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APCO > CL Optics > Flashcards

CL Optics Flashcards

1
Q

How is the CL optic zone typically designed?

A

Greater than pupil size in low light

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2
Q

What happens in CL design and performance for presbyopes?

A

It is strongly influenced by pupil size reduction with age and near work

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3
Q

Why does so many MF SCL use centre near optics, rather than centre distance?

A

Bcoz of the limitations of pupil size/ optic zone interaction

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4
Q

Where are CL placed vs spectacles?

A

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
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5
Q

What is the equation used to correct VD?

A 
F(cl) = [F(sp)/ (1-dxF(sp)]
*d = distance from back vertex of specs lens to corneal apex
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6
Q

Sometimes you can estimate the VD from px’s anatomy.

What are the VD for Asians and Caucasians?

A

Asians: 10-14mm
Caucasians: 12-15mm

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7
Q

In the correction tables, at what power does it usually start to need to have some corrections?

A

+/-4D

Values below 4D don’t need to correct for VD

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8
Q

FOr Rx above +/-4D,

  • A hyperope will need?
  • A myope will need?
A
  • Hyperope: a higher powered CL

- Myope will need a lower powered CL

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9
Q

Work out the CL Rx from Spectacles Rx:

  1. Spx Rx: -4.00D (12mm)
  2. Spx Rx: +4.50D (12mm)
  3. Spx Rx: -6.00D (14mm)
  4. Spx Rx: +6.50D (14mm)
A
  1. CL Rx: -3.75D
  2. CL Rx: +4.75D
  3. CL Rx: -5.50D
    4, CL Rx: +7.25D
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10
Q

What is spectacle magnification?

A
  • Ratio of the image size in corrected ametropic eye to “uncorrected” eye
  • usually up to 20% minification/ magnification
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11
Q

Do you get spectacle magnification when you wear CL?

A

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
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12
Q

What will myopes experience with spectacle magnification?

A
  • 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
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13
Q

Why are CL useful to aniseikonic px? (Anisometropes)

A

Bcoz of reduction in unwanted magnification and prismatic effects (Prentice’s Rule: P=cF)

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14
Q

Difference between Accommodative Demands in myopes and hyperopes when wearing specs and CL

A

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
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15
Q

Difference between Convergence Demands in myopes and hyperopes when wearing specs and CL

A

Myopes:

  • wear specs: BI prisms => less CD
  • wear CL: => more CD

Hyperopes:

  • wear specs: BO Prisms => more CD
  • wear CL: => less CD
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16
Q

Difference between myopes and hyperopes going from specs->CL, do they have problems with it?

A

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
17
Q

Difference between Field of View in myopes and hyperopes when wearing specs and CL

A

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
18
Q

Optically, what are CL considered as?

A

THICK Lenses (from an optics POV) and are made in meniscus form

Their thickness, compared to their short radii of curvature, is optically significant

19
Q

The position of the 2nd principal focus from the back vertex of the CL gives what?

A

The Back Vertex Power (BVP) of the lens

20
Q

What are the plus power & minus power CL principle points?

A
  • 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

21
Q

What are some optical limitations for CL manufactures?

A
  1. Fewer degrees of freedom in lens design compared to specs lenses
  2. Narrow range of refractive indices available in CL materials
  3. SCL lens shape on the eye: largely determined by conformance to corneal shape - no tear lens
  4. RGP optics are more predictable
  5. RGP tear lens can mask corneal cylinder but lens movement means can lead to non-coaxial optics
22
Q

Why and how does pupil size impact SCL design?

A
  • 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
23
Q

How do you correct spectacle lenses effectively in spherical CL prescription?

A

?? 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 ?

24
Q

What is the CL Rx impact on image magnification?

A
  • In myopes: increase retinal image size with CL

- In hyperopes: decrease retinal image size with CL

25
Q

What is the CL Rx impact on AD & CD

A

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
26
Q

What is the CL Rx impact on FOV and eye movement

A

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

27
Q

Why do we use BVP to describe CL power?

A

we usually only deal with vertex powers since that’s the power the focimeter measures

APCO (13 decks)

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