Color

Summary

Colour (Commonwealth English) or color (American English) is the visual perception based on the electromagnetic spectrum. Though colour is not an inherent property of matter, colour perception is related to an object's light absorption, reflection, emission spectra and interference. For most humans, colours are perceived in the visible light spectrum with three types of cone cells (trichromacy). Other animals may have a different number of cone cell types or have eyes sensitive to different wavelength, such as bees that can distinguish ultraviolet, and thus have a different colour sensitivity range. Animal perception of colour originates from different light wavelength or spectral sensitivity in cone cell types, which is then processed by the brain.

coloured pencils

Colours have perceived properties such as hue, colourfulness (saturation) and luminance. Colours can also be additively mixed (commonly used for actual light) or subtractively mixed (commonly used for materials). If the colours are mixed in the right proportions, because of metamerism, they may look the same as a single-wavelength light. For convenience, colours can be organised in a colour space, which when being abstracted as a mathematical colour model can assign each region of colour with a corresponding set of numbers. As such, colour spaces are an essential tool for colour reproduction in print, photography, computer monitors and television. The most well-known colour models are RGB, CMYK, YUV, HSL and HSV.

Because the perception of colour is an important aspect of human life, different colours have been associated with emotions, activity, and nationality. Names of colour regions in different cultures can have different, sometimes overlapping areas. In visual arts, colour theory is used to govern the use of colours in an aesthetically pleasing and harmonious way. The theory of colour includes the colour complements; colour balance; and classification of primary colours (traditionally red, yellow, blue), secondary colours (traditionally orange, green, purple) and tertiary colours. The study of colours in general is called colour science.

Physical properties edit

 
The visible spectrum perceived from 390 to 710 nm wavelength

Electromagnetic radiation is characterised 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 390 nm to 700 nm), it is known as "visible light".[1]

Most light sources emit light at many different wavelengths; a source's spectrum is a distribution giving its intensity at each wavelength. Although the spectrum of light arriving at the eye from a given direction determines the colour sensation in that direction, there are many more possible spectral combinations than colour sensations. In fact, one may formally define a colour 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. This effect can be visualised by comparing the light sources' spectral power distributions and the resulting colours.

Spectral colours edit

The familiar colours of the rainbow in the spectrum—named using the Latin word for appearance or apparition by Isaac Newton in 1671—include all those colours that can be produced by visible light of a single wavelength only, the pure spectral or monochromatic colours. The spectrum above shows approximate wavelengths (in nm) for spectral colours in the visible range. Spectral colours have 100% purity, and are fully saturated. A complex mixture of spectral colours can be used to describe any colour, which is the definition of a light power spectrum.

The spectral colours form a continuous spectrum, and how it is divided into distinct colours linguistically is a matter of culture and historical contingency.[2] Despite the ubiquitous ROYGBIV mnemonic used to remember the spectral colours in English, the inclusion or exclusion of colours is contentious, with disagreement often focused on indigo and cyan.[3] Even if the subset of colour terms is agreed, their wavelength ranges and borders between them may not be.

The intensity of a spectral colour, relative to the context in which it is viewed, may alter its perception considerably according to the Bezold–Brücke shift; for example, a low-intensity orange-yellow is brown, and a low-intensity yellow-green is olive green. In colour models capable of representing spectral colours,[4] such as CIELUV, a spectral colour has the maximal saturation. In Helmholtz coordinates, this is described as 100% purity.

colour of objects edit

The physical colour of an object depends on how it absorbs and scatters light. Most objects scatter light to some degree and do not reflect or transmit light specularly like glasses or mirrors. A transparent object allows almost all light to transmit or pass through, thus transparent objects are perceived as colourless. Conversely, an opaque object does not allow light to transmit through and instead absorbing or reflecting the light it receives. Like transparent objects, translucent objects allow light to transmit through, but translucent objects are seen coloured because they scatter or absorb certain wavelengths of light via internal scatterance. The absorbed light is often dissipated as heat.[5]

Colour vision edit

Development of theories of colour vision edit

 
The upper disk and the lower disk have exactly the same objective colour, and are in identical gray surroundings; based on context differences, humans perceive the squares as having different reflectances, and may interpret the colours as different colour categories; see checker shadow illusion.

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 colours in which he provided a rational description of colour experience, which 'tells us how it originates, not what it is'. (Schopenhauer)

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 marvelous investigator achieved in advance of his time, remained unnoticed until Maxwell directed attention to it."[6]

At the same time as Helmholtz, Ewald Hering developed the opponent process theory of colour, noting that colour blindness and afterimages typically come in opponent pairs (red-green, blue-orange, yellow-violet, and black-white). Ultimately these two theories were synthesised 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.[7]

In 1931, an international group of experts known as the Commission internationale de l'éclairage (CIE) developed a mathematical colour model, which mapped out the space of observable colours and assigned a set of three numbers to each.

