Prism - Solving for imbalances Flashcards
What is the general rule for soling for vertical imbalance?
You are trying to get both the eyes to see the same amount of prism so there is no imbalance.
Which lens do you choose to alter?
You always choose the lens with the stronger power at 90.
What is a trick for solving prism imbalances?
You are trying to match the prims amount, and you want to also match the prism base - to do this you move the OC to the opposite side of the eye by the same amount
How much vertical imbalance can people handle at the reading level?
1.5 ∆ at the READING level, but the goal is 0.5 ∆
What are 5 ways to solve Vertical imbalance at the reading level? Which one is the best?
- Avoid bifocals altogether
- Prism segs - grind prism into the reading segment
- Compensating “Ribbon” Segs
- Dissimlar segs- intentionally use different segs in each lens
- ** Bicentric grinding (Slab off) - Bottom half of the lens has prism ground into it
Why are Prism segs not ideal?
They are not great cosmetically. They stick out more than an executive
When do ribbon segs work for solving vertical imbalance at the reading level?
When the vertical imbalance is 1.5 ∆ or less
What is a bonus for dissimilar segs?
They are a good price conscious option
What are some benefits to using slab off?
- Good cosmetics
- Grind BD prism into plastic lenses (BU in glass)
What are the rules for Slab off? What lens material are these rules for?
- Give Slab off to the coldest lens - based on the power at 90
- Amount of prism ground in has to be rounded down to the nearest 0.50 D of total VI @ RL
- Always Base UP
SLAB OFF is for GLASS -
You use you normally use reverse slab off rules for plastic!
What are the rules for Reverse Slab off
- Give slab off to the warmest lens (based on power 90)
- Round prism DOWN to the nearest 0.50 D
- Always BASE DOWN
How do you calculate prism imbalance between lenses
If the images are going in the same direction, SUBTRACT prism values
If the images are going in the opposite directions, ADD prism values
What is the formula for calculating Vertical Imbalance at the reading level?
VI @ RL = (difference in power @ 90 x RL ) / 10
How do you select the right ribbon segs?
of ribbon seg = mm the optical centre is from the top edge of the segment.. you compare this number to how far the eye drops into the segment when reading
You want to choose the ribbon segs whose OC is difference is closest to the value of X
What is a short cut for determining if you nee slab off?
VI @ RL = (Difference in Power @ 90 x 9)/10
If this value is less that 1.5 then you do not need slab off
