform Color SpaceA color space in which equivalent numericaldifferences represent equivalent visual differences,regardless of location within the color space. A trulyuniform color space has been the goal of color scien-tists for many years. Most color spaces, though notperfectly uniform, are referred to as uniform colorspaces, since they are more nearly uniform whencompared to the chromaticity diagram.

V

Value (in Munsell)An attribute of color used in the Munsell color sys-tem to indicate the lightness of a specimen viewed indaylight, on a scale from 0 for the ideal black to 10for ideal white, in steps that are visually approxi-mately equal in magnitude.

Visible Spectrum (Visible Region)That portion of the electromagnetic spectrumbetween 380 nm and 770 nm that can be seenby the human eye.

Volume of AcceptabilityThe volume of acceptability is used to represent thelocation of acceptable trials in that color space. It isa three-dimensional region that surrounds a stan-dard and is defined by tolerance values. If a trialfalls within this volume, it is acceptable according tothe tolerance criteria. If it falls outside this volume,it is unacceptable according to the tolerance criteria.

W

White Reflectance StandardA physical white standard of an imperfectly diffusingmaterial, such as white ceramic, that is calibrated inreference to the perfect diffuser.

WhitenessAttribute by which an object color is judged to ap-proach the preferred white.

Wide Band FluorescentGeneric term given to those fluorescent lamp productssuch as Cool White and Warm White that producewide bands of visible light as a function of theirphosphor blend.

X

XYZThe set of tristimulus values for numerically describ-ing a color, calculated using ASTM E308-85.

xyY (Chromaticity Coordinates)A nonuniform color space in which colors arelocated within a three-dimensional rectangularcoordinate system; x and y describe the chromaticity(hue and chroma) of a color. Y describes the lumi-nosity (lightness or brightness) of a color.

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Kelly, K.L., and Judd, D.B. Color Universal Languageand Dictionary of Names. Washington D.C.: NationalBureau of Standards (U.S.) Spec. Publication 440.,U.S. Government Printing Office, 1976.

MacAdam, David L. Color Measurement, Theme andVariation. New York: Springer Verlag, 1981.

MacAdam, David L. Sources of Color Science.Cambridge, MA: The MIT Press, 1970.

McDonald, Roderick (Editor). Colour Physics forIndustry. Bradford, England: Society of Dyers andColourists, 1987.

McLaren, Keith. The Colour Science of Dyes andPigments, Second Edition. Bristol, England:Adam Hilger Ltd., 1986.

Munsell, A.H. A Color Notation. New Windsor, NY:Macbeth Division of Kollmorgen, 14th Edition 1979.

Overheim, R. Daniel, and Wagner, David L.Light and Color. New York: John Wiley & Sons, 1982.

Rea, Mark S. (Editor). Lighting Handbook. New York:Illuminating Engineering Society of North America,8th Edition 1993.

Wright, W.D. The Measurement of Colour, FourthEdition. New York: D. Van Nostrand Co., 1969.

Wyszecki, G.W., and Stiles, W.S. Color Science.New York: John Wiley and Sons, 2nd Edition 1982.

ASTM Standards on Color and Appearance Measurement.Philadelphia, PA: ASTM, 4th Edition 1994.

Berger Schunn, Anni. Practical Color Measurement,A Primer for the Beginner, A Reminder for the Expert.New York: John Wiley & Sons, 1994.

Billmeyer, F.W., Jr., and Saltzman, M. Principles of ColorTechnology. New York: John Wiley and Sons, 2ndEdition 1981.

Committee on Colorimetry, Optical Society ofAmerica. The Science of Color. Washington D.C.:Optical Society of America, 1963.

Evans, Ralph M. An Introduction to Color. New York:John Wiley and Sons, 1948.

Evans, Ralph M. The Perception of Color. New York:John Wiley and Sons, 1974.

FSCT Inter-Society Color Council Committee, andFSCT Definitions Committee. Glossary of Color Terms.Blue Bell, PA: Federation of Societies for CoatingsTechnology, 1981.

Hardy, Arthur C. Handbook of Colorimetry.Cambridge, MA: The Technology Press, 1936.

Hunter, Richard S. and Harold, R.W. The Measurementof Appearance. New York: John Wiley and Sons, 2ndEdition 1987.

Judd, D.B., and Wyszecki, G.W. Color in Business,Science, and Industry. New York: John Wiley and Sons,3rd Edition 1975.

References References

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