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Brian Vohnsen; Modeling Photoreceptor Imaging as Light Scattering by Arrayed Dipolar Antennas. Invest. Ophthalmol. Vis. Sci. 2014;55(13):5205.
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© ARVO (1962-2015); The Authors (2016-present)
Light propagation in the retina is commonly described by a combination of guided modes in the photoreceptor cones and unguided modes in the intracellular matrix. Nonetheless the layered outer segments perturb waveguiding via the modulation of its nonuniform refractive index. Here a different approach is taken by considering each single membrane infolding of cones and outer-segment discs of rods as light scattering elements. The aim is to gain insight into its role for high-resolution retinal imaging to visualize the individual cones and rods.
The retinal cone photoreceptor mosaic is modeled as a hexagonal array of outer segments that consist of 1000 pigment-containing membrane infoldings. Individual pigments are modeled as induced dipolar light scattering elements whose packing and ordering form the layers and arrangement of the outer segments. For flood illumination, the outer segments are illuminated by a plane wave and images are calculated in the plane of the retina. For scanning laser ophthalmoscopy, a focused Gaussian beam is raster-scanned across the outer segments and a confocal image is build up from light that exits through an 8 mm pupil. Both a uniform and an annular pupil are considered for the incident beam.
The calculated photoreceptor mosaic images are in good agreement with experimental images reported in the literature for high-resolution cone and rod photoreceptors from fovea to parafovea. The appearance is wavelength dependent. For densely packed cones the model shows that parasitic light scattering may give erroneous appearances of rod-like structures which is best avoided by imaging at different wavelengths. A dark ring can appear around each cone and some cones may appear entirely dark.
Numerical analysis shows that scattering by the layered structure of photoreceptor outer segments corresponds well to details in high-resolution cone and rod retinal images. This shows that scattering may be a better model than waveguiding for the analysis of light interaction with photoreceptors. Only by inclusion of guided as well as unguided components in forward and backward directions would a waveguide model become identical to an entirely scattering-based model.
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