Tear Lens Calculations

Question 1

if the cornea was a sphere (e=0), what would be the only variable?

Answer 1

radius of curvature

Question 2

if the cornea is an ellipse (e~0.5), what are the variables?

Answer 2

• radius of curvature

- eccentricity (rate of flattening)

Question 3

if cornea was spherical, what would the alignment base curve be = to and what would that mean?

Answer 3

alignment BC would equal K

• alignment means BC contours cornea
• no localized bearing
• plano tear lens for alignment

Question 4

what does an aspheric surface mean?

Answer 4

radius of curvature continually changes as you move along the curve

Question 5

shape if eccentricity e=1

Answer 5

parabola

Question 6

shape if eccentricity e>1

Answer 6

hyperbola

Question 7

what is the touch result when a spherical base curve CL rests on an aspheric cornea?

Answer 7

3 points of touch result (or 1 or 2 points of touch)

Question 8

to achieve alignment, the BC needs to be ____ than the central K curvature (because the cornea is an ellipse)

Answer 8

flatter

Question 9

what does the tear lens contribute?

Answer 9

contributes power to the optical system

Question 10

assuming e=0.5 and OZ= 7.6mm, what does the BC have to be in comparison to the K value?

Answer 10

BC needs to be about 0.75 D flatter than K

Question 11

what does it mean if the tear lens is minus

Answer 11

flatter than -0.75

-touch

Question 12

what does it mean if tear lens =0

Answer 12

plano, too steep on cornea?

Question 13

what does it mean if tear lens is plus

Answer 13

steeper than -0.75 D

-clearance