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The CIELAB color space, also referred to as ''L*a*b*'' , 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 ...
defined by the International Commission on Illumination (abbreviated CIE) in 1976. (Referring to CIELAB as "Lab" without asterisks should be avoided to prevent confusion with
Hunter Lab 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 ...
). It expresses color as three values: ''L*'' for perceptual lightness and ''a*'' and ''b*'' for the four unique colors of human vision: red, green, blue and yellow. CIELAB was intended as a perceptually uniform space, where a given numerical change corresponds to a similar perceived change in color. While the LAB space is not truly perceptually uniform, it nevertheless is useful in industry for detecting small differences in color. Like the
CIEXYZ 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 defin ...
space it derives from, CIELAB color space is a device-independent, "standard observer" model. The colors it defines are not relative to any particular device such as a computer monitor or a printer, but instead relate to the CIE standard observer which is an averaging of the results of color matching experiments under laboratory conditions. The CIELAB space is three-dimensional and covers the entire gamut (range) of human color perception. It is based on the opponent color model of human vision, where red and green form an opponent pair and blue and yellow form an opponent pair. The lightness value, ''L*'', also referred to as "Lstar," defines black at 0 and white at 100. The ''a*'' axis is relative to the green–red opponent colors, with negative values toward green and positive values toward red. The ''b*'' axis represents the blue–yellow opponents, with negative numbers toward blue and positive toward yellow. The ''a*'' and ''b*'' axes are unbounded and depending on the reference white they can easily exceed ±150 to cover the human gamut. Nevertheless, software implementations often clamp these values for practical reasons. For instance, if integer math is being used it is common to clamp ''a*'' and ''b*'' in the range of −128 to 127. CIELAB is calculated relative to a reference white, for which the CIE recommends the use of CIE Standard illuminant D65. D65 is used in the vast majority industries and applications, with the notable exception being the
printing industry Printing is a process for mass reproducing text and images using a master form or template. The earliest non-paper products involving printing include cylinder seals and objects such as the Cyrus Cylinder and the Cylinders of Nabonidus. The ...
which uses D50. The
International Color Consortium The International Color Consortium (ICC) was formed in 1993 by eight vendors in order to create an open, vendor-neutral color management system which would function transparently across all operating systems and software packages. Overview The ...
largely supports the printing industry and uses D50 with either CIEXYZ or CIELAB in the Profile Connection Space, for v2 and v4 ICC profiles.International Color Consortium,
Specification ICC.1:2004-10 (Profile version 4.2.0.0)
Image technology colour management — Architecture, profile format and data structure,'' (2006).
While the intention behind CIELAB was to create a space that was more perceptually uniform than CIEXYZ using only a simple formula, CIELAB is known to lack
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 ...
, particularly in the area of blue hues. The lightness value, ''L*'' in CIELAB is calculated using the cube root of the relative luminance with an offset near black. This results in an ''effective'' power curve with an exponent of approximately 0.43 which represents the human eye's response to light under daylight (
photopic Photopic vision is the vision of the eye under well-lit conditions (luminance levels from 10 to 108  cd/m2). In humans and many other animals, photopic vision allows color perception, mediated by cone cells, and a significantly higher visual ...
) conditions.


Advantages

Unlike the
RGB The RGB color model is an additive color model in which the red, green and blue primary colors of light are added together in various ways to reproduce a broad array of colors. The name of the model comes from the initials of the three additi ...
and
CMYK The CMYK color model (also known as process color, or four color) is a subtractive color model, based on the CMY color model, used in color printing, and is also used to describe the printing process itself. The abbreviation ''CMYK'' refers ...
color models, CIELAB is designed to approximate human vision. The ''L*'' component closely matches human perception of lightness, though it does not take the
Helmholtz–Kohlrausch effect The Helmholtz–Kohlrausch effect (after Hermann von Helmholtz and V. A. Kohlrausch) is a perceptual phenomenon wherein the intense saturation of spectral hue is perceived as part of the color's luminance. This brightness increase by saturation, ...
into account. CIELAB is less uniform in the color axes, but is useful for predicting small differences in color. The CIELAB coordinate space represents the entire gamut of human photopic (daylight) vision and far exceeds the gamut for sRGB or CMYK. In an integer implementation such as TIFF, ICC or Photoshop, the large coordinate space results in substantial data inefficiency due to unused code values. Only about 35% of the available coordinate code values are inside the CIELAB gamut with an integer format. Using CIELAB in an 8-bit per channel integer format typically results in significant quantization errors. Even 16-bit per channel can result in clipping, as the full gamut extends past the bounding coordinate space. Ideally, CIELAB should be used with floating-point data to minimize obvious quantization errors. CIE standards and documents are copyright by the CIE and must be purchased; however, the formulas for CIELAB are available on the CIE website.


