Introduction
Light is a form of radiant energy that is absorbed by sensory cells in the retina of the eye. The absorbing cells, called photoreceptors, convert light into the electrical energy of nerve impulses. The impulses generated by photoreceptors travel along the optic nerves to the optic lobes of the brain, where they are integrated into perception of a visual image.
The energy of light follows a wave path through space; the distance from crest to crest in a wave path is called the wavelength. The wavelengths of light that are visible to humans fall between about 400 nanometers (seen as blue light) and 750 nanometers (seen as red light). Wavelengths outside this range are invisible to humans because human photoreceptors are not “tuned” to receive and convert them to electrical energy.
The cornea and lens of the eye, acting together, focus light rays reflected from objects in the environment into a picturelike image that falls on the retina of the eye. The retina contains the photoreceptors of the eye, called rods and cones because of their elongated shapes. The cones, which are shorter in length than the rods and conically shaped at their outer tips, are the photoreceptors responsible for color vision. The retina contains about 110 to 120 million rods and 6 million cones. More than half the cones are concentrated in the fovea, where rods are completely absent.
Cones
There are three types of cones in the retina, distinguished by the kind of pigment molecules they contain. Different pigments absorb light at different wavelengths. One pigment, cyanolabe, absorbs maximally at approximately 440 nanometers (blue light near the border of the spectrum with violet); one, chlorolabe, at approximately 545 nanometers (green light); and one, erythrolabe, at approximately 580 nanometers (yellow light near the border with orange), though the exact absorption maxima of each type varies from person to person.
Each type of cone cell contains only one of the three pigments. As a result, there is one population of cones in the retina that absorbs blue light maximally, one population absorbing green, and one absorbing yellow. By convention, these different cone cells are referred to as blue, green, and red cones, respectively (the prefix "erythr-," as in "erythrolabe," means "red"). The three types of cones are mixed intimately in the fovea, the region of clearest vision in the retina.
Each photoreceptor type also absorbs other wavelengths near their absorption maxima, although less efficiently. For example, red cones, which absorb maximally at yellow wavelengths, actually absorbs wavelengths beginning at about 460 nanometers and extending to nearly 700. As wavelengths are encountered farther from a cone's absorption maximum, light absorption becomes progressively less efficient. This results in a smooth absorption curve that starts near zero on each side and peaks at the 580-nanometer wavelength.
The total ranges absorbed by the three cone types overlap, so that light at any wavelength in the visible range is likely to be absorbed by and stimulate at least two of the three photoreceptor types. For example, orange light at 590 nanometers is absorbed by and stimulates both the green and red cones, but not the blue cones. The green and red cones, however, are stimulated to a different extent: at 590 nanometers, the red cones are stimulated almost maximally, while the green are stimulated to only about 40 percent of their maximum.
Color Perception
This difference in the absorption and stimulation of cones by light of a given wavelength is considered to underlie human perception of color. For example, when light stimulates the red cones at 99 percent, the green cones at 40 percent, and the blue cones at 0 percent, the color is perceived as orange. A wavelength stimulating the blue and green cones at 50 percent of their maxima and the red cones at 5 percent is perceived as a blue-green color.
Light at wavelengths above about 620 nanometers stimulates only the red cones at or below 70 percent of their maximum; these wavelengths are perceived as red colors. This is why the photoreceptors absorbing maximally in the yellow wavelengths are typically identified as red, rather than yellow, cones. Similarly, light at about 420 nanometers stimulates only the blue photoreceptors and is perceived as a deep blue. Light that stimulates all three cone types equally is perceived as white. White is strictly a perceived color; there is no wavelength of light corresponding to white.
In response to absorbing light at various levels nearer or farther from their maxima, the photoreceptors generate nerve impulses. When absorbing at its maximum, a photoreceptor generates impulses at the highest frequency; at levels farther from the maximum, the frequency of impulses is proportionately reduced. The impulses sent by the three types of cones at various frequencies are partially integrated into color perception in the complex nerve circuitry of the retina, which may be considered as an extension of the brain into the eye, and partly in the optic lobes at the rear of the cerebral cortex. When objects are viewed in bright light, the total integration reconstructs the image focused in the fovea of the retina as a full-color perception of the scene viewed.
Each cone in the fovea has essentially a straight-line connection through neurons to the optic lobes. As a result, each detail of light, shade, and color in the image is likely to register as differences in stimulation between neighboring cones in the fovea and to be registered and transmitted separately to the visual area of the brain. This arrangement specializes the cones in the fovea for the detection of minute details in full color.
Dark Adaptation
Color reception by the cones is most efficient in bright light. As light intensity falls during and after sunset, stimulation of the cones drops off rapidly. (Compared to the rods of the retina, cones have relatively little ability to adapt to dark.) The red cones drop out first, so that colors in the yellow, orange, and red wavelengths fade and, in deep twilight, appear gray or black. The blue and green cones still retain some sensitivity at twilight, so that blues and greens can still be perceived. In deepest twilight, only the blue photoreceptors are stimulated, so that if any color can be perceived at all, the scene appears blue-black. The shift in color sensitivity toward the greens and blues in reduced light also explains why green fields and trees look so rich in color, and reds and yellows so dull, on overcast or rainy days.
Adaptation to darkness occurs through an increase in the amount of pigment molecules in both the cone and rod photoreceptors. The ability of the cone cells to increase their quantities of pigment molecules is limited as compared to the rods, which can greatly increase their pigment quantities and their sensitivity to light. As a result, as light intensity decreases, visual perception shifts from the cones to the rods, which detect light but are not stimulated differentially by different wavelengths. This produces the perception of images of grays and blacks rather than color. Because the rods are outside the region of sharp vision in the fovea, objects are perceived only as relatively unfocused, fuzzy images in light of very low intensity.
