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Dispensing A Flashcards

1
Q

Lens power formulae

A
  1. F=1/f’
  2. F= (n’-n)/r
  3. F= F1+F2
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2
Q

What test can be used to differentiate spherical surface form?

A

Straight edge test with a ruler

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3
Q

What does steepness of radius of curvature tell about the steepness of the surface?

A

Steeper RoC means a steeper surface

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4
Q

What are flat forms

A

The two centers of curvatures are on opposite sides of the lens

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5
Q

What is a meniscus lens?

A

One surface is convex and other is concave.

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6
Q

Where are the centers of curvature for a meniscus lens?

A

Same side of the lens.

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7
Q

What is base curves?

A

The lowest numerical surface power.

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8
Q

Why are meniscus chosen rather than flat forms?

A

Meniscus minimize lens aberrations.

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9
Q

How to position lens?

A

Most convex or least concave= Front
Most concave or least convex= Back (closer to eye)

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10
Q

Why is how you position the lens important?

A

Optimises image quality and reduces aberrations

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11
Q

Transposition:
Cross-cyl to +ve Sphcyl

A
  1. Sphere= most negative/least positive power
  2. Cyl= difference between cyl powers (positive value)
  3. Axis of 2nd cyl
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12
Q

Transposition:
Cross-cyl to -ve Sphcyl

A
  1. Sphere= most positive/ least negative power
  2. Cyl= difference between cyl powers (negative values)
  3. Axis of 1st cyl
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13
Q

Transposition:
+/- Sphycyl to Cross-cyl

A
  1. 90 deg to axis, sph as 1st cyl
  2. 2nd cyl = sph + cyl. Axis remains the same
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14
Q

+/- sph Cyl conversion

A
  1. Add sphere to cylinder to give new sph
  2. change sign of cyl (power remains the same)
  3. Change axis by 90 degs
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15
Q

Convention to write toric prescription

A 
               Sphere curve
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16
Q

Toric to Sphcyl

A
  1. Add sphere curve to first power of toroidal surface = this gives the first power and axis of Cross-cyl
  2. Add sphere curve to second power of toroidal surface = this gives second power and axis of Cross-cyl
  3. Convert Cross-cyl to Sph-cyl form
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17
Q

Formula of curve variation factor

A

CVF = (n’-1)/ (nc-1)

nc is the refactive index of crown glass 1.523

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18
Q

Formula to find the real surface power

A

Real power= CVF x Lens measure power

19
Q

Movement of image through plus lens

A

Against. Image moves up as lens moved down

20
Q

Movement of image through minus lens

A

With. Image moves down as lens moved down

21
Q

What are the methods to measure prim power?

A
  1. Neutralisation
  2. Tangent scale
  3. Focimeter
  4. Thickness difference
22
Q

Why is thin edge substance added to apex of prims?

A

Prisms are meniscus and so thin edge substance is added to prevent knife edge (prevent it being pointy and sharp)

23
Q

Thick edge substance equation
and equation to find g

A

Thick edge substance= Thin edge substance + g

g= P x d / 100 x (n’-1)

P= prism power
d= diameter in meters

24
Q

Differential prism
Additive

A

OUT + OUT
IN + IN
DOWN + UP

25
Differential prism Subtractive
OUT + IN DOWN + DOWN UP + UP
26
What is angle kappa?
When the visual axis does not pass though the center of pupil
27
How to measure interpupillary distance (PD) of one eyed patient?
Measure outer canthus to inner canthus
28
How to measure interpupillary distance (PD) of a patient with squint?
Cover one eye, measure, then cover other eye and measure.
29
Formula of near centration distance
NCD= PD x Working distance
30
Why is effectivity (vertex distance) important?
1. British standard requires you to quote back vertex distance for prescriptions with power of 5DS. Also need to specify whether the BVD was measured from the back of the lens to the cornea or closed eyelid. 2. Contact lenses have a BVD of zero which would change the power of correction.
31
Formula for effectivity (vertex distance)
Fe'= F/ 1- d x F Fe'= New power d= distance in meters Don't forget signs
32
What is the hypermetropic and myopic correction if lens further from eye?
Hypermetropic correction (+lens) needs to be made weaker if further from eye Myopic correction (-lens) needs to be made stronger if further from eye.
33
Prentice's rule
P = c x F P= prismatic effect c= distance from optical center in centimeters
34
Define anisometropic
different prescription for each eye
35
When the patient moves their eyes to no longer be in line with optical center (they are using the peripheral part of lens) prismatic effect occurs. When will this not be an issue?
1. If eyes are same prescription 2. Horizontal movement = range of 20 to 30 diopters of convergence/divergence
36
When the patient moves their eyes to no longer be in line with optical center (they are using the peripheral part of lens) prismatic effect occurs. When will this be an issue?
Vertical movement = range of 1 dioptert
37
The further the eyes move from the optical center the more likely to experience diplopia. What are the solutions for this?
1. Advise patient to move head rather than than eyes. 2. Prescribe bicentric lenses 3. Contact lenses (may not be suitable for all patients)
38
How do bicentric lenses work?
1. Minus lenses - Can remove base down. So remove the amount of differential diopters 2. Plus lenses - remove base down in other eye to increase prism to make both eyes equal.
39
What are the steps to find total decentration?
1. Place lens on focimeter to locate optical center 2. center target over center of graticule 3. Mark point using focimeter maker pens 4. Place lens over city rule scale B 5. Move so that the extreme edges of lens are parallel to horizontal and vertical lines of scale B 6. Mark boxed center as the scale center 7. Record position of optical center relative to boxed center 8. Each dot separation on scale B is equal to 2mm decentration 9. Decentration per eye = (BCD - PD)/2 10. Take into account: decentration for pupil position in frame, and decentration for creating prismatic effect. Add these two together to get total decentration
40
What is a blank?
A moulded piece of ophthalmic material before completion of the surfacing processes.
41
What is an uncut / finished lens?
A spectacle lens that has been surfaced on both sides to the required power and thickness and is still in uncut form.
42
True or false: A surface with a RoC of +120.00 mm is flatter than a surface with a RoC of +80.00 mm
True
43
What is curved lens form?
Meniscus lens

Year 1 (1 decks)

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