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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 CIELAB space is three-dimensional, and covers the entire range of human color perception, or gamut. 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.
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Like the CIEXYZ space it derives from, CIELAB colorspace is a device-independent, "standard observer" model. While the LAB space is not truly perceptually uniform, it nevertheless is useful in industry for detecting small differences in color. CIELAB was intended as a perceptually uniform space, where a given numerical change corresponds to similar perceived change in color. (Referring to CIELAB as "Lab" without asterisks should be avoided to prevent confusion with Hunter Lab.) 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. 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. Each axis of each square ranges from −128 to 127. The CIE 1976 ( L*, a*, b*) color space (CIELAB), showing only colors that fit within the sRGB gamut (and can therefore be displayed on a typical computer display).