Like L*u*v*, L*a*b* (often abbreviated as LAB or Lab) is a colorspace defined in 1976 by CIE with the goal of perceptual uniformity. L*a*b* is also designed to be a device-independent description of color.

L* defines lightness, which is the corrected form of luminance. a* and b* define green-red and yellow-blue balance, respectively, with the origin where the two axes cross being fully desaturated. Negative a* is green, and negative b* is blue.

L*a*b* is defined in terms of CIE XYZ as follows. The terms of XYZ are first divided by the XYZ coordinates of the chosen reference white to create X_{n} Y_{n} Z_{n}. (This is usually the D65 illuminant.)

Then, for each component, a prime is calculated by a piecewise function:

x' = X_{n}^{(1/3)} for X_{n} > 0.008856

x' = (7.787 × X_{n}) + (16 / 116) for X_{n} ≤ 0.008856

(Repeat using Y_{n} and Z_{n} to obtain y' and z'.)

Finally, we can calculate L*a*b*:

L* = (116 × y') - 16

a* = 500 × (x' - y')

b* = 200 × (y' - z')

The definition of a* and b* in particular are based on the opponent-process theory of vision. However, since humans tend to think about color differently than it is actually perceived, it is difficult to use L*a*b* to create a particular color. As such, this colorspace is not much used for raw input; it is instead used by humans for running filters that are most effective in Lab mode. Lab is also a superset of most other color spaces, so it also finds use as an intermediate space while converting between others, e.g. RGB↔CMYK. The most famous application to use Lab is Photoshop.