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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.