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Chapter 11 ∣ Color
Copyright Goodheart-Willcox Co., Inc.
Exactly how the brain makes us aware of colors is
still much of a mystery.
Anatomy of the Eye and Vision
When light reaches the eye, it is focused onto
the retina, a layer of light-sensitive cells at the back
of the eye. See Figure 11-28. This occurs mainly by
refraction at the front of the cornea, a transparent
window at the front of the eye. A special lens behind
the pupil and the eye’s muscles control adjustments
needed for focusing. The iris controls the amount
of light entering the eye by altering the size of the
pupil—from 0.08″ (2 mm) in bright sunlight to 0.3″ ″
(8 mm) at night.
Our perception of light and color is determined
by the light-sensitive nerve cells of the retina,
known as rods and cones. There are about twelve
million rods and seven million cones in the human
eye. The rods detect light intensity and are most
sensitive to black, white, and shades of gray. Very
little light is needed to stimulate the rods, so they
are particularly important for night vision.
The retina has three types of cones that
detect both intensity and wavelength (color). The
cones contain light-sensitive chemicals called
photopigments that respond to red, green, or blue.
All colors are mixtures of signals from these three
types of cone receptors. In other words, colors are
perceived according to which cones are stimulated
by wavelengths of light entering the eye. This color-
mixing ability allows us to perceive a multitude
of colors derived from the three primaries. For
Goodheart-Willcox Publisher
Figure 11-28. The rods and cones are located in the retina of the human eye.
Cornea
Iris
Optic Nerve
Cones
Rods
Pupil
H T LIGHT LI LI G G
©George Deal
Figure 11-27. Newer spectrodensitometers read very small
details on plates.