Theories of Color Vision

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Chapter 3 Sensation and Perception
Certain cells in the brain’s cerebral cortex are called feature detectors because they
respond to specific characteristics of objects in the visual world (Hubel & Wiesel, 1979).
For example, one type of feature detector specializes in responding to straight lines.
Others respond to corners, to angles, or to some other feature. The combined responses
of several types of feature-detecting cells allow us to sense the shapes of objects, such
as rectangles or triangles. Most people can also detect color. Let’s explore how color
vision works.
Seeing Color
Like beauty, color is in the eye of the beholder. Many animals see only shades of gray,
even when they look at a rainbow, but for humans, color is an important feature of vision.
At a given intensity, each wavelength of
light is sensed as a certain color. However, the eye rarely, if ever, encounters pure light
of a single wavelength. Sunlight, for example, is a mixture of all wavelengths of light. When
sunlight passes through a droplet of water, each different wavelength of light bends to
a different degree, separating into a colorful rainbow. The spectrum of color found in
the rainbow illustrates an important concept: The sensation produced by a mixture of
different wavelengths of light is not the same as the sensations produced by separate
wavelengths.
The sensation of a color results from features of the wavelength mixtures striking
the eye. The three separate aspects of this sensation are hue, saturation, and brightness. These are psychological dimensions that correspond roughly to the physical
properties of light. Hue, the essential “color,” is determined by the dominant wavelength in the mixture of the light. Black, white, and gray are not considered hues,
because they do not have a dominant wavelength. Saturation is related to the purity
of the color. A color is more saturated (purer) if just one wavelength is more
intense—contains more energy—than other wavelengths. The yellow of a school bus
and the red of a stop sign are saturated colors. Add in many other wavelengths, and
the color is said to be desaturated. Pastels are colors that have been desaturated by
the addition of whiteness. Brightness refers to the overall intensity of the wavelengths making up light.
The color circle shown in Figure 3.9 arranges hues according to their perceived similarities. Mix two different light wavelengths of equal intensity, and the color you
sense is midway between the two original colors on the color circle. This process is
called additive color mixing, because the effects of the wavelengths are added together.
Keep adding different colored lights and you eventually get white, which is the combination of all wavelengths. You are probably more familiar with a different form of
color mixing, called subtractive color mixing, which occurs when paints are combined.
Paint, like other physical objects, reflects certain wavelengths and absorbs others.
Grass is green because it absorbs all wavelengths except wavelengths perceived as
green. White objects appear white because they reflect all wavelengths. So if you keep
combining different colored paints, all of the wavelengths will eventually be subtracted, resulting in black.
Wavelengths and Color Sensations
feature detectors Cells in the cortex
that respond to a specific feature of an
object.
hue The essential color determined by
the dominant wavelength of a light.
saturation
The purity of a color.
brightness The overall intensity of the
wavelengths making up light.
trichromatic theory
A theory of color
vision stating that information from
three types of visual elements combines
to produce the sensation of color.
Theories of Color Vision
Psychologists have long tried to explain how color vision works, but only two theories
have stood the test of time: trichromatic (or “three-color”) theory and opponentprocess theory.
The Trichromatic Theory of Color Vision In the early 1800s, Thomas Young
and, later, Hermann von Helmholtz proved that by mixing pure versions of blue, green,
and red light in different ratios, they could produce any other color. Their theory of
color vision is called the trichromatic theory.
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Seeing
FIGURE
3.9
NONSPECTRAL HUES
No single wavelength
produces these colors.
The Color Circle
Arranging colors according to their psychological similarities creates a color circle that predicts the result of additive
mixing of two colored lights. For example,
mixing equal amounts of pure green and
pure red light will produce yellow, the
color that lies at the midpoint of the line
connecting red and green. (Note: nm
stands for nanometers, the unit in which
light wavelengths are measured.)
420
nm
700 nm Red
Blue
470 nm
600 nm
Green
500 nm
570
nm Yellow
510
nm
These colors are produced by either
a single wavelength or a mixture.
SPECTRAL HUES
Support for trichromatic theory comes from research on cones in the retina. There
are three types of cones, and each is most sensitive to particular wavelengths. Shortwavelength cones respond most to light in the blue range. Medium-wavelength cones
are most sensitive to light in the green range. Long-wavelength cones respond best to
light in the reddish-yellow range, but, by tradition, they are known as “red” cones. No
single cone by itself can signal the color of a light. It is the ratio of the activities of the
three types of cones that determines what color will be sensed. As you can see in Figure 3.10, the exact mixture of these three cone types can differ from person to person.
The trichromatic theory was applied in the creation of color television screens, which
contain microscopic elements of red, green, and blue. A television broadcast excites
these elements to varying degrees, mixing their colors to produce many other colors.
You see color mixtures, not patterns of red, green, and blue dots, because the dots are
too small and close together to be seen individually.
The
vivid array of colored powders offered by
this vendor in India allows him to create
virtually any combination of hue (color),
saturation (purity), and brightness that a
customer might request.
THE SENSATION OF COLOR
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