- Adams chromatic valence color space
Adams chromatic valence color spaces are a class of
color spaces suggested by Elliot Quincy Adams.cite conference|title=Chromatic Valence as a Correlate of Munsell Chroma|authorlink=Elliot Quincy Adams|first=Elliot Quincy|last=Adams|booktitle=Proceedings of the Twenty-Eighth Annual Meeting of the Optical Society of America|pages=683|year=1943|month=October|location=Pittsburg, PA|volume=33|issue=12|url=http://www.opticsinfobase.org/abstract.cfm?URI=josa-33-12-679 ] Two important Adams chromatic valence spaces are CIELUV and Hunter Lab.
Chromatic value/valence spaces are notable for incorporating the opponent process model, and the empirically-determined 2½ factor in the red/green vs. blue/yellow chromaticity components (such as in
In 1942, Adams suggested chromatic "value" color spaces.cite journal|first=Elliott Quincy|last=Adams|authorlink=Elliot Quincy Adams|title=X-Z planes in the 1931 I.C.I. system of colorimetry|journal=
JOSA|volume=32|issue=3|pages=168–173|month=March|year=1942| url=http://www.opticsinfobase.org/abstract.cfm?id=49502] [cite book
title=The Measurement of Appearance|first=Richard Sewall|last=Hunter|coauthors=Harold, Richard Wesley|publisher=
Wiley- IEEE|isbn=0471830062|year=1987] Chromatic value, or "chromance" , refers to the intensity of the opponent processresponses, and is derived from Adams' theory of color vision. [cite book|url=http://books.google.com/books?id=T1NS5wI3H4UC&pg=PA6&lpg=PA6&dq=%22chromatic+valence%22+%22chromatic+response%22&source=web&ots=uWUKtqWkk9&sig=VnC6RNW_yvwReikrnh0_JkN0E8w#PPA6,M1|title=Color in Electronic Displays|first=Heino|last=Widdel|coauthors=Post, David Lucien|pages=5-6|isbn=0306441918|publisher= Springer|year=1992] [cite book|url=http://books.google.com/books?id=-fNJZ0xmTFIC&pg=RA1-PA161&dq=%22chromatic+response%22+%22color+matching%22&ei=IEKhR8KkOJmmiQGMn_CmCg&ie=ISO-8859-1&sig=EnGz2p5FzNtDc2V3zSVhGuzN1cg#PRA1-PA161,M1|title=The Science of Color|page=161|isbn=0444512519|year=2003|publisher= Elsevier|first=Steven K.|last=Shevell] [cite journal|title=A Theory of Color Vision|first=Elliot Quincy|last=Adams|month=January|year=1923|volume=30|issue=1|journal=Psychological Review| url=http://content.apa.org/journals/rev/30/1/56.pdf|doi=10.1037/h0075074|pages=56]
A chromatic value space consists of three components:
* the Munsell-Sloan-Godlove value function:
* , the red-green chromaticity dimension, where is the value function applied to instead of Y
* , the blue-yellow chromaticity dimension, where is the value function applied to instead of Y
A chromatic value diagram is a plot of (horizontal axis) against (vertical axis). The 2½ scale factor is intended to make radial distance from the
white pointcorrelate with the Munsell chroma along any one hue radius (i.e., to make the diagram perceptually uniform). For achromaticsurfaces, and hence , . In other words, the white point is at the origin.
Constant differences along the chroma dimension did not "appear" different by a corresponding amount, so Adams proposed a new class of spaces, which he termed chromatic "valence". These spaces have "nearly equal radial distances for equal changes in Munsell chroma".
In chromaticity scales, lightness is factored out, leaving two dimensions. Two lights with the same
spectral power distribution, but different luminance, will have identical chromaticitycoordinates. The familiar CIE ("x", "y") chromaticity diagramis very perceptually non-uniform; small perceptual changes in chromaticity in greens, for example, translate into large distances, while larger perceptual differences in chromaticity in other colors are usually much smaller.
Adams suggested a relatively simple uniform chromaticity scale in his 1942 paper: and
where are the chromaticities of the reference white object (the "n" suggests
normalized). (Adams had used smoked magnesium oxideunder CIE Illuminant C but these would be considered obsolete today. This exposition is generalized from his papers.)
Objects which have the same chromaticity coordinates as the white object usually appear neutral, or fairly so, and normalizing in this fashion ensures that their coordinates lie at the origin. Adams plotted the first one the horizontal axis and the latter, multiplied by 0.4, on the vertical axis. The scaling factor is to ensure that the contours of constant chroma (saturation) lie on a circle. Distances along any radius from the origin are proportional to colorimetric purity.
The chromance diagram is not invariant to brightness, so Adams normalized each term by the Y tristimulus value: and
These expressions, he noted, depended only on the chromaticity of the sample. Accordingly, he called their plot a "constant-brightness chromaticity diagram". This diagram does not have the white point at the origin, but at (1,1) instead.
Chromatic valence spaces incorporate two relatively perceptually uniform elements: a
chromaticityscale, and a lightness scale. The lightness scale, determined using the Newhall-Nickerson-Judd value function, forms one axis of the color space:
The remaining two axes are formed by multiplying the two uniform chromaticity coordinates by the lightness, VJ:
This is essentially what Hunter used in his
Lab color space. As with chromatic value, these functions are plotted with a scale factor of 2⅛ to give nearly equal radial distance for equal changes in Munsell chroma.
Adams' color spaces rely on the
Munsellvalue for lightness. Defining chromatic valence components and , we can determine the difference between two colors as:cite journal|title=Evaluation of Single-Number Expressions of Color Difference|first=Angela C.|last=Little|year=1963|month=February|volume=53|issue=2|pages=293–296|journal= JOSA|url=http://www.opticsinfobase.org/abstract.cfm?URI=josa-53-2-293 ]
where VJ is the Newhall-Nickerson-Judd value function and the 0.4 factor is incorporated to better make differences in WX and WZ perceptually correspond to one another.
In chromatic value color spaces, the chromaticity components are and . The difference is:
where the Munsell-Sloan-Godlove value function is applied to the tristimulus value indicated in the subscript. (Note that the two spaces use different lightness approximations.)
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