CLM - Optics of Contact Lenses II - Week 3 Flashcards
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 contact lens and the anterior surface of the eye
Thickness is 10-20μm
Is the tear lens involved in the optical system to produce a retinal image?
Yesd
Comparing the BOZR to the K readings of the cornea, what is the power of the tear lens?
BOZR > K, then negative tear lens
BOZR = K, then plano tear lens
BOZR < K, then positive tear lens
For every what difference between the BOZR and K, there is a what change of tear lens power?
When does this approximation fail (2)?
For every 0.05mm difference between the BOZR and K, there is 0.25D of tear lens power
Very high (steep) radii like keratoconus
Large changes in BOZR
Define over-refraction.
Determination of a residual error of refraction of the eye while the patient is wearing spectacles or contact lenses
If the BOZR of the trial lens is the same as which you are going to order, what is the power of the ordered lens (formula)?
BVP (CL) = BVP (trial) + over-refraction
Consider astigmatism with RGP lenses. What percentage of the front surface astigmatism of the cornea will be neutralised by the back surface of the tear lens? What results in neutralisation of the remainder?
90%
The radii of the back surface of the cornea are slightly smaller than the front surface, resulting in a mild neutralisation of approximately 10%
Consider that sperical rigid lenses might optically correct astigmatism. Does it always fit the cornea adequately? What levels of astigmatism does it become a problem?
It might not always fit the cornea adequately
For astigmatic corneas >2.00D, toroidal lenses are required
Do RGPs correct for all the astigmatism in an eye? Explain (5).
No, there may be residual astigmatism
Residual astigmatism = ocular astigmatism (prescription) - corneal astigmatism
Can be lenticular, retinal, or both
RGPs neutralise all of them except residual astigmatism
Define resigual astigmatism.
The astigmatic prescrition that remains after a rigid contact lens is placed on the eye
What are two benefits of using a toroidal lens?
Improved fit in cases of high astigmatism
Stabilises 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 (will reduce it)
A cylindrical component must therefore be incorporated, equal to the degree by which the astigmatic component of the tear lens has been reduced
Is the tear lens of a soft contact lens focal or afocal?
Afocal
What is the formula for BVP (CL) with soft contact lenses?
BVP (trial) + over-prescription
Is corneal astigmatism masked by a soft contact lens?
No
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 three things that can cause a change in lens power for soft contact lenses (aside from power).
Lens flexure
Changes in temperature
Evaporation
Can you use a minus trial lens for a hyperope and vice versa (soft contact lenses)?
No, must use minus for myopes and plus for hyperopes
Within what power should a trial soft contact lens be of their prescription?
Within 3.00D
Define lens flexure.
When a soft contact lens is placed on the eye, it takes up the same curvature as the central cornea
This change compared to normal is the lens flexure
Is lens flexure important when prescribing soft contact lenses? Explain.
Not very important and generally insignificant except where the lenses are of high positive power
What happens to a soft contact lens when there is a change in temperature (especially when placing it on the eye)? Explain (2).
With the contact lens being ~20C and the eye being ~37C, the change in temperature can cause a slight steepening of the contact lens and a slight increase in the negative power
List two ways that evaporation can affect the lens power of a contact lens.
Small increase in refractive index
Steepening of the lens
Both tend to increase the negative power of the lens
Should the tear lens of a soft contact lens be focal or afocal?
Should be afocal
Between plus and minus soft contact lenses, which can entrap a greater volume of tears? What effect does this have?
Minus entrap low volume and minimal power
Plus lenses can entrap a greater volume and significant power
Plus tend to lose power when placed on the eye
