Lecture 5

Question 1

what causes some scattering to occur in the transparent vitreous?

Answer 1

fibrous structures (collagen fibers) - scatter is relatively small in young eyes and increases with age

Question 2

how can you view scatter in the vitreous?

Answer 2

dark-field illumination (or with ophthalmoscope in older eyes)

Question 3

what is the total transmittance of light through the eye?

Answer 3

Total = direct + forward scattered

Question 4

why are the cornea and aqueous opaque in visible light but not infrared light?

Answer 4

they have strong absorption of visible light and not of infrared

Question 5

of the 79% of the light to arrive at the retina, how much is used to form an image, what happens to the rest?

Answer 5

42% forms an image on retina and 37% reduces the contrast due to forward scatter

Question 6

what wavelengths do the cornea absorb? what about the lens?

Answer 6

cornea = <3.0 microns (UVC and some UVB)
Lens = 3.0-4.0 microns (UVA and some UVB)

Question 7

what structures in the retina absorb light?

Answer 7

blood vessels, macula (short wavelengths), photoreceptors, and RPE

Question 8

does the retina scatter light?

Answer 8

very little in young, healthy eyes

Question 9

what structures in the retina produce reflection?

Answer 9

photoreceptors, RPE and choroid

Question 10

what are 3 solutions to minimize scatter from light going through the retina to hit the photoreceptors?

Answer 10

1. homogenous refractive index between neural layers
2. at fovea, neural tissue is pushed to the side
3. avascular zone formed at the fovea

Question 11

which cones, foveal or peripheral, are more sensitive to scatter?

Answer 11

foveal = smaller cone size in the fovea (2.5 micron diameter and high density number)

Question 12

how does light get captured by a photoreceptor?

Answer 12

by total internal reflection (amount of light captured depends on the incident angle)

Question 13

what is the critical angle for TIR for a cone receptor?

Answer 13

83.77 degrees

Question 14

what is phototropism?

Answer 14

reaction of certain plants and animals to move towards or away from a source of light - photoreceptors do this too

Question 15

what is the stiles-crawford effect?

Answer 15

peripheral rays are less efficiently absorbed by cones - visual sensitivity varies with pupil entry point (center = bright and periphery = dim)

Question 16

does an individual photoreceptor have visual acuity?

Answer 16

no - VA is realized by sampling the retinal image with an array of photoreceptors

Question 17

what is the pigment that is above the macula?

Answer 17

yellow pigment = xanthophyll (2mm diameter or 6.7 degree area)

Question 18

what does the xanthophyll pigment do over the macula?

Answer 18

absorbs 1/3 of short-wavelength light (blue) and leads to Maxwell’s spot (entopic phenomena)

Question 19

what are 3 possible functions of having the xanthophyll pigment?

Answer 19

1. increases contrast of retinal image by filtering out of focus blue light
2. reduce blue light exposure - prevent onset of macular degeneration
3. by-product of increased metabolic activity in cone-rich fovea

Question 20

what sources of the retina reflect the most amount of light (appear darkest on the OCT image)?

Answer 20

photoreceptors and RPE/choroid

Question 21

which light is reflected directionally and which is reflected diffusely?

Answer 21

directional = reflected light guided by photoreceptors
diffuse = all other light reflected

Question 22

what percentage of reflected light is in the visible spectrum?

Answer 22

5-20% (20-45% in the infrared spectrum)

Question 23

why do we use infrared light to do retinal imaging?

Answer 23

much brighter reflection, less bothersome to patient, penetrates further into tissue (deeper layers are seen)

Question 24

what is the tapetum lucidum?

Answer 24

between the RPE and choroid in nocturnal animals - biological mirror that reflects light which passes through rods and cones

Question 25

what is the purpose of the tapetum lucidum? what is a disadvantage?

Answer 25

purpose = increased sensitivity by passing un-absorbed light to photoreceptors a second time
disadvantage = increases scatter and reduces image contrast