Ocular Structures 2 Flashcards
Whats the total optical power of eye?
60 Diopters
How much power does the lens have?
20 Diopters
What makes the epithelial layer transparent?
- Cells have a homogeneous refractive index.
- Cells are void of opacities and pigments that absorb light.
- Cells are packed tightly together with no extracellular fluid in between.
Edematous epithelial layer of cornea is caused by what?
This condition can be caused by too much exposure to ultraviolet radiation. It is considered a type of “sunburn”, albeit of the cornea rather than the skin.
The difference in refractive index between the fluid and cells redirects some of the incident light causing ?
a reduction in the transparency of the layer
Edema can reside throughout the entire thickness of the corneal epithelial layer with….
reflections potentially occurring at many depths.
What layer of cornea is 90% the thickness?
stroma
Each lamella of corneal stroma contains what?
many collagen fibrils, regularly spaced.
Regular spacing in stroma causes what?
transparency
Irregular spacing in stroma causes what?
opaqueness
How many lamella are in the corneal stroma?
~200 Lamella, limbus to limbus 1 to 2μm each
Collagen fibrils
dia=30 nm (about 1/17 of wavelength)
spacing=64 nm (about 1/8 wavelength)
n fibrils= 1.55 fibrils
n spacing= 1.35
What condition causes irregular spacing in stroma?
edematous corneal stroma
If there is a tear in descemet’s membrane what will happen?
negatively impacts the pumping action of the endothelium
2 conditions causing stroma transparency
small fibrils
regular spacing
Why do scars cause opacity on cornea?
New collagen fibrils are generally much larger in diameter (e.g., 5 times) and arranged in a random pattern.
Why is the sclera opaque, while cornea is transparent even tho both consist of collagen fibers?
collagen fibrils in the sclera form irregular bundles and are much thicker
direct light
Light rays that form image on the retina
Why are there no blood vessels in Cornea, Lens or Vitreous?
blood is not transparent like water, we wouldn’t want it in our eye because it will degrade the retinal image
Water is highly transparent in visible light, but highly absorbent in what type of light?
infrared light (wavelengths above 1.1 microns)
What amount of power does the cornea contribute?
2/3 power of eye
What parameters of the eye influence the power of the cornea?
d=thickness
r=curvature
n=refractive index
Refractive Surgery will cause what parameter of the eye to change?
curvature (has more effect than thickness) and thickness
Diabetes will cause what parameter of the eye to change?
refractive index (glucose levels will change aqueous and vitreous); power changes b/c refractive index changes
Divergence has negative or positive power?
negative
Convergence has negative or positive power?
positive
If your TV is 5 meters away from your eye, and you have no refractive error what vergence does the TV have to your eye?
-0.2 Diopters
use reciprocal of distance: 1/5
If patient is a -3 Diopter without correction and TV is 1 M away, how much correction is needed to view TV clearly?
-4 Diopters?
object vergence (L) + surface power (F)=
image vergence (L’)
When using a keratometer, what type of power are we reading?
back vertex power
Methods for measuring axial distances within the optics of the eye.
- Simple optical pachometer (not clinically viable)
- Optical pachometry using a slit lamp biomicroscope
- Orbscan (computer-controlled slit lamp)
- Ultrasonic pachometry
- Optical coherence tomography
Where is the AC?
b/w cornea and lens
What is the AC filled with?
aqueous humour
98% water content
refractive index of aqueous humour
1.336
What are the 2 key properties of AC that affect the optical power of the eye?
refractive index and depth
How does depth affect total power of eye?
If depth increases, total optical power decreases!
*inverse relationship
When does the AC depth decrease?
when we accommodate
*this is why we have more power
1 mm change in depth produces how much total optical power of the eye?
0.64 D
1 mm change in depth produces how much refractive error of the eye?
1.3 D
