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color, effect produced on the eye and its associated nerves by light waves of different wavelength or frequency. Light transmitted from an object to the eye stimulates the different color cones of the retina, thus making possible perception of various colors in the object.
The Visible Spectrum
Apparent Color of Objects
Properties of Colors
Symbolic Uses of Color
See G. Wyszecki and W. S. Stiles, Color Science (1967); M. W. Levine and J. M. Shefner, Fundamentals of Sensation and Perception (1991).
That aspect of visual sensation enabling a human observer to distinguish differences between two structure-free fields of light having the same size, shape, and duration. Although luminance differences alone permit such discriminations to be made, the term color is usually restricted to a class of differences still perceived at equal luminance. These depend upon physical differences in the spectral compositions of the two fields, usually revealed to the observer as differences of hue or saturation.
Color discriminations are possible because the human eye contains three classes of cone photoreceptors that differ in the photopigments they contain and in their neural connections. Two of these, the R and G cones, are sensitive to all wavelengths of the visible spectrum from 380 to 700 nanometers. (Even longer or shorter wavelengths may be effective if sufficient energy is available.) R cones are maximally sensitive at about 570 nm, G cones at about 540 nm. The ratio R/G of cone sensitivities is minimal at 465 nm and increases monotonically for wavelengths both shorter and longer than this. This ratio is independent of intensity, and the red-green dimension of color variation is encoded in terms of it. The B cones, whose sensitivity peaks at about 440 nm, are not appreciably excited by wavelengths longer than 540 nm. The perception of blueness and yellowness depends upon the level of excitation of B cones in relation to that of R and G cones. No two wavelengths of light can produce equal excitations in all three kinds of cones. It follows that, provided they are sufficiently different to be discriminable, no two wavelengths can give rise to identical sensations.
Different complex spectral distributions usually, but not always, look different. Suitable amounts of short-, middle-, and long-wavelength lights, if additively mixed, can for example excite the R, G, and B cones exactly as does a light containing equal energy at all wavelengths. As a result, both stimuli look the same. This is an extreme example of the subjective identity of physically different stimuli known as chromatic metamerism. Additive mixture is achievable by optical superposition, rapid alternation at frequencies too high for the visual system to follow, or (as in color television) by the juxtaposition of very small elements which make up a field structure so fine as to exceed the limits of visual acuity. See Light
Although colors are often defined by appeal to standard samples, the trivariant nature of color vision permits their specification in terms of three values. Ideally these might be the relative excitations of the R, G, and B cones. Because too little was known about cone action spectra in 1931, the International Commission on Illumination (CIE) adopted at that time a different but related system for the prediction of metamers (the CIE system of colorimetry). This widely used system permits the specification of tristimulus values X, Y, and Z, which make almost the same predictions about color matches as do calculations based upon cone action spectra. If, for fields 1 and 2, X1 = X2, Y1 = Y2, and Z1 = Z2, then the two stimuli are said to match (and therefore have the same color) whether they are physically the same (isometric) or different (metameric).
Colors are often specified in a two-dimensional chart known as the CIE chromaticity diagram, which shows the relations among tristimulus values independently of luminance. In this plane, y is by convention plotted as a function of x, where y = Y/(X + Y + Z) and x = x/(x + Y + Z). [The value z = Z/(X + Y + Z) also equals 1 - (x + y) and therefore carries no additional information.] Such a diagram is shown in the illustration, in which the continuous locus of spectrum colors is represented by the outermost contour. All nonspectral colors are contained within an area defined by this boundary and a straight line running from red to violet. The diagram also shows discrimination data for 25 regions, which plot as ellipses represented at 10 times their actual size. A discrimination unit is one-tenth the distance from the ellipse's center to its perimeter. Predictive schemes for interpolation to other regions of the CIE diagram have been worked out.
