8. CL VIII - CL Optics II Flashcards
Tear Lens
When a contact is placed on the eye, a ... is trapped between ... of the contact and the ... of the cornea and acts as .... It’s usually ...-...microns thick.
Tear Lens
When a contact is placed on the eye, a thin fluid layer is trapped between posterior surface of the contact and the anterior surface of the cornea and acts as another optical element. It’s usually 10-20microns thick.
Tear Lens power
The power depends on the relationship between ... and ....
If ... = ... -> ... (...)
If ... = ... -> ... (...)
If ... = ... -> ... (...)
The tear lens can add ..., ..., or .. power to the overall .../... system. Given that the lens thickness approaches ..., we can use ....
Tear Lens power
The power depends on the relationship between BOZR and corneal curvature.
If BOZR = K -> plano tear lens (on K)
If BOZR < K -> positive tear lens (steeper than K)
If BOZR > K -> negative tear lens (flatter than K)
The tear lens can add positive, negative, or zero power to the overall CL/eye system. Given that the lens thickness approaches zero, we can use thin lens theory.
Calculating the Tear Lens power
Power of tear lens (FV’) = F1 + F2 = ... + ...
F1 is ... and F2 is ....
Simplified to BVPtears = ... - ... or BVPtears = ... - ..., as the numerator was multiplied by ... so we can use ... as mm.
Calculating the Tear Lens power
Power of tear lens (FV’) = F1 + F2 = (ntears-1)/r1 + (1-ntears)/r2
F1 is front surface power and F2 is back surface power.
Simplified to BVPtears = 336/r1 - 336/r2 or BVPtears = 336/BOZR - 336/K, as the numerator was multiplied by 1000 so we can use radius of curvature as mm.
What is the Tear Lens Approximation and when does it fail?
For every 0.05mm difference between the BOZR and K, there is a 0.25D of tear lens power.
Fails for very steep curvatures (short radii of curvature) or when there are large differences between BOZR and K (0.5mm).
Rigid Corneal Lens Trial Fitting
After putting trial lens in, assuming perfect fit, apply:
BVPCL = ... + ....
If the fit is not perfect, need to use:
BVPCL = ... + ... - same, where same = ... (ordered CL) - ....
If ... is very high, must ... using ....
Rigid Corneal Lens Trial Fitting
After putting trial lens in, assuming perfect fit, apply:
BVPCL = BVPtrial + Over Rx.
If the fit is not perfect, need to use:
BVPCL = BVPtrial + OverRx - ΔBVPtears, where ΔBVPtears (last part) = 336/BOZRCL (ordered CL) - 336/BOZRtrial.
If OverRx is very high, must convert to ocular plane BVP using effectivity equation.
Corneal Astigmatism/Tear Lens Relationship (1)
When a ... RCL is placed on the cornea, the ... of the tear lens will neutralise about ...% of the cornea’s ....
How?
1. Calculate ... -> ... (Fcornea = ..., where ncornea = 1.376).
2. Calculate ... by finding power of each meridian (BVPtears = ... - ...) and combining.... between 1 and 2 is the over percentage.
Corneal Astigmatism/Tear Lens Relationship (1)
When a spherical RCL is placed on the cornea, the back surface of the tear lens will neutralise about 90% of the cornea’s front surface astigmatism.
How?
1. Calculate corneal cyl -> difference in power between each meridian (Fcornea = (1.376-1)/r, where ncornea = 1.376).
2. Calculate tear lens power by finding power of each meridian (BVPtears = 336/BOZR - 336/K) and combining.Difference between 1 and 2 is the over percentage.
Corneal Astigmatism/Tear Lens Relationship (2)
If ... RCLs can neutralise about ...% of the cornea’s ..., what about the remaining percentage? It’s accounted for by the ....
The cornea’s ... surface has radii of curvature slightly ... than the ... surface and the refractive index is much .... These differences neutralise about ...% of the cornea’s astigmatism and therefore the problem is solved.
Corneal Astigmatism/Tear Lens Relationship (2)
If spherical RCLs can neutralise about 90% of the cornea’s front surface astigmatism, what about the remaining percentage? It’s accounted for by the difference between Δn or air/cornea and air/tear lens.
The cornea’s posterior surface has radii of curvature slightly steeper than the anterior surface and the refractive index is much less. These differences neutralise about 10% of the cornea’s astigmatism and therefore the problem is solved.
Clinical implications of the corneal astigmatism/tear lens relationship
... corneal astigmatism, ... corneal astigmatism, and even ... corneal astigmatism can be masked by a ... RGP, due to the tear lens. However, the ... may not be appropriate if the ... is greater than ...DC.
Clinical implications of the corneal astigmatism/tear lens relationship
Regular corneal astigmatism, irregular corneal astigmatism, and even general corneal astigmatism can be masked by a spherical RGP, due to the tear lens. However, the fit may not be appropriate if the corneal cyl is greater than -2.00DC.
