Additivity of Colour Equations

A colour can be matched by an additive mixture of three coloured lights, the most suitable being specified red, green and blue stimuli. If c1 units of one colour are matched by r1, g1 and b1 units of red, green and blue respectively, and c2 units of another colour by r2, g2 and b2 units, then according to the law of additivity of colour equations the combined colour may be matched by r1 + r2, g1 + g2 and b1 + b2 units respectively. It is the purpose of this work to investigate this law. Additivity of colour equations is implicit in all colorimetric calculations. The law is based on the results of early work done with the comparatively crude apparatus of the time, but recently these results have been questioned. The experimental work was performed on the Wright colorimeter, where colour matches were made on pairs of stimuli separately and in combination. The results showed some deviations from additivity, but on the whole these were small. Large deviations appear to be associated with poor discrimination.