THE EFFECT OF ROD ACTIVITY ON LARGE FIELD COLOUR MATCHING

Abstract

ABSTRACTColorimetric standards, such as the CIE 1931 System for a 2° field, and the 1964 System for a (large) 10° field, depend on the additivity principle both for their derivation and use. Tests have shown that while the principle can fairly safely be applied in the former case, this is not true of the latter, where for some colour combinations large discrepancies occur. The fact that deviations have been shown by many workers to apply specifically to the blue matching stimulus leads to a suggestion for a physiological model.A quite different aspect to the model is the question of how to remove these deviations, i.e., to find an additive system of large field colorimetry. The reason for the deviations is the presence of rod activity in all fields subtending more than 2°: the large field trichromatic match is distorted, causing additivity deviations, because this rod activity has not been equated. To do so an extra matching stimulus is required, resulting in a tetrachromatic colour match. The method proposed for achieving this match is the two luminance level, convergent technique. This has been shown to give a unique match which is generally valid. The tetrachromatic match obeys the additivity principle under conditions where the trichromatic does not: it is quick enough to perform and precise enough to form a starting point for large field additive colorimetry.