Lecture 5 - optics of a CL

Question 1

why is a CL considered a thick lens not a thin lens?

Answer 1

CL has a small thickness and small radii (thin lens = radii are large compared to thickness)

Question 2

what 3 things do you need to consider for the power calculation of a CL?

Answer 2

front and back surface radii, center thickness and refractive index

Question 3

what can you do to ovoid measurement errors when measuring the back vertex power of CL?

Answer 3

use a small lens stop

Question 4

how does the front vertex power compare to the back vertex power?

Answer 4

the front vertex power is always a little less than back vertex power (*except for high plus lenses)

Question 5

how do you use the effective power equation in sphero-cylinder lenses?

Answer 5

if one meridian is more than 4D = break equation into optical cross - apply equation to each meridian and then put back into Rx

Question 6

when can you use the square rule approximation for effective power calculations?

Answer 6

if the vertex distance given is 12mm

Question 7

how are diopters and mm related?

Answer 7

inversely related = when mm increase, diopters decrease

Question 8

how many diopters is 7.50mm?

Answer 8

45.00D

Question 9

if you change by 0.1mm - how much does it change in diopters?

Answer 9

0.1mm = 0.5D (1mm = 5D)

Question 10

what happens to the BCR of a soft lens when placed on the eye?

Answer 10

the back surface of the lens takes on the K-reading of the cornea (= no optically meaningful LL)

Question 11

when is a LL optically meaningful in GP lenses?

Answer 11

when the BCR is selected flatter or steeper than the K-readings

Question 12

what is the power of the LL if the BCR is flatter than K-reading?

Answer 12

minus power LL

Question 13

what is the power of the LL if the BCR is steeper than K-reading?

Answer 13

plus power LL

Question 14

if the patients Rx = -3.00D and GP fits 0.50D flatter than the Flat-K, what is the necessary GP lens power?

Answer 14

-3.00D - (-0.5D LL) = -2.50D GP lens

Question 15

if your patients Rx = +3.00 -2.00 x 180 and GP fits 0.25D steeper than Flat-K, what is the necessary power of the GP lens?

Answer 15

*use spherical component

+3.00 - (+0.25D LL) = +2.75D GP lens

Question 16

what is the SAM-FAP rule?

Answer 16

if GP fits steeper = add minus

if GP fits flatter = add plus

Question 17

what astigmatism does a spherical soft CL correct for?

Answer 17

none (after 0.50D use toric soft lens)

Question 18

what astigmatism does a spherical GP correct?

Answer 18

almost all of the corneal astigmatism (about 10.6% remains - corrects about 90%)

Question 19

when should you use a toric GP lens?

Answer 19

for more than 2.50D of corneal astigmatism

Question 20

what is internal astigmatism?

Answer 20

the refractive astigmatism - corneal astigmatism (K’s)

Question 21

do spherical GP CL correct internal astigmatism?

Answer 21

no - only corneal

Question 22

if a patient has 1.50D x 180 corneal astigmatism and 2.50D x 180 refractive astigmatism, should they use a spherical GP lens?

Answer 22

2.50 - 1.50 = 1.00D x 180 internal astigmatism (no more than 0.75D residual)

Question 23

what type of lens can be used to compensate for residual astigmatism?

Answer 23

front toric GP lenses or bitoric lenses

Question 24

what is the reference location for checking for unwanted rotation of a toric lens?

Answer 24

the 6 o’clock position

Question 25

if a lens rotates to the left - how do you compensate to avoid poor vision?

Answer 25

left rotation = deviation is added to original axis
(right rotation = deviation is subtracted to original axis)
LARS rule

Question 26

if a toric lens Rx = -2.00 -0.75 x 180 and rotated 15 degrees to the left, what should the new Rx be?

Answer 26

-2.00 -0.75 x 015 (subtract 15 from 180)

Question 27

is the accommodative demand greater for a myope wearing glasses or CL?

Answer 27

accommodative demand is greater with CL

Question 28

is the accommodative demand greater for a hyperope wearing glasses or CL?

Answer 28

the accommodative demand is greater with glasses

Question 29

do CL induce prismatic effects when converging?

Answer 29

no - CL should always be centered on the pupil

Question 30

what are prism-ballasted CL?

Answer 30

created by varying thickness from superior to inferior - so lenses are stabilized against rotation (1.5-3.0 BD)

Question 31

how do you calculate prism-ballasted CL?

Answer 31

use P = 100 g(n-1)/l

Question 32

when does unintentionally induced prism occur?

Answer 32

when a lens de-centers on the eye (use prentice rule)

Question 33

what is the spectacle magnification in CL correction?

Answer 33

close to 1

Question 34

which type of ametropia do CL help reduce magnification?

Answer 34

refractive (axial = specs)