Chapter 2:
Origin of Color
What produces the color sensation?
Light
EM Waves
Dispersion
700 nm
550 nm
Stream of Photons
(Energy: a
measurable quantity)
400 nm
Direct
Source
(Illuminant)
Indirect
Object
Color sensation depends on:
•Spectral composition of source / object
•Intensity of light
•Source type
•Spectral sensitivity of the eye
Spectral Energy Distribution
or Spectral Composition
Relative amounts of light from different parts of the spectrum
Measurement of Light
Physical (Radiometric) Units
Luminous (Photometric) Units
Light as a form of energy
Visual effect produced by 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.
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:
Description
Physical Units
Luminous Units
Total energy (light)
output
Radiant flux
(Watts)
Luminous flux
(Lumens)
Light reaching a unit
area
Irradiance or Intensity
(Watts/m2)
Illuminance
(Lumens/m2 = 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)?
0.0073 W
•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
Review Question
100 Watts
1400 Lumens
100 Watts
1900 Lumens
•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.
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…
ww2.unime.it/dipart/i_fismed/
wbt/ita/physlet/blackbody/
corponero.htm
Features:
1. Stephan’s Law:
Total energy output  T4
2. Wein’s displacement
law:
Peak wave length (nm) 
2.89 x 106
T ( K)
Review Problems
1. Calculate the peak wavelength at which you radiate
light (your body temperature is about 3100K).
9323 nm
2. How hot would a blackbody need to be in order to
have its peak wavelength at 550 nm?
5255 0K
Color Temperature
Describes the kind of light produced by a blackbody source.
Higher color temperature  abundant in blue
Lower color temperature  abundant in red
Solar Spectrum (Blackbody Source)
Bright Line Sources
•Generally single elements,
characterized by discontinuous
line spectra.
Examples: Sodium street light,
mercury lamp, neon sign, laser…
http://mo-www.harvard.edu/Java/MiniSpectroscopy.html
Hydrogen
Helium
Carbon
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
E2
E2
E1
E1
Jump to a higher level
Jump to a lower level
Energy equal to or greater
than (E2-E1) must be supplied
Excess energy (E2-E1)
is released as a photon
Example: Hydrogen Atom
•Energy levels are given by:
•Ground state: E1 = -13.6 eV
•Higher states: E2 = -3.4 eV
E3 = -1.5 eV….
 13.6 
En   2  eV
 n 
The Hydrogen Spectrum
 13.6 
En   2  eV
 n 
1240 eV - nm
λ
E
n=5
n=4
-0.54 eV
-0.85 eV
n=3
-1.5 eV
n=2
-3.4 eV
n=1
Transition
Photon Energy
Wavelength
Color
E 3  E2
1.9 eV
653 nm
Red
E 4  E2
2.55 eV
486 nm
Blue
E 5  E2
2.86 eV
434 nm
Violet 1
-13.6 eV
Visible lines in the hydrogen spectrum
Reflection, Transmission & Absorption
Object
Incident Light
Transmitted Light
Reflected Light
•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).
Spectral Energy Curves & Reflectance Curves
Percent of light reflected
Rel. intensity
White
Bright
100 %
Gray
50 %
Black
Dim
400
500
600
Lights
0%
700
(nm)
400
500
600
Surfaces
700
(nm)
Reflection & Transmission
Perceived Color
Selective reflectivity or
transmission of object
Rel. intensity
% Reflectance
+
Blue light
400
Spectral content of
source
700 nm
Rel. intensity
Red surface
400
=
Dark appearance
700 nm
400
•Important Rule: For each wavelength,
 Reflected or   Fraction 

 
  Incident
 transmitte d    reflected or  x 
 intensity
  transmitte d   intensity

 




http://www.cs.brown.edu/exploratories/freeSoftware/repository/edu/brown/
cs/exploratories/applets/spectrum/reflection_guide.html
700 nm
Review Problem
Calculate the transmitted spectrum from the following data:
Incident light intensity
% Transmission of filter
Rel. intensity
% Transmission
10
100
5
50
0
400
500
600
700
0
400
(nm)
500
600
Rel. intensity
10
Transmitted Spectrum
5
0
400
500
600
700
(nm)
700
(nm)
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?
Rel. intensity
•Ideal white light source:
Produces equal energy in
all parts of the visible spectrum!
400
500
600
700
(nm)
•Additive primaries: Divide the ideal source into three equal
parts.
Rel. intensity
Rel. intensity
Green
Blue
400 500 600 700
Rel. intensity
(nm)
400 500 600 700
Red
(nm)
400 500 600 700
(nm)
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
Rel. intensity
+
Red
400 500 600 700
(nm)
Rel. intensity
=
Green
400 500 600 700
(nm)
Yellow
400 500 600 700
http://www.cbu.edu/~jvarrian/applets/color1/colors_g.htm
(nm)
Subtractive Mixing
•Subtractive primaries: Yellow, Cyan , and Magenta.
Rel. intensity
Rel. intensity
400 500 600 700
Yellow or - Blue
(nm)
400 500 600 700
Rel. intensity
(nm)
Cyan or - Red
400 500 600 700
(nm)
Magenta or - Green
•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.
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