Lowenstein-prismatic properties of bifocals

Question 1

what is the pole

Answer 1

optical center of a lens

Question 2

what is the MRP

Answer 2

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

Question 3

is the MRP is OC of the lens?

Answer 3

not always the case

Question 4

what is NVP

Answer 4

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

Question 5

what is jump

Answer 5

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

Question 6

what is displacement

Answer 6

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

Question 7

what does jump depend on

Answer 7

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

Question 8

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

Answer 8

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

Question 9

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

Answer 9

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

Question 10

why is there a prismatic effect from the seg OC

Answer 10

seg OC is below the top of the seg

-prismatic effect = jump and displacement

Question 11

what is the effect of jump

Answer 11

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

Question 12

is jump always the same for both eyes?

Answer 12

yes