The CIE 1976 (L*,u*,v*) system, abbreviated to CIELuv, is an improvement of the CIE 1964 (U*,V*,W*) system which it supersedes. The rectangular coordinates L*,u*,v* of this system are calculated from the CIE X,Y,Z tristimulus values by equations. Lightness forms the central axis of the system with the u and v axes perpendicular to the lightness axis and each other. Planes of constant lightness form uv chromaticity planes.
The u axis approximates to the blue-yellow axis and varies in extent based on Lightness.
The v axis approximates to the green-red axis and varies in extent based on Lightness.