Editing Primary color (section)

== Color model primaries ==

A [[color model]] is an abstract model intended to describe the ways that colors behave, especially in [[color mixing]]. Most color models are defined by the interaction of multiple primary colors. Since most humans are [[trichromacy|trichromatic]], color models that want to [[color reproduction|reproduce]] a meaningful portion of a human's perceptual [[gamut]] must use at least ''three'' primaries.<ref>{{cite book |last1=Westland |first1=Stephen |last2=Cheung |first2=Vien |editor1-last=Chen |editor1-first=Janglin |editor2-last=Cranton |editor2-first=Wayne |editor3-last=Fihn |editor3-first=Mark |title=Handbook of visual display technology |date=2012 |publisher=Springer |location=Cham, Switzerland |isbn=978-3-540-79567-4 |pages=155 |edition=2nd|quote="Color vision is based upon the responses of three classes of cones in the retina, each of which has broadband sensitivity but maximum sensitivity at different wavelengths. A consequence of this is that color reproduction is trichromatic – the use of three primaries allows a wide range of colors to be reproduced."}}</ref> More than three primaries are allowed, for example, to increase the size of the gamut of the color space, but the entire human perceptual gamut can be reproduced with just three primaries (albeit imaginary ones as in the [[CIE XYZ color space]]).

Some humans (and most mammals<ref>{{cite journal |last1=Bowmaker |first1=James K |title=Evolution of colour vision in vertebrates |journal=Eye |date=May 1998 |volume=12 |issue=3 |pages=543 |doi=10.1038/eye.1998.143|pmid=9775215 |doi-access=free }}</ref>) are [[dichromacy|dichromats]], corresponding to specific forms of [[color blindness]] in which color vision is mediated by only two of the types of color receptors. Dichromats require only two primaries to reproduce their entire gamut and their participation in color matching experiments was essential in the determination of cone fundamentals leading to all modern color spaces.<ref>{{cite book |last1=Stockman |first1=Andrew |title=Encyclopedia of Color Science and Technology |chapter=Cone Fundamentals |date=2016 |pages=541–546 |doi=10.1007/978-1-4419-8071-7_85|isbn=978-1-4419-8070-0 }}</ref> Despite most vertebrates being [[tetrachromacy|tetrachromatic]],<ref>{{cite journal |last1=Scholtyßek |first1=C. |last2=Kelber |first2=A. |title=Farbensehen der Tiere: Von farbenblinden Seehunden und tetrachromatischen Vögeln |journal=Der Ophthalmologe |date=November 2017 |volume=114 |issue=11 |pages=978–985 |doi=10.1007/s00347-017-0543-6|pmid=28752388 |doi-access=free }}</ref> and therefore requiring four primaries to reproduce their entire gamut, there is only one scholarly report of a functional human [[tetrachromacy|tetrachromat]], for which trichromatic color models are insufficient.<ref>{{cite journal
| last1   = Jordan
| first1  = G.
| last2   = Deeb
| first2  = S. S.
| last3   = Bosten
| first3  = J. M.
| last4   = Mollon
| first4  = J. D.
| title   = The dimensionality of color vision in carriers of anomalous trichromacy
| journal = Journal of Vision
| date    = 20 July 2010
| volume  = 10
| issue   = 8
| pages   = 12
| doi     = 10.1167/10.8.12
| pmid = 20884587
| doi-access= free
}}</ref>

=== Additive models ===
{{See also|RGB color model}}
[[File:AdditiveColor.svg|thumb|Additive primary color model]]
[[File:LCD pixels RGB.jpg|thumb|upright=1.4|A photograph of the red, green, and blue elements (subpixels) of an [[Liquid-crystal display|LCD]]. Additive mixing explains how light from these colored elements can be used for photorealistic color image reproduction.]]