Colour in the eye edit

 
Normalised typical human cone cell responses (S, M, and L types) to monochromatic spectral stimuli

The ability of the human eye to distinguish colours is based upon the varying sensitivity of different cells in the retina to light of different wavelengths. Humans are trichromatic—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 is perceived as blue or blue-violet, with wavelengths around 450 nm; cones of this type are sometimes called short-wavelength cones or S cones (or misleadingly, blue cones). The other two types are closely related genetically and chemically: middle-wavelength cones, M cones, or green cones are most sensitive to light perceived as green, with wavelengths around 540 nm, while the long-wavelength cones, L cones, or red cones, are most sensitive to light that is perceived as greenish yellow, with wavelengths around 570 nm.

Light, no matter how complex its composition of wavelengths, is reduced to three colour components by the eye. Each cone type adheres to the principle of univariance, which is that each cone's output is determined by the amount of light that falls on it over all wavelengths. 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 amounts of stimulation are sometimes called tristimulus values.[8]

The response curve as a function of wavelength varies for each type of cone. 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 colour space. It has been estimated that humans can distinguish roughly 10 million different colours.[9]

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.[10] On the other hand, in dim light, the cones are understimulated leaving only the signal from the rods, resulting in a colourless 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. These effects, combined, are summarised also in the Kruithof curve, which describes the change of colour perception and pleasingness of light as a function of temperature and intensity.

Colour in the brain edit

While the mechanisms of colour vision at the level of the retina are well-described in terms of tristimulus values, colour processing after that point is organised differently. A dominant theory of colour 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 humans cannot perceive a "reddish green" or "yellowish blue", and it predicts the colour wheel: it is the collection of colours for which at least one of the two colour channels measures a value at one of its extremes.

The exact nature of colour 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—a type of qualia—is a matter of complex and continuing philosophical dispute.[citation needed]

 
The visual dorsal stream (green) and ventral stream (purple) are shown. The ventral stream is responsible for colour perception.

From the V1 blobs, colour information is sent to cells in the second visual area, V2. The cells in V2 that are most strongly colour tuned are clustered in the "thin stripes" that, like the blobs in V1, stain for the enzyme cytochrome oxidase (separating the thin stripes are interstripes and thick stripes, which seem to be concerned with other visual information like motion and high-resolution form). Neurones in V2 then synapse onto cells in the extended V4. This area includes not only V4, but two other areas in the posterior inferior temporal cortex, anterior to area V3, the dorsal posterior inferior temporal cortex, and posterior TEO.[11][12] Area V4 was initially suggested by Semir Zeki to be exclusively dedicated to colour,[13] and he later showed that V4 can be subdivided into subregions with very high concentrations of colour cells separated from each other by zones with lower concentration of such cells though even the latter cells respond better to some wavelengths than to others,[14] a finding confirmed by subsequent studies.[11][15][16] The presence in V4 of orientation-selective cells led to the view that V4 is involved in processing both colour and form associated with colour[17] but it is worth noting that the orientation selective cells within V4 are more broadly tuned than their counterparts in V1, V2 and V3.[14] colour processing in the extended V4 occurs in millimeter-sised colour modules called globs.[11][12] This is the part of the brain in which colour is first processed into the full range of hues found in colour space.[18][11][12]

Nonstandard colour perception edit

Colour vision deficiency edit

A colour vision deficiency causes an individual to perceive a smaller gamut of colours than the standard observer with normal colour vision. The effect can be mild, having lower "colour resolution" (i.e. anomalous trichromacy), moderate, lacking an entire dimension or channel of colour (e.g. dichromacy), or complete, lacking all colour perception (i.e. monochromacy). Most forms of colour blindness derive from one or more of the three classes of cone cells either being missing, having a shifted spectral sensitivity or having lower responsiveness to incoming light. In addition, cerebral achromatopsia is caused by neural anomalies in those parts of the brain where visual processing takes place.

Some colours that appear distinct to an individual with normal colour vision will appear metameric to the colour blind. The most common form of colour blindness is congenital red–green colour blindness, affecting ~8% of males. Individuals with the strongest form of this condition (dichromacy) will experience blue and purple, green and yellow, teal and gray as colours of confusion, i.e. metamers.[19]

Tetrachromacy edit

Outside of humans, which are mostly trichromatic (having three types of cones), most mammals are dichromatic, possessing only two cones. However, outside of mammals, most vertebrate are tetrachromatic, having four types of cones, and includes most, birds,[20][21][22] reptiles, amphibians and bony fish.[23][24] An extra dimension of colour vision means these vertebrates can see two distinct colours that a normal human would view as metamers. Some invertebrates, such as the mantis shrimp, have an even higher number of cones (12) that could lead to a richer colour gamut than even imaginable by humans.

