Lowenstein-prismatic properties of bifocals Flashcards Preview

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Flashcards in Lowenstein-prismatic properties of bifocals Deck (12)
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1

what is the pole

optical center of a lens

2

what is the MRP

major reference point
-the intersection of the line of sight for dist viewing w/ the spectacle plane
-this is the point on the lens through which the wearer looks when looking far

3

is the MRP is OC of the lens?

not always the case

4

what is NVP

near visual point
-the intersection of the line of sight for near viewing w/ the spectacle plane
-point on the lens through which the wearer looks when reading w/ a bifocal

5

what is jump

the prismatic effect at the top of the seg in a bifocal, attributable to the seg alone
-dist the lens doesnt enter into jump
-always base down

6

what is displacement

the total prismatic effect in a bifocal at the near visual point
-vertical prismatic effect at the reading point usually taken as 8mm below the dist OC

7

what does jump depend on

seg power and the dist from the seg top of the OC

8

what is the distances to the seg OC for:
round seg
flat top
executive
ultex

round: 1/2 seg diameter
flat top: 5 mm
executive: 0
ultex A: 19mm

9

why is displacement more important in bifocals than in SV lenses?

wearers of SV often lower head to read w/ the result that they are still looking through the lens at or near the OC so little prism occurs
-bifoal = head has to be held up and the eyes depressed making a prismatic effect from the dist portion of the lens as well as the seg

10

why is there a prismatic effect from the seg OC

seg OC is below the top of the seg
-prismatic effect = jump and displacement

11

what is the effect of jump

sudden appearance of a BD prism when the eye crosses the seg line produces an image jump up
-annoying and can lead to vert dipolopia

12

is jump always the same for both eyes?

yes