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CIELAB is one of two uniform colour spaces (UCS) introduced in 1976 by the Commission Internationale de L'Éclairage (CIE, or translated to English, International Commission on Illumination, or ICI). The other uniform colour space, CIELUV, has fallen into disuse.
CIELAB has three colour components - L* (ranging 0 through 100), and a* and b* (ranging very roughly ±100); the space is also denoted L*a*b*. By uniform, or perceptually uniform, we mean that a small change to any component value produces approximately equally perceptible change in colour anywhere in the three-dimensional space.
Perceptibility of a colour difference can be estimated as the Euclidean distance - that is, the square root of the sum of the squared differences - between two "points" in LAB colour space. Image scientists generally consider delta-E less than one to be imperceptible. Delta-E of 4 or greater is likely to be perceptible anywhere in colour space. For delta-E between 1 and 4, perceptibility depends upon the location in LAB space.