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| == Color perception == | |||
| ] responses (S, M, and L types) to monochromatic spectral stimuli]] | |||
| === Development of theories of color vision === | |||
| : ''Main article: ]'' | |||
| Although ] and other ancient scientists had already written on the nature of light and ], it was not until ] that light was identified as the source of the color sensation. In 1810, ] published his comprehensive '']''. In 1801 ] proposed his ], based on the observation that any color could be matched with a combination of three lights. This theory was later refined by ] and ]. As Helmholtz puts it, "the principles of Newton's law of mixture were experimentally confirmed by Maxwell in 1856. Young's theory of color sensations, like so much else that this marvellous investigator achieved in advance of his time, remained unnoticed until Maxwell directed attention to it."<ref>Hermann von Helmholtz, ''Physiological Optics – The Sensations of Vision'', 1866, as translated in ''Sources of Color Science'', David L. MacAdam, ed., Cambridge: MIT Press, 1970.</ref> | |||
| At the same time as Helmholtz, ] developed the ] theory of color, noting that ] and afterimages typically come in opponent pairs (red-green, blue-yellow, and black-white). Ultimately these two theories were synthesized in 1957 by Hurvich and Jameson, who showed that retinal processing corresponds to the trichromatic theory, while processing at the level of the ] corresponds to the opponent theory.<ref name ="Palmer99">Palmer, S.E. (1999). ''Vision Science: Photons to Phenomenology'', Cambridge, MA: MIT Press. ISBN 0-262-16183-4.</ref> | |||
| In 1931, an international group of experts known as the Commission Internationale d'Eclairage (]) developed a mathematical color model, which mapped out the space of observable colors and assigned a set of three numbers to each. | |||
| === Color in the eye === | |||
| {{main|Color vision}} | |||
| ]|series=Wiley Series in Pure and Applied Optics|edition=third edition|location=New York|year= 1975|page=388|isbn=0471452122}}</ref> | |||
| ]] | |||
| The ability of the human ] to distinguish colors is based upon the varying sensitivity of different cells in the ] to light of different wavelengths. The retina contains three types of color receptor cells, or ]. One type, relatively distinct from the other two, is most responsive to light that we perceive as violet, with wavelengths around 420 ]. (Cones of this type are sometimes called ''short-wavelength cones'', ''S cones'', or, misleadingly, ''blue cones''.) The other two types are closely related genetically and chemically. One of them (sometimes called ''long-wavelength cones'', ''L cones'', or, misleadingly, ''red cones'') is most sensitive to light we perceive as yellowish-green, with wavelengths around 564 nm; the other type (sometimes called ''middle-wavelength cones'', ''M cones'', or, misleadingly, ''green cones'') is most sensitive to light perceived as green, with wavelengths around 534 nm. | |||
| Light, no matter how complex its composition of wavelengths, is reduced to three color components by the eye. For each location in the visual field, the three types of cones yield three signals based on the extent to which each is stimulated. These values are sometimes called ''tristimulus values''. | |||
| The response curve as a function of wavelength for each type of cone is illustrated above. Because the curves overlap, some tristimulus values do not occur for any incoming light combination. For example, it is not possible to stimulate ''only'' the mid-wavelength (so-called "green") cones; the other cones will inevitably be stimulated to some degree at the same time. The set of all possible tristimulus values determines the human ''color space''. It has been estimated that humans can distinguish roughly 10 million different colors.<ref>{{cite book|first=Deane B.|last=Judd|coauthors=Wyszecki, Günter|title=Color in Business, Science and Industry|publisher=]|series=Wiley Series in Pure and Applied Optics|edition=third edition|location=New York|year= 1975|page=388|isbn=0471452122}}</ref> | |||
| The other type of light-sensitive cell in the eye, the ], has a different response curve. In normal situations, when light is bright enough to strongly stimulate the cones, rods play virtually no role in vision at all.<ref>"Under well-lit viewing conditions (photopic vision), cones ...are highly active and rods are inactive." {{cite conference|last=Hirakawa|first=K.|coauthors=Parks, T.W. |title=Chromatic Adaptation and White-Balance Problem |conference=IEEE ICIP |year=2005|doi=10.1109/ICIP.2005.1530559| |url=http://www.accidentalmark.com/research/papers/Hirakawa05WBICIP.pdf}}</ref> On the other hand, in dim light, the cones are understimulated leaving only the signal from the rods, resulting in a colorless response. (Furthermore, the rods are barely sensitive to light in the "red" range.) In certain conditions of intermediate illumination, the rod response and a weak cone response can together result in color discriminations not accounted for by cone responses alone. | |||
| === Color in the brain === | |||
| {{main|Color vision}} | |||
| ] (green) and ] (purple) are shown. The ventral stream is responsible for color perception.]] | |||
