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CIE 202 – Reappraisal of Colour Matching and Grassmann's Laws

The laws of additivity and proportionality of colour matches, Grassmann’slaws, are the basis of all colour theory, but are not axiomatically true. Theextent of departure of human vision from Grassmann’s laws has beenperiodically examined. One exploration, by W. A. Thornton, foundconsiderable failure of transformability of primaries – a symptom ofGrassmann additivity failure. In the 14 years since Thornton’s finding, severalgroups have formed to replicate and understand Thornton’s results and thelimitations of Grassmann’s laws. CIE TC 1-56 is the latest of these. During theten years of this committee’s existence, statistical simulations indicated thatreplicate matches by the same observer (not present in Thornton’s data) arerequired to suppress random errors, and accordingly three laboratoriesgenerated intra-observer matching results in three different luminancedomains. Two of the studies, respectively conducted at 300 cd*m-2 and30 cd*m-2, confirm Grassmann additivity, but the third study shows failure ofadditivity at 3 cd*m-2. In addition, Maxwell and maximum-saturation colourmatches have long been known to be inconsistent even at high luminancelevels and with intra-observer match replication to suppress noise. A practicalconsequence of the failure of additivity could be problems observed in crossmediacolour matching, although cross-media studies also have other wellknown sources of imprecision when the colour-matching is asymmetric. Somesuggestions are made for a covering theory of Grassmann’s laws that mightaccommodate both Maxwell and maximum-saturation match data while stillmaintaining consistency with high-luminance success in experiments such asreported recently. Further investigations are indicated for a successor to TC 1-56.