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Colour

2007 Schools Wikipedia Selection. Related subjects: General Physics

   Color is an important part of the visual arts.
   Enlarge
   Colour is an important part of the visual arts.

   Colour (or colour, see spelling differences) is the visual perceptual
   property corresponding in humans to the categories called red, yellow,
   white, etc. Colour 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 colour 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 400 nm to 700 nm,
   roughly) are included, thereby objectively relating the psychological
   phenomenon of color to its physical specification. Since perception of
   colour 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 colour
   appearance.

   The science of colour is sometimes called chromatics. It includes the
   perception of color by the human eye and brain, the origin of colour in
   materials, colour theory in art, and the physics of electromagnetic
   radiation in the visible range (that is, what we commonly refer to
   simply as light).

Physics of colour

                         CAPTION: The colors of the visible light spectrum

                             colour wavelength interval frequency interval
                              red      ~ 625–740 nm       ~ 480–405 THz
                             orange    ~ 590–625 nm       ~ 510–480 THz
                             yellow    ~ 565–590 nm       ~ 530–510 THz
                             green     ~ 500–565 nm       ~ 600–530 THz
                              cyan     ~ 485–500 nm       ~ 620–600 THz
                              blue     ~ 440–485 nm       ~ 680–620 THz
                             violet    ~ 380–440 nm       ~ 790–680 THz

   Continuous optical spectrum. Designed for monitors with gamma 1.5.
   Continuous optical spectrum. Designed for monitors with gamma 1.5.
   Computer "spectrum". The narrow red, green and blue bars below the main
   bar show the relative intensities of the three primary colors mixed to
   make the color directly above.
   Enlarge
   Computer "spectrum". The narrow red, green and blue bars below the main
   bar show the relative intensities of the three primary colors mixed to
   make the colour directly above.

         CAPTION: Colour, wavelength, frequency and energy of light

    Colour \lambda \,\! /nm \nu \,\! /10^14 Hz \nu_b \,\! /10^4 cm^−1 E
                        \,\! /eV E \,\! /kJ mol^−1
                                     Infrared >1000 <3.00 <1.00 <1.24 <120
                                                Red 700 4.28 1.43 1.77 171
                                             Orange 620 4.84 1.61 2.00 193
                                             Yellow 580 5.17 1.72 2.14 206
                                              Green 530 5.66 1.89 2.34 226
                                               Blue 470 6.38 2.13 2.64 254
                                             Violet 420 7.14 2.38 2.95 285
                                   Near ultraviolet 300 10.0 3.33 4.15 400
                               Far ultraviolet <200 >15.0 >5.00 >6.20 >598

   Electromagnetic radiation is characterized by its wavelength (or
   frequency) and its intensity. When the wavelength is within the visible
   spectrum (the range of wavelengths humans can perceive, approximately
   from 380  nm to 740 nm), it is known as "visible light."

   A given light source may emit light at many different wavelengths (and
   most do); its spectrum is then a distribution giving its intensity at
   each wavelength. Although the spectrum of light arriving at the eye
   from a given direction determines the color perceived in that
   direction, there are many more possible spectral combinations than
   color sensations. In fact, one may formally define a color as a class
   of spectra that give rise to the same colour sensation, although such
   classes would vary widely among different species, and to a lesser
   extent among individuals within the same species. In each such class
   the members are called metamers of the colour in question.

Spectral colors

   The familiar colors of the rainbow in the spectrum – named from the
   Latin word for appearance or apparition by Isaac Newton in 1671 –
   contains all those colors that can be produced by visible light of a
   single wavelength only, the pure spectral or monochromatic colors. The
   colour table at right shows approximate frequencies (in terahertz) and
   wavelengths (in nanometers) for various pure spectral colors. The
   wavelengths are measured in vacuum (see refraction).

   The colour table should not be interpreted as a definitive list – the
   pure spectral colors form a continuous spectrum, and how it is divided
   into distinct colors is a matter of culture, taste, and language.
   Newton added a seventh colour, indigo, between blue and violet, but
   most people are not able to distinguish it and most color scientists do
   not recognize it as a separate colour; it is sometimes designated as
   wavelengths of 420–440 nm. Furthermore, the intensity of a spectral
   colour may alter its perception considerably; for example, a
   low-intensity orange-yellow is brown, and a low-intensity yellow-green
   is olive-green.

