Chapter 2:

Origin of Color

What produces the color sensation?

Light

Stream of Photons(Energy: a

measurable quantity)

EM Waves

Dispersion

700 nm 550 nm 400 nm

Color sensation depends on:

•Spectral composition of source / object

•Intensity of light

•Source type

•Spectral sensitivity of the eye

Source(Illuminant)

Direct

Object

Indirect

Spectral Energy Distribution

or Spectral Composition

Relative amounts of light from different parts of the spectrum

Measurement of Light

Physical Units:

•Joule (J): SI unit of energy. Energy required to lift a 1 kg object by 10.2 cm at sea level.

•Watt (W): Rate at which energy is transformed (or work is done). One Watt is the same as one Joule/second.

•Electron-Volt (eV): More useful unit of energy when dealing with atoms. 1 eV is equal to 1.6 x 10-19 J.

Physical (Radiometric) Units

Light as a form of energy

Luminous (Photometric) Units

Visual effect produced by light

Measurement of Light (Contd.)

Luminous Units:

Take into account the sensitivity

of the eye at different wavelengths.

So physical units must be scaled

up or down!

Units of Illumination:

DescriptionDescription Physical UnitsPhysical Units Luminous UnitsLuminous Units

Total energy (light) Total energy (light) outputoutput

Radiant flux Radiant flux

(Watts)(Watts)Luminous fluxLuminous flux

(Lumens)(Lumens)

Light reaching a unit Light reaching a unit areaarea

Irradiance or Irradiance or Intensity (Watts/mIntensity (Watts/m22))

Illuminance Illuminance

(Lumens/m(Lumens/m2 2 = Lux)= Lux)

Photometric Conversion

•Formula to convert physical units to luminous units:

Luminous Units = Physical Units x RLE x 685

•Example: How many watts of power are required to produce 1 lux of illuminance by…

•Red light (650 nm)?

•Green light (550 nm)?

•Useful Information:•Dark Night: 0.0001 lux

•Star light: 0.001 lux

•Moon: 0.1 lux

•Office: 300 lux

•Cloudy day: 1000 lux

0.0015 W

0.0073 W

Review Question

•Both bulbs radiate the same amount of total energy. 100 Watts = 100 Joules per second.

•1900 Lumens appears brighter because it radiates more energy in the “useful” part of the spectrum.

100 Watts1400 Lumens

100 Watts1900 Lumens

Sources of Light

•Depending on their spectra, light sources can be divided into two main categories.

•Blackbody sources

•Bright line sources

Blackbody Sources

• “Hot” objects characterized by continuous spectra.Examples: Sun, candle light, incandescent lamp…

Features:

1. Stephan’s Law:

2. Wein’s displacement

law:

ww2.unime.it/dipart/i_fismed/wbt/ita/physlet/blackbody/corponero.htm

4T output energy Total

K)( T

10x 2.89 (nm) length Peak wave

6

Review Problems

1.Calculate the peak wavelength at which you radiate light (your body temperature is about 3100K).

2.How hot would a blackbody need to be in order to have its peak wavelength at 550 nm?

Color Temperature

Describes the kind of light produced by a blackbody source.

Higher color temperature abundant in blue

Lower color temperature abundant in red

9323 nm

5255 0K

Solar Spectrum (Blackbody Source)

Bright Line Sources

•Generally single elements,

characterized by discontinuous

line spectra.

Examples: Sodium street light,

mercury lamp, neon sign, laser…

Hydrogen

Helium

Carbon

http://mo-www.harvard.edu/Java/MiniSpectroscopy.html

How do atoms emit / absorb light?

Model of an Atom

•Atoms = Nucleus (protons + neutrons) + Electrons.

•Electrons in neutral atoms occupy definite energy levels (orbits) around the nucleus.

•Electrons can jump between energy levels by absorbing or emitting energy.

Electronic Transitions

Example: Hydrogen Atom

•Energy levels are given by:

•Ground state: E1 = -13.6 eV

•Higher states: E2 = -3.4 eV

E3 = -1.5 eV….

