Superficial lipid layer is how thick?
- Lipid ~0.1 microns
- Aqueous ~10 microns
- Microvilli ~0.5 microns into tears
- Mucus ~0.01-0.1 microns
Main optical role of Tear film?
Make the optical rough surface of from the microvilli to a smooth optical surface.
Which layer in the eye has the worst relfectance, 100x more than any other layer.
- Tear film ~2.3%
- the rest ~0.03%
Name 7 common subj symptoms of dry eye
- sandy/gritty sensation
- FB sensation
- excess tearing
- red, irritated eyes
- blurred vision
- ocular fatigue
Name 3 things why the K epithelium is transparent.
1) cells have homogeneous refractive index.
2) Cells are void of opacities and pigments that absorb light.
3) Cells are packed tightly together w/ no extracellular fluid in b/w.
Fibrils act like a grating. What's the equation for grating?
- sin (0) = (m*lambda)/ d
- if d<lambda, 0 = 90deg
- ie no diffraction/scattering
K scars are made of what?
How much thicker than regular k fibrils?
- Collagen fibrils
- 5x thicker.
Define Rayleigh scatter. Example too.
When short wL light is scatttered more than long wL light. As seen in scleral thinning.
Equivalent Power equation
- Ftot= F1 + F2 - (d/n)*F1*F2
- d=diameter of lens
- F1 = (n2-n1)/r1
- r os +/- depending on radius direction.
Transfer Rule define + equation
Lx = La/[ 1 - La (d/n) ]
- It means if you move from point A to a closer spot point X, there is a change in vergence power L.
- As one goes closer then L increases.
Refract Rule. Equation please.
L' = L + F
Shows change of vergence @ reflective surface when changing from one medium to another.
Back Vertex Power Equation.
Fv = F2 + F1 / [ 1 - (d/n) F1 ]
Measures K thickness by seeing the image of the posterior K surface.
K thickness is about ___ larger than what appears when viewed through a pachometer.
1/3rd larger. The image lies w/i the K.
K thickness measurement requires knowledge of 2 things in pachometry.
- 1) K refractive index
- 2) radius of the anterior K surface.
Objects behind the K are closer/farther than they appear.
farther b/c the K acts like a magnifier.
What is avg K thickness and A/C thickness?
Perceived and acutal.
- K: 0.37mm perc / 0.5mm actual
- A/C: 0.3 perc / 0.35 actual
How does Ultrasonic Pachometer work?
Sends high freq pulses of 1/10th of a microsecond -5 to 25MHz. Echos are recorded and thickness is found by (velocity * time)/2.
Ultrasonic Pachometer measures what 4 objects.
- K, ant/post lens & retina. Measures thickness as well as distance.
Adv and dAdv of ultrasonic pachometry.
4 total & compare results to optical pachometry.
Adv: provides more accurate thickness than optical pachometers
- dAdv: 1) separate instrument is required unlike optical pach which only needs an attachment on the SL
- 2) Invasive
- 3) accurate thickness measurement requires knowing the velocity of sound waves in ocular tissue. (in optical we need to know refractive index)
define OCT (optical coherence tomography).
Measures coherence properties of light via optical interferometer to determine the thickness/distances of the eye.
OCT has a resolution of less than __.
- 10 microns
How can one measure the ant chamber angle w/o physical contact.
Q1.The image of the back surface of a patient's cornea is located 0.5 mm behind the front surface. Assuming the refractive index of the cornea to be 1.376, and radius of curvature of front surface to be 7.8 mm, what is the actual thickness of the patient's cornea? Express the difference between measured and actual thickness as a percentage of the measured thickness. Sketch the problem before solving.
Q2. The image of the front surface of a patient's crystalline lens is located 3.05 mm behind the front surface of the cornea. Treating the cornea as a single refracting surface of radius 7.8 mm that separates air (n = 1) from aqueous humor (n = 1.336), what is the actual depth of the anterior chamber of the patient's eye? Express the difference between measured and actual depth as a percentage of the measured depth. Sketch the problem before solving.
Q3. The image of the same patient's iris indicates a 4 mm pupil diameter. What is the actual diameter of the patient's pupil? Express the difference between measured and actual diameter as a percentage of the measured diameter.