Week 4: Prisms & Lens Fitting

Question 1

Define/describe prism

Answer 1

• A lens “whose surfaces are angled with respect to one another in order to produce a given amount of deviation of incident light” (Grosvenor, 1996)
• Light is deviated towards the prism base
• The image is displaced towards the prism apex

Question 2

What is the direction of base in?

Answer 2

base directed towards midline of face

Question 3

What is the direction of base out?

Answer 3

base directed away from the midline of the face

Question 4

What is the direction of base up?

Answer 4

base directed superiorly

Question 5

What is the direction of base down?

Answer 5

base directed inferiorly

Question 6

Explain resolving prism

Answer 6

• Process of expressing a single oblique prism as two perpendicular components

Question 7

What happens if minus lens decentred out?

Answer 7

Pupil is looking through base in prism

Question 8

What happens if plus lens decentred out?

Answer 8

Pupil is looking through base out prism

Question 9

What if minus lens decentred in

Answer 9

Pupil is looking through base out prism

Question 10

What if plus lens decentred in?

Answer 10

Pupil is looking through base in prism

Question 11

How do we obtain stereopsis?

Answer 11

We need both of our eyes to be directed towards the object we are looking at

Question 12

Why do we use prism?

Answer 12

• Prism can be used to decrease the demand placed on the muscles of the eyes when a high phoria or tropia is present, or if a very high addition lens is prescribed

Question 13

Describe phoria

Answer 13

• Eyes have a tendency to point away from the object being viewed, however the muscles are capable of overcoming this tendency
• Usually do not prescribe prism

Question 14

Describe tropia

Answer 14

• Eyes have a tendency to point away from the object being viewed, and muscles are incapable of overcoming this tendency
• Prism can be prescribed to assist

Question 15

What are the two vertical deviation?

Answer 15

• Vertical deviation = hyper/hypo (phoria/tropia)
• Horizontal deviation = exo/eso (phoria/tropia)

Question 16

Explain lens neutralisation

Answer 16

• Method of measuring the power of an ophthalmic lens
• Two lenses neutralise each other if, when placed in contact, total refracting power is ZERO
• +ve lens = against movement
• -ve lens = with movement

Question 17

Explain lens decentration

Answer 17

• If centration distance (CD) between the lenses is not equal to the pupillary distance (PD), then the eye will not be looking through the optical centre of the lens

Question 18

What does Prentice’s Rule find?

Answer 18

• Knowing the amount of decentration along with the power of the lens, we can then calculate the induced prism

Question 19

List out all of F by approximation

Answer 19

Refer to table on pg 2

Question 20

Explain differential prismatic effects

Answer 20

• Different powers will result in different prismatic effects for the same distance from the optic centre (typically at Near Visual Point)
• The brain must work harder to try and fuse the images

Question 21

When to Correct Vertical Differential Prism?

Answer 21

• Consider for: vertical meridian difference > 1.50 D difference
• Must for: vertical meridian difference > 2-3 D difference

Question 22

When do you add or subtract with vertical & horizontal prisms?

Answer 22

• Vertical (base up/down)
- Opposite direction = add
- Same direction = subtract

• Horizontal (base in/out)
- Opposite direction = subtract
- Same direction = add

Question 23

How does unwanted decentration occur?

Answer 23

• If through poor lens fitting the eye is not looking through the optical centre of the lens, then there may be unwanted prism, causing strain on the eyes

Question 24

What are the symptoms of unwanted decentration?

Answer 24

• Tired eyes
• Headaches
• Eyestrain
• Double vision

Question 25

Accurate frame measurements are essential for the correct…

Answer 25

• Ordering the correct size lens
• Edging & fitting of the lens to the frame
• Positioning of optical centre of the lens along with the lens height (for bifocals & progressive lenses)

Question 26

What are the two different methods of taking frame measurements?

Answer 26

1. Datum system
2. Boxing system

Question 27

Briefly describe Datum system

Answer 27

• Older system
• Line midway between a tangent to upper edge of the bevel to a tangent to the lowest edge of bevel

Question 28

Briefly describe boxing system

Answer 28

• More recent
• Measurements for the box are taken from the inside bevel of the frame from the highest, lowest, most nasal, and most temporal parts of the frame

Question 29

Describe Geometric centre

Answer 29

located in the geometric centre of the box for each eye

Question 30

Describe Frame size

Answer 30

Is equal to the sum of the eyesize + DBL (bridge size)

Question 31

Describe Effective diameter (ED)

Answer 31

Equal to double the distance between the geometric centre of the lens shape and the furthest edge on the lens shape

Question 32

List advantages and disadvantages of plastic frames

Answer 32

Advantages:
- Allow for more colour, and a variety of styles
- Thicker rims can hide thick edges of lenses

Disadvantages:
- Harder to adjust than metal frames (require heating)
- Less durable
- Shorter lifespan
- Thicker rim can restrict peripheral vision

Question 33

What are the four main types of metal frames?

Answer 33

1. Gold-filled
2. Gold-plated
3. Nickel Silvers
4. Titanium