Color
Clearly, then, interaction among
sensory elements can shape the sensory input. In par-ticular, this process can
highlight elements such as boundaries and moments of change that are of
particular interest to the organism. This pattern of interaction is also
evident when we consider a different aspect of vision—namely, the perception of
color.
A person with normal color vision
can distinguish over 7 million shades of color. But fortunately, this
staggering number of colors can be classified in terms of just three
dimensions. First, hue is the
attribute that distinguishes blue from green from red; it’s also the attribute
shared by, say, a bright orange, a middle orange, and a dark orange. This
dimension corresponds closely to the way we use the word color in everyday life. Hue varies with wavelength (Figure 4.32),
so that a wavelength of 465 nanometers is perceived as unique blue, a blue that’s judged to have no trace of red or green
in it; a wavelength of about 500 nanometers is perceived as unique green (green with no blue or
yellow); and a wavelength of 570 nanometers is perceived as unique yellow (yellow with no green or
red).
Second, brightness is the dimension of color that differentiates black (low
brightness) from white (high brightness) and distinguishes the various shades
of gray in between. Black, white, and all of the grays are the achromatic colors; they have no hue. But
bright-ness is also a property of the chromatic
colors (purple, red, yellow, and so forth). Thus, ultramarine blue is
darker (i.e., has a lower brightness) than sky blue, just as charcoal gray is
darker than pearl gray (Figure 4.33).
The third dimension, saturation, is the “purity” of a
color—the extent to which it is chromatic rather than achromatic. The more gray
(or black or white) that’s mixed with a color, the less saturation it has.
Consider the bottom row of the grid shown in Figure 4.33. All five of these
squares have the same hue (blue), and all have the same bright-ness. The
patches differ only in one way: the proportion of blue as opposed to that of
gray.
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