Contributions to the theory of invariance of color under the condition of varying illumination

Any visual-processing algorithm aimed at attaining color constancy will in fact attain it only for restricted spectral classes of illuminants and reflectances. These classes constitute implicit heuristics for the physical world, in an artificial-intelligence sense. In the present work, physically reasonable spectral classes are presented which insure that von Kries's law of chromatic adaptation will — in its simplest form — restore object colors in human tristimulus space to illuminant-invariant positions in the space. Algebraic functions of the adapted tristimulus values are presented which are illuminant-invariant for some departures from the spectral heuristics. These functions, a hierarchy of invariants, may be useful in developing lighting and pigment standards for partially controlled viewing environments.