CIELAB coordinates

The three coordinates of CIELAB represent the lightness of the color (''L*'' = 0 yields black and ''L*'' = 100 indicates diffuse white; specular white may be higher), its position between red and green (''a*'', where negative values indicate green and positive values indicate red) and its position between yellow and blue (''b*'', where negative values indicate blue and positive values indicate yellow). The asterisks (*) after ''L*'', ''a*,'' and ''b*'' are pronounced ''star'' and are part of the full name to distinguish ''L''*''a''*''b''* from Hunter's ''Lab'', described below. Since the ''L*a*b*'' model has three axes, it requires a three-dimensional space to be represented completely. Also, because each axis is non-linear, it is not possible to create a two-dimensional chromaticity diagram. Also, it is important to understand that the visual representations shown in the plots of the full CIELAB gamut on this page are an approximation, as it is impossible for a monitor to display the full gamut of LAB colors. The red-green and yellow-blue opponent channels relate to the human vision system's opponent color process. This makes CIELAB a Hering opponent color space. The nature of the transformations also characterizes it as an chromatic value color space. A related color space, the CIE 1976 ''L''*''u''*''v''* color space (a.k.a.
CIELUV In colorimetry, the CIE 1976 ''L''*, ''u''*, ''v''* color space, commonly known by its abbreviation CIELUV, is a color space adopted by the International Commission on Illumination (CIE) in 1976, as a simple-to-compute transformation of the 1931 CI ...
), preserves the same ''L*'' as ''L*a*b*'' but has a different representation of the chromaticity components. CIELAB and CIELUV can also be expressed in cylindrical form (CIELCh ab and CIELChuv, respectively), with the chromaticity components replaced by correlates of chroma and hue. Since the work on CIELAB and CIELUV, the CIE has been incorporating an increasing number of
color appearance phenomena Color (American English) or colour (British English) is the visual perceptual property deriving from the spectrum of light interacting with the photoreceptor cells of the eyes. Color categories and physical specifications of color are associa ...
into their models and difference equations to better predict human color perception. These
color appearance model A color appearance model (CAM) is a mathematical model that seeks to describe the perceptual aspects of human color vision, i.e. viewing conditions under which the appearance of a color does not tally with the corresponding physical measurement of ...
s, of which CIELAB is a simple example, culminated with
CIECAM02 In colorimetry, CIECAM02 is the color appearance model published in 2002 by the International Commission on Illumination (CIE) Technical Committee 8-01 (''Color Appearance Modelling for Color Management Systems'') and the successor of CIECAM97s. ...
.


Perceptual differences

The nonlinear relations for ''L*'', ''a*'' and ''b*'' are intended to mimic the nonlinear response of the visual system. Furthermore, uniform changes of components in the ''L*a*b*'' color space aim to correspond to uniform changes in perceived color, so the relative perceptual differences between any two colors in ''L*a*b*'' can be approximated by treating each color as a point in a three-dimensional space (with three components: ''L*'', ''a*'', ''b*'') and taking 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, therefore o ...
between them.


RGB and CMYK conversions

In order to convert
RGB The RGB color model is an additive color model in which the red, green and blue primary colors of light are added together in various ways to reproduce a broad array of colors. The name of the model comes from the initials of the three additi ...
or
CMYK The CMYK color model (also known as process color, or four color) is a subtractive color model, based on the CMY color model, used in color printing, and is also used to describe the printing process itself. The abbreviation ''CMYK'' refers ...
values to or from ''L*a*b*'', the RGB or CMYK data must be linearized relative to light. The reference illuminant of the RGB or CMYK data must be known, as well as the RGB primary coordinates or the CMYK printer's reference data in the form of a color lookup table (CLUT). In color managed systems,
ICC profile In color management, an ICC profile is a set of data that characterizes a color input or output device, or a color space, according to standards promulgated by the International Color Consortium (ICC). Profiles describe the color attributes of ...
s contain these needed data, which are then used to perform the conversions.


