2 - Optics of CLs

Question 1

Why do we have to convert from spec rx to ocular rx

Answer 1

Bceause the BVD goes from 12mm to 0mm

Question 2

How to calculate ocular Rx

Answer 2

• if above 4Ds then use table
• if there is a cyl, calculate the difference between the 2 sph values and that is the new cyl power

Question 3

when convertimg, always prescribe the

Answer 3

least negative

Question 4

where is refractive astigmatism located?

Answer 4

• cornea
  or
• crystalline lens

Question 5

how is corneal astigmatism calculated

Answer 5

from keratometry or typography
(instruments used to determien shape of cornea)

Question 6

what does keratometry measure

Answer 6

the curvature along the 2 principle meridians of cornea over 3-4mm

Question 7

what does it mean when the CA is a bigger number?

Answer 7

• circle is bigger
• the curvature is flatter

Question 8

what does it mean when the number is smaller

Answer 8

• circle is smaller
• curvature is steeper

Question 9

0.05mm difference between 2 principle meridians means

Answer 9

there is 0.25D of corneal astigmatisn

Question 9

0.05mm difference between 2 principle meridians means

Answer 9

there is 0.25D of corneal astigmatisn

Question 10

calculate the CA wtih 7.50mm and 7.30mm

Answer 10

7.50 - 7.30 = 0.20mm
= 1.00 CA

0.20 / 0.05 = 4
4 X 0.25 = 1

Question 11

CA when its 8.00mm and 7.60mm & what is sign

Answer 11

0.4mm difference
0.4 / 0.05 = 7
8 x 0.25 = 2 CA

negative cyl = flattest k = bigger circle

Question 12

WTR astigmatism

Answer 12

verically steeper ( so smaller radius)

Question 13

ATR astigmatism

Answer 13

horizontally steeper

Question 14

if all astigmatism is located cornea it means…

Answer 14

px is excellent candidate for RGP

Question 15

Air/tear layer interface calculations

Answer 15

F = (1-n) /r

Question 16

RGP CL choice for astigmatism

Answer 16

• spherical CL if CA up to 2.50D
• back surface toric if >2.50
• front surface toric for any remaining (i.e. lenticular astigmatism)

Question 17

SCL choice for astigmatism

Answer 17

-toric to correct 0.75 and over
- needs to be stabilised to avoid rotation

Question 18

what do you do if cyl is >0.75 SCL

Answer 18

meann spherical equivalent: add half the cyl to the sphere

Question 19

7.60mm & 7.80mm
ocular rx: -3.75/-2.00 x 90

cl choice

Answer 19

CA= 1.00 ATR because steeper meridian is horizontal

order RGP front surface toric to correct ocular sphere
plus lenticular astigmatism (-3.75/-1.00 x 90)

Question 20

spec rx is -2.75/-1.00 x 90

7.60mm and 8.00mm
lenticular astigmatism is?