   As discussed in the section on color vision, a light source need not
   actually be of one single wavelength to be perceived as a pure spectral
   colour.

   For discussion of non-spectral colors, see below.

Colour of objects

   Setting aside illuminant adaptation and contextual effects, surfaces
   appear to have the color of the light leaving them in the direction of
   the eye. Since the composition of this light may depend on the
   orientation of the surface and lighting conditions, the perceived
   colour of an object also depends on these factors. However, some
   generalizations can be drawn.

   Light arriving at an opaque surface is either reflected "specularly"
   (that is, in the manner of a mirror), scattered (that is, reflected
   with diffuse scattering), or absorbed – or some combination of these.

   Opaque objects that do not reflect specularly (which tend to have rough
   surfaces) have their colour determined by which wavelengths of light
   they scatter more and which they scatter less (with the light that is
   not scattered being absorbed). If objects scatter all wavelengths, they
   appear white. If they absorb all wavelengths, they appear black.

   Opaque objects that specularly reflect light of different wavelengths
   with different efficiencies look like mirrors tinted with colors
   determined by those differences. An object that reflects some fraction
   of impinging light and absorbs the rest may look black but also be
   faintly reflective; examples are black objects coated with layers of
   enamel or lacquer.

   Objects that transmit light are either translucent (scattering the
   transmitted light) or transparent (not scattering the transmitted
   light). If they also absorb (or reflect) light of varying wavelengths
   differentially, they appear tinted with a colour determined by the
   nature of that absorption (or that reflectance).

   Objects may emit light that they generate themselves, rather than
   merely reflecting or transmitting light. They may do so because of
   their elevated temperature (they are then said to be incandescent), as
   a result of certain chemical reactions (a phenomenon called
   chemoluminescence), or for other reasons (see the articles
   Phosphorescence and List of light sources).

   Objects may absorb light and then as a consequence emit light that has
   different properties. They are then called fluorescent (if light is
   emitted only while light is absorbed) or phosphorescent (if light is
   emitted even after light ceases to be absorbed; this term is also
   sometimes loosely applied to light emitted due to chemical reactions).

   For further treatment of the colour of objects, see the section
   Structural colour, below.

   To summarize, the color of an object is a complex result of its surface
   properties, its transmission properties, and its emission properties,
   all of which factors contribute to the mix of wavelengths in the light
   leaving the surface of the object. The perceived color is then further
   conditioned by the nature of the ambient illumination, and by the
   colour properties of other objects nearby (see the article Colour
   constancy); and finally, by the permanent and transient characteristics
   of the perceiving eye and brain.

Colour perception

   Normalized typical human cone responses (and the rod response) to
   monochromatic spectral stimuli
   Enlarge
   Normalized typical human cone responses (and the rod response) to
   monochromatic spectral stimuli

Development of theories of colour vision

   Although Aristotle and other ancient scientists had already written on
   the nature of light and colour vision, it was not until Newton that
   light was identified as the source of the colour sensation. In 1810,
   Goethe published his comprehensive Theory of Colors. In 1801 Thomas
   Young proposed his trichromatic theory, based on the observation that
   any colour could be matched with a combination of three lights. This
   theory was later refined by James Clerk Maxwell and Hermann von
   Helmholtz. As Helmholtz puts it, "the principles of Newton's law of
   mixture were experimentally confirmed by Maxwell in 1856. Young's
   theory of colour sensations, like so much else that this marvellous
   investigator achieved in advance of his time, remained unnoticed until
   Maxwell directed attention to it."

   At the same time as Helmholtz, Ewald Hering developed the opponent
   process theory of color, noting that colour blindness and afterimges
   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 lateral
   geniculate nucleus corresponds to the opponent theory.

   In 1931, an international group of experts known as the Commission
   Internationale d'Eclairage ( CIE) developed a mathematical colour
   model, which mapped out the space of observable colors and assigned a
   set of three numbers to each.