The perception elicited by multiple light sources co-stimulating the same area of the retina is [[additive color|additive]], i.e., predicted via summing the [[spectral power distribution]]s (the intensity of each wavelength) of the individual light sources assuming a color matching context.<ref name="Williamson1983">{{cite book |last1=Williamson |first1=Samuel J. |title=Light and color in nature and art |date=1983 |publisher=Wiley |location=New York |isbn=0471083747 |url=https://archive.org/details/lightcolorinnatu00will/page/17/mode/2up |access-date=28 April 2021}}</ref>{{rp|17–22}} For example, a [[purple]] spotlight on a dark background could be matched with coincident [[blue]] and [[red]] spotlights that are both dimmer than the purple spotlight. If the intensity of the purple spotlight was doubled it could be matched by doubling the intensities of both the red and blue spotlights that matched the original purple. The principles of additive color mixing are embodied in [[Grassmann's law (optics)|Grassmann's laws]].<ref>{{cite book
| last1      = Reinhard
| first1     = Erik
| last2      = Khan
| first2     = Arif
| last3      = Akyuz
| first3     = Ahmet
| last4      = Johnson
| first4     = Garrett
| title      = Color imaging : fundamentals and applications
| date       = 2008
| publisher  = A.K. Peters
| location   = Wellesley, Mass
| isbn       = 978-1-56881-344-8
| pages      = 364–365
| url        = https://books.google.com/books?id=suLqBgAAQBAJ&q=grassman%27s+laws+additive&pg=PA364
| access-date = 31 December 2017
}}</ref> Additive mixing is sometimes described as "additive color matching"<ref>{{cite book |last1=Berns |first1=Roy S. |title=Billmeyer and Saltzman's principles of color technology |date=2019 |location=Hoboken, NJ |isbn=9781119367192 |pages=54–64 |edition=Fourth}}</ref> to emphasize the fact the predictions based on additivity only apply assuming the color matching context. Additivity relies on assumptions of the color matching context such as the match being in the [[Fovea centralis|foveal]] field of view, under appropriate luminance, etc.<ref>{{cite book |last1=Brill |first1=Michael H. |last2=Robertson |first2=Alan R. |chapter=Open Problems on the Validity of Grassmann's Laws |title=Colorimetry |date=27 July 2007 |pages=245–259 |doi=10.1002/9780470175637.ch10|isbn=9780470175637 | quote="Grassmann’s laws are known not to be exactly true in human color matching. Symmetry could be called into question by color difference formulas, such as CIE94,3 that are asymmetric between batch and standard. Transitivity can be considered to be violated if we take the term ‘‘color match’’ to mean that two colors are within a just-noticeable difference of each other. In this case, adding two subthreshold differences together could produce a combined difference that is above thresh- old. Proportionality and additivity can also be compromised. Besides the three cone types that herald the trichromacy of vision at high (photopic) light intensities, a fourth photoreceptor type (rods) contributes to vision at low (mesopic and scotopic) light intensities and away from the center of vision (fovea). At very high light intenities, unbleached photopigments deplete and, in aggregate, change their action spectrum. At still higher light intensities, a photopigment molecule can absorb multiple photons but respond as if it absorbed only one photon. All these effects compromise Grassmann’s laws, but the successful application of the laws, for example, in photography and television, has led us to believe that the compromises are not serious."}}</ref>

Additive mixing of coincident spot lights was applied in the experiments used to derive the [[CIE 1931]] colorspace (see [[#Color space primaries|color space primaries section]]). The original ''[[monochromatic radiation|monochromatic]]'' primaries of the wavelengths of 435.8&nbsp;nm ([[violet (color)|violet]]), 546.1&nbsp;nm ([[green]]), and 700&nbsp;nm (red) were used in this application due to the convenience they afforded to the experimental work.<ref>{{cite journal
| last1   = Fairman
| first1  = Hugh S.
| last2   = Brill
| first2  = Michael H.
| last3   = Hemmendinger
| first3  = Henry
| title   = How the CIE 1931 color-matching functions were derived from Wright-Guild data
| journal = Color Research & Application
| date    = February 1997
| volume  = 22
| issue   = 1
| pages   = 11–23
| doi     = 10.1002/(SICI)1520-6378(199702)22:1<11::AID-COL4>3.0.CO;2-7
| quote = "The first of the resolutions offered to the 1931 meeting defined the color-matching functions of the soon-to-be-adopted standard observer in terms of Guild’s spectral primaries centered on wavelengths 435.8, 546.1, and 700nm. Guild approached the problem from the viewpoint of a standardization engineer. In his mind, the adopted primaries had to be producible with national-standardizing-laboratory accuracy. The first two wavelengths were mercury excitation lines, and the last named wavelength occurred at a location in the human vision system where the hue of spectral lights was unchanging with wavelength. Slight inaccuracy in production of the wavelength of this spectral primary in a visual colorimeter, it was reasoned, would introduce no error at all."
}}</ref>

