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)( T10x 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 TemperatureDescribes 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 6.132
n
En
The Hydrogen Spectrum
eV 6.132
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
Enm-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
intensityIncident
x dtransmitte
or reflectedFraction
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 ProblemCalculate the transmitted spectrum from the following data:
Rel. intensity
400 700 (nm)500 6000
5
10
Incident light intensity % Transmission of filter
% Transmission
400 700 (nm)500 6000
50
100
Transmitted Spectrum
Rel. intensity
400 700 (nm)500 6000
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
Review1. 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