E2

E1

E2

E1

Jump to a higher level

Energy equal to or greater than (E2-E1) must be supplied

Jump to a lower level

Excess energy (E2-E1) is released as a photon

eV 6.13

2

n

En

The Hydrogen Spectrum

eV 6.13

2

n

En

-13.6 eV

-1.5 eV

-3.4 eV

-0.85 eV-0.54 eV

Visible lines in the hydrogen spectrum

TransitioTransitionn

Photon Photon EnergyEnergy

WavelengthWavelength ColorColor

EE3 3 E E22 1.9 eV1.9 eV 653 nm653 nm RedRed

EE44 E E22 2.55 eV2.55 eV 486 nm486 nm BlueBlue

EE55 E E22 2.86 eV2.86 eV 434 nm434 nm Violet Violet 11

E

nm-eV 1240λ

n=4n=5

n=3

n=2

n=1

Reflection, Transmission & Absorption

•Incident Energy = Transmitted + Reflected + Absorbed

•Colored objects can selectively reflect or transmit some part of the incident spectrum.

•Absolute amount of reflected or transmitted light depends on:

•Reflection / Transmission curve

•Intensity of incident light at each wavelength (spectral composition).

Transmitted Light

Reflected Light

Incident LightObject

Spectral Energy Curves & Reflectance Curves

Rel. intensity

400 700 (nm)

Lights

500 600

Dim

Bright

Percent of light reflected

400 700 (nm)500 600

Black

White

Gray

Surfaces

50 %

100 %

0 %

Reflection & Transmission

•Important Rule: For each wavelength,

Perceived Color

Spectral content of source

Selective reflectivity or transmission of object

intensity

Incidentx

dtransmitte

or reflected

Fraction

intensity

dtransmitte

or Reflected

Rel. intensity Rel. intensity% Reflectance

+ =

400 400400700 nm 700 nm700 nm

Blue light Red surface Dark appearance

http://www.cs.brown.edu/exploratories/freeSoftware/repository/edu/brown/cs/exploratories/applets/spectrum/reflection_guide.html

Review Problem

Calculate the transmitted spectrum from the following data:

Rel. intensity

400 700 (nm)500 600

0

5

10

Incident light intensity % Transmission of filter

% Transmission

400 700 (nm)500 600

0

50

100

Transmitted Spectrum

Rel. intensity

400 700 (nm)500 600

0

5

10

Absorption

•Absorbed energy raises the temperature of the object.

•Dark objects absorb more energy.

•The Greenhouse Effect:Absorbed light is converted

to heat (IR) which is

trapped by the greenhouse

because glass is opaque

to IR.

Color Mixing

•Where do colors like pink, brown, silver…come from?

•Ideal white light source:

Produces equal energy in

all parts of the visible spectrum!

•Additive primaries: Divide the ideal source into three equal parts.

Rel. intensity

400 700 (nm)500 600

Rel. intensity

400 700 (nm)500 600

Rel. intensity

400 700 (nm)500 600

Rel. intensity

400 700 (nm)500 600

Blue RedGreen

Additive Mixing

•Additive primaries: Red, Green , and Blue.

•Each primary is 1/3 of the spectrum.

•Colors are produced by “adding” spectra.

•Need three sources of light to produce colors.

•Applications: Color TV, stage lighting…etc.

•Example:

Rel. intensity

400 700 (nm)500 600

Rel. intensity

400 700 (nm)500 600

RedRed GreenGreen

Rel. intensity

400 700 (nm)500 600

YellowYellow+ =

http://www.cbu.edu/~jvarrian/applets/color1/colors_g.htm

Subtractive Mixing

•Subtractive primaries: Yellow, Cyan , and Magenta.

•Each primary is 2/3 of the spectrum.

•Colors are produced by “subtracting” part of the spectrum from white light source (i.e. by overlapping filters).

•Need one white light source to produce colors.

•Applications: Pigments, dyes, color printing…etc.

Rel. intensity

400 700 (nm)500 600

Magenta or - GreenMagenta or - Green

Rel. intensity

400 700 (nm)500 600

Cyan or - RedCyan or - Red

Rel. intensity

400 700 (nm)500 600

Yellow or - BlueYellow or - Blue

http://lite.bu.edu/vision/applets/Color/Color/Color.html

Complementary Colors

• Pair of colors that produce white when mixed additively.

• Example: Yellow + Blue

Cyan + Red

Green + Magenta

Review

1. Explain how you would obtain the following colors by combining various intensities of the additive primaries:

a) Yellow b) Pink

c) White d) Orange

e) Purple f) Light cyan

2. Explain how you would obtain the following colors by combining subtractive primary filters:

a) Red b) Green c) Blue

d) Black e) White f) Pink

g) Orange

http://www.cs.brown.edu/courses/cs092/2000/py27/cmatchapp.html