Range of coordinates

As mentioned previously, the ''L''* coordinate nominally ranges from 0 to 100. The range of ''a''* and ''b''* coordinates is technically unbounded, though it is commonly clamped to the range of −128 to 127 for use with integer code values, though this results in potentially clipping some colors depending on the size of the source colorspace. The gamut's large size and inefficient utilization of the coordinate space means the best practice is to use floating-point values for all three coordinates.


Converting between CIELAB and CIEXYZ coordinates


From CIEXYZ to CIELAB

:\begin L^\star &= 116 \ f\!\left(\frac\right) - 16\\ a^\star &= 500 \left(f\!\left(\frac\right) - f\!\left(\frac\right)\right)\\ b^\star &= 200 \left(f\!\left(\frac\right) - f\!\left(\frac\right)\right) \end where, being ''t'' = , , or : :\begin f(t) &= \begin \sqrt & \text t > \delta^3 \\ \dfrac + \frac & \text \end \\ \delta &= \tfrac \end ''X, Y, Z'' describe the color stimulus considered and ''X''n, ''Y''n, ''Z''n describe a specified white achromatic reference illuminant. for the CIE 1931 (2°) standard colorimetric observer and assuming normalization where , the values are: For Standard Illuminant D65: :\begin X_&=95.0489,\\ Y_&=100,\\ Z_&=108.8840 \end For illuminant D50, which is used in the printing industry: :\begin X_&=96.4212,\\ Y_&=100,\\ Z_&=82.5188 \end The division of the domain of the function into two parts was done to prevent an infinite slope at . The function was assumed to be linear below some and was assumed to match the part of the function at ''t''0 in both value and slope. In other words: :\begin \sqrt &= m t_0 + c & \text\\ \tfrac13 \left(t_0\right)^ &= m & \text \end The intercept was chosen so that would be 0 for : . The above two equations can be solved for and : :\begin m &= \tfrac13\delta^ &= 7.787037\ldots\\ t_0 &= \delta^3 &= 0.008856\ldots \end where .


From CIELAB to CIEXYZ

The reverse transformation is most easily expressed using the inverse of the function ''f'' above: :\begin X &= X_ f^\left(\frac + \frac\right)\\ Y &= Y_ f^\left(\frac\right)\\ Z &= Z_ f^\left(\frac - \frac\right)\\ \end where :f^(t) = \begin t^3 & \text t > \delta \\ 3\delta^2\left(t - \tfrac\right) & \text \end and where .


CIEHLC cylindrical model

The "CIELCh" or "CIEHLC" space is a color space based on CIELAB, which uses the
polar coordinates In mathematics, the polar coordinate system is a two-dimensional coordinate system in which each point on a plane is determined by a distance from a reference point and an angle from a reference direction. The reference point (analogous to th ...
''C''* (chroma, relative saturation) and ''h''° (hue angle, angle of the hue in the CIELAB color wheel) instead of the
Cartesian coordinates A Cartesian coordinate system (, ) in a plane is a coordinate system that specifies each point uniquely by a pair of numerical coordinates, which are the signed distances to the point from two fixed perpendicular oriented lines, measured in ...
''a''* and ''b''*. The CIELAB lightness L* remains unchanged. The conversion of ''a''* and ''b''* to ''C''* and ''h''° is performed as follows: : C^\star = \sqrt, \qquad h^\circ = \operatorname\left(/\right) Conversely, given the
polar coordinates In mathematics, the polar coordinate system is a two-dimensional coordinate system in which each point on a plane is determined by a distance from a reference point and an angle from a reference direction. The reference point (analogous to th ...
, conversion to Cartesian coordinates is achieved with: : a^\star = C^\star \cos(h^\circ), \qquad b^\star = C^\star \sin(h^\circ) The LCh (or HLC) color space is not the same as the HSV, HSL or HSB color models, although their values can also be interpreted as a base color, saturation and lightness of a color. The HSL values are a polar coordinate transformation of what is technically defined RGB cube color space. LCh is still perceptually uniform. Further, ''H'' and ''h'' are not identical, because HSL space uses as primary colors the three additive primary colors red, green and blue (''H'' = 0, 120, 240°). Instead, the LCh system uses the four colors red, yellow, green and blue (''h'' = 0, 90, 180, 270°). Regardless the angle ''h'', ''C'' = 0 means the achromatic colors (non saturated), that is, the gray axis. The simplified spellings LCh, LCh(ab), LCH, LCH(ab) and HLC are common, but the letter presents a different order. HCL color space (Hue-Chroma-Luminance) on the other hand is a commonly used alternative name for the L*C*h(uv) color space, also known as the ''cylindrical representation'' or ''polar
CIELUV In colorimetry, the CIE 1976 ''L''*, ''u''*, ''v''* color space, commonly known by its abbreviation CIELUV, is a color space adopted by the International Commission on Illumination (CIE) in 1976, as a simple-to-compute transformation of the 1931 CI ...
''. This name is commonly used by
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, ...
practitioners who want to present data without the bias implicit in using varying saturation. The name Lch(ab) is sometimes used to differentiate from L*C*h(uv).


