Visual system-response functions and estimating reflectance.

The almost-illuminant-independent achromatic variable zeta [J. Opt. Soc. Am A 11, 1003 (1994)] is supplemented by two achromatic variables, both almost illuminant-independent. The results are numerically verified and obtained by means of so-called (human) visual system-response functions, defined as those linear combinations of the color-matching functions that constitute the best possible approximations to delta functions. It is argued that they and not the cone sensitivities are basic to understanding the visual system as a color-constant signal detection system. The three variables solve the color-constancy problem and define a chromatic adaptation transform for blackbody radiators of temperature T. The generalization of the results to arbitrary well-behaved illuminants is discussed.