The dependence of the colour and brightness of a monochromatic light upon its angle of incidence on the retina.

The changes in brightness and colour of a monochromatic test light as its angle of incidence on the retina was changed from normal (pupil centre traverse) to oblique (3.5 mm temporal pupil traverse), was measured by matching it with three normally incident primaries. Results on two normal trichromats were generally in accord with published data on the Stiles-Crawford intensity and colour effects. One observer was also the subject of the preceding paper (Alpern & Kitahara, 1983) in which the field sensitivities of his foveal IIj(mu) (j = 3, 4, 5) mechanisms for normally, and obliquely, incident backgrounds were reported. For normal incidence, the colour matching functions are in rough accord with expectation if the action spectra of the three cone mechanisms, which provide the photoreceptor basis for his trichromacy, were the same IIj mechanisms for normal incidence. A unified theory is developed for both Stiles-Crawford intensity and colour effects, assuming that the same visual pigments in the same set of univariantly signalling cones absorbs both the normal incident primaries and the obliquely incident test. Given no free parameters for curve fitting, the Stiles-Crawford intensity effect data are in reasonable agreement with the theory if the photoreceptor basis of these matches were the normally and obliquely incident IIj(mu) mechanisms. The Stiles-Crawford colour effect data contradict the expectations of the unified theory applied with these same IIj(mu) mechanisms. Either II3(mu) is not a valid operational definition of the action spectrum of his short-wave sensitive photoreceptors or at least one assumption of the unified theory is false.