Where CIELAB is used

Some systems and software applications that support CIELAB include: * CIELAB is used by Datacolor spectrophotometers, including the related
color difference 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 ...
calculations. * CIELAB is used by the PantoneLive library. * CIELAB is used extensively by XRite as a color space with their hardware and software color measuring systems. * CIELAB D50 is available in
Adobe Photoshop Adobe Photoshop is a raster graphics editor developed and published by Adobe Inc. for Microsoft Windows, Windows and macOS. It was originally created in 1988 by Thomas Knoll, Thomas and John Knoll. Since then, the software has become the indu ...
, where it is called "Lab mode". * CIELAB is available in Affinity Photo by changing the document's Colour Format to "Lab (16 bit)". The white point, which defaults to D50, can be changed by ICC profile. * CIELAB D50 is available in
ICC profile In color management, an ICC profile is a set of data that characterizes a color input or output device, or a color space, according to standards promulgated by the International Color Consortium (ICC). Profiles describe the color attributes of ...
s as a profile connection space named "Lab color space". * CIELAB (any white point) is a supported color space in TIFF image files.
TIFF: Revision 6.0
'' Adobe Developers Association, 1992
* CIELAB (any white point) is available in
PDF Portable Document Format (PDF), standardized as ISO 32000, is a file format developed by Adobe in 1992 to present documents, including text formatting and images, in a manner independent of application software, hardware, and operating systems ...
documents, where it is called the "Lab color space". * CIELAB is an option in Digital Color Meter on macOS described as "L*a*b*". * CIELAB is available in the
RawTherapee RawTherapee is application software for processing photographs in raw image formats, as created by many digital cameras. It comprises a subset of image editing operations specifically aimed at non-destructive post-production of raw photos and is ...
photo editor, where it is called the "Lab color space". * CIELAB is used by GIMP for the hue-chroma adjustment filter, fuzzy-select and paint-bucket. There is also a LCh(ab) color picker. * Future support for CIELAB is planned for
CSS Cascading Style Sheets (CSS) is a style sheet language used for describing the presentation of a document written in a markup language such as HTML or XML (including XML dialects such as SVG, MathML or XHTML). CSS is a cornerstone technolo ...
level 4, however at present it is only supported in Safari 15.


Hunter Lab

The Hunter Lab color space, defined in 1948 (Proceedings of the Winter Meeting of the Optical Society of America) (Proceedings of the Thirty-Third Annual Meeting of the Optical Society of America) by Richard S. Hunter, is another color space referred to as "Lab". Like CIELAB, it was also designed to be computed via simple formulas from the CIEXYZ space, but to be more perceptually uniform than CIEXYZ. Hunter named his coordinates ''L'', ''a'' and ''b.'' The CIE named the coordinates for CIELAB as ''L*'', ''a*'', ''b*'' to distinguish them from Hunter's coordinates.


