250
Graphic Communications
Copyright Goodheart-Willcox Co., Inc.
example, despite yellow’s pure appearance, it is not
a pure color. There is no retinal receptor sensitive to
the yellow frequency. The color yellow is perceived
through the combined activity of the red-sensitive
and green-sensitive cones.
With three colors—red, green, and blue—it
would seem logical that the cones would have
an even distribution of 33% for each color. The
reality is that we have 64% red cones, 32% green,
and only 2% blue. The missing 2% accounts for
damaged cones. Using red and green for stoplights
is not an arbitrary decision.
The macula is a small hollow in the middle
of the retina. The cones are concentrated in the
macula and diminish in concentration toward the
edges of the retina. At the center of the macula
is the fovea, the most sensitive part of the retina.
The fovea is packed with cone cells and is the area
of sharpest vision. Unlike rods, the cones need
high levels of light. This is why colors often appear
muted or absent at night.
Brightness and Intensity
The cones and rods of the retina are extremely
sensitive light detectors. Even the smallest amount
of radiant energy stimulates them. The intensity
of light entering the eye causes the sensation of
brightness.
Bright light bleaches the color-sensitive cones
of the retina. This bleaching stimulates the nerves.
After exposure to bright light, it takes time for the
photochemical activity of the eye to return to normal.
A good example of this effect is when a person is
temporarily blinded by a camera flash. During the
time a region of the retina is bleached, that region
is less sensitive than surrounding regions. This
can cause both positive and negative afterimages.
Afterimages are created when the eye attempts
to restore equilibrium. Gazing for some time at a
green square and then closing your eyes causes
a red square to appear as an afterimage. If the
square is red, a green square will be the afterimage.
This experiment demonstrates that, with any color,
the afterimage is always the complementary color,
Figure 11-29. The technical name for this color-
vision effect is successive contrast.
Opponent Color Theory
Scientists have established how additive and
subtractive colors work and how they have solidified
the additive color wheel. Referring to Figure 7-20,
note that magenta is the opposite or complementary
color to green. Yet magenta is the only color that
does not appear in the prism and does not have
a wavelength. The only way to get magenta is to
eliminate all traces of green. Magenta is the “anti-
green.” So when there is no green, rather than
seeing actual magenta, our brains merely perceive
magenta.
Afterimages are a perfect example of Ewald
Hering’s opponent-process theory of color vision,
first proposed in 1878. The opponent-process
color theory suggests that there are three basic
systems of opposing neurons that define our color
perception. These pairs are red and green, blue and
yellow, and white and black. See Figure 11-30. Our
eye sees the wavelengths. It sees more red and
green, as the number of cones far exceeds those
for blue, and the neurons carry the positive red
(excitatory) or negative green (inhibitory) responses
to the brain.
The color pairs should sound familiar. The most
accurate and even distribution of colors comes from
the CIELAB color space, which is device-independent.
These dimensions are represented by these
designations:
• L. Lightness, black and white.
• A. Red to green values where positive a* values
(+a*) appear reddish, and negative a* values
(–a*) appear greenish.
Goodheart-Willcox Publisher
Figure 11-29. To see an afterimage, stare at the center of
this flag for about 30 seconds. Then, look at a white sheet of
paper. You will see an image of the flag with its proper colors.