Visual Optics
10/27/03
I.
II.
V023a
Some Review
a. Slide 159 pg 99
i. The image forms on the apertures of the inner segments to get a sharp
image.
b. Mechanisms to minimize scatter
i. All the ganglion cell bodies and neural tissue b/t have similar refractive
indices.
1. by Snell’s law there will not be a change in angle
ii. Some light still scatters, this will be very damaging if the photoreceptors
are very close
1. Slide 161 pg 100: Cone density decreases with retinal
eccentricity from fovea
a. At the fovea, they are tightly packed but at the peripheral
edge the cones are bigger and further apart.
b. Cones at the foveal center are most sensitive to scattered
light due to their high number density. Scattering here
will decrease the contrast of the image.
c. In the peripheral retina the same indices give little
enough scatter NOT to effect visual performance. This
is not the case in the fovea.
iii. Slide 158: Transverse and cross-sectional views of retina.
1. In the fovea the mechanism to correct the scatter is by getting rid
of all the neural tissue in front of the photoreceptors
a. But we don’t want to waste it so let’s just push it to the
side and form the foveal pit.
New stuff
a. Slide 162 pg 101: Fluorescein angiogram showing capillary-free area of the
fovea
i. Another obstacle is the capillaries all over the retina that will scatter or
absorb light.
ii. From the entopic lab we know we can see the shadow patterns via ‘Thru
the pupil method’
iii. To help with this we have a well defined avascular zone (the enlarged
pic) that prevents capillaries from scattering light at the fovea.
b. The cornea has three mechanisms to minimize scatter
i. Indices of all the neural tissue in front of the photoreceptors is about the
same.
ii. The neural tissue is pushed away from the fovea, so light won’t have to
propagate thru as much neural tissue.
iii. An avascular area in the fovea.
c. Slide 163: How does light get captured by a photoreceptor?

i. Given: Let ninnersegment  1.42 , noutsideinn
ersegment  1.38 (these are
hypothetical numbers only),  the angle going in and   the angle
coming out.
ii. The arrow is a ray of light coming into the inner segment from the
vitreous. It will reach the side wall and one of two things will happen:
either it will pass thru or reflect off (if it passes thru it can’t get to the
pigment and the photoreceptor can’t detect the light)
iii. We want to be the most efficient and the light reflected off will be the
light captured by the photopigment, so we want 100% reflectance
Visual Optics
10/27/03
V023b
1. We do this using the concept of total internal reflection: n> n
which means   >  . With total internal reflection this will
happen when    90 
2. We want to find the CRITICAL angle in which this will happen:
a. Using Snell’s law n sin   n sin  
b. n sin   n (1)
c.  critical  sin 1 ( nn )
d. = 7 6 
3. Whenever    critical Total reflectance will occur
4. So if we know the indices we can find the critical angle and
which rays will be completely captured.
iv. Answer:
1. By total internal reflection the amount of light captured depends
on the incident angle:
a.   critical  100% of light is reflected (captured)
b.   critical  only a % of light is reflected  
d. Slide 164 pg 102: Light collection by photoreceptor waveguides is dependent on
the incident angle
i. Given n> n
ii. At a steep angle (large  ) the light will be strongly captured and
reflected, but if it’s less than the critical angle it will be only partially
captured, and finally if it is at a shallower angle light may not be
captured at all.
e. Demo 1: Let’s magnify
i. Does this really happen? All the optical stuff is happening at the
interface. Let’s see if this we can duplicate this:
1. A cone photoreceptor is about 2.5 m in diameter and 60 m
long.
2. Magnified 100 times the photoreceptor will be 250 m in
diameter and 6mm long.
3. Magnified 10,000 times it will be 1inch in diameter and 2 feet
long.
4. Magnified 30,000 times it will be 3 inches in diameter and 6 feet
long.
f. Demo 2: A 6 foot photoreceptor
i. The refractive index of the acrylic rod is higher than air so optically it is
just like a photoreceptor in our eye.
ii. If we can shine some light into the photoreceptor at the critical angle it
will reflect off the wall and will come out the other end (since we don’t
have any pigment to absorb).
iii. Because the index of plastic is so much larger than air, the critical angle
is very large (he says very large but if you do an example it is actually
very small) so just about any angle will give total internal reflectance.
iv. We see the light reflected onto a piece of paper!
v. At the top edge you can’t see any light coming thru.
vi. The edge is very sensitive to light so the retinal image needs to form
right at that edge or not much light will be transferred.
Visual Optics
10/27/03
V023c
vii. Index is the key for photoreceptors b/c the capturing of light is based on
total internal reflection.
g. Demo 3: What does an individual photoreceptor actually see?
i. Let’s put our eye to a representative photoreceptor.
1. What’s the VA for an individual photoreceptor?
a. Not very good but there is a better comparison if we put
a letter right up to the front edge b/c what is on the
photoreceptor is an image of the object. Can see
individual letters this way.
b. The photoreceptor does not distinguish what part of the
photoreceptor the light is getting absorbed in.
c. The photoreceptor acts as a point detector and can
resolve shades of gray but doesn’t provide any spatial
resolution.
2. The light getting into the photoreceptor varies with regards to
how it is oriented to the pupil
a. We saw this by looking thru the photoreceptor centered
at his paper pupil. As he moves it we focused where the
light is maximum so we rotate photoreceptor while he
rotated the pupil.
h. Slide 165: Phototropism
i. Reaction of certain plants and animals to move towards or away from a
source of light (with respect to our eye it is the pupil)
1. ex: Sunflowers
ii. Photoreceptors also have this feature (orient at nodal point or generally
the pupil)
i. Slide 166 Pg 103: Receptors directed towards center of pupil
i. Photoreceptors like sunflowers all face the pupil