Light waveguiding in the Retina

TYPEBiophysics and Optics
Speaker:Idan Mishlovsky
Affiliation:Physics Department, Technion
Location:Lidow Asher Peres (502)
Remark:MSc Seminar
Abstract:Subject: Studies of Light Scattering from the Retina.
Keywords: vision, retina, light guiding, BPM, retina scattering, acuity
In vertebrate eyes, the retina is structured from layers of cells organized in a specific and apparent “reverse” order. The light sensitive photoreceptors are positioned at the back of the retina, forcing light that is focused on the inverted retina to pass through all retinal layers until it reaches the photoreceptors that convert that light to neural signals. Energetically, only a third of the incident light is transferred to electrical signals at the photoreceptors and although large fraction of light is absorbed to the retinal pigment epithelium (RPE), still a portion of light is scattered back to the eyeball.
Interlaced throughout the entire inter-layered structure of the retina are cylindrical-like Müller cells that widen into a conical funnel facing the vitreous homour (the eyeball gel). Measurements of these cells demonstrate a higher refractive index compared to their vicinity, which implies a wave-guiding phenomenon.
A comprehensive three-dimensional computer model of the retina was constructed based on measured optical and physical parameters describing both geometrical outline and refractive indices. Since analytical solution is inapplicable due to the complexity of the problem, Fast Fourier Transform Split-Step Beam Propagation Method (BPM) was used to solve the Helmholz equations of light travelling inward. Light was propagated from the vitreous homour to the outermost layer (RPE) of the retina. Next, using Spherical Wave Light Propagation Techniques and after adding a random phase to each scattering dot, light was scattered from both types of the photoreceptor layers and from the RPE simultaneously, and their intensities were summed incoherently back at the vitreous homour.
Scattering results created an image that indicates strong spatial correlation between the back-propagated light intensities at the center of the cones at the bottom of the retina and the corresponding scattered light intensities at the vitreous homour, which can be corroborated with actual measurements. The rods being small and positioned randomly at the photoreceptors layers created the corresponding indirect scattered image. These results, which demonstrate direct light scattering from the cones and indirect light scattering from the rods, support the hypothesis that Müller cells guide light and advocates for directionality in light propagation through the retina for improved visual acuity.
Authors: Idan Mishlovsky, Erez N. Ribak.
Affiliation: Physics Department, Technion - Israel Institute of Technology, Haifa 32000, Israel