L15 Single Vision Dispensing Flashcards

1
Q

Spectacle Magnification

A

Ratio of retinal image size in the corrected eye, to that in the uncorrected eye

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2
Q

Spectacle Magnification depends on

A
  • power factor = power of the lens
  • shape factor = form and thickness of the lens
  • BVD
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3
Q

SM EQUATION

A

SM=[1/(1-dF’V)] x [1/(1-t/nF1)]

d is from the back of the glasses to the entrance point of the pupil

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4
Q

SM for myope/Hypermetrope

A
  • for a hyperope it is greater than 1 = magnification
  • for a myope it is less than 1 = minification
  • has no units
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5
Q

How to increase spectacle magnification

A
  • increase power of F1 - but can’t really change it
  • increase thickness - by altering curves
  • decrease n
  • increase d for hypermetrope ( increase vertex distance)
  • decrease d for a myope ( decrease vertex distance)
  • a thin lens makes magnification less
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6
Q

Why calculate spectacle magnification?

A
  • anisometropia

- differential prism

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7
Q

Anisometropia

A

= an unequal refractive power ( around 2D) bet the 2 eyes

- results in aniseikonia - a diff on the retinal image size bet the eyes resulting from the difference in rx

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8
Q

Differential prism

A

If prism encountered by each eye is different then the px will experience differential prismatic effect
- can cause Diplopia, eye strain, headaches and general discomfort

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9
Q

Near vision point

A

=point to which eyes go to read
- 8 to 10mm down and 2mm in - in is due to vergence

P=cF

  • from this relationship we can see that for every 1Ds of anisometropia there will be 1 prism diopter of differential prism
  • we can cope more vertically with differential prism than horizontally
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10
Q

Size/ orthogonal lenses for Anisokonia

A
  • if there is an intolerance in the variation in SM bet the 2 eyes then we can dispense a size lens
  • these lenses have a back surface power of 0Ds so don’t change the rx
  • here, the back vertex power and the vertex distance are fixed, meaning we can’t change the power factor
  • alters the shape factor
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11
Q

Magnification if shape factor of an a afocal size lens

A

-tF2/10n

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12
Q

Solutions for differential prism

A

1) separate pairs
2) franklin splint
3) bonded Plano prism
4) slab off

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13
Q

Franklin split

A
  • not usually dispensed
  • 2 half lenses cemented together in the frame
  • ugly, thick
  • info in first half of the lens could be diff to second half
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14
Q

Bonded Plano prism

A
  • looks like a bifocal

- stuck on top of lens

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15
Q

Slab off

A
  • way of working prismatic effect into one half of the lens
  • works vertically and not horizontally
  • can remove/ add prism to parts of lenses
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