Light Radiated from Myoids for Oblique Incidences upon Foveal Cones

Abstract

Purpose: : Analyzing data from the optical Stiles-Crawford (S-C) effect provides three independent parameters: (1) the position of the S-C peak in the eye’s pupil, (2) the directionality factor ρ of photoreceptors and (3) the ratio of powers carried by the directional and diffuse components. The third parameter has been shown to decrease as the angle of incidence θ upon the myoid increases. But the dependence of the two other parameters on θ is badly documented; we have studied whether the position of the S-C peak and the directionality factor remain steady when θ varies.

Methods: : The directional characteristics of foveal photoreceptors are determined by measuring the spatial distribution in the eye’s pupil of light radiated from the retina. The distribution of light in the pupil is measured with a CCD camera cooled by liquid nitrogen (Princeton Instruments, Trenton, NJ). The angle of incidence upon photoreceptors is controlled by two stepping motors. We have measured the alignment characteristics of foveal photoreceptors (sample field of 2 deg) as a function of the angle of incidence in a group of 20 normal young subjects. Incident light (532 nm) was linearly polarized.

Results: : Let I be the pupil intercept of photoreceptor axes, and I’ the position of the peak measured for a location of J (entrance pupil) apart from I. The directionality factor ρ was maximal when J was aligned to photoreceptor axes (mean value: 0.233 mm^-2, SD: 0.030 mm^-2), and decreased as the distance JI increased; its mean values were 0.208 mm^-2 (SD: 0.033 mm^-2) and 0.185 mm^-2 (SD: 0.038 mm^-2) for IJ equal to 0.9 mm and 1.3 mm, respectively. For each of the subjects, the position of I’ was shifted toward J when JI increased; the ratio I’I/JI was on average 0.31 (SD: 0.06).

Conclusions: : The pattern radiated from myoids to the eye’s pupil depends on the angle of incidence θ of light upon the photoreceptors. When the entrance pupil is aligned to photoreceptor axes, the incident wave launches the only mode HE₁₁. But for oblique incidences, the modes TE₀₁, TM₀₁ and HE₂₁ are also excited. Light guided inwards is reflected by the ellipsoids and the outer segment tips, then guided backwards. The spatial extent of modes TE₀₁, TM₀₁ and HE₂₁ is greater than the extent of mode HE₁₁, so the directional factor is likely to decrease with θ.

Keywords: optical properties • photoreceptors • macula/fovea