Colour in the eye

   The ability of the human eye to distinguish colors is based upon the
   varying sensitivity of different cells in the retina to light of
   different wavelengths. The retina contains three types of colour
   receptor cells, or cones. One type, relatively distinct from the other
   two, is most responsive to light that we perceive as violet, with
   wavelengths around 420 nm. (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 colour 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/"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 colour
   space. It has been estimated that humans can distinguish roughly 10
   million different colors.

   The other type of light-sensitive cell in the eye, the rod, 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. On the other hand, in dim light, the cones are
   understimulated leaving only the signal from the rods, resulting in a
   monochromatic 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 colour discriminations not accounted for by cone responses
   alone.

Colour in the brain

   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 colour information is transmitted out of
   the eye by three opponent processes, 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 colour experience. Specifically, it explains why we
   cannot perceive a "reddish green" or "yellowish blue," and it predicts
   the colour wheel: it is the collection of colors for which at least one
   of the two colour 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 colour 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
   qualia).

Nonstandard colour perception

Colour deficiency

   If one or more types of a person's colour-sensing cones are missing or
   less responsive than normal to incoming light, that person can
   distinguish fewer colors and is said to be colour deficient or colour
   blind (though this latter term can be misleading; almost all color
   deficient individuals can distinguish at least some colors). Some kinds
   of colour deficiency are caused by anomalies in the number or nature of
   cones in the retina. Others (like central or cortical achromatopsia)
   are caused by neural anomalies in those parts of the brain where visual
   processing takes place.

Tetrachromacy

   While most humans are trichromatic (having three types of colour
   receptors), many animals, known as tetrachromats, have four types.
   These include some species of spiders, most marsupials, birds,
   reptiles, and many species of fish. Other species are sensitive to only
   two axes of color or do not perceive colour 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 a half of all women, but only a
   small percentage of men, are retinal tetrachromats. 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). For some of these retinal tetrachromats, colour
   discriminations are enhanced, making them functional tetrachromats.

Synesthesia

   In certain forms of synesthesia, perceiving letters and numbers (
   grapheme → colour synesthesia) or hearing musical sounds (music → color
   synesthesia) will lead to the unusual additional experiences of seeing
   colors. Behavioural and functional neuroimaging 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 colour percepts, albeit evoked through a non-standard route.

Afterimages

   After exposure to strong light in their sensitivity range,
   photoreceptors 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 colour 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 complementary colour.

   Afterimage effects have also been utilized by artists, including
   Vincent van Gogh.

Colour constancy

   There is an interesting phenomenon which occurs when an artist uses a
   limited colour palette: the eye tends to compensate by seeing any grey
   or neutral color as the color which is missing from the colour 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.

   The trichromatric theory discussed above is strictly true only if the
   whole scene seen by the eye is of one and the same colour, 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 Edwin Land in the 1970s and
   led to his retinex theory of colour constancy.

Colour naming

   Different cultures have different terms for colors, and may also assign
   some colour names to slightly different parts of the spectrum: for
   instance, the han character 青 (rendered as qīng in Mandarin and ao in
   Japanese) has a meaning that covers both blue and green; blue and green
   are traditionally considered shades of "青." In more contemporary terms,
   they are 藍 (lán, in Mandarin) and 綠 (lǜ, in Mandarin) respectively. For
   example, in Japan, although the traffic lights have the same colored
   lights that other countries have, the green light is called using the
   same word for blue, "aoi", because green is considered a shade of blue.

   Similarly, languages are selective when deciding which hues are split
   into different colors on the basis of how light or dark they are. Apart
   from the black-grey-white continuum, English splits some hues into
   several distinct colors according to lightness: such as red and pink or
   orange and brown. To English speakers, these pairs of colors, which are
   objectively no more different from one another than light green and
   dark green, are conceived as totally different. A Russian will make the
   same red-pink and orange-brown distinctions, but will also make a
   further distinction between sinij and goluboj, which English speakers
   would simply call dark and light blue. To Russian speakers, sinij and
   goluboj are as separate as red and pink or orange and brown.

