Graphics Reference
In-Depth Information
One last note about spectral power distributions: Ordinary incandescent lights
(especially those with a clear glass bulb) have spectral power distributions that
are quite similar to the blackbody radiation described in Chapter 26, because
they produce light by heating a piece of metal (tungsten, typically) to a very
high temperature—such as 2500 C—by pushing electric current through it; the
resultant emission begins to approximate the blackbody curve, even though the
tungsten itself is not matte black. (The sun, by contrast, has a surface tempera-
ture of around 6000 C.) One important characteristic of this radiation is that the
SPD is quite smooth, rather than being very “spiky.” This makes simple summary
descriptors like “dominant wavelength” and “excitation purity” work quite well
for such smooth SPDs.
28.3 The Phenomenon of Color Perception
and the Physiology of the Eye
People with unimpaired vision perceive light; they describe their sensations of it
in various terms like “brightness” and “hue” and with a great many individual
words (“saffron,” “teal,” “indigo,” “aqua,” ...) that capture individual sensations
of color.
Our perception of color is also influenced by a gestalt view of the world: We
use different words to describe the color of things that emit light and to describe
those that reflect light. People will describe an object as “brown,” but they will
almost never speak of a “brown light.”
This same gestalt view allows us to understand the “colors of objects.” One
might say that a yellow book, in a completely dark closet, is black, but people are
more inclined to say that it's yellow but not lit right now. Certainly in a dimly lit
room, the light leaving the yellow book's surface is different from that leaving the
surface in a well-lit room, and yet we describe the topic as “yellow” in both cases.
Our ability to detect something about color in a way that's partly independent of
illumination is termed color constancy.
Of course, one can imagine an experiment in which one looks through a peep-
hole and sees something behind it. The something might be a glowing yellow
bulb, or it might be a yellow piece of paper reflecting the light from an incan-
descent bulb. When the object is seen from a distance, and without other objects
nearby for comparison, one cannot tell the difference between the two. So the
distinction between “emitters” and “reflectors” is not one that's captured by the
physics of the light entering the eye, but by the overall context in which the light
is seen.
By the way, to experiment with color, it turns out that “color matching” is
different from “color naming”: Saying the name of a color is more complex than
matching a color with another during an experiment.
It's commonplace to say that “intensity” is independent of “hue”: One can
have a bright blue light or a dim blue light, and the same goes for red and yel-
low and orange and green. In the same way, the degree of “saturation” of a
color—Is it really red, or is it pinkish, or a grayish-red?—appears independent
of both intensity and hue. But it's difficult to think of a fourth property of color
that's independent of these three. This suggests that perhaps color is defined by
three independent characteristics, which we'll later see is true. Just which three
 
 
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