Residual astigmatism
RGPs ... all of an eye’s astigmatism all the time, there may be residual. Residual astigmatism = ... - ..., and is most commonly ... but may be due to the .... Therefore, RGPs neutralise ... and not ....
Residual astigmatism
RGPs don't correct all of an eye’s astigmatism all the time, there may be residual. Residual astigmatism = ocular astigmatism - corneal astigmatism, and is most commonly lenticular astigmatism but may be due to the posterior cornea. Therefore, RGPs neutralise corneal astigmatism and not residual astigmatism.
Toric RGPs’ BOZR
Often necessary for ... reasons due to ... or for providing ... when correcting residual astigmatism.
In a case with high corneal cyl (e.g. -5.00DC):
* A spherical RGP will yield a ... overrefraction but ... fit.
* A toric RGP with K’s matching the corneal will yield a ... fit but poor vision (as the tear lens is ...), so a ... RGP would be required.
* A toric RGP with flatter BOZR than ... meridian will have a ... fit and ... vision as there is ... to neutralise the ... cyl
All in all, a toric BOZR reduces ..., so we need to incorporate a ... on the RCL to .... BVPCL = ... + ... - (... - ...)
Toric RGPs’ BOZR
Often necessary for fitting reasons due to high corneal astigmatism or for providing stability of axis when correcting residual astigmatism.
In a case with high corneal cyl (e.g. -5.00DC):
* A spherical RGP will yield a zero overrefraction but terrible fit.
* A toric RGP with K’s matching the corneal will yield a great fit but poor vision (as the tear lens is plano), so a bitoric RGP would be required.
* A toric RGP with flatter BOZR than steeper meridian will have a good fit and good vision as there is tear lens cyl to neutralise the corneal cyl
All in all, a toric BOZR reduces tear lens astigmatism, so we need to incorporate a cylindrical component on the RCL to neutralise the corneal cyl. BVPCL = BVPtrial + OverRx - (336/BOZRCL - 336/BOZRtrial)
Soft Contact Lens optics
SCLs ... of the front surface of the cornea, so the tear lens is .... Therefore, no ... relating to tear lens power. Has the disadvantage that there is ... and may require a ....
BVPCL = ... + ... = ..., adjusted to ..., which is the ... as for RGPs. However, if we change the BOZR there will be ....
Soft Contact Lens optics
SCLs conform to the shape of the front surface of the cornea, so the tear lens is generally afocal. Therefore, no involved calculations relating to tear lens power. Has the disadvantage that there is no masking of corneal astigmatism by soft lenses and may require a toric SCL for good vision.
BVPCL = BVPtrial + OverRx = Ocular Rx, adjusted to corneal plane, which is the same as for RGPs. However, if we change the BOZR there will be no change to BVPCL.
On-eye changes in BVP in SCLs
Minor changes in lens power on the eye due to: ..., ..., ..., and .... Must use ... for myopes and ... for hyperopes, and the trial should be within ...D of Ocular Rx.
On-eye changes in BVP in SCLs
Minor changes in lens power on the eye due to: lens flexure, tear lens (in high plus), change in temperature, and evaporation. Must use minus trials for myopes and plus trials for hyperopes, and the trial should be within 3.00D of Ocular Rx.
SCL lens flexure
When SCLs are placed on the eye, the ... takes up the same curvature as the .... This ... (i.e. ... is different ... and ...) is referred to as lens flexure. Only really important for ... as this power change is ... otherwise.
SCL lens flexure
When SCLs are placed on the eye, the centre of the back surface takes up the same curvature as the central cornea. This curvature change (i.e. back surface curvature is different on-eye and off-eye) is referred to as lens flexure. Only really important for high positive power as this power change is generally insignificant otherwise.
SCLs Tear Lens
They are normally ... but studies demonstrate that:
* Minus lenses entrap tear layers of ... and ...
* Plus lenses can entrap tear layers of ... and ...; both effects of which ... as ... increases. i.e. ... lose ... when placed on the eye. Don’t need ..., just an ...
SCLs Tear Lens
They are normally afocal but studies demonstrate that:
* Minus lenses entrap tear layers of low volume and minimal power
* Plus lenses can entrap tear layers of greater volume and significant minus power; both effects of which increase as BVP increases. i.e. high plus lenses lose power when placed on the eye. Don’t need calculations, just an over-refraction
Changes in Temperature effect on SCLs
Cornea is hotter than room temp, so change in temp after placing a CL on eye leads to: ..., ..., and ...
Changes in Temperature effect on SCLs
Cornea is hotter than room temp, so change in temp after placing a CL on eye leads to: cold feeling, slight steepening of CL, and slight increase in negative power of lens
