Designers Light Forum

Effective Light Measurement and Data Use

Kevin Willmorth

Credit(s) earned on completion of this course will be reported to AIA CES for AIA members. Certificates of Completion for both AIA members and non-AIA members are available upon request.

This course is registered with AIA CESfor continuing professional education. As such, it does not include content that may be deemed or construed to be an approval or endorsement by the AIA of any

material of construction or any method or manner ofhandling, using, distributing, or dealing in any material or product.___________________________________________Questions related to specific materials, methods, and services will be addressed at the conclusion of this presentation.

LearningObjectives

1. Determine what can be expected from light meters2. Apply methods for increasing accuracy of

measurements 3. Determine uses of relevant data as a valuable tool4. Set up an affordable small informal lab to evaluate

product in an office5. Determine practices for tracking luminaire

performance over time (lumen depreciation and other factors.

At the end of the this course, participants will be able to:

EFFECTIVE LIGHT MEASUREMENT AND DATA USE

Lazy Metering

What’s wrong with your metering methodology?

You’re not using one!

Seeing vs. Perceiving

The human visual system has zero accuracy – cannot be calibrated to a known standard

Variable, uncalibrated optical receiver parts with adaptive features that change with age and non-uniform perception 100

9590

8580

7570

6560

5550

4540

3530

2520

1510

50

100 97 95 92 89 87 8481

7774

7167

6359

5550

4539

32

22

00

10

20

30

40

50

60

70

80

90

100

100 95 90 85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0

Measured Light Percieved Light

Relative Perceived Illuminance

Rela

tive

Actu

al

Illum

inan

ce

Visual Performance Response

• Human response to spectral color balance is not flat• Varies by age, health, and individual preference

CCT S/P Ratio EVE Factor

PerceivedIlluminance

(20Fc Base @ 3500K CCT)

2700K 0.8 1.56 12.82

3000K 1.25 1.09 18.35

3500K 1.4 1.00 20.00

4000K 1.6 0.90 22.22

5000K 2.0 0.75 26.67

Subjectivity

It’s too dark in here Wrong, too bright, I can still see you

Let there be Meters

• Changes observation from subjective (looks good) to objective (numeric value)

• Allows comparisons and communication– Between observers– Over time

• Calibration = verifiable accuracy

Electrophot1931

Weston 6171932

Why Meter

• Evaluate a product• Survey a lighting condition• Verify performance against a product specification• Verify performance against a calculated prediction• Monitor performance over time (color)• Track lumen depreciation• Evaluate an environment to be lighted

– Solve problems or capture design data• Understand more about light in a space

Testo 540

What We Measure

• Illuminance• Color and color

characteristics• Modulation (Flicker)• A wide range of pecific

applicable characteristics– Brightness (luminance)– X-y coordinate– Duv shift– Etc…

Data Extrapolation

• Color Accuracy– CRI Ra and specific R values– TM30 Rf and Rg– McAdam Steps from standard center point

(product to product consistency)• Visual performance• Human factors• Energy vs. Power• Consistency comparison• S/P ratio• Horticultural

– PPFD (Photosynthetic Photon Flux Density)– PAR (Photosynthetic Active Radiation)

Derivatives

• With accurate objective data comparisons can be made and conclusions drawn between parties

• Subjective variables can be considered within the bounds of real data

• Perceptive variables can be factored mathematically from objective uniform data

METERS AT WORK

Visual Response Factoring

• Meters are designed to match human visual response– Without this, meters just measure total energy collected

at the photo receptor, whether or not it can be “seen”

Cosine Correction

• Cosine correction for illuminance measurements• Does not provide luminance or spot measurement

Lambert’s cosine law

Accuracy vs Precision

• Accuracy is how close a meter will produce a result against a known reference or standard

• Precision is how tight readings are to one another, sometimes referred to as repeatability

Accuracy vs Precision

Not acceptable in professional measurement applications

Acceptable for relative measurements and comparisons only

Ideal Result

LED and Analog Meters

Mismatch in technology delivers very low accuracy and low

precision

If it doesn’t use a battery and has an analog gage- it is likely useless

Low Cost Digital Meters

Diffuser (Cosine Corrector)

Gage (Voltmeter w/firmware)

Photosensor (under diffuser)

Detachable sensor head is an excellent feature

Hundreds of acceptable meters at a wide range of price points

Generally offer high precision, but are widely diverse in accuracy from product to product

Quality Digital Meters

Detachable sensor head

Array of multiple sensors

Offer durability and versatility – not necessarily any gain in precision

When calibrated properly, offer higher accuracy over low cost alternatives

Other Meters

Many older digital light Meters –from fluorescent era forward may still be usable.

