Week 4: Prisms & Lens Fitting Flashcards
Define/describe prism
- 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
What is the direction of base in?
base directed towards midline of face
What is the direction of base out?
base directed away from the midline of the face
What is the direction of base up?
base directed superiorly
What is the direction of base down?
base directed inferiorly
Explain resolving prism
- Process of expressing a single oblique prism as two perpendicular components
What happens if minus lens decentred out?
Pupil is looking through base in prism
What happens if plus lens decentred out?
Pupil is looking through base out prism
What if minus lens decentred in
Pupil is looking through base out prism
What if plus lens decentred in?
Pupil is looking through base in prism
How do we obtain stereopsis?
We need both of our eyes to be directed towards the object we are looking at
Why do we use prism?
- 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
Describe phoria
- 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
Describe tropia
- 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
What are the two vertical deviation?
- Vertical deviation = hyper/hypo (phoria/tropia)
- Horizontal deviation = exo/eso (phoria/tropia)
Explain lens neutralisation
- 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
Explain lens decentration
- 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
What does Prentice’s Rule find?
- Knowing the amount of decentration along with the power of the lens, we can then calculate the induced prism
List out all of F by approximation
Refer to table on pg 2
Explain differential prismatic effects
- 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
When to Correct Vertical Differential Prism?
- Consider for: vertical meridian difference > 1.50 D difference
- Must for: vertical meridian difference > 2-3 D difference
When do you add or subtract with vertical & horizontal prisms?
• Vertical (base up/down)
- Opposite direction = add
- Same direction = subtract
• Horizontal (base in/out)
- Opposite direction = subtract
- Same direction = add
How does unwanted decentration occur?
- 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
What are the symptoms of unwanted decentration?
- Tired eyes
- Headaches
- Eyestrain
- Double vision
Accurate frame measurements are essential for the correct…
- 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)
What are the two different methods of taking frame measurements?
- Datum system
- Boxing system
Briefly describe Datum system
- Older system
- Line midway between a tangent to upper edge of the bevel to a tangent to the lowest edge of bevel
Briefly describe boxing system
- 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
Describe Geometric centre
located in the geometric centre of the box for each eye
Describe Frame size
Is equal to the sum of the eyesize + DBL (bridge size)
Describe Effective diameter (ED)
Equal to double the distance between the geometric centre of the lens shape and the furthest edge on the lens shape
List advantages and disadvantages of plastic frames
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
What are the four main types of metal frames?
- Gold-filled
- Gold-plated
- Nickel Silvers
- Titanium