Small red, green, and blue elements (with controllable brightness) in electronic displays mix additively from an appropriate viewing distance to synthesize compelling colored images. This specific type of additive mixing is described as ''partitive mixing''.<ref name="Williamson1983"/>{{rp|21–22}} Red, green, and blue light are popular primaries for partitive mixing since primary lights with those hues provide a large [[color triangle]] ([[gamut]]).<ref>{{cite book |last1=Tooms |first1=Michael S. |title=Colour Reproduction in Electronic Imaging Systems: Photography, Television, Cinematography |date=26 January 2016 |publisher=John Wiley & Sons |isbn=978-1-119-02176-6 |pages=22 |url=https://books.google.com/books?id=nQslCgAAQBAJ |access-date=25 February 2021 |language=en |quote=If we now define the primaries in terms of the three colours which together in various ratios produce the largest gamut of colours in the eye–brain complex, then, as reasoned above, the primary colours are red, green and blue.}}</ref>

The exact colors chosen for additive primaries are a compromise between the available technology (including considerations such as cost and power usage) and the need for large chromaticity gamut. For example, in 1953 the [[NTSC]] specified primaries that were representative of the [[phosphor]]s available in that era for color [[Cathode-ray tube|CRT]]s. Over decades, market pressures for brighter colors resulted in CRTs using primaries that deviated significantly from the original standard.<ref>{{cite web |last1=Poynton |first1=Charles |title=Frequently Asked Questions about Color |url=http://poynton.ca/PDFs/ColorFAQ.pdf |archive-url=https://web.archive.org/web/20180219004614/http://poynton.ca/PDFs/ColorFAQ.pdf |archive-date=2018-02-19 |url-status=live |website=Charles Poynton, PhD |access-date=26 April 2021 |quote=The NTSC in 1953 specified a set of primaries that were representative of phosphors used in color CRTs of that era. But phosphors changed over the years, primarily in response to market pressures for brighter receivers, and by the time of the first the videotape recorder the primaries in use were quite different from those “on the books”. So although you may see the NTSC primary chromaticities documented, they are of no use today.}}</ref> Currently, [[Rec. 709|ITU-R BT.709-5]] primaries are typical for [[high-definition television]].<ref>{{cite book |last1=Westland |first1=Stephen |last2=Cheung |first2=Vien |editor1-last=Chen |editor1-first=Janglin |editor2-last=Cranton |editor2-first=Wayne |editor3-last=Fihn |editor3-first=Mark |title=Handbook of visual display technology |date=2016 |publisher=Springer |location=Cham, Switzerland |isbn=978-3-319-14347-7 |pages=171–177 |edition=2nd}}</ref>
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=== Subtractive models ===
{{See also|CMYK color model}}
[[File:SubtractiveColor.svg|thumb|Subtractive primary color model]]
[[File:Halftoningcolor.svg|thumb|upright=1.4|A magnified representation of small partially overlapping spots of cyan, magenta, yellow, and key (black) [[halftone]]s in [[CMYK]] process printing. Each row represents the pattern of partially overlapping ink "rosettes" so that the patterns would be perceived as blue, green, and red when viewed on white paper from a typical viewing distance. The overlapping ink layers mix subtractively while additive mixing predicts the color appearance from the light reflected from the rosettes and white paper in between them.]]

