9 - Prism in Multifocals - Exam 2 Flashcards
(24 cards)
Does image jump depend on the distance power or the distance OC location?
No
Only depends on near OC and add power
What is the equation for image jump?
(Seg power)(distance from seg top to seg OC) = image jump
What is another phrase for object displacement?
Total prismatic effect
What 4 pieces of information must be given to compute object displacement?
- Distance OC in relation to segment
- Seg drop
- Seg OC
- Location of NRP
What is the relationship between seg inset and distance Rx?
More minus distance Rx - smaller seg inset
More plus distance Rx - larger seg inset
What is anisometropia?
Significant difference in power between eyes
What is anisophoria?
Unequal prismatic effects resulting from looking away from the optical centers of anisometropic lenses
Patients who have convergence problem need what kind of prism at near?
Base in prism
What are 3 conditions that would indicate a patient needs near prism?
- Too high or too low of AC/A ratio
- Periodic strabismus at near
- Nonconcomitant strabismus
What are 4 ways to obtain horizontal prism at near?
- Two separate pairs of glasses (one for near, one for far)
- Stick-on Fresnel prism
- Prism segment bifocals (very noticeable)
- Increase the seg inset
Where is unequal prism more troublesome, in the horizontal or vertical direction?
Vertical direction
What is VPI?
Vertical Prism Imbalance
Difference in vertical prism between the two eyes
Who will most likely experience VPI?
An anisometropic presbyope
How much of a difference in Rx is generally tolerated in terms of VPI?
Up to 1.50 D of anisometropia is tolerated
What is the only thing that VPI depends on?
The distance Rx
What are 7 ways to correct VPI?
- Contact lenses
- Two separate pairs of glasses
- Drop the distance optical center
- Raise the seg height
- Fresnel prism
- Dissimilar segs
- Slab off lens
How far below the geometrical center should the optical center be in a near vision pair of glasses?
5 mm lower
What type of prism is removed in a slab off lens?
Base down prism removed
Which lens does slab off occur in?
The most minus (or least plus) lens with respect to the vertical meridian
What is the name of the process for slab off lenses?
Bicentric grinding
Where is the slab taken off for plastic lenses?
The back surface
Where is the slab taken off for glass lenses?
The front surface
What is reverse slab off?
Base down ADDED to most plus (or least minus) lens
What is the equation for calculating the vertical distance between seg centers for dissimilar segs?
Z = (c) x (ADD)