The existence of human tetrachromats is a contentious notion. As many as half of all human females have 4 distinct cone classes, which could enable tetrachromacy.[25] However, a distinction must be made between retinal (or weak) tetrachromats, which express four cone classes in the retina, and functional (or strong) tetrachromats, which are able to make the enhanced colour discriminations expected of tetrachromats. In fact, there is only one peer-reviewed report of a functional tetrachromat.[26] It is estimated that while the average person is able to see one million colours, someone with functional tetrachromacy could see a hundred million colours.[27]

Synesthesia edit

In certain forms of synesthesia, perceiving letters and numbers (grapheme–colour synesthesia) or hearing sounds (chromesthesia) will evoke a perception of colour. Behavioral and functional neuroimaging experiments have demonstrated that these colour experiences lead to changes in behavioral tasks and lead to increased activation of brain regions involved in colour perception, thus demonstrating their reality, and similarity to real colour percepts, albeit evoked through a non-standard route. Synesthesia can occur genetically, with 4% of the population having variants associated with the condition. Synesthesia has also been known to occur with brain damage, drugs, and sensory deprivation.[28]

The philosopher Pythagoras experienced synesthesia and provided one of the first written accounts of the condition in approximately 550 BCE. He created mathematical equations for musical notes that could form part of a scale, such as an octave.[29]

Afterimages edit

After exposure to strong light in their sensitivity range, photoreceptors of a given type become desensitised.[30][31] For a few seconds after the light ceases, they will continue to signal less strongly than they otherwise would. Colours observed during that period will appear to lack the colour component detected by the desensitised 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 used by artists, including Vincent van Gogh.

Colour constancy edit

When an artist uses a limited colour palette, the human eye tends to compensate by seeing any gray or neutral colour as the colour which is missing from the colour wheel. For example, 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 gray will appear bluish.[32]

The trichromatic theory is strictly true when the visual system is in a fixed state of adaptation.[33] In reality, the visual system is constantly adapting to changes in the environment and compares the various colours in a scene to reduce 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 colours in the scene appear relatively constant to us. This was studied by Edwin H. Land in the 1970s and led to his retinex theory of colour constancy.[34][35]

Both phenomena are readily explained and mathematically modeled with modern theories of chromatic adaptation and colour appearance (e.g. CIECAM02, iCAM).[36] There is no need to dismiss the trichromatic theory of vision, but rather it can be enhanced with an understanding of how the visual system adapts to changes in the viewing environment.

Reproduction edit

 
The CIE 1931 colour space xy chromaticity diagram with the visual locus plotted using the CIE (2006) physiologically relevant LMS fundamental colour matching functions transformed into the CIE 1931 xy colour space and converted into Adobe RGB. The triangle shows the gamut of Adobe RGB. The Planckian locus is shown with colour temperatures labeled in Kelvins. The outer curved boundary is the spectral (or monochromatic) locus, with wavelengths shown in nanometers. The colours in this file are being specified using Adobe RGB. Areas outside the triangle cannot be accurately rendered since they are outside the gamut of Adobe RGB, therefore they have been interpreted. The colours depicted depend on the gamut and colour accuracy of your display.

Colour reproduction is the science of creating colours for the human eye that faithfully represent the desired colour. It focuses on how to construct a spectrum of wavelengths that will best evoke a certain colour in an observer. Most colours are not spectral colours, meaning they are mixtures of various wavelengths of light. However, these non-spectral colours are often described by their dominant wavelength, which identifies the single wavelength of light that produces a sensation most similar to the non-spectral colour. Dominant wavelength is roughly akin to hue.

There are many colour perceptions that by definition cannot be pure spectral colours 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 colours are the achromatic colours (black, gray, and white) and colours such as pink, tan, and magenta.

Two different light spectra that have the same effect on the three colour receptors in the human eye will be perceived as the same colour. They are metamers of that colour. This is exemplified by the white light emitted by fluorescent lamps, which typically has a spectrum 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 the colour rendering index of each light source may affect the colour of objects illuminated by these metameric light sources.

Similarly, most human colour perceptions can be generated by a mixture of three colours 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 colours. Each method has its advantages and disadvantages depending on the particular application.

