Editing Color (section)

=== Color in the eye<span class="anchor" id="Colour in the eye"></span> ===
{{main|Color vision#Cone cells in the human eye}}
[[File:Cones SMJ2 E.svg|thumb|upright=1.2|Normalized typical human [[cone cell]] responses (''S'', ''M'', and ''L types'') to monochromatic spectral stimuli]]

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 [[wavelength]]s. Humans are [[trichromatic]]—the retina contains three types of color receptor cells, or [[cone cell|cone]]s. 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&nbsp;[[nanometre|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&nbsp;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&nbsp;nm.

Light, no matter how complex its composition of wavelengths, is reduced to three color 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''.<ref>{{Cite web |last=Magazine |first=Nicola Jones, Knowable |title=Color Is in the Eye, and Brain, of the Beholder |url=https://www.scientificamerican.com/article/color-is-in-the-eye-and-brain-of-the-beholder/ |access-date=2022-11-08 |website=Scientific American |language=en}}</ref>

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 ''color space''. It has been estimated that humans can distinguish roughly 10 million different colors.<ref name="business">{{cite book|last1=Judd|first1=Deane B.|title=Color in Business, Science and Industry|last2=Wyszecki|first2=Günter|publisher=[[Wiley-Interscience]]|year=1975|isbn=978-0471452126|edition=3rd|series=Wiley Series in Pure and Applied Optics|location=New York|page=388}}</ref>

The other type of light-sensitive cell in the eye, the [[rod cell|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.<ref>"Under well-lit viewing conditions (photopic vision), cones&nbsp; ...are highly active and rods are inactive."{{cite conference|last=Hirakawa|first=K.|author2=Parks, T.W.|title=IEEE International Conference on Image Processing 2005|chapter=Chromatic Adaptation and White-Balance Problem|conference=IEEE ICIP|year=2005|pages=iii-984|doi=10.1109/ICIP.2005.1530559|isbn=0780391349|chapter-url=http://www.accidentalmark.com/research/papers/Hirakawa05WBICIP.pdf|url-status=dead|archive-url=https://web.archive.org/web/20061128184104/http://www.accidentalmark.com/research/papers/Hirakawa05WBICIP.pdf|archive-date=November 28, 2006}}</ref> On the other hand, in dim light, the cones are understimulated leaving only the signal from the rods, resulting in a [[black-and-white|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. These effects, combined, are summarized also in the [[Kruithof curve]], which describes the change of color perception and pleasingness of light as a function of temperature and intensity.