Lecture 5 - optics of a CL Flashcards

1
Q

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

A

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

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2
Q

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

A

front and back surface radii, center thickness and refractive index

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3
Q

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

A

use a small lens stop

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4
Q

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

A

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

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5
Q

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

A

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

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6
Q

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

A

if the vertex distance given is 12mm

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7
Q

how are diopters and mm related?

A

inversely related = when mm increase, diopters decrease

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8
Q

how many diopters is 7.50mm?

A

45.00D

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9
Q

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

A

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

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10
Q

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

A

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

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11
Q

when is a LL optically meaningful in GP lenses?

A

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

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12
Q

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

A

minus power LL

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13
Q

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

A

plus power LL

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14
Q

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

A

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

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15
Q

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?

A

*use spherical component

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

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16
Q

what is the SAM-FAP rule?

A

if GP fits steeper = add minus

if GP fits flatter = add plus

17
Q

what astigmatism does a spherical soft CL correct for?

A

none (after 0.50D use toric soft lens)

18
Q

what astigmatism does a spherical GP correct?

A

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

19
Q

when should you use a toric GP lens?

A

for more than 2.50D of corneal astigmatism

20
Q

what is internal astigmatism?

A

the refractive astigmatism - corneal astigmatism (K’s)

21
Q

do spherical GP CL correct internal astigmatism?

A

no - only corneal

22
Q

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

A

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

23
Q

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

A

front toric GP lenses or bitoric lenses

24
Q

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

A

the 6 o’clock position

25
Q

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

A

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

26
Q

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

A

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

27
Q

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

A

accommodative demand is greater with CL

28
Q

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

A

the accommodative demand is greater with glasses

29
Q

do CL induce prismatic effects when converging?

A

no - CL should always be centered on the pupil

30
Q

what are prism-ballasted CL?

A

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

31
Q

how do you calculate prism-ballasted CL?

A

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

32
Q

when does unintentionally induced prism occur?

A

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

33
Q

what is the spectacle magnification in CL correction?

A

close to 1

34
Q

which type of ametropia do CL help reduce magnification?

A

refractive (axial = specs)