Hunter Lab Math

''L'' is a correlate of
lightness Lightness is a visual perception of the luminance (L) of an object. It is often judged relative to a similarly lit object. In colorimetry and color appearance models, lightness is a prediction of how an illuminated color will appear to a stan ...
and is computed from the ''Y'' tristimulus value using Priest's approximation to Munsell value: : L = 100\sqrt\frac where ''Y''n is the ''Y'' tristimulus value of a specified white object. For surface-color applications, the specified white object is usually (though not always) a hypothetical material with unit reflectance that follows Lambert's law. The resulting ''L'' will be scaled between 0 (black) and 100 (white); roughly ten times the Munsell value. Note that a medium lightness of 50 is produced by a luminance of 25, due to the square root proportionality. ''a'' and ''b'' are termed opponent color axes. ''a'' represents, roughly, Redness (positive) versus Greenness (negative). It is computed as: : a = K_\left(\frac\right) where ''K''a is a coefficient that depends upon the illuminant (for D65, ''K''a is 172.30; see approximate formula below) and ''X''n is the ''X'' tristimulus value of the specified white object. The other opponent color axis, ''b'', is positive for yellow colors and negative for blue colors. It is computed as: : b = K_\left(\frac\right) where ''K''b is a coefficient that depends upon the illuminant (for D65, ''K''b is 67.20; see approximate formula below) and ''Z''n is the ''Z'' tristimulus value of the specified white object.Hunter Labs (1996). "Hunter Lab Color Scale". ''Insight on Color'' 8 9 (August 1–15, 1996). Reston, VA, USA: Hunter Associates Laboratories. Both ''a'' and ''b'' will be zero for objects that have the same
chromaticity 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 s ...
coordinates as the specified white objects (i.e., achromatic, grey, objects).


Approximate formulas for ''K''a and ''K''b

In the previous version of the Hunter ''Lab'' color space, ''K''a was 175 and ''K''b was 70. Hunter Associates Lab discovered that better agreement could be obtained with other color difference metrics, such as CIELAB (see above) by allowing these coefficients to depend upon the illuminants. Approximate formulae are: : K_ \approx \frac(X_ + Y_) : K_ \approx \frac(Y_ + Z_) which result in the original values for Illuminant ''C'', the original illuminant with which the ''Lab'' color space was used.


As an Adams chromatic valence space

Adams chromatic valence color space Adams chromatic valence color spaces are a class of color spaces suggested by Elliot Quincy Adams. Two important Adams chromatic valence spaces are CIELUV and Hunter Lab. Chromatic value/valence spaces are notable for incorporating the opponent p ...
s are based on two elements: a (relatively) uniform lightness scale and a (relatively) uniform
chromaticity 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 s ...
scale. If we take as the uniform lightness scale Priest's approximation to the Munsell Value scale, which would be written in modern notation as: : L = 100\sqrt and, as the uniform chromaticity coordinates: : c_\mathrm = \frac - 1 = \frac : c_\mathrm = k_ \left(1 - \frac\right) = k_\mathrm\frac where ''k''e is a tuning coefficient, we obtain the two chromatic axes: : a = K\cdot L\cdot c_\mathrm = K\cdot 100\frac and : b = K\cdot L\cdot c_\mathrm = K\cdot 100 k_\mathrm \frac which is identical to the Hunter ''Lab'' formulas given above if we select and . Therefore, the Hunter Lab color space is an
Adams chromatic valence color space Adams chromatic valence color spaces are a class of color spaces suggested by Elliot Quincy Adams. Two important Adams chromatic valence spaces are CIELUV and Hunter Lab. Chromatic value/valence spaces are notable for incorporating the opponent p ...
.


See also

*
Color theory In the visual arts, color theory is the body of practical guidance for color mixing and the visual effects of a specific color combination. Color terminology based on the color wheel and its geometry separates colors into primary color, seconda ...
* Opponent color theory *
HSL and HSV HSL (for hue, saturation, lightness) and HSV (for hue, saturation, value; also known as HSB, for hue, saturation, brightness) are alternative representations of the RGB color model, designed in the 1970s by computer graphics researchers to more ...
*
RGB color model The RGB color model is an additive color model in which the red, green and blue primary colors of light are added together in various ways to reproduce a broad array of colors. The name of the model comes from the initials of the three addit ...
*
CMYK color model The CMYK color model (also known as process color, or four color) is a subtractive color model, based on the CMY color model, used in color printing, and is also used to describe the printing process itself. The abbreviation ''CMYK'' refers ...
*
CIECAM02 In colorimetry, CIECAM02 is the color appearance model published in 2002 by the International Commission on Illumination (CIE) Technical Committee 8-01 (''Color Appearance Modelling for Color Management Systems'') and the successor of CIECAM97s. ...
* HCL color space


References


External links


Demonstrative color conversion applet

CIE Colorimetry 15-3
CIE Technical Report Colorimetry 15 third edition (2004). An authoritative reference.
Whitepaper on understanding colors
by X-rite. {{Color space Color space 1976 introductions