Editing Chromaticity

{{Short description|Specification of color hue and saturation}}
{{For|the album by Tony MacAlpine|Chromaticity (album)}}

[[File:PlanckianLocus.png|right|thumb|250px|The [[CIE 1931 color space|CIE 1931]] xy chromaticity space, also showing the chromaticities of black-body light sources of various temperatures, and lines of constant [[Color temperature#Correlated color temperature|correlated color temperature]]]]

[[File:SRGB gamut within CIExyY color space mesh.webm|alt=3D Chromaticity Diagram of the WideGamutRGB color space|thumb|sRGB gamut plotted in CIE xyY color space. '''''x'' and ''y'' are the chromaticity axes''', the ''Y'' axis represents (linear) [[Relative luminance|luminance]].]]

'''Chromaticity''' is an objective specification of the quality of a [[color]] regardless of its [[luminance]]. Chromaticity consists of two [[dimension|independent parameters]], often specified as ''[[hue]]'' (''h'') and ''[[colorfulness]]'' (''s''), where the latter is alternatively called ''saturation'', ''chroma'', ''intensity'',<ref>In modern terminology the word ''intensity'' may refer to lightness, not colorfulness.</ref> or ''[[excitation purity]]''.<ref>{{cite book | title = Progress in Optics | author = Emil Wolf | publisher = North Holland Pub. Co | year = 1961 | url = https://books.google.com/books?id=5n7yXmBusL0C&q=chromaticity+hue+purity}}</ref><ref>{{cite book | title = The Focal Encyclopedia of Photography | author = Leslie D. Stroebel, Richard D. Zakia | publisher = Focal Press | year = 1993 | isbn = 978-0-240-51417-8 | url = https://archive.org/details/focalencyclopedi00lesl| url-access = registration | page = [https://archive.org/details/focalencyclopedi00lesl/page/124 124] | quote = chromaticity hue saturation chroma colorfulness purity. }}</ref> This number of parameters follows from [[trichromacy]] of vision of most humans, which is assumed by most models in [[color science]].

==Quantitative description==
In color science, the [[white point]] of an illuminant or of a display is a neutral reference characterized by a chromaticity; all other chromaticities may be defined in relation to this reference using [[Polar coordinate system|polar coordinates]]. The ''hue'' is the angular component, and the ''purity'' is the radial component, normalized{{clarify|date=June 2013|reason=note that the chromaticity space does not have a canonical affine structure, so there are no invariantly defined ratios of lengths even along the same line}} by the maximum radius for that hue.

''Purity'' is roughly equivalent to the term ''[[Colorfulness#Saturation|saturation]]'' in the [[HSL and HSV|HSV color model]]. The property ''[[hue]]'' is as used in general color theory and in specific [[color model]]s such as [[HSL and HSV]] color spaces, though it is more [[Color difference#Tolerance|perceptually uniform]] in color models such as [[Munsell color system|Munsell]], [[CIELAB color space|CIELAB]] or [[CIECAM02]].

Some [[color space]]s separate the three dimensions of color into one [[relative luminance|luminance]] dimension and a pair of chromaticity dimensions. For example, the white point of an [[sRGB]] display is an {{mvar|x}}, {{mvar|y}} chromaticity of (0.3127, 0.3290), where {{mvar|x}} and {{mvar|y}} coordinates are used in the xyY space.

[[Image:CIE 1976 UCS.png|right|thumb|240px|{{math|(''u′'', ''v′'')}}, the chromaticity in [[CIELUV]], is a fairly [[Color difference#Tolerance|perceptually uniform]] presentation of the chromaticity as (another than in CIE 1931) planar [[Euclidean space|Euclidean]] shape. This presentation is a [[projective transformation]] of the CIE 1931 chromaticity diagram above.]]

These pairs determine a chromaticity as [[affine coordinates]] on a [[triangle]] in a [[Plane (mathematics)|2D-space]], which contains all possible chromaticities. These {{mvar|x}} and {{mvar|y}} are used because of simplicity of expression in [[CIE 1931 color space|CIE 1931]] (see below) and have no inherent advantage. Other [[coordinate system]]s on the same X-Y-Z triangle, or other [[color triangle]]s, can be used.

On the other hand, some color spaces such as [[RGB color spaces|RGB]] and [[CIE 1931 color space#Definition of the CIE XYZ color space|XYZ]] do not separate out chromaticity, but chromaticity is defined by a [[projectivization|mapping that normalizes out intensity]], and its coordinates, such as {{mvar|r}} and {{mvar|g}} or {{mvar|x}} and {{mvar|y}}, can be calculated through the [[division (mathematics)|division]] operation, such as {{math|1=''x'' =}}&nbsp;{{sfrac|''X''|''X'' + ''Y'' + ''Z''}}, and so on.

The xyY space is a cross between the CIE XYZ and its normalized chromaticity coordinates xyz, such that the luminance Y is preserved and augmented with just the required two chromaticity dimensions.<ref>{{cite book | title = Digital Video and HDTV: Algorithms and Interfaces | author = Charles A. Poynton | publisher = Morgan Kaufmann | year = 2003 | isbn = 978-1-55860-792-7 | url = https://books.google.com/books?id=ra1lcAwgvq4C&q=chromaticity+xyy&pg=RA1-PA219 }}</ref>

==See also==
{{commons category|Chromatic diagrams}}
* [[CIE xyY]] (chromaticity diagram)
* [[Chrominance]]
* [[rg chromaticity]]
* [[Impossible color]]
* [[Color index]] in astronomy

==References==
{{Reflist}}

== External links ==
* Stanford University CS 178 [http://graphics.stanford.edu/courses/cs178-10/applets/threedgamut.html interactive Flash demo ] explaining chromaticity diagrams.
* [https://sites.google.com/view/juddofelt/ JOES application software for calculation and plotting of CIE 1931 and 1976 from spectra]<ref name=joes>{{cite journal|last1=Ćirić|first1=Aleksandar|last2=Stojadinović|first2=Stevan|last3=Sekulić|first3=Milica|last4=Dramićanin|first4=Miroslav D.|title=JOES: An application software for Judd-Ofelt analysis from Eu<sup>3+</sup> emission spectra|journal=Journal of Luminescence|date=January 2019|volume=205|pages=351–356|doi=10.1016/j.jlumin.2018.09.048|bibcode=2019JLum..205..351C |s2cid=105828989 }}</ref>

{{color topics}}

[[Category:Color]]
[[Category:Photometry]]