The rods are completely insensitive to red light. Therefore, it is possible to become completely dark-adapted even if relatively bright red light is used as a source of illumination. For this reason, persons who must work under conditions of reduced light, such as pilots flying at night, commonly use red light for required illumination.
Color Blindness
Individuals who are color-blind
carry gene mutations that reduce or inhibit the synthesis of one or more of the three color-absorbing pigments of the cones in the retina. A protanope, an individual who carries a mutation inhibiting synthesis of the erythrolabe, or yellow-absorbing pigment, is insensitive to red, orange, and yellow wavelengths and perceives all these colors as the same gray or greenish hue. Typically, such individuals cannot distinguish between green and red. A deuteranope, a person lacking the chlorolabe, or green-absorbing pigment, is also unable to distinguish between red, orange, yellow, and green. Since their inability to distinguish among red, orange, yellow, and green is similar, both protanopes and deuteranopes are classified as red-green color-blind. A tritanope, an individual deficient in the cyanolabe, or blue-absorbing pigment, cannot distinguish between blue and green. Persons deficient in all three pigments cannot perceive color and see the world only in shades of gray. Mutations affecting synthesis of the chlorolabe and erythrolabe pigments, producing green and red color blindness, are most common. About 2 percent of men are deficient in the erythrolabe pigment, and about 6 percent of men are deficient in the chlorolabe pigment, giving a total of about 8 percent of men who are red-green color-blind. The total red-green color blindness among women is about 2 percent. Blue color blindness is relatively rare in the human population; only about one in as many as sixty-five thousand people is deficient in cyanolabe.
Color blindness affects males more often than females—about twenty times more frequently—because it is a sex-linked, recessive trait. A color-blind father cannot pass the trait to any of his sons. A color-blind mother who has children with a man with normal vision will pass the trait to all of her sons. Her daughters will have normal vision but will be carriers of the trait. The sons of a female carrier of the trait and a male with normal vision have a 50 percent chance of being color-blind; all the daughters are expected to have normal vision, but they have a 50 percent chance of being carriers of the trait. Deficiencies in color vision are presently uncorrectable.
Color Commentary
The beginnings of an understanding of color vision go back to 1801, when the English physicist Thomas Young
proposed that the human eye has only three different kinds of receptors for color. According to Young, the ability to sense the hundreds of different colors that humans can recognize depends on the interaction of the three receptor types. Young based his idea on the fact that painters can mix any color by starting from only three primary colors: red, blue, and yellow. Orange, for example, can be mixed from equal quantities of red and yellow. Young’s highly perceptive explanation for this was that wavelengths in the orange range are not actually produced when light is reflected from mixed red and yellow pigments. Instead, he proposed that the mixture of red and yellow stimulates red and yellow receptors in the eye equally. This equal stimulation is summed and interpreted in the brain as the color orange. Young also proposed that the sensation of white is produced by equal stimulation of all three receptors. These proposals, which turned out to be essentially correct, were later expanded by the German physicist and physiologist Hermann von Helmholtz into what is now known as the Young-Helmholtz trichromatic theory of color vision.
Two lines of later research confirmed Young and Helmholtz's proposals that there are only three types of color photoreceptors in the eye. One series of experiments, carried out in the 1960s by Paul K. Brown and George Wald at Harvard University and Edward F. MacNichol, William H. Dobelle, and William B. Marks at Johns Hopkins University, measured the wavelengths of light stimulating individual cones to generate nerve impulses. This work revealed that there are actually only three different types of cones, each absorbing light maximally at the blue, green, or yellow-orange wavelengths. These colors differ to some extent from the red, blue, and yellow photoreceptors proposed by Young and Helmholtz, whose ideas were derived primarily from mixing painter’s pigments; however, they are exactly the colors used if colored lights rather than painter’s pigments are used to mix additional colors from three primary colors.
The second line of major supporting evidence came from experiments carried out by Wald, William A. H. Rushton, and others that identified and isolated the pigments responsible for light absorption in the eye. Some of these experiments were done by the simple but elegant technique of shining a white or colored light into the eye and then analyzing the light reflected from the retina. The reflected light was missing the colors absorbed by the pigments in the eye. These experiments detected only three different pigments in the cones—cyanolabe, chlorolabe, and erythrolabe—just as predicted by Young and Helmholtz so many years before.
Bibliography
Albers, Josef. Interaction of Color. 50th anniv. ed. New Haven: Yale UP, 2013. Print.
Ball, Philip. Bright Earth: Art and the Invention of Color. Chicago: U of Chicago P, 2003. Print.
Eckstut, Joann, and Arielle Eckstut. The Secret Language of Color: Science, Nature, History, Culture, Beauty of Red, Orange, Yellow, Green, Blue & Violet. New York: Black Dog, 2013. Print.
Koenderink, Jan. Color for the Sciences. Cambridge: MIT P, 2010. Print.
Hunt, R. W. G., and M. R. Pointer. Measuring Colour. 4th ed. Chichester: Wiley, 2011. Print.
Koeppen, Bruce M., and Bruce A. Stanton, eds. Berne & Levy Physiology. 6th ed. Philadelphia: Mosby, 2010. Print.
Gegenfurtner, Karl R., and Lindsey T. Sharpe, eds. Color Vision: From Genes to Perception. New York: Cambridge UP, 2001. Print.
Malacara, Daniel. Color Vision and Colorimetry: Theory and Applications. 2nd ed. Bellingham: SPIE, 2011. Print.
Orna, Mary Virginia. The Chemical History of Color. Heidelberg: Springer, 2013. Print.
Palmer, Stephen E. Vision Science: Photons to Phenomenology. Cambridge: MIT P, 1999. Print.
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