A chromaticity diagram has some very convenient properties. Chief among them is the fact that additive mixtures of colors plot along straight lines connecting the chromaticities of the colors being mixed. Although it is sometimes convenient to visualize colors in terms of the chromaticity chart, it is important to realize that this is not a psychological color diagram. Rather, the chromaticity diagram makes a statement about the results of metameric color matches, in the sense that a given point on the diagram represents the locus of all possible metamers plotting at chromaticity coordinates x, y. However, this does not specify the appearance of the color, which can be dramatically altered by preexposing the eye to colored lights (chromatic adaptation) or, in the complex scenes of real life, by other colors present in nearby or remote areas (color contrast and assimilation). Nevertheless, within limits, metamers whose color appearance is thereby changed continue to match.
For simple, directly fixated, and unstructured fields presented in an otherwise dark environment, there are consistent relations between the chromaticity coordinates of a color and the color sensations that are elicited. Therefore, regions of the chromaticity diagram are often denoted by color names, as shown in the illustration.
Although the CIE system works rather well in practice, there are important limitations. Normal human observers do not agree exactly about their color matches, chiefly because of the differential absorption of light by inert pigments in front of the photoreceptors. Much larger individual differences exist for differential colorimetry, and the system is overall inappropriate for the 4% of the population (mostly males) whose color vision is abnormal. The system works only for an intermediate range of luminances, below which rods (the receptors of night vision) intrude, and above which the bleaching of visual photopigments significantly alters the absorption spectra of the cones.
Color (quantum mechanics)
A term used to refer to a hypothetical quantum number carried by the quarks which are thought to make up the strongly interacting elementary particles. It has nothing to do with the ordinary, visual use of the word color.
The quarks which are thought to make up the strongly interacting particles have a spin angular momentum of one-half unit of h (Planck's constant). According to a fundamental theorem of relativity combined with quantum mechanics, they must therefore obey Fermi-Dirac statistics and be subject to the Pauli exclusion principle. No two quarks within a particular system can have exactly the same quantum numbers. See Exclusion principle, Fermi-Dirac statistics
However, in making up a baryon, it often seemed necessary to violate this principle. The &OHgr;- particle, for example, is made of three strange quarks, and all three had to be in exactly the same state. O. W. Greenberg is responsible for the essential idea for the solution to this paradox. In 1964 he suggested that each quark type (u, d, and s) comes in three varieties identical in all measurable qualities but different in an additional property, which has come to be known as color. The exclusion principle could then be satisfied and quarks could remain fermions, because the quarks in the baryon would not all have the same quantum numbers. They would differ in color even if they were the same in all other respects. See Baryon, Elementary particle, Meson, Quarks
in art, man’s artistic expression of his ability to perceive reality in the whole wealth of color. Color is associated with all elements of artistic form, for example, composition, spatial perspective, texture, and palette. It permeates the entire material embodiment of a work of art.
Color may define the degree of distance of an object in a picture’s space (color perspective) or the relationship of an object with other objects and its surroundings. It may express the material characteristics of a particular object or its parts, as well as the general emotional tone of the artistic image. Color may form conventional systems, having a symbolic meaning (especially evident in medieval art and in art produced in the early stages of a culture). Each period in the development of world art has had its own ideas on the use of color. Color is related to style, school, and creative method.
REFERENCESMatsa, I. L. “Problema tsveta v iskusstve,” Iskusstvo, 1933, nos 1–2.
Regel, G. Grundfragen des farbigen Gestaltens. Berlin, 1961.
V. S. TURCHIN
a property of physical objects perceived as a conscious visual sensation. The various colors are “conferred” by humans on an object as the object is perceived visually.
