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B. Vohnsen; Photoreceptor Waveguiding and the Hue Shift of the Second Stiles Crawford Effect. Invest. Ophthalmol. Vis. Sci. 2009;50(13):2742.
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The visibility of a narrow beam of light entering the eye depends strongly on its exact location at the pupil due to the first Stiles Crawford effect (SCE-I). This is commonly expressed by a Gaussian distribution function and attributed to waveguiding of light in individual cone photoreceptors. Less known, the second Stiles Crawford effect (SCE-II) describes a minor hue change that accompanies the SCE-I. Both effects depend on the pupil entrance point and on the wavelength of illumination. The SCE-II has commonly been attributed to screening by retinal pigments. Here, I propose that only by inclusion of photoreceptor waveguiding mechanisms can a unified understanding be obtained for both effects.
The cone photoreceptors are modeled as weakly-guiding optical fibers containing S, M, and L pigments, and the coupling of an incident plane wave at the retina (representative of a Maxwellian illumination configuration at the pupil) is studied with special attention to spectral features. The SCE-II has traditionally been characterized by observing quasi-monochromatic light of bandwidth in the range of 10 to 20 nm. Here the spectral variation of light coupling and hue shift is studied to elucidate the influence of waveguiding on common patterns for the SCE-II.
Analysis of the Gaussian distribution function and the associated directionality parameter characteristic of the SCE-I suffices to derive main characteristics of the SCE-II when a finite bandwidth illumination source is used. The SCE-II is peaked at the same point in the pupil as the SCE-I, and it increases with off-axis position until near the pupil rim where it levels off in agreement with the waveguide model. At a fixed off-axis pupil point the spectral variation of the SCE-II shows a complicated pattern that is derived from full knowledge of the spectral dependence of the SCE-I directionality parameter.
A waveguide model for retinal cone photoreceptors has been extended to include spectral variations of pigments to elucidate the impact of waveguiding on the directionality parameter characteristic of the SCE-I and its relation to the SCE-II. It is found that the Gaussian distribution function commonly used for the SCE-I suffices to derive main features of the SCE-II with regard to both pupil and spectral dependencies. Such modeling is an essential step towards an improved understanding of the role played by biophotonic properties of the retina for vision in the healthy eye as well as in eyes with retinal abnormalities.
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