Editing Primary color (section)

=== 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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