CIELAB color space

The CIELAB color space (also known as CIE L*a*b* or sometimes abbreviated as simply "Lab" color space) is a color space defined by the International Commission on Illumination (CIE) in 1976. It expresses color as three numerical values, L* for the lightness and a* and b* for the green–red and blue–yellow color components. CIELAB was designed to be perceptually uniform with respect to human color vision, meaning that the same amount of numerical change in these values corresponds to about the same amount of visually perceived change.

One of the most important attributes of the CIELAB model is that, with respect to a given white point, it is device-independent—it defines colors independent of how they are created or displayed. The CIELAB color space is typically used when graphics for print have to be converted from RGB to CMYK, as the CIELAB gamut includes both the gamuts of the RGB and CMYK color models.

The space itself is a three-dimensional real number space, allowing an infinite number of possible representations of colors. In practice, the space is usually mapped onto a three-dimensional integer space for digital representation, and thus the L*, a*, and b* values are usually absolute, with a pre-defined range. The lightness value, L*, represents the darkest black at L* = 0, and the brightest white at L* = 100. The color channels, a* and b*, represent true neutral gray values at a* = 0 and b* = 0. The a* axis represents the green–red component, with green in the negative direction and red in the positive direction. The b* axis represents the blue–yellow component, with blue in the negative direction and yellow in the positive direction. The scaling and limits of the a* and b* axes will depend on the specific implementation, as described below, but they often run in the range of ±100 or −128 to +127 (signed 8-bit integer).

The CIELAB color space was derived from the prior "master" CIE 1931 XYZ color space, which predicts which spectral power distributions will be perceived as the same color (see metamerism), but is not particularly perceptually uniform. Strongly influenced by the Munsell color system, the intention behind CIELAB was to create a space that can be computed via simple formulas from the CIEXYZ space but is more perceptually uniform than CIEXYZ. When storing color values using limited precision, using a perceptually uniform color space can improve the reproduction of tones.

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