| While the mechanisms of color vision at the level of the retina are well-described in terms of tristimulus values (see above), color processing after that point is organized differently. A dominant theory of color vision proposes that color information is transmitted out of the eye by three ], or opponent channels, each constructed from the raw output of the cones: a red-green channel, a blue-yellow channel and a black-white "luminance" channel. This theory has been supported by neurobiology, and accounts for the structure of our subjective color experience. Specifically, it explains why we cannot perceive a "reddish green" or "yellowish blue," and it predicts the ]: it is the collection of colors for which at least one of the two color channels measures a value at one of its extremes. | |||
| The exact nature of color perception beyond the processing already described, and indeed the status of color as a feature of the perceived world or rather as a feature of our ''perception'' of the world, is a matter of complex and continuing philosophical dispute (see ]). | |||
| === Nonstandard color perception === | |||
| ==== Color deficiency ==== | |||
| {{main|Color blindness}} | |||
| If one or more types of a person's color-sensing cones are missing or less responsive than normal to incoming light, that person can distinguish fewer colors and is said to be ''color deficient'' or '']'' (though this latter term can be misleading; almost all color deficient individuals can distinguish at least some colors). Some kinds of color deficiency are caused by anomalies in the number or nature of cones in the retina. Others (like ''central'' or ''cortical'' '']'') are caused by neural anomalies in those parts of the brain where visual processing takes place. | |||
| ==== Tetrachromacy ==== | |||
| {{main|Tetrachromacy}} | |||
| While most humans are ''trichromatic'' (having three types of color receptors), many animals, known as '']s'', have four types. These include some species of ], most ], ], ], and many species of ]. Other species are sensitive to only two axes of color or do not perceive color at all; these are called ''dichromats'' and ''monochromats'' respectively. A distinction is made between ''retinal tetrachromacy'' (having four pigments in cone cells in the retina, compared to three in trichromats) and ''functional tetrachromacy'' (having the ability to make enhanced color discriminations based on that retinal difference). As many as half of all women, but only a small percentage of men, are retinal tetrachromats.<ref name=Jameson/> The phenomenon arises when an individual receives two slightly different copies of the gene for either the medium- or long-wavelength cones, which are carried on the x-chromosome, accounting for the differences between genders.<ref name=Jameson/> For some of these retinal tetrachromats, color discriminations are enhanced, making them functional tetrachromats.<ref name=Jameson>{{cite journal |author=Jameson, K. A., Highnote, S. M., & Wasserman, L. M. |year=2001 |title=Richer color experience in observers with multiple photopigment opsin genes. | doi = 10.1038/351652a0 <!--Retrieved from Yahoo! by DOI bot-->|journal=Psychonomic Bulletin and Review |volume=8 |issue=2 |pages=244–261 |url=http://www.klab.caltech.edu/cns186/papers/Jameson01.pdf |format=PDF}}</ref> | |||
| ==== Synesthesia ==== | |||
| In certain forms of ], perceiving letters and numbers (]) or hearing musical sounds (music–color synesthesia) will lead to the unusual additional experiences of seeing colors. Behavioral and ] experiments have demonstrated that these color experiences lead to changes in behavioral tasks and lead to increased activation of brain regions involved in color perception, thus demonstrating their reality, and similarity to real color percepts, albeit evoked through a non-standard route. | |||
| <!-- | |||
| === Complementary colors === | |||
| to be written --> | |||
| === Afterimages === | |||
| After exposure to strong light in their sensitivity range, ]s of a given type become desensitized. For a few seconds after the light ceases, they will continue to signal less strongly than they otherwise would. Colors observed during that period will appear to lack the color component detected by the desensitized photoreceptors. This effect is responsible for the phenomenon of afterimages, in which the eye may continue to see a bright figure after looking away from it, but in a ]. | |||
| Afterimage effects have also been utilized by artists, including ]. | |||
| === Color constancy === | |||
| {{main|Color constancy}} | |||
| There is an interesting phenomenon which occurs when an artist uses a limited ]: the ] tends to compensate by seeing any grey or neutral color as the color which is missing from the color wheel. E.g., in a limited palette consisting of red, yellow, black and white, a mixture of yellow and black will appear as a variety of green, a mixture of red and black will appear as a variety of purple, and pure grey will appear bluish.{{Fact|date=October 2007}}<!-- not due to black pigment being dark blue therefore reflecting more blue light? This is a real physical phenomenon and not a perceptual one. Is this paragraph not irrelevant to color constancy? --> | |||
| The trichromatic theory discussed above is strictly true only if the whole scene seen by the eye is of one and the same color, which of course is unrealistic. In reality, the brain compares the various colors in a scene, in order to eliminate the effects of the illumination. If a scene is illuminated with one light, and then with another, as long as the difference between the light sources stays within a reasonable range, the colors of the scene will nevertheless appear constant to us. This was studied by ] in the 1970s and led to his retinex theory of ]. | |||
| === Color naming === | |||
| {{main|Color naming}} | |||
| Colors vary in several different ways, including ] (red vs. orange vs. blue), ], ], and ]. Some color words are derived from the name of an object of that color, such as "orange" or "salmon", while others are abstract, like "red". | |||
