The Stiles summation index applied to heterochromatic brightness matching.

The summation index technique introduced by Stiles has been applied to heterochromatic brightness matching in order to explain individual differences in luminous efficiency functions for brightness. Such functions were obtained for 2-deg and 10-deg fields from twelve subjects, and the difference between the two fields was compared with the macular pigment density tabulated in Wyszecki and Stiles's book. A summation index was obtained for red and green stimuli combinations and the results are expressed in the form of an equation with nonlinear coefficients. The same equation is applied to the brightness luminous efficiency functions and the individual differences in the efficiency function are interpreted as the result of different contributions of opponent chromatic channels to the perception of brightness.