Color sensation arises in the great majority of cases as a result of the effect produced on the eye by fluxes of electromagnetic radiation from the range of wavelengths in which such radiation can be perceived by the eye (the visible range of wavelengths is from 380 to 760 nanometers). Color sensation sometimes occurs without the action of a radiant flux on the eye as a result of pressure on the eyeball, a blow, electrical stimulation, mental association with other sensations (such as sound and heat), or the working of the imagination. Differently colored objects, differently illuminated parts of the objects, different light sources, and the illumination created by light sources induce a variety of color sensations. Moreover, even with identical relative spectral compositions of radiant fluxes, color perception may differ depending on whether radiation from light sources or from nonluminous objects strikes the eye. However, the same terms are used to designate the colors of these two different kinds of objects. Most of the objects that elicit color sensations are nonluminous objects, which merely reflect or transmit the light emitted by light sources. The color of an object is dependent, in general, on the following factors: the intrinsic coloration and properties of the object’s surface, the optical properties of the light source and of the medium through which the light is propagated, and the properties of the visual analyzer and the characteristics of the still inadequately studied process by which visual impressions are analyzed in the brain centers.
The capacity to perceive color evolved for the purpose of identifying objects along with the capacity to perceive other properties, such as size, hardness, and warmth, and movements in space. This enabled organisms to detect and recognize individual objects by their intrinsic coloration in crucial situations, regardless of any changes in lighting or the condition of the environment. This need to recognize objects is the main reason why the perceived color of an object is determined largely by the object’s intrinsic coloration; under the usual viewing conditions for humans, the identification of a color depends only slightly on illumination because the observer unconsciously makes corrections for different illuminations. For example, a green leaf of a tree is acknowledged to be green even in the reddish light of a sunset. The qualification of usual (in the broad sense) viewing conditions is very important, for if conditions are highly unusual, human judgments about the color of objects and, consequently, color sensations become unsure or erroneous. For example, descriptions of the colors of sunrises and sunsets viewed in space and attempts by various astronauts to reproduce the colors differ markedly from one another and from the colors rendered by the objective methods of color photography.
There is a persistent conception rooted in the human consciousness that a given color is an inseparable characteristic of the ordinary objects observed. Referred to as the phenomenon of color constancy, this psychological property of “belonging” is most apparent when nonluminous objects are viewed. It is due to the fact that in daily life we look at many objects simultaneously, subconsciously comparing their colors or comparing color sensations from differently colored or differently illuminated parts of the objects. The color constancy of nonluminous objects is so pronounced that even under unfavorable viewing conditions the color of an object is perceived from the recognition of the object by other features. The names of many colors originated from the names of objects whose coloration is very marked, such as raspberry, rose, and emerald. Sometimes even the color of a light source is described by the color of some characteristic nonluminous object, for example, the blood-red disk of the sun. Color constancy is not as strong for light sources, because under ordinary conditions (not related to their manufacture) they are rarely compared with other sources and the visual analyzer adapts well to the lighting conditions. An example is the vagueness of the notion of white light compared with the very definite concept of the white color of the surface of a nonluminous object (the color of a surface on all of whose parts the absorption of light is minimal over the entire visible spectrum and identical in relative intensity).
Color perception may be partly influenced by the psychophysiological condition of the observer. For example, it intensifies in dangerous situations and diminishes after the onset of fatigue. Despite the eye’s ability to adapt to light, color perception may differ markedly from ordinary conditions after a change in the intensity of radiation of the same relative spectral composition, a phenomenon discovered by the German scientists W. von Bezold and E. W. von Brücke in the 1870’s. It is clearly demonstrable in binocular colorimetry, which is based on the adaptation of one eye independently of the other. All the above factors are indicative of the important role played by the brain centers responsible for color perception and of their proper experiential training (given that the photochemical apparatus of color vision is unchanged).
The colors of the radiations whose wavelengths lie within specified intervals of the range of visible light around the wavelength of some monochromatic radiation are called spectral colors. Radiations with wavelengths ranging from 380 to 470 nanometers (nm) are violet and blue, those from 480 to 500 nm are bluegreen, those from 510 to 560 nm are green, those from 570 to 590 nm are yellow-orange, and those from 600 to 760 nm are red (in the smaller sections of these intervals the colors of the radiations correspond to different shades of these colors, many of which can be easily distinguished by a trained observer).
