wk3: CLM - Optics of CLs 2 Flashcards
Is the tear lens involved in the optical system producing the retinal image?
yes
Compare BOZR with the K values of the cornea and describe how their relationship affects the power of the tear lens (3)
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”)
In terms of back vertex power (BVP), how do you calculate ocular refraction for a rigid lens? [formula]
Ocular refraction = BVPcl + BVPtearlens
Given the following information, calculate the front surface “power” (F1) of the tear lens:
- K = 8mm, BOZR = 8.40mm, ntears = 1.336
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
Given the following information, calculate the back surface “power” (F2) of the tear lens:
- K = 8mm, BOZR = 8.40mm, ntears = 1.336
Back surface of tear lens (r2) = corneal curvature (K) = 8.00mm
So, F2 = (1-ntears)/r2
= (1-1.1336)/0.0080m
= -42.00 D
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
Fv’ = F1 + F2 = -2.00D
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’)?
BVPtears = Fv’ = 336/r1 - 336/r2, OR
BVPtears = Fv’ = 336/BOZR - 336/K
For every what difference between the BOZR and K there is a what difference in tear lens power? When can this approximation fail? (2)
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)
Define over-refraction
determination of a residual error of the refraction of the eye while the patient is wearing spectacles or contact lenses
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?
BVPcl = BVPtrial + OverRx
How do you calculate the power of the ordered lens if the BOZR is different to the lens that you wish to order?
Need to take into account the tear lens power:
BVPcl = BVPtrial + OverRx - deltaBVPtears
where;
deltaBVPtears = 336/BOZRcl - 336/BOZRtrial
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)
deltaBVPtears = 336/BOZRcl - 336/BOZRtrial = -0.62D
BVPcl = BVPtrial + OverRx - deltaBVPtears = -1.88D
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?
90%
How does the remaining residual front corneal astigmatism not eliminated by the tear lens get eliminated?
Neutralised by the radii of the back surface of the cornea being slightly smaller than the front surface
Why is all the astigmatism measured by a keratometer neutralised with a tear lens?
keratometers assume that the posterior cornea has about 10% of the anterior surface power, and excludes that amount based on this assumption
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?
It may not fit the cornea adequately. Can be a problem for corneas with astigmatism over 2.00DC, where toric lenses are required
Do RGPs correct for all the astigmatism in an eye?
not necessarily. There may be residual astigmatism
Define residual astigmatism
residual astigmatism = ocular astigmatism - corneal astigmatism. Is the astigmatic prescription remaining after a rigid contact lens is placed on the eye
Is residual astigmatism lenticular or retinal?
can be either or both
Define lenticular
relating to the lens
What are 2 benefits of a toric BOZR?
Improve fit in cases of high corneal astigmatism
Stabilise lens when correcting residual astigmatism
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?
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
What are the 2 methods of calculating BVPcl?
- empirically
- trial lens over-refraction using formula
Is the tear lens for a soft contact lens focal or afocal? Why?
Afocal. Because soft lenses conform to the shape of the front surface of the cornea.
What is the formula for BVPcl for soft contact lenses?
BVPcl = BVPtriallens + Over Rx = Ocular Rx
(remember, deltaBVPtears in this case = plano)
Is corneal astigmatism masked by a soft contact lens?
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
In what kind of soft contact lenses can the tear lens cause changes to lens power? Is it minor or major?
High plus, causing minor changes
List 4 minor things that can cause a change in lens power for soft contact lenses
Lens flexure
Change in temperature
Evaporation
Tear lens
Define the tear lens. How ‘thicc’ is it?
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
