NOTE There are numerous formulae for calculating a colour difference from the colour co-ordinates of two specimens. For various reasons the results given by such formulae, including the one defined here, do not in all cases agree satisfactorily with visual perception nor are they likely to agree with each other. The Commission Internationale de l'Eclairage ICIE) recommended in 1976 two formulae for general use. One of these, the CIE 1976 (L*a*b*) colour difference formula (see CIE Publication No. 15 Supplement No. 2) has proved to be of practical value for the colorimetric evaluation of paint films and is specified for this application in this part of ISO 7724.