4 - Color Mixing
Spectral Power Distribution (SPD)
Perfect absorber at all wavelengths = “Blackbody”
(if T = 0 K)
Blackbody with T = 3000 K
F 
F 
2 hc
Units?
2
1
hc  kT 
 e
3
2 h
5
1

c
e
1
h  Planck 's constant
2
h
1
W/(m2 Hz steradian)
kT
W/(m2 m ster)
W/(m2 m ster)
W/(m2 nm ster)
etc.
k  Stefan  Boltzmann constant
e  2.718......
Color Perception &
Color Mixing
•Hue – the color
•Saturation – how deep the color is. Higher
saturation means less “whiteness”
•Brightness – the intensity of the light
Additive Color Mixing
What does the human eye see when mixing
roughly “equal” amounts of Red, Yellow, Green,
Cyan, Blue, and Magenta?
G+R=Y
B+R=M
B+G=C
B+G+R=W
(unequal amounts
will produce other
hues)
Primary Colors
Any 3 colors that add up to make white light can be
“primary” colors, but RGB seems to produce the widest
range of other colors, so we usually refer to them as
THE primary colors for additive mixing of light.
Example:
M + Y = (B + R) + (G + R) = (B + G + R) + R = W + R
Newton’s Color Wheel
originally contained red, orange, yellow,
green, blue, indigo, and violet.
To do correctly, we
need to quantify
“brightness”
saturation
Different Types of Color Addition
(colored lights)
Superposition
Partitive Mixing - close spatially
Visual Persistence - close temporally
Subtractive
Color Mixing
(paints, filters,
“what’s left?”)
Rule for combining
filters:





T final  T filter1 x T filter2

Result of passing
light with a given
SPD through a filter
(or series of filters):





SPD final  SPD initial x T filter

Filter Types
Kodak Wratten 44A Filter
High transmission
We see this
but not this
Spectral “Purity”
“Bandpass” -  (such as Full-Width-Half-Maximum)
“Spectral Resolution” - R = /
“Nonspectral”
R ~ a few
“Spectral”
R ~ 10
“Monochromatic” R > 100
Sun with“H” filter,  =
656.3 nm, R~10,000
Neutral Density Filters - These have “flat”
transmission curves, or R << 1
Building
colored
images from
filtered ones
Colored Pigments (Paints)
Relies on combinations of additive
and subtractive color mixing.
When layered, can depend in
transmission, reflection, partitive
color mixing, etc.