Week 7 - Prisms, Bifocals and PALs

Question 1

What is a prism dioptre?

Answer 1

1 prism dioptre gives a displacement of 1 cm at 100cm

Question 2

What is the effect of prisms?

Answer 2

Light bends towards the base
Image towards the apex
for e.g.
Exotropia – prescribe BO to shift image in

Question 3

what about if we prescribe 3^ BU and 4^BI?

Answer 3

Question 4

What is the Prentice’s rule?

Answer 4

Prism = Decentration (cm) x power (D)

e.g. how much prism induced by +5.00D lens if we decentre the lens 4mm?
== 0.4cm x 5D = 2^

Question 5

Prismatic effects of lens decentration

Answer 5

Question 6

Prismatic effects of cyl lens decentration

Answer 6

Question 7

E.g. of Prentice’s Rule

Answer 7

Question 8

What if you move the lens up or down

Answer 8

Question 9

What if you have both base up

Answer 9

Question 10

What if you have both lenses decentred temporally?

Answer 10

Question 11

Inducing prismatic effect by looking away from the centre

Answer 11

R lens sitting 15mm higher than pupil centre
RE) 1.5cm x 2 = 3^ BD
LE) 1.5cm x 7 = 10.5^ BD
Total = Prism effect in the same direction
10.5 - 3 = 7.5^ BDL.
The image in the LE is 7.5^ higher than RE

Question 12

What is DPE

Answer 12

Differential prismatic effect = difference between prism induced away from the pupil centre of the eye
Anisometropia: Difference between the eye’s prescription >1D

Antimetropia: One eye myopic and the other hyperopic

Question 13

DPE example

Answer 13

Question 14

Frame depth of 48mm and pupil height of 38mm (14mm above geometrical centre). Calculate amount of prism if lenses were fitted on datum

Answer 14

Remember:
Prism in the same vertical direction ==>subtract the difference

Question 15

DPE tolerances

Answer 15

Differential prism = different eye rotation to maintain binocular vision and avoid diplopia

Max vertical differential prism tolerable for long periods 2^

Emsley said 1^

Question 16

Different types of bifocal design

Answer 16

Question 17

Advantages and disadvantages of fused bifocals

Answer 17

Advantages:
- less noticeable than CR39 equivalent
- more scratch resistant

Disadvantages
- heavier and less impact resistant
- low Abbe segment material = chromatic aberration at the top of the segment

Question 18

Bifocal terminology

Answer 18

Question 19

Does the Prentice’s law apply to bifocals

Answer 19

Yes
The segment lens is always plus which causes BD prism meaning the image is displaced upwards.
The amount of BD depends upon the power of the add and the distance from the optical centre of the segment

E.g. A gives maximum prism and C no prism

Question 20

Image jump?

Answer 20

Image displaced upwards when you look down

Question 21

Calculating Image Jump e.g.

Answer 21

Question 22

How do you eliminate image jump?

Answer 22

Choose executive style bifocals

Question 23

Anisometropia induced prism in bifocals

Answer 23

Question 24

Long/short corridors PAL design

Answer 24

Question 25

Use Prentice rule: looking through near portion of PAL that is 11mm down OC

Answer 25

Question 26

What if you use a shorter corridor lens

Answer 26

The DPE will be less compared to the longer corridor

Question 27

How to reduce anisometropia in PAL

Answer 27

1) PAL with shorter corridor
Will reduce distance to the NVP and hence DPE

2) prism slab-off applied to PAL
This eliminates the DPE at NVP but at the cost of an obvious line across one lens

3) Prism slab-off bifocal for prolonged near tasks
A shorter corridor PAL could be prescribed for normal wear and for extended reading a slab off bifocal will eliminate the DPE for reading