The ability to sense colors evolved with the formation of a special system of color vision consisting of three types of color-sensitive photoreceptors in the center of the retina (cones) with maximum spectral sensitivities in three different spectral regions: red, green, and blue. Rods, the fourth type of receptor, do not possess primary sensitivity to any one spectral color. They are situated along the periphery of the retina and play a major role in the creation of achromatic visual images. The frequently underestimated importance of the rods in the mechanism of color recognition becomes greater, the less illuminated the observed objects are. The action of fluxes of radiant energy differing in spectral composition and intensity on these four types of retinal receptors is the physicochemical basis of different perceptions of color. The combinations of photoreceptor stimuli of different intensities that are analyzed both in the peripheral conducting nerve pathways and in the brain visual centers are responsible for the great variety of color sensations.
The total spectral sensitivity of the eye caused by the action of photoreceptors of all types is highest in the green region (wavelengths of approximately.555 nm), but it shifts to the blue-green region when illumination is poor. The previously assumed reducibility of all color sensations to combinations of different stimuli of only three types of color-sensitive elements served as the basis for devising methods of expressing colors quantitatively in the form of sets of three numbers. A similar approach (see below) is based on a rational conception of the problem, but the influence of variations in illumination and in the intensity of radiation, the very important role played by the visual centers of the brain, and the general psychophysiological condition of the observer could not be taken into account in the development of such methods.
Three subjective attributes of color are used in a precise qualitative description: hue, saturation, and brightness. The division of color into these interrelated constituents is the result of an intellectual process largely dependent on habit and training. The most important attribute, hue, is associated in the mind with the dependence of the coloration of an object on the prescence of a certain type of pigment, paint, or dye. For example, a green hue is assigned to objects with a coloring similar to that of natural green containing chlorophyll. Saturation characterizes the degree, level, or strength of expression of a hue. This attribute is associated with the amount or concentration of a pigment, paint, or dye. Gray hues are called achromatic (colorless); they are considered as lacking in saturation and differing from each other only in brightness. Brightness is usually related to the amount of black or white pigment present; less commonly, to illumination. The brightness of differently colored objects is judged by comparison with achromatic objects. The achromaticity of nonluminous objects is more or less due to the uniform, identical reflection of radiations of all wavelengths within the visible spectrum.
The color of achromatic surfaces reflecting a maximum of light is called white. Even though objects that produce different color sensations when compared directly may be white according to this definition, white occupies a special position among achromatic colors of nonluminous objects. Surfaces with a white coloration often serve as standards. They are always recognized immediately, and comparison with such standards, together with adaptation of the eye, helps the observer make an unconscious correction for the given illumination. Even if only white objects are observed, the color of the illumination itself is identified from the objects. The color-shade relationships play a decisive part in the recognition of the colors of objects in the absence of standard white surfaces; these relationships are obtained from comparison of objects differing in brightness and hue with achromatic objects.
The saturation and brightness of nonluminous objects are interrelated because the intensification of selective spectral absorption with increasing quantity or concentration of a dye is invariably accompanied by a decrease in the intensity of the reflected light; this produces a sensation of decreased brightness. For example, a rose of a more saturated purple color is perceived as darker than a rose of the same, but less saturated hue.
The simultaneous viewing of identical nonluminous objects or light sources by several observers with normal color vision under identical viewing conditions reveals an unambiguous correspondence between the spectral composition of the radiations being compared and the color sensations produced. This is the basis of colorimetry. Although the correspondence is unambiguous, it is not reciprocally unambiguous, for radiant fluxes of different spectral compositions can produce identical color sensations. There are many definitions of color as a physical quantity, but even the best of them from the colorimetric standpoint does not state that this nonreciprocal unambiguity is achieved only under standardized viewing and illumination conditions. They also fail to account for changes in color perception that occur after changes in the intensity of radiation of the same spectral composition (Bezold-Brücke phenomenon) or to consider the phenomenon of color adaptation. Therefore, the diversity of color sensations arising under actual illumination conditions—with variations in the angular dimensions of elements comparable in color, with the fixation of images on different parts of the retina, and with different psychophysiological states of the observer—is always richer than the colorimetric color diversity. For example, colors that in everyday life are perceived, depending on brightness, as “grayish brown,” “chestnut brown,” “brown,” “chocolate,” “olive,” and so on are determined in colorimetry as orange or yellow. In one of the best attempts at defining color, that proposed by E. Schrödinger, the difficulties of the task are eliminated by the simple omission of any indication of the dependence of color sensations on numerous concrete viewing conditions. According to Schrödinger, color is a property of the spectral composition of radiations, which is common for radiations that cannot be distinguished by a human being.
