Week 7 - Prisms, Bifocals and PALs Flashcards
What is a prism dioptre?
1 prism dioptre gives a displacement of 1 cm at 100cm
What is the effect of prisms?
Light bends towards the base
Image towards the apex
for e.g.
Exotropia – prescribe BO to shift image in
what about if we prescribe 3^ BU and 4^BI?
What is the Prentice’s rule?
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^
Prismatic effects of lens decentration
Prismatic effects of cyl lens decentration
E.g. of Prentice’s Rule
What if you move the lens up or down
What if you have both base up
What if you have both lenses decentred temporally?
Inducing prismatic effect by looking away from the centre
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
What is DPE
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
DPE example
Frame depth of 48mm and pupil height of 38mm (14mm above geometrical centre). Calculate amount of prism if lenses were fitted on datum
Remember:
Prism in the same vertical direction ==>subtract the difference
DPE tolerances
Differential prism = different eye rotation to maintain binocular vision and avoid diplopia
Max vertical differential prism tolerable for long periods 2^
Emsley said 1^
Different types of bifocal design
Advantages and disadvantages of fused bifocals
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
Bifocal terminology
Does the Prentice’s law apply to bifocals
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
Image jump?
Image displaced upwards when you look down
Calculating Image Jump e.g.
How do you eliminate image jump?
Choose executive style bifocals
Anisometropia induced prism in bifocals
Long/short corridors PAL design
Use Prentice rule: looking through near portion of PAL that is 11mm down OC
What if you use a shorter corridor lens
The DPE will be less compared to the longer corridor
How to reduce anisometropia in PAL
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
