CIELAB

CIELAB, or CIE L*a*b*, is a color space defined by the Commission Internationale de l'Éclairage (CIE) in 1976. It provides a device-independent representation of color derived from the CIE XYZ tristimulus values and is intended to be perceptually uniform, meaning equal changes in coordinates roughly correspond to equal perceived color differences when viewed under standard conditions. Lab is tied to a reference white via the illuminant and observer used in the XYZ calculation.

A color in Lab is represented by three coordinates: L* for lightness, ranging from 0 (black) to 100 (white); a* for the green–red axis; and b* for the blue–yellow axis. Positive a* is red, negative is green; positive b* is yellow, negative is blue. The axes do not define a cylindrical gamut; Lab is not a device color space but a transformation of XYZ.

Lab values are obtained by transforming XYZ values (X, Y, Z) using a nonlinear function f(t) applied to the normalized tristimulus values, with a reference white Xn, Yn, Zn and a standard observer and illuminant (commonly D65 and 2°). The L*, a*, b* formulas: L* = 116 f(Y/Yn) − 16; a* = 500 [f(Y/Yn) − f(X/Xn)]; b* = 200 [f(Z/Zn) − f(Y/Yn)]. The function f(t) is piecewise to approximate perceptual response near dark values.

In practice, Lab is used for color quality control, color matching, and as a profile connection space in color management (ICC). Color differences are computed as ΔE*ab between two Lab colors; several improved metrics exist (ΔE*94, ΔE*2000). Limitations include imperfect perceptual uniformity in some regions and dependency on viewing conditions. Variants such as CIE LCh* express Lab coordinates in cylindrical form (L*, C*, h°).