|M.Sc Student||Idan Mishlovsky|
|Subject||Studies of Light Scattering from the Human Retina|
|Department||Department of Physics||Supervisor||Dr. Ribak Erez|
|Full Thesis text|
In vertebrate eyes, the retina is made of neural 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, which convert it to neural signals. Energetically, only a third of the incident light is converted to electrical signals by the photoreceptors. A large fraction of light is absorbed in the outermost layer of the retina, the retinal pigment epithelium (RPE); however, there remains a portion of light that is scattered back to the eyeball. Interlaced throughout the entire multi-layered structure of the retina are cylindrical Müller cells that widen into a conical funnel facing the vitreous humour (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 an analytical solution is inapplicable due to the complexity of the problem, I used the Fast Fourier Transform Split-Step Beam Propagation Method (BPM) to solve the Helmholz equations of light travelling inward. Light was propagated from the vitreous humour to the RPE. Next, using Spherical Wave Light Propagation techniques, while adding a random phase to each scattering point, 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 humour.
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 humour. The rods being small and positioned randomly at the photoreceptors layers created the corresponding indirect scattered image. The results of this analysis demonstrate direct light scattering from the cones and indirect light scattering from the rods, supporting the hypothesis that Müller cells guide light and advocating for directionality in light propagation through the retina for improved visual acuity. As opposed to previous simulations, this resultant light pattern is corroborated by actual measurements, to achieve a fine qualitative behavior.