HOW STRONG METAMERISM DISTURBS COLOR SPACES

Systems for arranging and describing color include ''color spaces'' an d ''color order systems.'' In a color space, tristimulus values R, G, and B are computable for every light (every point in the space). In fa miliar color spaces, such computation makes use of three functions of wavelength (the color-matching functions that define one of the CIE St andard Observers), one function corresponding to each of R, G, and B. In the presence of strong metamerism (marked spectral difference betwe en the spectral power distributions of a pair of visually matching lig hts), the color-matching functions may report that one light of the pa ir has art entirely different color from that of the other member of t he visually matching pair of lights. The CIE Standard Observer embodyi ng those color-matching functions ''sees'' the two visually matching l ights as entirely different in color, that is, it reports entirely dif ferent sets of R, G, and B for the two visually matching lights, and t hus, an entirely different chromaticity. In an example given here, eac h of the CIE Standard Observers assigns a strong green color to lights that are seen by normal human observers as a visual match to a hueles s reference white. On the other hand, color order systems comprising s ets of real objects in a specified illuminant, and which are assembled (visually arranged) by normal observers, as are the Munsell and OSA s ets, do not suffer from the type of trouble discussed here. Color spac es depending on mathematical functions of R, G, and B are at risk: bot h Standard Observers are shown to plot visually identical lights at wi dely varying points in familiar color spaces (e.g., delta E-ab = 40-5 0). (C) 1998 John Wiley & Sons, Inc.