lecture 21 - directional sensitivity - properties of rod and cone photoreceptors

Question 1

how do letters appear with no glare?

Answer 1

with no glare letters at different distances appear to have same contrast

Question 2

how do letters with glare appear?

Answer 2

with glare source on, we will get a lot of forward scatter which is spread uniformly over the whole image

• nearest letter - contrast is reduced due to scattered light being high
• furthest letter - amount of scattered light decreases so the contrast increases

Question 3

where is the amount of scattered light the largest?

Answer 3

• amount of scattered light is largest the closer you are to the scatter source
• nearest letter - contrast is reduced due to scattered light being high
• furthest letter - amount of scattered light decreases so the contrast increases

Question 4

what is disability glare?

Answer 4

• the wiping out of fine details in the image as a result of forward scatter in the eye
• reduces retinal image contrast- can no longer resolve the spatial details on the retina

Question 5

what happens to the light captured by the eye ?

Answer 5

• light captured by the eye follows the geometrical optics and ends up at the corresponding image point

Question 6

what happens to some photons?

Answer 6

• some photons will be scattered and they end up around the image point

Question 7

where are the rod photoreceptors?

Answer 7

rod photoreceptors are further away from the centre foveal region

Question 8

are cone photoreceptors larger than rods?

Answer 8

cone photoreceptors are larger with the exception of the very centre part where cone receptors are very small 2um

Question 9

where do cone and rod photoreceptor point towards ?

Answer 9

the point towards the centre of the pupil
- this means light that passes through the centre of pupil will be directed towards the corresponding cone photoreceptors in the retina and will be travelling along the axis of the cone

Question 10

where is photon more likely to be absorbed?

Answer 10

a photon is more likely to be absorbed in a cone photoreceptor when it enters the eye through the centre of the pupil

Question 11

explain the Stiles-Crawford effect?

Answer 11

• when the cones point towards the centre of the pupil , light is more likely to be absorbed then the light from the periphery
• this means that photons that pass through the periphery of the pupil are less likely to be absorbed by cone photoreceptors

Question 12

what does a large pupil have the benefit of?

Answer 12

• a large pupil has the benefit of large retinal illuminance
• it has small diffraction

Question 13

what does a large pupil produce?

Answer 13

• large pupil produces a large amount of aberrations

Question 14

what happens when pupil size is reduced?

Answer 14

• the probability of light being absorbed over the pupil is more uniform
• amount of diffraction decreases and some aberration

Question 15

when is a photon more likely to be absorbed in a cone receptor?

Answer 15

a photon is more likely to be absorbed in a cone receptor when its direction matches the axis of the cone

Question 16

what happens when pupil size is very small?

Answer 16

smallest pupil turns the eye into a pinhole camera

• lot of diffraction
• small aberration
• small retinal illuminance -

Question 17

where do most aberrated rays come from ?

Answer 17

they come from the periphery of the pupil

• rays form periphery have the largest amount of spherical aberration
• rays from periphery focus in front of retinal and decrease the retinal image contrast as a result of aberration

Question 18

how does the directional sensitivity of cone photoreceptors affect aberrations and visual performance?

Answer 18

• peripheral rays are less likely to be absorbed and therefore the directional sensitivity reduces the effects of spherical aberration and coma because these two aberrations increase rapidly with height (pupil size )
• effects of scattered light
  1. forward scatter
  2. back scatter - from cornea and lens
  3. internal scatter within the eye ball which spreads the light over the surface of the eye ball
• directional sensitivity doesn’t protect the eye from forward scatter
• directional sensitivity protects the eye against internal scatter because internal scatter in the eye ball can be at any angle - contrast of retinal image is not as affected because it’s not seen by cone receptor

Question 19

what do rod photoreceptors not exhibit? why is this an advantage?

Answer 19

• rod photoreceptors do not exhibit directional sensitivity -
  this is an advantage because rod photoreceptors are efficient at low light levels
• when light level is low and we rely on rod vision the spatial resolution of retina is vey poor so we are no longer worried by high order aberrations - we are concerned about capturing as many photons as possible over the pupil to increase the amount of light received by rod photoreceptors
• in the absence of directional sensitivity is a great advantage for scotopic vision

Question 20

what are some properties of the central foveal region in photopic vision ?

Answer 20

• rod-free region of the retina
• protected by the macular pigment- absorbs blue light
• S-cone limited-
• smallest cone diameter [<2um]
• highest L and M photoreceptor density
• maps onto the largest amount of visual cortex- brain power
• highest visual acuity and contrast sensitivity
• highest flicker, colour and motion sensitivity

Question 21

what do cone photoreceptors respond best to ?

Answer 21

respond best to blue-light

Question 22

what are the principal differences between rods and cones?

Answer 22

• rods peak in numbers - 16 to 20 deg in the periphery
• rods are much thinner than peripheral cones
• rods exhibit a vary large spatial summation ( high sensitivity to light )
• rods do not exhibit directional sensitivity- every photon that enters through the pupil is equally likely to be absorbed by rod photoreceptors
• rods vision is characterised by poor VA and extremely low contrast sensitivity

Question 23

what is the major advantage of having rod photoreceptors at very low light levels?

Answer 23

is that it allows us to see very small amounts of light