A colour‐difference formula for surface colours under illuminant A

Abstract

The available experimental data for small colour differences between surface colours under illuminant A have been analysed in a manner similar to that used earlier for the comparable results under daylight. Chromaticity discrimination ellipses were calculated from the results for each colour centre. The size of the ellipses varied with chromaticity in an irregular manner suggesting that the visual results for the different centres were effectively on different scales. New experiments, carried out using a grey scale method for the visual assessments, allowed the relative sizes of ellipses to be adjusted. After adjustment the size of the ellipses varied much more systematically with chromaticity. Similar adjustments allowed all the results to be combined together and used to develop a new colour‐difference formula suitable for assessments under illuminant A. Values of ΔE from the new formula gave a better fit to the visual results than those from other formulae. Earlier formulae were intended to be used with results for daylight. Using chromatic adaption formulae to transform the illuminant A results to illuminant D65 improved the agreement, but the results were still not as good as those obtained with the new formula.