Basic
Color
Theory
Susan Farnand
farnand@cis.rit.edu
Tell me about color:
How would you define color?
How does color happen?
Where does color happen?
Why do things look colorful?
At least two things needed for
color to happen:
A source…
…and a
sensor
Visible Light Spectrum
400 nm
700 nm
Light Sources Compared
350
typical daylight
incandescent
mercury vapor lamp
300
6500K fluorescent
Relative Spectral Power
250
200
150
100
50
0
300
400
500
600
Wavelength, nm
700
800
Basic Structure of
the Human Eye
Schematic of the
Retina
Photoreceptors
Horizontal,
bipolar, and
amacrine cells
Ganglion cells
Incoming light
Relative Spectral Sensitivity of Cones & Rods
Relative Sensitivity
100
S
rods
M
L
50
0
350
450
550
Wavelength, nm
650
750
Spectral sensitivity
Rods
S
Cones
R ML
Color Mixing–Additive
 Adding light
 Mixing light sources
 Red, Green, Blue primaries
Additive Color Mixing:
Color Mixing–Additive
 Adding light
 Mixing light sources
 Red, Green, Blue primaries
 Can you think of examples of
systems using additive light
Color Mixing– Subtractive
 Removing light
 Mixing dyes, pigments or other
colorants
 Cyan, magenta, & yellow
primaries
 Can you think of examples of
systems using subtractive light?
What happens to the light?
Transmitted
Absorbed
Reflected
Subtractive Color Mixing
Measuring Color
Are these two patches the same?
Reflected Light
Reflectacne
100
50
0
300
350
400
450
500
550
600
Wavelength, nm
650
700
750
800
Sensation of Color
Light Source
Object
Observer
Light Sources Compared
350
typical daylight
incandescent
mercury vapor lamp
300
6500K fluorescent
Relative Spectral Power
250
200
150
100
50
0
300
400
500
600
Wavelength, nm
700
800
Cone mosaic data from experiment by
Hofer, Singer, and Williams (2005).
Different sensations from cones with the same photopigment.
Journal of Vision, 5(5):5, 444–454.
CIE
Commission Internationale de l’Eclairage
(International Commission on Illumination), an
international organization that establishes and
maintains standards of light and color. Its
system of describing color is based on
standardization of illuminants and observers,
not physical samples.
CIE Illuminants
300
Illuminant A
illuminant C
250
illuminant D50
Relative Spectral Power
illuminant D65
illuminant D75
200
150
100
50
0
300
400
500
600
Wavelength, nm
700
800
Color Matching Experiment
Color Matching Functions
Color Matching Experiment
Color Matching Functions
1931 Standard Observer
2
CIE 2° x
1.8
CIE 2° y
1.6
CIE 2° z
1.4
1.2
1
0.8
0.6
0.4
0.2
0
380
480
580
Wavelength, nm
680
Computing CIE Tristimulus Values
Illuminant
Object
Observer
The CIE tristimulus values X, Y, and Z are obtained by multiplying
together the power (P) of a CIE standard illuminant, the
reflectance (R) of the object, and the standard observer
functions and then summing the products.
Computing CIE Tristimulus Values
Doing the math…
The CIE tristimulus values X, Y, and Z are
obtained by multiplying together the power of a
CIE standard illuminant, the reflectance of the
object, and the standard observer functions and
then summing the products.
Colorimetric Match
Colorimetric match is defined as when the
tristimulus values for two stimuli match:
X1 = X2
Y1 = Y2
Sample 1
Z1 = Z2
Sample 2
Relative Sensitivity
Metamerism
10
8
6
Patch 1
4
Patch 2
2
0
350
450
550
650
750
Wavelength (nm)
Two colored patches that have different reflectance
curves may appear the same when viewed under
one illuminant, but may appear different when
viewed under a second illuminant.
Colorimetric Match
Colorimetric match is defined as when the
tristimulus values for two stimuli match:
X1 = X2
Y1 = Y2
Sample 1
Z1 = Z2
Sample 2
Chromaticity Values and the
Chromaticity Diagram
The CIE tristimulus values X, Y, and Z are can
be transformed to chromaticity values (x, y)
and displayed on a unit plane is known as the
chromaticity diagram.
Chromaticity values are calculated by:
x = X/(X+Y+Z), y = Y/(X+Y+Z)
It is also possible to calculate a chromaticity for z, where
z=Z/(X+Y+Z). Since x+y+z = 1, it is redundant (i.e. z=1-x-y)
Chromaticity Diagram
Color Differences
on the 1931 CIE
Chromaticity Diagram