Photographers light meters-with spot attachment

Old color meters are either accurate or precise enough

Verify against known meterMay not produce a linear responseHigh precision, variable accuracy

Low accuracy, high precision –useful for relative comparisons only – extrapolating Fc data is impossible

Modern Meter Accuracy

Vs.

<$100Illuminance Only

>$2,000Illuminance + 89 other Parameters

Meters do not need to be expensive to be precise and accurate – requires verification

210215220225230235240245250255260

2700K 3000K 3500K 4000K 5000K

Illum

inan

ce

LED CCT

Meter Comparison

>$2000 <$100

All readings within +/-0.98%

Accuracy Comparison

Vs.

Analog meters disagree by >24% to one another = Poor Accuracy

A B C D EIncand 36.6 36.1 36.4 29 22Error Base 1.4% 0.5% 20.8% 39.9%

Assensetek LM-200LED Minolta T1H GE 214 GE 2175000 LED 40.1 39.6 39.9 30 22

Error Base 1.2% 0.5% 25.2% 45.1%

Vs. Vs.Vs.

A B C D E

Color Meter Error

Vs. Vs.

Older photographic color meters (C) lack accuracy (reference) and precision (repeatability) when reading LED light sources

-600

-500

-400

-300

-200

-100

02700 3000 3500 4000 5000

Color Deviation

A B C Low Light C High Light

A B C

LED - Meter Error

Vs. Vs.Vs.

2700K 3000K 3500K 4000K 5000KDW58 10 22 32 45 60LM-200LED 84 84 84 84 84Minolta T-1H 83.7 84 85.1 84.1 84GE 214 45.5 47 50 50 50

0102030405060708090

Illum

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A B C D

ABCD

Meter “D” lacks accuracy, but has good precision

Meter “A” lacks both accuracy and precision

App Base MetersDiffuser (Cosine Corrector)

Software driven readout – iOS or Android application

Multiple spectral sensor

Calibration using software

Generally offer good accuracy (if calibrated) and acceptable precision

Remote Sensor Advantages

• Reduce operator interference

• Allow meter to be read with sensor at any angle/orientation

• Less physical interference between target surface and sensor

Integrated Meters

Fixed sensor head can be an issue in task measurements

High end products, calibrated annually, offer high accuracy and precision

Delivered Data

CCTCRI(Ra)(R1-R8)Re(R1-15)R1~R15CQSIlluminanceFoot CandleCIE 1931CIE 1976Spectrum DiagramC78.377-2008IEC-SDCMTM-30-15(Rf, Rg & Diagram)Peak Wavelength (λp)Dominant Wavelength (λD)PurityDuvSP RatioPPFD (380~780 nm)Flicker Frequency(5-200 Hz)Flicker PercentageFlicker IndexTemperatureRelative Humidity

90+ Parameters

Lab Meters – PC Connected

Properly set up lab instruments, calibrated regularly, deliver the highest accuracy and precision due to elimination of external interference from observers and field conditions

Software Interface

CCTCRI(Ra)(R1-R8)Re(R1-15)R1~R15CQSIlluminanceFoot CandleCIE 1931CIE 1976Spectrum DiagramC78.377-2008IEC-SDCMTM-30-15(Rf, Rg & Diagram)Peak Wavelength (λp)Dominant Wavelength (λD)PurityDuvSP RatioPPFD (380~780 nm)Flicker Frequency(5-200 Hz)Flicker PercentageFlicker IndexTemperatureRelative Humidity

Desktop software evaluation tools

USING METERS

Core Principles

• Accuracy is attained by using meters of a known quality related to a reference– Use same meter for side-by-side comparisons– Use calibrated meters to eliminate conflicts with

results between different testers• Precision is attained by employing routines

and care in measuring that eliminate variables– Distance variables and precision– Control of light noise from surrounding sources– Record keeping

Photometric Field Verification

• Meters reading Fc or Lux can be used to generate candela data for comparisons and evaluation

Distance “D” (25’ or 7.62M)

Candela = Fc x D² (in feet)Lux = Fc x D² (in meters)Knowing the exact distance “D” is critical

Meter must be aligned normal to incident angle

12Fc x 25² = 7,500cd129.17Lux x 7.62² = 7,500cd

12Fc129.17Lux

55°

Transmission

Transmission =FC or Lux through material / FC or Lux no materialCare must be taken to avoid stray light entering sensor

