Adding Colors
• Mixing the broad distributions of green and red
yields yellow. Although the resulting spectral
distribution is very different from spectral yellow.
• If one mixes roughly equal amounts of
monochromatic green and red, the result also
looks yellow, although the spectral yellow is
completely absent.
• Two colors which look alike even though they
have different intensity distribution curves are
called metamers.
Simple Additive Rules
Red
• Green + Red = Yellow
Magenta
Yellow
W hite
• Blue + Green = Cyan
Green
Blue
Cyan
• Blue + Red = Magenta (purple)
• Blue + Green + Red = White
The primary
additive colors
are red, green,
and blue.
Complementary Colors
• Two complementary colors combine to
produce white
• Cyan + Red = W
(C = B + G)
• Blue + Yellow = W (Y = G + R)
• Magenta + Green = W (M = B + R)
• Some spectral colors have complementary
spectral colors. For instance, the
complementary of the orange (600 nm) is
bluish cyan (488 nm).
• Not all spectral colors have complementary
spectral colors. For instance, the complement
of green (530 nm) has to be a double humped
distribution of red and blue (magenta).
Chromaticity Diagram
• Question: if given spectral blue, green and
red, can any color (hue, saturation,
brightness) be matched by mixing them?
• Almost! You can’t match spectral cyan.
• If you choose as your three colors: red
(650nm), green (530nm), and blue (460nm)
the relative amount of mix to get the spectral
color is shown in Fig. 9.10
• A chromaticity diagram, a horseshoe shaped diagram, displays how any
two colors can combine to form other colors.
• The information in a chromaticity diagram is similar to, but more
quantitative than, that in the layer of branches at the top of the Munsell
color tree. Note that white, not gray or black, lies at the center of the
chromaticity diagram.
• Spectral colors (saturated, monochromatic) lie on the boundary of the
horseshoe.
• All colors inside the horseshoe are unsaturated. The degree of
saturation increases from 0% at white to 100% at the horseshoe
boundary.
• The line of purples (or magentas) connects the two ends of the
horseshoe. These are not spectral colors.
More about the Chromaticity
Diagram
• The straight-line between two of the colors passes
through all the colors that one can get by mixing
the two.
• The complement of any color is found by
extending a straight line from that color through
white to the opposite side of the horseshoe.
1931 CIE Chromaticity Diagram
Chromaticity
x,y (z=1-x-y)
Why do surfaces appear to have
color?
• Most light sources contain all possible
wavelengths in the visible light range (the sun,
light-bulbs,…)
• We see colored surfaces because they absorb the
light of certain wavelengths very strongly. The
reflected light then has a colors complementary to
the color spectrum of the absorption.
– A red surface absorbs cyan very strongly.
Subtractive Mixing
• When you mix red and green paint, you
don’t get yellow. You get black! This is
because the red paint absorbs cyan, and green
absorbs magenta. When mixed together, they
absorb all wavelengths.
• Therefore the three primary colors for
subtractive mixing (in pigments or filters)
are not red, blue and green, they are cyan,
yellow, and magenta. Let’s see why.
Ideal Subtractive Filters
Transmittance
Yellow filter
100%
Pass green & red
Absorb blue
0%
400 nm
500 nm
600 nm
700 nm
Transmittance
Magenta filter
100%
Pass blue
0%
400 nm
Absorb green
500 nm
Pass red
600 nm
700 nm
Transmittance
Cyan filter
100%
Pass blue & green
0%
400 nm
500 nm
Absorb red
600 nm
700 nm
Combining Subtractive Filters
Cyan
filter
Magenta
filter
Blue Light
White
Light
Cyan Light
(B+G+R)
(B+G)
Cyan
filter
Yellow
filter
Green Light
White
Light
Cyan Light
(B+G)
(B+G+R)
Magenta
filter
Yellow
filter
White
Light
(B+G+R)
Red Light
Yellow Light
(G+R)
Simple Subtractive Rules
• Mixing Cyan + Magenta
pigments, one gets Blue
• Mixing Cyan + Yellow
pigments, one gets Green
• Mixing Magenta + Yellow
pigments, one gets Red
• Mixing Cyan + Yellow +
Magenta pigments, one
gets Black
Magenta
Blue
Red
Black
Cyan
Green
Yellow
Dependence of subtractive color
on the light source
• The color of the light reflected from an
object also depends on the light source.
– Under the “golden white” sodium lamps on
highways, some objects lose their color because
there is little green and red light in it.
• To find the color of an object under a nonwhite light source, we need to know the
intensity distribution of the light source and
the absorption spectrum of the surface.
• Even if two illuminating lights look the
same, an object may still appear different
colors in them.
– Under a white light consisting of two narrow
bands of cyan and red, the yellow object can
only appears as red or black.
– However, the same yellow object appears
yellow under sun light.
Partitive Mixing
• Placing small separate sources close to each
other. Your eyes do not see the separate
sources, but the mixed color.
– Color TV: one picture tube but three
electron guns. Electrons from different
guns are directed to different points on the
screen. The screen consists of dots of three
different phosphors, each of which will
produce one of the additive colors.
• Pointillist painters: put small dabs of different
color paint near each other, and look at the
picture from a distance
–
–
–
–
Mosaic
Stained glass
Trees in the fall
Textiles
A Sunday on La Grande Jatte
by Georges Seurat - 1884
Positive afterimage
• Put the colors “near each other” in time.
The colors change so rapidly that the
positive afterimage of one mixes additively
in your eye with the image of the next.
– Rapidly rotating wheel with different color
segments (Color wheel)
Your eyes are exposed to pictures of different
colors!