| Different cultures have different terms for colors, and may also assign some ]s to slightly different parts of the spectrum: for instance, the ] 青 (rendered as ''qīng'' in ] and '']'' in ]) has a meaning that covers both ]; blue and green are traditionally considered shades of "青." | |||
| In the 1969 study ], ] and ] describe a pattern in naming "basic" colors (like "red" but not "red-orange" or "dark red" or "blood red", which are "shades" of red). All languages that have two "basic" color names distinguish dark/cool colors from bright/warm colors. The next colors to be distinguished are usually red and then yellow or green. All languages with six "basic" colors include black, white, red, green, blue and yellow. The pattern holds up to a set of twelve: black, grey, white, pink, red, orange, yellow, green, blue, purple, brown, and ] (distinct from blue in ] and ] but not English). | |||
| == Associations == | == Associations == | ||
Revision as of 20:48, 8 December 2008
This article is about the perceptual property. For other uses, see Color (disambiguation). For usage of color in templates and Misplaced Pages pages, see Misplaced Pages:Colors.
Color or colour is the visual perceptual property corresponding in humans to the categories called red, yellow, blue and others. Color derives from the spectrum of light (distribution of light energy versus wavelength) interacting in the eye with the spectral sensitivities of the light receptors. Color categories and physical specifications of color are also associated with objects, materials, light sources, etc., based on their physical properties such as light absorption, reflection, or emission spectra.
Typically, only features of the composition of light that are detectable by humans (wavelength spectrum from 380 nm to 740 nm, roughly) are included, thereby objectively relating the psychological phenomenon of color to its physical specification. Because perception of color stems from the varying sensitivity of different types of cone cells in the retina to different parts of the spectrum, colors may be defined and quantified by the degree to which they stimulate these cells. These physical or physiological quantifications of color, however, do not fully explain the psychophysical perception of color appearance.
The science of color is sometimes called chromatics. It includes the perception of color by the human eye and brain, the origin of color in materials, color theory in art, and the physics of electromagnetic radiation in the visible range (that is, what we commonly refer to simply as light).
Associations
Individual colors have a variety of cultural associations such as national colors (in general described in individual color articles and color symbolism). The field of color psychology attempts to identify the effects of color on human emotion and activity. Chromotherapy is a form of alternative medicine attributed to various Eastern traditions.
Measurement and reproduction of color
Relation to spectral colors
Most light sources are mixtures of various wavelengths of light. However, many such sources can still have a spectral color insofar as the eye cannot distinguish them from monochromatic sources. For example, most computer displays reproduce the spectral color orange as a combination of red and green light; it appears orange because the red and green are mixed in the right proportions to allow the eye's red and green cones to respond the way they do to orange.
A useful concept in understanding the perceived color of a non-monochromatic light source is the dominant wavelength, which identifies the single wavelength of light which produces a sensation most similar to the light source. Dominant wavelength is roughly akin to hue.
There are many color perceptions that by definition cannot be pure spectral colors due to desaturation or because they are purples (mixtures of red and violet light, from opposite ends of the spectrum). Some examples of necessarily non-spectral colors are the achromatic colors (black, gray and white) and colors such as pink, tan, and magenta.
Two different light spectra which have the same effect on the three color receptors in the human eye will be perceived as the same color. This is exemplified by the white light that is emitted by fluorescent lamps, which typically has a spectrum consisting of a few narrow bands, while daylight has a continuous spectrum. The human eye cannot tell the difference between such light spectra just by looking into the light source, although reflected colors from objects can look different. (This is often exploited e.g. to make fruit or tomatoes look more brightly red in shops.)
Similarly, most human color perceptions can be generated by a mixture of three colors called primaries. This is used to reproduce color scenes in photography, printing, television and other media. There are a number of methods or color spaces for specifying a color in terms of three particular primary colors. Each method has its advantages and disadvantages depending on the particular application.
No mixture of colors, though, can produce a fully pure color perceived as completely identical to a spectral color, although one can get very close for the longer wavelengths, where the chromaticity diagram above has a nearly straight edge. For example, mixing green light (530 nm) and blue light (460 nm) produces cyan light that is slightly desaturated, because response of the red color receptor would be greater to the green and blue light in the mixture than it would be to a pure cyan light at 485 nm that has the same intensity as the mixture of blue and green.
