Civil Engineering Reference
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differ in terms of the type of light absorbing pigment they contain. These cones are “tuned” to different
portions of the visible spectrum (e.g., Soderquist, 2002; Purves and Lotto, 2003):
.
Cones that absorb best at the relatively long wavelengths peaking at 575 nm (leading to the percep-
tion of red)
.
Cones with a peak absorption at 535 nm (leading to the perception of green)
.
Cones with a peak absorption at 445 nm (leading to the perception of blue)
A number of predictable contrast and fatigue effects are associated with color vision. These effects can
be demonstrated, for example, by fixating a red square that is placed on a green surrounding. After a few
moments, we begin to see a greenish tinge surrounding the red. Also, an intense green light induces a
reddish afterimage. The same effect can be observed also for a blue square on a yellow background,
or, more generally, for any complementary hues, that is, hues that, when mixed, form neutral grays.
While the fovea contains almost exclusively cones, the periphery of the retina is inhabited by both rods
and cones, with the number of cones declining rapidly with increasing eccentricity (Figure 23.3). As a
result, color discrimination is degraded at eccentricities beyond 20 to 30
. For example, the relative
brightness and perceived hues change, causing red and green to appear yellow. Also, colors that are per-
ceived in the periphery tend to be less saturated. This gradual reduction in the ability to discriminate
colors in the periphery can be compensated for, to some extent, by increasing the size, luminance, or sat-
uration of the stimulus. However, complete color blindness starts at around 40 to 50
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of visual angle.
Another factor that can limit color vision is color blindness or color deficiency. Complete color blind-
ness in people is extremely rare. Only about 0.005% of the population is truly color blind, that is, they
completely lack at least one of the photopigments used to transmit color information (Cornsweet, 1970).
However, for about 8% of males and 0.5% of females, some color distinctions are absent or at least not as
pronounced. In these cases, all photopigments are present but their responses are slightly altered. People
with color deficiencies most often have trouble distinguishing between red and green, a form of color
deficiency that is referred to as “protanopia”. They discriminate between the two hues based on perceived
brightness instead. For example, red colors appear darker to a person with a deficiency of “red” photo-
pigment cones (Murch, 1984). The need to account for users with color blindness or deficiency can be
considered one of the reasons for the general design recommendation to create monochrome displays
first and add color only at a later stage (Shneiderman, 1998).
As mentioned earlier, cones are not only responsible for color vision; they also support high acuity
vision. Their ability to resolve detail is much greater than for rods which, in turn, display greater
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FIGURE 23.3 Distribution of rods and cones in the retina. (Adapted from Rosenzweig, M.R., Leiman, A.L., and
Breedlove, S.M., Biological Psychology, Sunderland, MA, Sinauer Associates Inc., 1996.)
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