Light Propagation in the Retina
A section through the retina and its layers. Except for the gray absorbing layers at the bottom, all parts are transparent and were coloured here for demonstration purposes only. The neurons take up the blue volume, and their nuclei are coloured pink, brown and red. Light arrives from the eye’s pupil (at the top) and is guided along the glia cells (green). Each such cell is attached to a colour-sensitive cone (violet), surrounded by many light-sensitive rods (orange).
1. Adaptive optics system for the eye
2. Ocular wave front sensing
3. Acousto-caustic modulation for removing speckle from the wave front sensor
4. High-resolution imaging of the retina, and immersion optics for reduction of corneal aberrations
5. Light propagation in the retina: glial cells sort colors onto cones and rods
The retina (experiment): Light arrives from above and is concentrated by the glial cells in the direction of the photoreceptors. These cells function as fibre optics and separate the incoming light according to colour (red) before arriving at the photoreceptors (blue). A movie is available here.
1. Ocular aberrations
a) Construction of geometric model of neural layers, glial (Muller) cells in the parafovea, outside the central high-sensitivity macula
b) Attachment of relevant refractive index to layers
c) Usage of the split-step Fourier-transform beam propagation method
1. Verification test on cones
2. Good match to analytic results
d) Propagation of light through retinal layers
1. Incidence angles from zero to maximum permitted through pupil
2. Wave lengths from blue to near infra red
e) Results so far
1. Rejection of background and clutter: scattered light from light paths or from other directions does not reach into cones, responsible for colour vision
2. Rejection of aberrations: high modes (very tilted wave fronts, as a result of chromatic, other aberrations) are scattered off
3. Scattered light which did not arrive in cones can be detected by intervening rods, responsible for high sensitivity (but colour blind)
4. Good fit to experimental results by Franze et al. (2007) for glial cells
5. Might explain why the retina is inverted. If cones came first and neural layers behind, then the previous results would not have been valid:
I. Background light would have been too high
II. Most of the light would have reached the rods, not the cones
III. There would have been a higher sensitivity to aberrations in the optics of the eye
IV. There was no colour separation – green and red to the cones, blue and far red to the rods
(Left) Light intensity impinging on glial cell array, at the entrance to the funnel. Green light hits at 6 degrees to the right. (Right) After propagation and concentration along the glial cells light arrives at the cones at the bottom of the retina. Some of the light leaks and is scattered to the nearby rods. Notice different scales for incoming and outgoing intensities.
Light field propagating down the retina, getting locked in glial cells
Notice that movies show electromagnetic field, while images on left show intensity (=|field|2) which is more concentrated. The eye is sensitive to intensity, not field.
For comparison, the light intensity in the experiment mp4
Simulation performed by Amichai Labin, Erez Ribak (physics dept.)
(Please see previous work in publications page)