CL Optics Flashcards
How is the CL optic zone typically designed?
Greater than pupil size in low light
What happens in CL design and performance for presbyopes?
It is strongly influenced by pupil size reduction with age and near work
Why does so many MF SCL use centre near optics, rather than centre distance?
Bcoz of the limitations of pupil size/ optic zone interaction
Where are CL placed vs spectacles?
CL: Directly on the cornea (VD = ~0mm)
Specs: spectacle prescriptions neutralise the far point of an eye at a given VD (usually 12mm)
- specs and CL powers slightly differ to correct for this difference in VD
- sometimes some rounding is needed to meet 0.25 step power
What is the equation used to correct VD?
F(cl) = [F(sp)/ (1-dxF(sp)] *d = distance from back vertex of specs lens to corneal apex
Sometimes you can estimate the VD from px’s anatomy.
What are the VD for Asians and Caucasians?
Asians: 10-14mm
Caucasians: 12-15mm
In the correction tables, at what power does it usually start to need to have some corrections?
+/-4D
Values below 4D don’t need to correct for VD
FOr Rx above +/-4D,
- A hyperope will need?
- A myope will need?
- Hyperope: a higher powered CL
- Myope will need a lower powered CL
Work out the CL Rx from Spectacles Rx:
- Spx Rx: -4.00D (12mm)
- Spx Rx: +4.50D (12mm)
- Spx Rx: -6.00D (14mm)
- Spx Rx: +6.50D (14mm)
- CL Rx: -3.75D
- CL Rx: +4.75D
- CL Rx: -5.50D
4, CL Rx: +7.25D
What is spectacle magnification?
- Ratio of the image size in corrected ametropic eye to “uncorrected” eye
- usually up to 20% minification/ magnification
Do you get spectacle magnification when you wear CL?
NO
CL doesn’t experience the prismatic effects of spec lenses
- CL induces minimal magnification (<5%) as they are on the cornea and much closer to the pupil
What will myopes experience with spectacle magnification?
- They will experience an increase in retinal image size with CL
- So… their VA may improve too
- Eyes will also look bigger to the observer/ someone looking at them
Why are CL useful to aniseikonic px? (Anisometropes)
Bcoz of reduction in unwanted magnification and prismatic effects (Prentice’s Rule: P=cF)
Difference between Accommodative Demands in myopes and hyperopes when wearing specs and CL
High myopes:
- increase AD when wearing CL than specs
- may need adds sooner before presbyopia age
High hyperopes:
- decrease AD with CL than specs
- may delay adds with presbyopia
Difference between Convergence Demands in myopes and hyperopes when wearing specs and CL
Myopes:
- wear specs: BI prisms => less CD
- wear CL: => more CD
Hyperopes:
- wear specs: BO Prisms => more CD
- wear CL: => less CD
Difference between myopes and hyperopes going from specs->CL, do they have problems with it?
Myopes from glasses -> CL
- increase AD
- increase CD
- Binocular balance won’t be disturbed
Hyperopes from glasses -> CL
- decrease AD
- decrease CD
- Binocular balance won’t be disturbed
- Myopes: if they are struggling to deal with CL, they may have binocular issues
- If the change in demand is smaller than FR, wearers will usually adapt
Difference between Field of View in myopes and hyperopes when wearing specs and CL
Spec frames and lenses impact FOV when the eys are stationary
CL doesn’t interfere with FOV
Myopes/ Minus:
- Prismatic effect of minus specs lens: can induce Ring of Diplopia (Image smaller than the actual object through specs)
- Minus CL: need larger eye movement to see the same width of field
Hyperopes/ Plus:
- Prismatic effect of plus spec lens: Ring of Scotoma (Image larger than the actual object through spec)
- Plus CL: need less eye movement to see the same width of field
Optically, what are CL considered as?
THICK Lenses (from an optics POV) and are made in meniscus form
Their thickness, compared to their short radii of curvature, is optically significant
The position of the 2nd principal focus from the back vertex of the CL gives what?
The Back Vertex Power (BVP) of the lens
What are the plus power & minus power CL principle points?
- For practical reasons, CL are made in meniscus form
PLUS:
- meniscus plus power CL: have their principal planes anterior to the lens
MINUS:
- principal planes of a meniscus minus power CL: lie behind the lens
What are some optical limitations for CL manufactures?
- Fewer degrees of freedom in lens design compared to specs lenses
- Narrow range of refractive indices available in CL materials
- SCL lens shape on the eye: largely determined by conformance to corneal shape - no tear lens
- RGP optics are more predictable
- RGP tear lens can mask corneal cylinder but lens movement means can lead to non-coaxial optics
Why and how does pupil size impact SCL design?
- usually optic zone of CL is bigger than pupil size
- For presbyopic px, pupil size is smaller with age and near work, so CL design is different
- Many MF SCL uses centre near optics bcoz of limitations of pupil size
How do you correct spectacle lenses effectively in spherical CL prescription?
?? CL is directly on cornea, spectacles usually has a vertex distance so we can use the formula to calculate the correct Rx. Myopes: lower CL power and hyperopes need higher CL power ?
What is the CL Rx impact on image magnification?
- In myopes: increase retinal image size with CL
- In hyperopes: decrease retinal image size with CL
What is the CL Rx impact on AD & CD
Myope: increase AD and CD
Hyperopes: decrease AD and CD
- Myopes tend to work with parallel, as long as they are within FR, you’re okay
- Myopes will also feel more tired with CL
What is the CL Rx impact on FOV and eye movement
Myopes: Minus CL need more EM to see the same width of field, CL will see less FOV
Hyperopes: plus CL need less EM to see the same FOV, CL will see more FOV
Why do we use BVP to describe CL power?
we usually only deal with vertex powers since that’s the power the focimeter measures
