Lecture 5 Flashcards
what causes some scattering to occur in the transparent vitreous?
fibrous structures (collagen fibers) - scatter is relatively small in young eyes and increases with age
how can you view scatter in the vitreous?
dark-field illumination (or with ophthalmoscope in older eyes)
what is the total transmittance of light through the eye?
Total = direct + forward scattered
why are the cornea and aqueous opaque in visible light but not infrared light?
they have strong absorption of visible light and not of infrared
of the 79% of the light to arrive at the retina, how much is used to form an image, what happens to the rest?
42% forms an image on retina and 37% reduces the contrast due to forward scatter
what wavelengths do the cornea absorb? what about the lens?
cornea = <3.0 microns (UVC and some UVB) Lens = 3.0-4.0 microns (UVA and some UVB)
what structures in the retina absorb light?
blood vessels, macula (short wavelengths), photoreceptors, and RPE
does the retina scatter light?
very little in young, healthy eyes
what structures in the retina produce reflection?
photoreceptors, RPE and choroid
what are 3 solutions to minimize scatter from light going through the retina to hit the photoreceptors?
- homogenous refractive index between neural layers
- at fovea, neural tissue is pushed to the side
- avascular zone formed at the fovea
which cones, foveal or peripheral, are more sensitive to scatter?
foveal = smaller cone size in the fovea (2.5 micron diameter and high density number)
how does light get captured by a photoreceptor?
by total internal reflection (amount of light captured depends on the incident angle)
what is the critical angle for TIR for a cone receptor?
83.77 degrees
what is phototropism?
reaction of certain plants and animals to move towards or away from a source of light - photoreceptors do this too
what is the stiles-crawford effect?
peripheral rays are less efficiently absorbed by cones - visual sensitivity varies with pupil entry point (center = bright and periphery = dim)
does an individual photoreceptor have visual acuity?
no - VA is realized by sampling the retinal image with an array of photoreceptors
what is the pigment that is above the macula?
yellow pigment = xanthophyll (2mm diameter or 6.7 degree area)
what does the xanthophyll pigment do over the macula?
absorbs 1/3 of short-wavelength light (blue) and leads to Maxwell’s spot (entopic phenomena)
what are 3 possible functions of having the xanthophyll pigment?
- increases contrast of retinal image by filtering out of focus blue light
- reduce blue light exposure - prevent onset of macular degeneration
- by-product of increased metabolic activity in cone-rich fovea
what sources of the retina reflect the most amount of light (appear darkest on the OCT image)?
photoreceptors and RPE/choroid
which light is reflected directionally and which is reflected diffusely?
directional = reflected light guided by photoreceptors diffuse = all other light reflected
what percentage of reflected light is in the visible spectrum?
5-20% (20-45% in the infrared spectrum)
why do we use infrared light to do retinal imaging?
much brighter reflection, less bothersome to patient, penetrates further into tissue (deeper layers are seen)
what is the tapetum lucidum?
between the RPE and choroid in nocturnal animals - biological mirror that reflects light which passes through rods and cones
