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J.-M. Gorrand, F. Bacin, M. Doly; Directional Characteristics of Foveal Photoreceptors and the Inner Limiting Membrane. Invest. Ophthalmol. Vis. Sci. 2010;51(13):6293.
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A variety of techniques have been developed to measure the optical Stiles-Crawford effect, but they provide values of the directionality factor which cover a wide range. We have measured the directional characteristics of foveal photoreceptors in a group of 50 normal young subjects at 532 nm (Nd:YAG laser), and studied the specular reflection of the inner limiting membrane (ILM) in the perifovea.
In the fovea the orientation of photoreceptors and their directional characteristics are determined by measuring the spatial distribution in the eye’s pupil of light radiated from the retina. At 6 deg eccentricity, the direction of the normal to the ILM is determined by measuring the position in the eye’s pupil of the spot reflected specularly by the ILM. Light in the pupil is captured by a CCD camera cooled by liquid nitrogen (Princeton Instruments).
The directionality factor ρ of photoreceptors inside the sample field (2 deg) had a mean value of 0.245 mm-2 (SD: 0.039 mm-2) at the fovea. The reflectance factor of the ILM was equal to 0.144 (SD: 0.057) at 6 deg in the temporal retina. The FWHM of the spot reflected by the ILM was on average 0.544 mm (SD : 0.172 mm) in the pupil. We found a strong correlation between the orientation of photoreceptor axes and the direction of the normal to the ILM (r = 0.783).
We have obtained directionality factors whose values were slightly higher than those from other authors. The correlation that we found between the orientation of photoreceptor axes and the direction of the normal to the ILM confirms the results of a previous study on middle-aged subjects (where the light source was incoherent instead of the present Nd:YAG laser). Therefore the alignment of photoreceptors is likely to depend on the direction of the normal to the underlying RPE, in addition to phototropic and apodization mechanisms.
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