In colorimetry, colors are designated by a set of three numbers. There are many systems, each with its own method of determining the three numbers. In one of the widely used systems, numerical values are assigned to the three subjective attributes of color described above. The numerical values are assigned either by the comparison method (by comparison with the standard colors found in color tables or atlases) or by the instrument-calculation method. In the latter, the hue is expressed by an objectively determinable wavelength (the wavelength of radiation that, in a mixture with white, reproduces the measured color), saturation is expressed by its purity (the ratio of the intensity of a monochromatic color in a mixture to white), and brightness is expressed by the objectively established brightness of the measured radiation (which is called heterochromatic), as determined experimentally or calculated from the curve of spectral luminous efficacy of the radiation (or its visibility curve). Quantitative expression of the subjective attributes of color is ambiguous because it is heavily dependent on differences between actual viewing and standardized colorimetric conditions. Hence, there are many formulas for determining brightness.
In colorimetry, special importance is attached to the measurement of spectral colors and the determination therefrom of equal-energy distribution curves characterizing the spectral sensitivity of the visual analyzer in terms of relative quantities of three radiations that, when mixed, produce the given color sensation. The colors of radiations of different spectral compositions that are perceived to be the same under identical viewing conditions are called metamers. The number of metamers of a given color increases with decreasing saturation; that is, the less saturated a color is, the more combinations there may be of mixtures of radiations of different spectral compositions that evoke the sensation of the given color. White colors exhibit the greatest number of metamers. The colors of any two radiations that create a white color in a mixture are called complementary colors. For example, blue-green and red monochromatic radiations with wavelengths of 490 nm and 595 nm, or 480 nm and 580 nm, respectively, are complementary for white obtained from a source with a color temperature of 4800°K.
An observer with normal color vision, in comparing differently colored objects or light sources, can, if he is attentive, distinguish a great many colors. A trained observer can distinguish about 150 colors by hue, about 25 by saturation, and from 64 (with good illumination) to 20 (with poor illumination) by brightness (we are referring here to “trained” brain centers responsible for color sensations). Fewer colors can be distinguished by individuals suffering from anomalies of color vision.
Approximately 90 percent of the population have normal color vision; about 10 percent are partly or completely color-blind. Characteristically, 95 percent of those with anomalies of color vision are males. There are three kinds of color vision anomalies; (1) red blindness (protanopia), the inability to distinguish red colors from dark blue complementary and achromatic colors close to them in brightness; (2) green blindness (deuteranopia), the inability to distinguish, or difficulty in distinguishing, green colors from complementary purple colors and achromatic colors close to them in brightness; and (3) blue blindness (tritanopia), the inability to distinguish blue colors from complementary dark yellow colors and achromatic colors close to them in brightness. Cases of total color blindness, in which only achromatic images are perceived, are very rare. Anomalies of color vision do not prevent the individuals affected from working. There are certain jobs, however, to which the color-blind are unsuited.
