On the spectral relation between the first and second Stiles-Crawford effect

The first Stiles-Crawford effect describes the psychophysical visibility dependence on pupil point for a narrow beam of light incident on the eye. Its distribution function commonly attributed to photoreceptor waveguiding is characterized by a point of maximum visibility typically near the pupil centre and a wavelength-dependent directionality parameter. The second Stiles-Crawford effect describes a minor hue shift of obliquely incident light commonly attributed to pigment screening that accompanies observations of the first Stiles-Crawford effect. Here the first Stiles-Crawford effect and the wavelength dependence of its directionality parameter are examined in terms of photoreceptor waveguiding. Its distribution function is analyzed to derive characteristic features of the second Stiles-Crawford effect directly from the spectrum of the directionality parameter. The results obtained are independent of the exact physical mechanisms underlying both effects but do support a major role played by waveguiding. Finally, the influence of bleaching is discussed in relation to the model and to pigment screening.