The background which led Stiles to the discovery that the color of a monochromatic ray of light varied with its angle of incidence on the retina, and the developments in the subject since 1937 when this discovery was reported are summarized. Stiles's original measurements of the 'hue shift' did not successfully quantify SCII in every part of the spectrum, since it provided only two degrees of freedom while the color changes sometimes require three. Nevertheless, they were the paradigm for most subsequent work for the next quarter of a century. Full quantification of the effect was first obtained by trichromatic matching on Stiles's NPL trichromator almost 25 years after the initial discovery. The phenomena were then also fully described quantitatively with an elaboration of Stiles's original theory ('self-screening' theory), on the assumption that the ordinary laws of additivity are valid for color matches of three primaries striking the retina at one angle of obliquity to a test incident at a different angle. More recent experiments suggest that this initial assumption may not be generally true and that 'self-screening' theory may not generally provide a satisfactory description of the color changes throughout the visible spectrum, even for measurements of the effect obtained under conditions in which the additivity assumption seems to be valid. However, the available data strongly imply that absorptions of photons obliquely incident on foveal cones depend upon spectra clearly different from those upon which absorptions of normally incident photons depend.

The Stiles-Crawford effect of the second kind (SCII): a review.

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....

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

We examined the effects of spatial frequency and colorimetric purity using a hue-matching technique. At constant retinal illuminance, hue matches of alternate stripes of square-wave gratings changed with spatial frequency. At high spatial frequencies the data did not tend towards tritanopic confusion loci. Several alternative hypotheses, including optical factors, chromatic induction, and changes in the relative sensitivity of the opponent-colors channels, are discussed. None of these mechanisms can explain all of the data.

Changes in constant-hue loci with spatial frequency

https://deepblue.lib.umich.edu/bitstream/handle/2027.42/27010/0000577.pdf?sequence=1

The change in color matches with retinal angle of incidence of the colorimeter beams.

Luminance contrast was found to affect high-spatial-frequency tritanopia despite the S-cone coordinates of the stimuli being kept constant. This proves that a traditional account of artificial tritanopias based on the spatial resolution differences between the S-cone channel and the M- and L-cone channels is not applicable here. We suggest that the lack of spatial resolution in one of the post-receptor (rather than receptor) spatio-chromatic channels could be a cause of high-spatial-frequency tritanopia.

https://www.gcu.ac.uk/media/gcalwebv2/theuniversity/academicschools/sls/psytimetables/PDF_9.pdf

R.A. Weale

Papers

On matching colours