Align and fix meter normal to incident angle and set distance

Fc through material = 112FcFc without material = 238Fc112/238 = 47% Transmission

Align material normal to incident angle and close to meter sensor – be wary of reflective losses

Reflectance

Reflectivity =FC or Lux off material (A) / FC or Lux direct (B)

Align and fix meter normal to incident angleFc off material = 112Fc

Fc direct = 238Fc112/238 = 47% Reflectance

Distance A+C and B+C must be identical

AB

C

Incident and reflectedangles should match

Simple Benchtop Goniometer

Mount for small fixtures/lamps

Rotate in X axis in any degree steps desired with controlleror manual crank

Mount any illuminance meter here

Can be also be done with two tripods, protractor and string – just make the measured distances and alignment as accurate as possible

Intensity and Scale

Benchtop Goniometer on steroids – the brighter and larger the source is, the larger the distances involved

Relative Measurement

• Comparisons and ratios do not require absolute values– Precision (repeatability) more important than accuracy– Distance to subject and optics must be equivelant

• Comparison of a known reference source to another

A:Known good sourceRef: 600lm Reads 36Fc

24” 24”

Reads 30Fc

600/30 = 20(Ratio of lumens per Fc measured)

B:Comparison sourceLumens???

20 x 36 = 720lm(Source B relative lumens)

Relative Measurement

Test Lumens Reading Multiplier

Referance 322 157 2.051

Reading Multiplier Lumens

Test 1 141 2.051 289.19

Test 2 172 2.051 352.77

Metric values are less important than ratios to reference source

Requires a precise light meter, but not necessarily accurate

Ref and Test sources must be of similar optical character

Relative Spot Measurement

A

B

C

DE

F

G

ID Ev Comp Ratio

A 10.1 A:C 1.4:1

B 5.7 A:B 1.9:1

C 7.2 E:C 1.7:1

D 8.1 A:D 1.2:1

E 11.9 E:A 1.2:1

F 9.1 A:F 1.1:1

G 8.0 A:G 1.3:1

Accuracy is less important than ratios between target surfaces measured with high precision

Brightness ratio using a spot meter

Common Measurement Errors

• Incorrect assumptions about what meter capability– Assumptions of accuracy– Misapplication causing poor precision

• Lack of care in setting up meter to capture measurement– Inconsistent measurement location– Inconsistent distance from source– Lack of preparation for repeating measurements

• Interference from surrounding sources or reflective surfaces• Mismatch between meter and light source• Observer interference

– White shirt effect on illuminance– Red shirt effect on color measurement

• Wrong meter setting• Poor equipment condition and lack of calibration

Field Measurement

• Make a map of the space and note of conditions• Establish a fixed x,y,z point that can be recaptured

later• Use a meter sensor mount vs. hand holding

– Tripod or monopod mount ideal• Measure at night for artificial sources• Measure without and without artificial light when

measuring daylight contribution– Capture artificial at night to differentiate contributions

clearly• Make field holding tools and keep them handy

Record keeping, accuracy and precision are critical in monitoring lumen depreciation

Field Measurement

Record Keeping

TOOLS AND ACCESSORIES

Handy Tools

Tape Measure

Level and Protractor

Reference source(s)

Color Checker and Gray Cards

Plans

Tripod/Monopod

Any tool that will reduce variables or eliminate interference is a good tool

Temperature Meters

• Contact temperature meter– K type thermocouples– Various contact types– 2 channels preferable

• Ambient• Light Source

– 4 is better • Housing/heat sink• Lens/diffuser

2 channel

4 channel

Note: Non-contact IR meters are useless for most lighting applications

1 channel

Simple Flicker Checker

If you see flicker, it exists – metrics won’t change that

Flicker Meter

Understand what the metrics are and why they are meaningful

Multi-purpose Meter:CCT/Lux/CRISVM calculations% and indexFrequencyModulationMagnitudeVisual reference

Subjective Color Reference

• Multiple CCTs– 2700K– 3000K– 3500K– 4000K– 5000K

• Dimmable• Provides visual

reference and subjective capture

Final Thought

Define a specific and simple procedure that controls variability, using easy to use meters appropriate to need, and stick with it consistently

In other words… K.I.S.S.

This concludes The American Institute of Architects Continuing Education Systems Course

Kevin Willmorth

Lumenique, LLC

www.lumenique.com

414-241-5124

kwillmorth@Lumenique.com