Adaptation of vision, one of the main properties of the visual analyzer, makes it possible to recognize objects by their color (as a result of the phenomenon of “belonging”) under a very wide range of illumination and viewing conditions. However, with a change in the spectral composition of the illumination, visually perceptible differences between some colors increase, and those between other colors decrease. For example, blue and green hues are harder to distinguish in the yellow illumination produced by incandescent lamps than are red and orange; red and orange hues are harder to distinguish in the bluish illumination of heavily overcast days. When the illumination is weak, all colors are harder to distinguish and are perceived to be less saturated (the twilight vision effect). When the illumination is very bright, colors are perceived to be less saturated and “whitened.” The characteristics of visual perception are widely used in the visual arts to create the illusion of particular types of illumination.
In private and public life. Color plays an exceptionally important role in the life and work of every person and of society as a whole: in industry, transport, art, and information transmission technology. Color and color combinations are extensively used in daily life and at work as symbols substituting for whole concepts in the rules of behavior. For example, traffic lights on main roads control the flow of vehicles, provide warnings, and catch the attention of motorists. In industry and other collective work, colors are used as symbols to mark pipelines carrying different substances or operating at different temperatures, various electric wires, all kinds of tags, labels, and counters, information cards, bank records, bank notes, and protective clothing.
Color is one of the main factors contributing to comfort at work and in the home. The psychological effects of certain color combinations, or color harmony, are the concern of color aesthetics. Color harmony is widely used in art and in the organization of production processes to create psychological emphases aimed at increasing labor productivity, decreasing fatigue, promoting comfort at home, and ensuring active and healthful relaxation.
Color is particularly important for increasing the quality and uniformity of industrial products. Color is indispensable as an indicator of high quality in cases where other objective or subjective methods cannot be used for one reason or another or when their use requires prolonged and labor-intensive work or costly equipment. As a result, wide use is made of comparison methods of identifying the colors of many food products and of the substances used in chemical production, food processing, and light industry, as well as in other branches of the economy. A variety of color tables, atlases, paint samples, comparators, colorimeters, color photometers, and densitometers are available for the practical application of such methods.
REFERENCESArtiushin, L. F. Osnovy vosproizvedeniia tsveta v fotografii, kino i poligrafii. Moscow, 1970.
Gurevich, M. M. Tsvet i ego izmerenie. Moscow-Leningrad, 1950.
Kustarev, A. K. Kolorimetriia tsvetnogo televideniia. Moscow, 1967.
Evans, R. M. Vvedeniie v teoriiu tsveta. Moscow, 1964. (Translated from English.)
Wyszecki, G., and W. S. Stiles. Color Science. New York-London-Sydney, 1967.
L. F. ARTIUSHIN
color calibrationThe matching of colors to a base color, such as a Pantone color, or from one device to another. Color calibration is widely used in print publishing in order to display on screen an accurate representation of the colors that will be printed on paper. Monitors use red, green and blue (RGB) to display the color gamut, while printers use cyan, magenta, yellow and black (CMYK) inks. See TV calibration, color gamut and color correction.
|Light Source's Colortron Color System includes this color calibrator, which is used to accurately determine the color from a sample. Using suction cups, it attaches to the monitor screen to calibrate the monitor so that designers see true representations of the final colors on screen. (Image courtesy of Light Source, Inc.)|
color temperatureThe color of light, expressed in Kelvins (K). The sun is about 5800 Kelvins. This is called a "temperature," because the hotter fire gets, the more it changes from a warm yellow glow to white.
The lower the Kelvin rating, such as the common incandescent/tungsten bulb, the warmer, softer and more yellow the light. The higher the K number, the cooler, whiter and bluer the light. See white balance.
Approximate TemperatureLight In Degrees ofSource Kelvin (K) Match/Candle 1,700-1,800 Warm/More Incandescent 2,600-3,000 Yellow| Halogen 2,800-3,400 || Fluorescent 3,000-4,300 Daylight| Outdoor sun 5,000-6,000 || Outdoor shade 6,000-7,000 Cool/Whiter/ North sky 10,000+ Bluer LED lights range approximately from 2,600 to 5,200 K
|From Warm to Cool|
|Increasingly, light bulb packages show the color temperature as a measure from warm to cool. This is a halogen bulb label.|