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The ability to discriminate light on the basis of wavelength composition. It is found in humans, in other primates, and in certain species of birds, fishes, reptiles, and insects. These animals have visual receptors that respond differentially to the various wavelengths of visible light. Each type of receptor is especially sensitive to light of a particular wavelength composition. Evidence indicates that primates, including humans, possess three types of cone receptor, and that the cones of each type possess a pigment that selectively absorbs light from a particular region of the visible spectrum. The trichromatic system of colorimetry, using only three primary colors, is based on the concept of cone receptors with sensitivities having their peaks, respectively, in the long, middle, and short wavelengths of the spectrum.
Color is usually presented to the individual by the surfaces of objects on which a more or less white light is falling. A red surface, for example, is one that absorbs most of the short-wave light and reflects the long-wave light to the eye. A set of primary colors can be chosen so that any other color can be produced from additive mixtures of the primaries in the proper proportions. Thus, red, green, and blue lights can be added together in various proportions to produce white, purple, yellow, or any of the various intermediate colors. Three-color printing, color photography, and color television are examples of the use of primaries to produce plausible imitations of colors of the original objects.
Colors lying along a continuum from white to black are known as the gray, or achromatic, colors. They have no particular hue. Whiteness is a relative term; white paper, paint, and snow reflect some 80% or more of the light of all visible wavelengths, while black surfaces typically reflect less than 10% of the light. The term white is also applied to a luminous object, such as a gas or solid, at a temperature high enough to emit fairly uniformly light of all visible wavelengths.
Color blindness is a condition of faulty color vision. It appears to be the normal state of animals that are active only at night. It is also characteristic of human vision when the level of illumination is quite low or when objects are seen only at the periphery of the retina. Under these conditions, vision is mediated not by cone receptors but by rods, which respond to low intensities of light. In rare individuals, known as monochromats, there is total color blindness even at high light levels. Such persons are typically deficient or lacking in cone receptors, so that their form vision is also poor.
Dichromats are partially color-blind individuals whose vision appears to be based on two primaries rather than the normal three. Dichromatism occurs more often in men than in women because it is a sex-linked, recessive hereditary condition. One form of dichromatism is protanopia, in which there appears to be a lack of normal red-sensitive receptors. Red lights appear dim to protanopes and cannot be distinguished from dim yellow or green lights. A second form is deuteranopia, in which there is no marked reduction in the brightness of any color, but again there is a confusion of the colors normally described as red, yellow, and green. A third and much rarer form is tritanopia, which involves a confusion among the greens and blues. See Human genetics
Many so-called color-blind individuals might better be called color-weak. They are classified as anomalous trichromats because they have trichromatic vision of a sort, but fail to agree with normal subjects with respect to color matching or discrimination tests. Protanomaly is a case of this type, in which there is subnormal discrimination of red from green, with some darkening of the red end of the spectrum. Deuteranomaly is a mild form of red-green confusion with no marked brightness loss. Nearly 8% of human males have some degree of either anomalous trichromatism or dichromatism as a result of hereditary factors; less than 1% of females are color-defective.
Color blindness is most commonly tested by the use of color plates in which various dots of color define a figure against a background of other dots. The normal eye readily distinguishes the figure, but the colors are so chosen that even the milder forms of color anomaly cause the figure to be indistinguishable from its background.
Techniques of microspectrophotometry have been used to measure the absorption of light by single cone receptors from the eyes of primates, including humans. The results confirm that three types of cone receptors are specialized to absorb light over characteristic ranges of wavelength, with maximum absorption at about 420, 530, and 560 nanometers. In addition there are rod receptors sensitive to low intensities of light over a broad range of wavelengths peaking at about 500 nm. In each of the four types of receptor there is a photosensitive pigment that is distinguished by a particular protein molecule. This determines the range and spectral location of the light which it absorbs.
Central nervous system factors are also evident. Color vision, like other forms of perception, is highly dependent on the experience of the observer and on the context in which the object is perceived. See Eye (invertebrate), Eye (vertebrate), Nervous system (vertebrate), Perception, Photoreception, Vision
the ability of the human eye and the eye of many diurnally active animals to distinguish colors, that is, to perceive differences in the spectral composition of visible light and in the coloring of objects.
The visible part of the spectrum includes various wavelengths perceived by the eye in the form of different colors. Color vision results from the combined functioning of several types of retinal photoreceptors differing in spectral sensitivity. The photoreceptors transform radiant energy into physiological excitation, which is perceived by the nervous system as different colors because different wavelengths excite the receptors unequally. The spectral sensitivity of the various photoreceptors varies with the spectrum of absorption of visual pigments. No photoreceptor can distinguish colors by itself. For any photoreceptor all types of light differ in a single parameter—luminosity—since light of any spectral composition has qualitatively the same physiological action on each of the light pigments. Thus, different types of light having a certain correlation of intensity cannot be completely differentiated from each other by a single receptor. If the retina has several receptors, the correlation of intensity that results in such equality for each will vary. Therefore, for a combination of several receptors many types of light cannot be equated with any set of intensities.
The underlying principles of current theories of human color vision were worked out in the 19th century by the English physicist T. Young and the German scientist H. von Helmholtz. According to the Young-Helmholtz trichromatic theory of color perception, the human retina has three types of receptors, or cones, sensitive in varying degrees to red, green, and blue light. However, the physiological mechanism of light perception does not make it possible to differentiate all types of light. For example, mixtures of red and green in certain proportions cannot be distinguished from yellow-green, yellow, or orange light. Mixtures of blue and orange can be equated with mixtures of red and blue or with blue-green. Some individuals suffer from a hereditary absence of one or two of the three photoreceptors. If two photoreceptors are absent, color vision is not possible.
Color vision characterizes many animals other than humans. Many vertebrates (for example, monkeys, fishes, amphibians) and insects (for example, honeybees and bumble bees) have trichromatic vision. Color vision is dichromatic, that is, it is based on the functioning of two types of photoreceptors, in susliks and many insect species. Birds and turtles may have four types of photoreceptors. For insects, the visible part of the spectrum is shifted toward shortwave light and includes the ultraviolet range. Thus, the insect world of colors is quite different from man’s.
The principal biological significance of color vision for humans and other animals existing in the world of non-self-luminous objects is correct recognition of coloration and not merely differentiation of light. The spectral composition of reflected light depends both on the coloring of the object and on the incident light. It therefore changes significantly with changes in lighting conditions. The capacity of the visual apparatus to identify the coloring of objects correctly from their reflective properties under changing lighting conditions is called constancy of color perception.
Color vision is an important element in the visual orientation of animals. In the course of evolution, many animals and plants developed various means of signaling that enable animal “observers” to perceive color. Examples are the brightly colored crowns of flowers that attract insect and bird pollinators and the bright coloration of fruits and berries that attract seed-scattering animals. In the animal world examples are the warning and repellent coloration of poisonous animals and species that mimic them, the “billboard” signaling coloring of many tropical fishes and lizards, the brilliant seasonal or constant nuptial dress of many fishes, birds, reptiles, and insects, and the special means of signaling used by birds and fish to facilitate relations between parents and offspring.
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