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This document was revised by the International Commission on Illumination (CIE) and is now published in cooperation with Technical Committee ISO/TC 274, Light and lighting, in collaboration with the European Committee for Standardization (CEN) Technical Committee CEN/TC 139, Paints and varnishes, in accordance with the Agreement on technical cooperation between ISO and CEN (Vienna Agreement).

This second edition cancels and replaces the first edition (ISO/CIE 11664-6:2014), of which it constitutes a minor revision. The changes are as follows:

- normative references updated;
- previous Clause 3 split into Clauses 3 and 4;
- Clause 6: previous NOTE 1 changed to body text;
- minor editorial changes.

The three-dimensional colour space produced by plotting CIE tristimulus values (X, Y, Z) in rectangular coordinates is not visually uniform. Neither is the (x, y, Y) space nor the two-dimensional CIE (x, y) chromaticity diagram. Equal distances in these spaces and the respective diagram do not represent equally perceptible differences between colour stimuli. For this reason, the CIE has standardized two more-nearly uniform colour spaces (known as CIELAB and CIELUV), whose coordinates are non-linear functions of X, Y and Z. Numerical values representing approximately the relative magnitude of colour differences can be described by simple Euclidean distances in these spaces or by more sophisticated colour-difference formulae that improve the correlation with the relative perceived size of differences. The purpose of this document is to define one such formula, the CIEDE2000 formula.