In
colorimetry
Colorimetry is "the science and technology used to quantify and describe physically the human color perception".
It is similar to spectrophotometry, but is distinguished by its interest in reducing spectra to the physical correlates of color ...
, the CIE 1976 ''L''*, ''u''*, ''v''* color space, commonly known by its abbreviation CIELUV, is a
color space
A color space is a specific organization of colors. In combination with color profiling supported by various physical devices, it supports reproducible representations of colorwhether such representation entails an analog or a digital represen ...
adopted by the
International Commission on Illumination
The International Commission on Illumination (usually abbreviated CIE for its French name, Commission internationale de l'éclairage) is the international authority on light, illumination, colour, and colour spaces. It was established in 1913 a ...
(CIE) in 1976, as a simple-to-compute transformation of the 1931
CIE XYZ color space
The CIE 1931 color spaces are the first defined quantitative links between distributions of wavelengths in the electromagnetic visible spectrum, and physiologically perceived colors in human color vision. The mathematical relationships that defi ...
, but which attempted
perceptual uniformity
In color science, color difference or color distance is the separation between two colors. This metric allows quantified examination of a notion that formerly could only be described with adjectives. Quantification of these properties is of great ...
. It is extensively used for applications such as computer graphics which deal with colored lights. Although additive mixtures of different colored lights will fall on a line in CIELUV's uniform
chromaticity diagram
Chromaticity is an objective specification of the quality of a color regardless of its luminance. Chromaticity consists of two independent parameters, often specified as hue (h) and colorfulness (s), where the latter is alternatively called ...
(called the ''CIE 1976 UCS''), such additive mixtures will not, contrary to popular belief, fall along a line in the CIELUV color space unless the mixtures are constant in
lightness.
Historical background
CIELUV is an
Adams chromatic valence color space and is an update of the
CIE 1964 (''U''*, ''V''*, ''W''*) color space (CIEUVW). The differences include a slightly modified
lightness scale and a modified uniform chromaticity scale, in which one of the coordinates, ''v''′, is 1.5 times as large as ''v'' in its
1960 predecessor. CIELUV and
CIELAB were adopted simultaneously by the CIE when no clear consensus could be formed behind only one or the other of these two color spaces.
CIELUV uses Judd-type (translational)
white point
A white point (often referred to as reference white or target white in technical documents) is a set of tristimulus values or chromaticity coordinates that serve to define the color "white" in image capture, encoding, or reproduction. Depending ...
adaptation (in contrast with CIELAB, which uses a
"wrong" Kries transform). This can produce useful results when working with a single illuminant, but can predict
imaginary color
Impossible colors are colors that do not appear in ordinary visual functioning. Different color theories suggest different hypothetical colors that humans are incapable of perceiving for one reason or another, and fictional colors are rou ...
s (i.e., outside the
spectral locus) when attempting to use it as a
chromatic adaptation transform Chromatic adaptation is the human visual system’s ability to adjust to changes in illumination in order to preserve the appearance of object colors. It is responsible for the stable appearance of object colors despite the wide variation of light w ...
.
[Mark D. Fairchild, ''Color Appearance Models''. Reading, MA: Addison-Wesley, 1998.] The translational adaptation transform used in CIELUV has also been shown to perform poorly in predicting corresponding colors.
[D. H. Alman, R. S. Berns, G. D. Snyder, and W. A. Larson, "Performance testing of color difference metrics using a color-tolerance dataset". ''Color Research and Application'', 21:174–188 (1989).]
XYZ → CIELUV and CIELUV → XYZ conversions
For typical images, ''u''* and ''v''* range ±100 %. By definition, .
The forward transformation
CIELUV is based on CIEUVW and is another attempt to define an encoding with uniformity in the perceptibility of
color differences.