Because of this, and because the primaries in color printing systems generally are not pure themselves, the colors reproduced are never perfectly saturated colors, and so spectral colors cannot be matched exactly. However, natural scenes rarely contain fully saturated colors, thus such scenes can usually be approximated well by these systems. The range of colors that can be reproduced with a given color reproduction system is called the gamut. The CIE chromaticity diagram can be used to describe the gamut.
Another problem with color reproduction systems is connected with the acquisition devices, like cameras or scanners. The characteristics of the color sensors in the devices are often very far from the characteristics of the receptors in the human eye. In effect, acquisition of colors that have some special, often very "jagged," spectra caused for example by unusual lighting of the photographed scene can be relatively poor.
Species that have color receptors different from humans, e.g. birds that may have four receptors, can differentiate some colors that look the same to a human. In such cases, a color reproduction system 'tuned' to a human with normal color vision may give very inaccurate results for the other observers.
The next problem is different color response of different devices. For color information stored and transferred in a digital form, color management technique based on ICC profiles attached to color data and to devices with different color response helps to avoid deformations of the reproduced colors. The technique works only for colors in gamut of the particular devices, e.g. it can still happen that your monitor is not able to show you real color of your goldfish even if your camera can receive and store the color information properly and vice versa.
Pigments and reflective media
Main article: PigmentPigments are chemicals that selectively absorb and reflect different spectra of light. When a surface is painted with a pigment, light hitting the surface is reflected, minus some wavelengths. This subtraction of wavelengths produces the appearance of different colors. Most paints are a blend of several chemical pigments, intended to produce a reflection of a given color.
Pigment manufacturers assume the source light will be white, or of roughly equal intensity across the spectrum. If the light is not a pure white source (as in the case of nearly all forms of artificial lighting), the resulting spectrum will appear a slightly different color. Red paint, viewed under blue light, may appear black. Red paint is red because it reflects only the red components of the spectrum. Blue light, containing none of these, will create no reflection from red paint, creating the appearance of black.
Structural color
Structural colors are colors caused by interference effects rather than by pigments. Color effects are produced when a material is scored with fine parallel lines, formed of a thin layer or of two or more parallel thin layers, or otherwise composed of microstructures on the scale of the color's wavelength. If the microstructures are spaced randomly, light of shorter wavelengths will be scattered preferentially to produce Tyndall effect colors: the blue of the sky, the luster of opals, and the blue of human irises. If the microstructures are aligned in arrays, for example the array of pits in a CD, they behave as a diffraction grating: the grating reflects different wavelengths in different directions due to interference phenomena, separating mixed "white" light into light of different wavelengths. If the structure is one or more thin layers then it will reflect some wavelengths and transmit others, depending on the layers' thickness.
Structural color is studied in the field of thin-film optics. A layman's term that describes particularly the most ordered or the most changeable structural colors is iridescence. Structural color is responsible for the blues and greens of the feathers of many birds (the blue jay, for example), as well as certain butterfly wings and beetle shells. Variations in the pattern's spacing often give rise to an iridescent effect, as seen in peacock feathers, soap bubbles, films of oil, and mother of pearl, because the reflected color depends upon the viewing angle. Numerous scientists have carried out research in butterfly wings and beetle shells, including Isaac Newton and Robert Hooke. Since 1942, electron micrography has been used, advancing the development of products that exploit structural color, such as "photonic" cosmetics.
Additional terms
- Colorfulness, chroma, or saturation: how "intense" or "concentrated" a color is; also known as chroma or purity.
- Hue: the color's direction from white, for example in a color wheel or chromaticity diagram.
- Shade: a color made darker by adding black.
- Tint: a color made lighter by adding white.
- Value, brightness, or lightness: how light or dark a color is.
Colors of the rainbow
Red = , orange = , yellow = , green = , blue = , indigo = , violet = .
See also
- International Color Consortium
- International Commission on Illumination
- List of colors
- Neutral color
- Primary color
- Secondary color
- Tertiary color
References
- See American and British English spelling differences.
- "Economic and Social Research Council - Science in the Dock, Art in the Stocks". Retrieved 2007-10-07.
External links and sources
- Bibliography Database on Color Theory, Buenos Aires University
- Color, Contrast & Dimension in News Design
- Barry Maund. "Color". In Zalta, Edward N. (ed.). Stanford Encyclopedia of Philosophy.
- Comparative article examining Goethean and Newtonian Color
- The Creation of Color in Eighteenth-Century Europe
- Why are things colored?
- Why Should Engineers and Scientists Be Worried About Color?
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