   Color terms evolve. It is argued that there are a limited number of
   universal "basic color terms" which begin to be used by individual
   cultures in a relatively fixed order. For example, a culture would
   start with only two terms, meaning roughly 'dark' (covering black, dark
   colors and cold colors such as blue ) and 'bright' (covering white,
   light colors and warm colors such as red), before adding more specific
   color names, in the order of red; green and/or yellow; blue; brown; and
   orange, pink, purple and/or gray. Older arguments for this theory also
   stipulated that the acquisition and use of basic colour terms further
   along the evolutionary order indicated a more complex culture with more
   highly developed technology.

   A somewhat dated example of a universal colour categories theory is
   Basic Colour Terms: Their Universality and Evolution (1969) by Brent
   Berlin and Paul Kay. A more recent example of a linguistic determinism
   theory might be Is colour categorisation universal? New evidence from a
   stone-age culture (1999) by Jules Davidoff et al. The idea of
   linguistically determined colour categories is often used as evidence
   for the Sapir-Whorf hypothesis (Language, Thought and Reality (1956) by
   Benjamin Lee Whorf).

   Additionally, different colors are often associated with different
   emotional states, values or groups, but these associations can vary
   between cultures. In one system, red is considered to motivate action;
   orange and purple are related to spirituality; yellow cheers; green
   creates cosiness and warmth; blue relaxes; and white is associated with
   either purity or death. These associations are described more fully in
   the individual colour pages, and under colour psychology.

   See also: National colors

Health effects

   When the colour spectrum of artificial lighting is mismatched to that
   of sunlight, material health effects may arise including increased
   incidence of headache. This phenomenon is often coupled with adverse
   effects of over-illumination, since many of the same interior spaces
   that have colour mismatch also have higher light intensity than
   desirable for the task being conducted in that space.

Measurement and reproduction of colour

Relation to spectral colors

   The CIE 1931 color space chromaticity diagram. The outer curved
   boundary is the spectral (or monochromatic) locus, with wavelengths
   shown in nanometers. Note that the colors depicted depend on the color
   space of the device on which you are viewing the image, and therefore
   may not be a strictly accurate representation of the color at a
   particular position, and especially not for monochromatic colors.
   Enlarge
   The CIE 1931 colour space chromaticity diagram. The outer curved
   boundary is the spectral (or monochromatic) locus, with wavelengths
   shown in nanometers. Note that the colors depicted depend on the colour
   space of the device on which you are viewing the image, and therefore
   may not be a strictly accurate representation of the colour at a
   particular position, and especially not for monochromatic 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 colour 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 colour 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.

   Of course, there are many colour 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.
   A color photo of a sunset
   Enlarge
   A colour photo of a sunset

   Two different light spectra which have the same effect on the three
   color receptors in the human eye will be perceived as the same colour.
   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 colour perceptions can be generated by a mixture
   of three colors called primaries. This is used to reproduce colour
   scenes in photography, printing, television and other media. There are
   a number of methods or colour spaces for specifying a colour 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 colour, 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 colour 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 colour 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 colour 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 colour 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 colour 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 colour vision may give very inaccurate results
   for the other observers.

   The next problem is different color response of different devices. For
   colour information stored and transferred in a digital form, colour
   management technique based on colour profiles attached to color data
   and to devices with different colour 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 colour information
   properly and vice versa.

Structural colour

   Structural colors are colors which are caused by interference effects
   rather than pigment. Colors are produced when a material is scored with
   fine parallel lines, formed of one or more thin parallel layers, or
   otherwise composed of microstructures on the scale of the colour'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, aerogel 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 white light into colors. If the
   structure is one or more thin layers then it will reflect some
   wavelengths and transmit others, depending on the thickness of the
   layer(s).

   Structural colour is responsible for the blues and greens of many bird
   feathers (example, blue jay feathers) 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
   colour depends upon the viewing angle.

   Structural colour is studied in the field of thin-film optics. A
   layman's term that describes particularly the most ordered structural
   colors is iridescence.

Additional terms

     * Hue: the colour's direction from white, for example in the CIE
       chromaticity diagram.
     * Saturation: how "dense" or "intense" or "concentrated" or "pure" a
       colour is.
     * Value: how light or dark a colour is.
     * Tint: a colour made lighter by adding white.
     * Shade: a colour made darker by adding black.

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   with only minor checks and changes (see www.wikipedia.org for details
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