wk3: CLM - Optics of CLs 2

Question 1

Is the tear lens involved in the optical system producing the retinal image?

Answer 1

yes

Question 2

Compare BOZR with the K values of the cornea and describe how their relationship affects the power of the tear lens (3)

Answer 2

If BOZR > K: negative tear lens (“flatter than K”)
If BOZR = K: plano tear lens (“on K”)
If BOZR < K: positive tear lens (“steeper than K”)

Question 3

In terms of back vertex power (BVP), how do you calculate ocular refraction for a rigid lens? [formula]

Answer 3

Ocular refraction = BVPcl + BVPtearlens

Question 4

Given the following information, calculate the front surface “power” (F1) of the tear lens:
- K = 8mm, BOZR = 8.40mm, ntears = 1.336

Answer 4

tear lens front surface curvature (r1) = BOZR = 8.40mm

Front surface power of tear lens (F1) = (ntears-1)/r1
= (1.336-1)/0.0084m = 40.00D

Question 5

Given the following information, calculate the back surface “power” (F2) of the tear lens:
- K = 8mm, BOZR = 8.40mm, ntears = 1.336

Answer 5

Back surface of tear lens (r2) = corneal curvature (K) = 8.00mm

So, F2 = (1-ntears)/r2
= (1-1.1336)/0.0080m
= -42.00 D

Question 6

Given the following information, calculate the power of the tear lens (Fv’):
Front surface power of tear lens (F1) = 40.00D
Back surface power of tear lens (F2) = -42.00D

Answer 6

Fv’ = F1 + F2 = -2.00D

Question 7

Since we know that ntears = 1.336, what is a simplified formula we can use to calculate BVPtears (i.e. power of tear lens, Fv’)?

Answer 7

BVPtears = Fv’ = 336/r1 - 336/r2, OR
BVPtears = Fv’ = 336/BOZR - 336/K

Question 8

For every what difference between the BOZR and K there is a what difference in tear lens power? When can this approximation fail? (2)

Answer 8

For every 0.05mm difference between the BOZR and K, there is 0.25D of tear lens power. This approximation fails for very high steep radii (e.g. keratoconus) or large changes in BOZR

(*another way to look at it: the tear lens power will change by 0.25D f every 0.05mm change in BOZR)

Question 9

Define over-refraction

Answer 9

determination of a residual error of the refraction of the eye while the patient is wearing spectacles or contact lenses

Question 10

If the BOZR of the trial lens is the same as that which you aregoing to order, what will be the power of the ordered lens?

Answer 10

BVPcl = BVPtrial + OverRx

Question 11

How do you calculate the power of the ordered lens if the BOZR is different to the lens that you wish to order?

Answer 11

Need to take into account the tear lens power:
BVPcl = BVPtrial + OverRx - deltaBVPtears
where;
deltaBVPtears = 336/BOZRcl - 336/BOZRtrial

Question 12

For the following information, what is the required BVP for this RGP?
- BVPtrial = -1.75D, BOZRtrial = 7.30mm, Over-Rx = -0.75D, and you want to order a BOZRcl = to 7.40mm [note: i don’t have to calculate this when studying card, just remember formula)

Answer 12

deltaBVPtears = 336/BOZRcl - 336/BOZRtrial = -0.62D
BVPcl = BVPtrial + OverRx - deltaBVPtears = -1.88D

Question 13

What percentage of front surface astigmatism of the cornea will be neutralised by the back surface of a tear lens with a spherical RGP in place?

Answer 13

90%

Question 14

How does the remaining residual front corneal astigmatism not eliminated by the tear lens get eliminated?

Answer 14

Neutralised by the radii of the back surface of the cornea being slightly smaller than the front surface

Question 15

Why is all the astigmatism measured by a keratometer neutralised with a tear lens?

Answer 15

keratometers assume that the posterior cornea has about 10% of the anterior surface power, and excludes that amount based on this assumption

Question 16

Consider that spherical rigid lenses might optically correct astigmatism. Does it always fit the cornea adequately? For what levels of astigmatism does it become a problem?

Answer 16

It may not fit the cornea adequately. Can be a problem for corneas with astigmatism over 2.00DC, where toric lenses are required

Question 17

Do RGPs correct for all the astigmatism in an eye?

Answer 17

not necessarily. There may be residual astigmatism

Question 18

Define residual astigmatism

Answer 18

residual astigmatism = ocular astigmatism - corneal astigmatism. Is the astigmatic prescription remaining after a rigid contact lens is placed on the eye

Question 19

Is residual astigmatism lenticular or retinal?

Answer 19

can be either or both

Question 20

Define lenticular

Answer 20

relating to the lens

Question 21

What are 2 benefits of a toric BOZR?

Answer 21

Improve fit in cases of high corneal astigmatism
Stabilise lens when correcting residual astigmatism

Question 22

What does making the BOZR toric do to the tear lens? What must be introduced to the RGP as a result? What is it equal to?

Answer 22

alters the degree of astigmatism in the tear lens (reduces it). A cylindrical component must therefore be incorporated, equal to the degree by which the astigmatic component of the tear lens has been reduced

Question 23

What are the 2 methods of calculating BVPcl?

Answer 23

1. empirically
2. trial lens over-refraction using formula

Question 24

Is the tear lens for a soft contact lens focal or afocal? Why?

Answer 24

Afocal. Because soft lenses conform to the shape of the front surface of the cornea.

Question 25

What is the formula for BVPcl for soft contact lenses?

Answer 25

BVPcl = BVPtriallens + Over Rx = Ocular Rx

(remember, deltaBVPtears in this case = plano)

Question 26

Is corneal astigmatism masked by a soft contact lens?

Answer 26

No. This means that the cylindrical component of an over-Rx over a spherical soft CL will generally equal the degree of astigmatism in the ocular refraction

Question 27

In what kind of soft contact lenses can the tear lens cause changes to lens power? Is it minor or major?

Answer 27

High plus, causing minor changes

Question 28

List 4 minor things that can cause a change in lens power for soft contact lenses

Answer 28

Lens flexure
Change in temperature
Evaporation
Tear lens

Question 29

Define the tear lens. How ‘thicc’ is it?

Answer 29

When a rigid lens is placed on the eye, a thin fluid layer is trapped between the posterior surface of the CL and the anterior surface of the cornea. This is the “tear lens”. Thickness is 10-20uM