Graphics Reference
In-Depth Information
40
B
35
30
25
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15
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5
G
R
0
0
0.8
1.0
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Figure 28.13: The set of responses associated to monospectral lights of equal brightnesses.
These form a curve in a plane of constant brightness. Three points, corresponding to per-
cepts of “red,” “green,” and “blue,” are marked on the curve.
For paints, there's something else going on: Red paint absorbs most light
with short wavelengths and reflects most light with longer wavelengths; when
illuminated with white light, it appears red. Similar statements apply to green and
blue paint. When we mix red and green paint, the red paint absorbs much of the
green light, and the green paint absorbs much of the red light, and what's reflected
is a spectral mix of red and green, but not much of either—we see brown.
This, by the way, is the formal explanation of the claim that “lights mix addi-
tively, while paints mix subtractively.”
But the statement that red, green, and blue are primary is only part true. Indeed,
choosing any three points on the curve above covers
some
portion of all possible
perceptual responses and misses others. To really do the job, you'd need infinitely
many “primaries” consisting of all the monospectral lights.
People are sometimes told that “purple isn't a real color,” because it doesn't appear
in the rainbow. (Purple, as we mentioned, is how we describe the color sensation
produced by a mix of red and blue-to-violet light, i.e., near the straight edge of
the horseshoe shape.) It's true that it's not a color sensation corresponding to a
monospectral source, but it certainly is a color sensation.
by Looking at Them in White Light
The claim that objects have colors that are revealed by exposing them to white
light can perhaps be better stated by saying that objects illuminated by sunlight
reflect light with a spectral distribution that provokes a color response in our
brains. But there are many kinds of “white light,” and every actor knows that the
white lights on stage and the white light of the sun are very different, and require
