Light Propagation in the Retina

Experiments and simulations

(בעברית: מעבר אור ברשתית)


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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. The light arrives from the pupil (above) and is captured in the funnels of the glia cells (green) where it is concentrated down to the colour photoreceptor (cones) in violet. The rest of the light is scattered and arrives in the gray-sensitive photoreceptors (rods) in orange. The thickness of the retina is one quarter to one half millimeter (one to two hundredth of an inch). 
Experiments

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 (home-built system), and immersion optics for reduction of corneal aberrations (ongoing work)


   

Simulations

1.   Ocular aberrations

2.   Retinal aberrations (ongoing work) scientific draft, layman introduction, more explanations

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

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(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 light leaked to the cones on the right and rod photoreceptors. The different colour bars at the bottom represent the intensities of the green light.

Example Movies:

Light field propagating down the retina, getting locked in glial cells

5 degrees incidence, blue (400nm) slow, avi (12MB), mov (3.3MB)

5 degrees incidence, blue (400nm) fast, avi (6MB), mov (1.7MB)

6 degrees incidence, green (580nm) slow, avi (12MB), mov (3.3MB)

6 degrees incidence, near-IR (700nm) fast, avi (4MB), mov (1.4MB)

10 degrees incidence, red (670nm) slow, avi (12MB), mov (3.3MB)

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.

 

Simulation performed by Amichai Labin, Erez Ribak

 

(Please see previous work in publications page)