The [[subtractive color]] mixing model predicts the resultant spectral power distribution of light filtered through overlaid partially absorbing materials, usually in the context of an underlying reflective surface such as white paper.<ref name="Williamson1983"/>{{rp|22–23}}<ref>{{cite book |last1=Berns |first1=Roy S. |title=Billmeyer and Saltzman's Principles of Color Technology |date=9 April 2019 |publisher=John Wiley & Sons |isbn=978-1-119-36722-2 |pages=195–209 |url=https://books.google.com/books?id=8GGLDwAAQBAJ |language=en}}</ref> Each layer partially absorbs some wavelengths of light from the illumination while letting others pass through, resulting in a colored appearance. The resultant spectral power distribution is predicted by the wavelength-by-wavelength product of the spectral reflectance of the illumination and the product of the spectral reflectances of all of the layers.<ref>{{cite web |last1=Levoy |first1=Marc |title=Additive versus subtractive color mixing |url=https://graphics.stanford.edu/courses/cs178-10/applets/colormixing.html |website=graphics.stanford.edu |access-date=4 November 2020 |quote="On the other hand, if you reflect light from a colored surface, or if you place a colored filter in front of a light, then some of the wavelengths present in the light may be partially or fully absorbed by the colored surface or filter. If we characterize the light as an SPD, and we characterize absorption by the surface or filter using a spectrum of reflectivity or transmissivity, respectively, i.e. the percentage of light reflected or transmitted at each wavelength, then the SPD of the outgoing light can be computed by multiplying the two spectra. This multiplication is (misleadingly) called subtractive mixing."}}</ref> Overlapping layers of ink in printing mix subtractively over reflecting white paper, while the reflected light mixes in a partitive way to generate color images.<ref name="Williamson1983"/>{{rp|30–33}}<ref>{{cite journal |last1=Kuehni |first1=Rolf |title=Color mixture |journal=Scholarpedia |date=2011 |volume=6 |issue=1 |pages=10686 |doi=10.4249/scholarpedia.10686|bibcode=2011SchpJ...610686K |doi-access=free }}</ref> Importantly, unlike additive mixture, the color of the mixture is not well predicted by the colors of the individual dyes or inks. The typical number of inks in such a printing process is 3 (CMY) or 4 ([[CMYK]]), but can commonly range to 6 (e.g., [[hexachrome|Pantone hexachrome]]). In general, using fewer inks as primaries results in more economical printing but using more may result in better color reproduction.<ref>{{cite book |last1=Sharma |first1=Abhay |title=Understanding color management |date=2018 |location=Hoboken, NJ |isbn=9781119223634 |page=235 |edition=2nd}}</ref>

[[Cyan]] (C), [[magenta]] (M), and [[yellow]] (Y) are good chromatic subtractive primaries in that filters with those colors can be overlaid to yield a surprisingly large chromaticity gamut.<ref>{{cite book |last1=Westland |first1=Stephen |last2=Cheung |first2=Vien |editor1-last=Chen |editor1-first=Janglin |editor2-last=Cranton |editor2-first=Wayne |editor3-last=Fihn |editor3-first=Mark |title=Handbook of visual display technology |date=2012 |publisher=Springer |location=Cham, Switzerland |isbn=978-3-540-79567-4 |page=155 |edition=2nd|quote="The optimum primaries of the subtractive color system are cyan, magenta, and yellow. The use of cyan, magenta, and yellow subtractive primaries allows a surprisingly large – albeit limited – gamut of colors to be reproduced."}}</ref> A black (K) ink (from the older "[[key plate]]") is also used in CMYK systems to augment C, M and Y inks or dyes: this is more efficient in terms of time and expense and less likely to introduce visible defects.<ref>{{cite web |last1=Poynton |first1=Charles |title=Color FAQ - Frequently Asked Questions Color |url=https://poynton.ca/notes/colour_and_gamma/ColorFAQ.html#RTFToC25 |website=poynton.ca | quote=Printing black by overlaying cyan, yellow and magenta ink in offset printing has three major problems. First, coloured ink is expensive. Replacing coloured ink by black ink – which is primarily carbon – makes economic sense. Second, printing three ink layers causes the printed paper to become quite wet. If three inks can be replaced by one, the ink will dry more quickly, the press can be run faster, and the job will be less expensive. Third, if black is printed by combining three inks, and mechanical tolerances cause the three inks to be printed slightly out of register, then black edges will suffer coloured tinges. Vision is most demanding of spatial detail in black and white areas. Printing black with a single ink minimizes the visibility of registration errors. |access-date=27 April 2021}}</ref> Before the color names ''cyan'' and ''magenta'' were in common use, these primaries were often known as blue and red, respectively, and their exact color has changed over time with access to new pigments and technologies.<ref>{{cite book
| title     = General Physics and Its Application to Industry and Everyday Life
| author    = Ervin Sidney Ferry
| publisher = John Wiley & Sons
| year      = 1921
| url       = https://books.google.com/books?id=3rYXAAAAIAAJ&q=date:0-1923+additive+color+mixing+primary&pg=PA621
}}</ref> Organizations such as [[Fogra]],<ref>{{cite web |title=FOGRA characterization data |url=https://www.color.org/chardata/fogra.xalter |website=International Color Consortium |access-date=26 April 2021}}</ref> [[European Color Initiative]] and [[Specifications for Web Offset Publications|SWOP]] publish [[colorimetry|colorimetric]] CMYK standards for the printing industry.<ref>{{cite book |last1=Homann |first1=Jan-Peter |title=Digital color management : principles and strategies for the standardized print production |date=2009 |publisher=Springer |location=Berlin |isbn=9783540693772}}</ref>
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