L8 - RGP over refration, Tear Lens, Fit Amendment

Question 1

Ocular refraction =

Answer 1

Power trail lens + over refraction

Question 2

Air, tear lens and cornea refractive index

Answer 2

Air n=1
Tear lens n=1.336
Cornea n=1.376

Question 3

Positive tear lens rule of thumb

Answer 3

Tear lens induces +0.25D for every 0.05mm that the BOZR of the lens is steeper than the corneal radius.

Question 4

Negative tear lens rule of thumb

Answer 4

Tear lens induces -0.25D for every 0.05mm that the BOZR of the lens is flatter than the corneal radius

Question 5

Adjusting BOZR rules of thumb (2)

Answer 5

For every increase of 0.05mm in BOZR, increase power by +0.25DS.

For every decrease of 0.05mm in BOZR, increase power by -0.25DS.

Question 6

Compensating rx for BVD equation

Answer 6

f = F/1-dF

Question 7

TD is increased to…

Answer 7

Stabilise fit
Decrease 3 and 9 o’clock staining
Increase lid attachment

Question 8

TD is decreased to…

Answer 8

Improve fit of a spherical lens on toric cornea

Decrease lid attachment

Question 9

TD rule of thumb

Answer 9

For every +0.5mm increase in TD (or BOZD), increase (steepen) BOZR by +0.05mm to maintain equivalent fit