20 - Image Jump and Horizontal Prism at Near Flashcards
What causes Image Jump? 2
Crossing the border of the segment, prism amount suddenly changes.
Abrupt change in prismatic effect causes objects to suddenly be displaced.
Practice finding image jump
Where is the OC in a round seg?
The middle = half the segment measurement
22mm Rseg = OC is 11mm
In a flat top segment how do you find the distance from the OC?
Seg depth - (Seg width/2) = distance from OC
What 3 things determine the amount of accommodation needed to see at near?
-Near-viewing distance
-Power of the distance spec lens rx
-Distance from the lens to the principal planes of the eye.
Who needs progressives first and why?
Hyperope as they have to accommdate almost an entire dioptre more at a 40cm distance compared to a myope
What happens when add power increases?
The amount of accommodation needed between the hyperope and myope decreases.
What do the Values in this Formula stand for?
Ld=1/(1/Fv’)-d (not on quiz)
Ld = the vergance of light at the corneal plane from infinity
FV’=the power of the lens
d=distance in meters from the reference position of the lens to the new reference position
Why would an Optician change the add power? 3
Depending on the distance they need to see.
Closer = stronger add
Further away = weaker add
Why do nonpresbyopes need a different cyl correction at near? 2
Large difference in powers = accommodate at different points
solves as px becomes presbyopic
How much do these +7.00D/-7.00D/0.00D rx accommodate?
(Based on spec plane of 40.14cm)
Myope - +2.00D
Hyperope- +2.96D
Emmetrope- +2.42D
How can you correct Horizontal prism at near? 5
1-2 pairs of glasses
2-Fresnell press on
3-Decentering Larger segment top lens
4-Split Lens
5-Cement segment construction
Why would near prism be needed?
-If prism is needed for distance, different amount for near
-High Accommodative convergance
-Intermittent near tropias
-Non committal strabismus
What are the 2 restrictions when decentering the segment for horizontal prism?
Too much decentration = lens size wont allow it
Segment too small = edge will intefer with distance vision
If the segment is moved nasalward/temporally when decentering for horizontal prism, what is the base directions?
Nasally - Base In
Temporally - Base Out
What is the seg inset formula?
Seg Inset= distance PD-NearPD/2
What is the formula for finding seg size needed when correcting horizontal prism?
Seg size = 2(10+additional seg inset)
What are the steps to verifying progressive lenses?
What 2 things should we check when dispensing progressives?
-fitting cross should be in front of pupil centre
-horizontal dashes should be exactly horizontal and not tilted
What are verifying measurements/markings in a progressive? 5
DRP
Fitting Cross
Horizontal Dashes
PRP
NRP
What is the DRP? 2
Distance Reference Point
At the center of the arc
Where does the fitting cross sit on the patient?
Ideally centered in the pupil
What do the two horizontal dashes do?
Tell us if the lens is tilted or level
What is the PRP? 4
-Prism reference point
-also known as major reference point (MRP)
-Centrally located
-used to verify prism power
What is NRP? 3
-Near Reference Point
-circle in lower part of the lens
-used to verify near power
