Opponent process additivity. II. Yellow/blue equilibria and nonlinear models.

A yellow/blue equilibrium Iight is one which appears neither yellowish nor bluish (i.e. uniquely red, uniquely green, or achromatic). The spectral locus of the monochromatic greenish equilibrium (around 500 nm) shows little, if any, variation over a luminance range of .! log,, units. Reddish equilibria are extraspectral. involving mixtures of short-and long-wave light. Their wavelength composition is noninvariant with luminance: a reddish equilibrium light turns bluish-red if luminance is increased with wavelength composition constant. The additive mixture of the reddish and greenish equilibria is again a yellow, blue equilibrium light. We conclude that yellow/blue equilibrium can be described as the zeroing of a nonlinear functional. which is, however, approximately linear in the short-wavelength (" blue ") and middle-wavelength (" green ") cone responsesand nonlinear only in the long-wavelength (" red ") cone response. The " red " cones contribute to yellowness but via a compressive function of luminance. This effect works against the direction of the Bezold-Briicke hue shift. The Jameson-Hurvich yellowiblue chromatic-response function is only approximately correct; the relative values of yellow/blue chromatic response for an equal energy spectrum must vary somewhat with the energy level.