[ The non-linear relations for ''L''*, ''u''*, and ''v''* are given below:]
:
The quantities ''u''′''n'' and ''v''′''n'' are the chromaticity coordinates of a "specified white object" – which may be termed the white point
A white point (often referred to as reference white or target white in technical documents) is a set of tristimulus values or chromaticity coordinates that serve to define the color "white" in image capture, encoding, or reproduction. Depending ...
– and ''Y''''n'' is its luminance. In reflection mode, this is often (but not always) taken as the of the perfect reflecting diffuser under that illuminant. (For example, for the 2° observer and standard illuminant
A standard illuminant is a theoretical source of visible light with a spectral power distribution that is published. Standard illuminants provide a basis for comparing images or colors recorded under different lighting.
CIE illuminants
The Inter ...
C, , .) Equations for ''u''′ and ''v''′ are given below:[''Colorimetry,'' second edition: CIE publication 15.2. Vienna: Bureau Central CIE, 1986.][
:
]
The reverse transformation
The transformation from to is:[
:
The transformation from CIELUV to XYZ is performed as follows:][
:
]
Cylindrical representation (CIELCh)
CIELChuv, or HCL color space
HCL ( Hue- Chroma-Luminance) or LCh refers to any of the many cylindrical color space models that are designed to accord with human perception of color with the three parameters. Lch has been adopted by information visualization practitioners to ...
(hue–chroma–luminance) is increasingly seen in the information visualization
Information is an abstract concept that refers to that which has the power to inform. At the most fundamental level information pertains to the interpretation of that which may be sensed. Any natural process that is not completely random, a ...
community as a way to help with presenting data without the bias implicit in using varying saturation
Saturation, saturated, unsaturation or unsaturated may refer to:
Chemistry
* Saturation, a property of organic compounds referring to carbon-carbon bonds
**Saturated and unsaturated compounds
** Degree of unsaturation
**Saturated fat or fatty aci ...
.
The cylindrical version of CIELUV is known as CIELChuv, or CIELChuv, CIELCh(uv) or CIEHLCuv, where ''C''*''uv'' is the chroma and ''h''''uv'' is the hue
In color theory, hue is one of the main properties (called color appearance parameters) of a color, defined technically in the CIECAM02 model as "the degree to which a stimulus can be described as similar to or different from stimuli that ...
:[
:
:
where atan2 function, a "two-argument arctangent", computes the polar angle from a Cartesian coordinate pair.
Furthermore, the saturation correlate can be defined as
:
Similar correlates of chroma and hue, but not saturation, exist for CIELAB. See Colorfulness for more discussion on saturation.
]
Color and hue difference
The color difference can be calculated using the Euclidean distance
In mathematics, the Euclidean distance between two points in Euclidean space is the length of a line segment between the two points.
It can be calculated from the Cartesian coordinates of the points using the Pythagorean theorem, therefor ...
of the coordinates. It follows that a chromaticity distance of corresponds to the same Δ''E''*''uv'' as a lightness difference of , in direct analogy to CIEUVW.
The Euclidean metric can also be used in CIELCh, with that component of Δ''E''*''uv'' attributable to difference in hue as[ , where .
]
See also
* YUV
YUV is a color model typically used as part of a color image pipeline. It encodes a color image or video taking human perception into account, allowing reduced bandwidth for chrominance components, compared to a "direct" RGB-representation. H ...
*CIELAB color space
The CIELAB color space, also referred to as ''L*a*b*'' , is a color space defined by the International Commission on Illumination (abbreviated CIE) in 1976. (Referring to CIELAB as "Lab" without asterisks should be avoided to prevent confusion ...
References
External links
Chromaticity diagrams, including the CIE 1931, CIE 1960, CIE 1976
MATLAB toolbox for color science computation and accurate color reproduction (by Jesus Malo and Maria Jose Luque, Universitat de Valencia). It includes CIE standard tristimulus colorimetry and transformations to a number of non-linear color appearance models (CIE Lab, CIE CAM, etc.).
{{Color space
Color space
1976 introductions