No mixture of colours, however, can produce a response truly identical to that of a spectral colour, although one can get close, especially for the longer wavelengths, where the CIE 1931 colour space chromaticity diagram 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 colours reproduced are never perfectly saturated spectral colours, and so spectral colours cannot be matched exactly. However, natural scenes rarely contain fully saturated colours, thus such scenes can usually be approximated well by these systems. The range of colours 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 colour reproduction systems is connected with the initial measurement of colour, or colourimetry. The characteristics of the colour sensors in measurement devices (e.g. cameras, scanners) are often very far from the characteristics of the receptors in the human eye.

A colour reproduction system "tuned" to a human with normal colour vision may give very inaccurate results for other observers, according to colour vision deviations to the standard observer.

The different colour response of different devices can be problematic if not properly managed. For colour information stored and transferred in digital form, colour management techniques, such as those based on ICC profiles, can help to avoid distortions of the reproduced colours. Colour management does not circumvent the gamut limitations of particular output devices, but can assist in finding good mapping of input colours into the gamut that can be reproduced.

Additive colouring edit

 
Additive colour mixing: combining red and green yields yellow; combining all three primary colours together yields white.

Additive colour is light created by mixing together light of two or more different colours.[37][38] Red, green, and blue are the additive primary colours normally used in additive colour systems such as projectors, televisions, and computer terminals.

Subtractive colouring edit

 
Subtractive colour mixing: combining yellow and magenta yields red; combining all three primary colours together yields black.
 
Twelve main pigment colours

Subtractive colouring uses dyes, inks, pigments, or filters to absorb some wavelengths of light and not others.[39] The colour that a surface displays comes from the parts of the visible spectrum that are not absorbed and therefore remain visible. Without pigments or dye, fabric fibers, paint base and paper are usually made of particles that scatter white light (all colours) well in all directions. When a pigment or ink is added, wavelengths are absorbed or "subtracted" from white light, so light of another colour reaches the eye.

If the light is not a pure white source (the case of nearly all forms of artificial lighting), the resulting spectrum will appear a slightly different colour. Red paint, viewed under blue light, may appear black. Red paint is red because it scatters only the red components of the spectrum. If red paint is illuminated by blue light, it will be absorbed by the red paint, creating the appearance of a black object.

The subtractive model also predicts the colour resulting from a mixture of paints, or similar medium such as fabric dye, whether applied in layers or mixed together prior to application. In the case of paint mixed before application, incident light interacts with many different pigment particles at various depths inside the paint layer before emerging.[40]

Structural colour edit

Structural colours are colours caused by interference effects rather than by pigments. Colour effects are produced when a material is scored with fine parallel lines, formed of one or more parallel thin 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 colours: the blue of the sky (Rayleigh scattering, caused by structures much smaller than the wavelength of light, in this case, air molecules), 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 colour is studied in the field of thin-film optics. The most ordered or the most changeable structural colours are iridescent. Structural colour 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 colour 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 colour, such as "photonic" cosmetics.[41]

Cultural perspective edit

Colours, their meanings and associations can play a major role in works of art, including literature.[42]

Associations edit

Individual colours have a variety of cultural associations such as national colours (in general described in individual colour articles and colour symbolism). The field of colour psychology attempts to identify the effects of colour on human emotion and activity. Chromotherapy is a form of alternative medicine attributed to various Eastern traditions. Colours have different associations in different countries and cultures.[43]

Different colours have been demonstrated to have effects on cognition. For example, researchers at the University of Linz in Austria demonstrated that the colour red significantly decreases cognitive functioning in men.[44] The combination of the colours red and yellow together can induce hunger, which has been capitalised on by a number of chain restaurants.[45]

colour plays a role in memory development too. A photograph that is in black and white is slightly less memorable than one in colour.[46] Studies also show that wearing bright colours makes you more memorable to people you meet.

Terminology edit

olours vary in several different ways, including hue (shades of red, orange, yellow, green, blue, and violet, etc), saturation, brightness. Some colour words are derived from the name of an object of that colour, such as "orange" or "salmon", while others are abstract, like "red".

In the 1969 study Basic colour Terms: Their Universality and Evolution, Brent Berlin and Paul Kay describe a pattern in naming "basic" colours (like "red" but not "red-orange" or "dark red" or "blood red", which are "shades" of red). All languages that have two "basic" colour names distinguish dark/cool colours from bright/warm colours. The next colours to be distinguished are usually red and then yellow or green. All languages with six "basic" colours include black, white, red, green, blue, and yellow. The pattern holds up to a set of twelve: black, gray, white, pink, red, orange, yellow, green, blue, purple, brown, and azure (distinct from blue in Russian and Italian, but not English).

See also edit

References edit

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