Lec 5: vitreous and retina Flashcards
Define the entrance pupil of the eye
virtual image of the pupil, in AIR
why is it in air? virtual image looks like its in aqueous, but the real pupil is in aqueous. therefore our virtual image is in air.
define the exit pupil of the eye
virtual image of the pupil (made by crystal lens), in vitreous
- The vitreous is largely transparent, but some scattering occurs due to fibrous structures (collagen fibers). Scatter is relatively small in young eyes and increases with age as the fibers degenerate.
- Scatter in the vitreous is best viewed using special technique (dark-field illumination). However, some scatter can be readily observed in older eyes with a standard ophthalmoscope.
human vitreous structure
Deviation of light in multiple directions, as may be produced by particles in the medium through which the light passes.
scatter
total transmittance of light through the eye
direct + forward scattered
visible wavelength range
400-700 nm
when light gets into eye, what percent gets into retina?
79%
*of that 79%, some is forward scattered tho
where is there a lot of absorption of uv light (<400 nm) in eye?
from anterior surface of lens to posterior surface of lens
above 1200 nm long wave length light (infrared light), where is this absorbed in eye?
vitreous and retina
The strong absorption spectra of the cornea and aqueous at certain infrared wavelengths make the anterior of the eye ________ at these wavelength.
opaque
The human eye consists largely of water. Water is highly transparent in visible light, but highly absorbent in the ____ at wavelengths above 1.1
microns.
infrared
Alien (Spectrum and brightness of their sun could differ from ours.
How would this change the evolutionary development of their eyes?
- Large optics (eyes) to collect more light.
2. Transparent in the infrared, but opaque at visible wavelengths.
direct transmittance
light that forms image on our retina
42% of direct light forms what?
retinal image
what amount of light is absorbed, reflected, or backward scattered by ocular tissue?
79%
what amount of forward scattered light that reaches the retina & forms a halo?
37%
a turbid cornea and crystalline lens that scatters off-axis light onto a foveal image, thus decreasing what?
image contrast.
The primary optical effect of ocular scatter is reduced what?
contrast of the retinal image
what percent of the incident light is forward scattered by the optics of the eye? Scattering increases with age and with certain types of surgery, e.g., refractive surgery.
40%
range for uv light
200-380 nm
what absorbs most of uva light?
crystal lens
what absorbs uv light <0.3 microns?
what pathology does this cause?
cornea; uv keratitis
what absorbs uv light b/w 0.3 micron to 0.4 microns? what pathology does this cause?
lens; cataracts
what pathology is caused b/c uv is reaching our retina?
age related maculopathy
uvc is absorbed by what?
ozone layer
uvb is absorbed by what?
cornea and lens
range for uva?
315-380 nm
range for uvb
280-315 nm
uvc range?
200-280 nm
What percent of uva and uvb reaches the eye?
5%
UV radiation is most abundant where?
whenever there is less atmosphere between you and the sun (e.g. mountain top, low latitudes, sun at zenith)
Optical properties of the retina
- Absorption (blood vessels, macula, photoreceptors, RPE)
- Scatter (neural tissue)
- Refraction (not really)
- Reflection (photoreceptors, RPE, and choriod)
- Transmit (neural tissue)
- Thin (>200 μm), transparent
- little scatter in young, healthy eyes
- macular pigment absorbs short wavelengths
- pigment deposits (drusen) common in old age
- dense capillary vasculature absorbs and scatters light; creates “Purkinje tree”
- photoreceptors are directional, phototropic
- spherical shape gives wide field of view, good focus
retina
- Where should the image form in the retina to provide best focus?
photoreceptors
*cones and rods capture light
- How does the retina minimize its own degradation of the retinal image?
(i. e. before the image is captured by the photoreceptors)
foveal cones are more sensi to scattered light than peripheral ones 1. Homogeneous refractive index between neural layers 2. At fovea, neural tissue is pushed to the side. 3. Avascular zone formed at fovea.
- How do the photoreceptors capture the retinal image? (i.e. what optical properties of the receptors are required?)
total internal reflection
While this is a strange way to design the retina, it permits what?
the RPE and choroid to provide the nutrients necessary to meet the high metabolic demand of the photoreceptors.
Unfortunately light passing through these layers can scatter and become problematic depending on the cone size.
Three solutions to minimize this problem are:
1. Homogeneous refractive index between neural layers 2. At fovea, neural tissue is pushed to the side. 3. Avascular zone formed at fovea.
where are larger cones?
in periphery
scattered ray enters 3 cones away in fovea. this will do what to the image?
degrade image
*fovea cones are more sensitive than peripheral retina
where is cone density high?
fovea
scattered ray enters 3 cones away in peripheral. this will do what to the image?
does not degrade image
how does light get captured by photoreceptor?
total internal reflection
an optical phenomenon that occurs when a ray of light strikes a medium boundary at an angle larger than a particular critical angle with respect to the normal to the surface. If the refractive index is lower on the other side of the boundary, no light can pass through and all of the light is reflected.
Light collection by photoreceptor is dependent on what?
on the incident angle
Reaction of certain plants and animals to move towards or away from a source of light
phototropism
