Experts needed

[Mick Luvin]

do bifocal segments have base up or base down prism and explain why

[obxeyeguy]

do bifocal segments have base up or base down prism and explain why

Way too broad of a question, No possible correct answer.

[drk]

Do your own homework, Mickey. ---the Teacher

[MakeOptics]

do bifocal segments have base up or base down prism and explain why

FTs and round exhibit base down. The real answer is much more complicated that that. Segmented bifocals have a optical center so to speak in the segment, for a FT if you subtract 1/2 the width from the height you get the measure from the segment top to that OC (more accurately the near reference point or NRP) round segs have the NRP at the center of the segment. Since all segments are plus powers when the eye rotates down towards the segment initially you experience prism base down when entering the segment, once your eye rotates below the NRP the prism reverses to base up again towards the center of the seg. To complicate things further the segments prism contribution should be summed with the lenses power as well to come up with the total prism present at any point being measured. If you are checking for imbalance and you restrict your lens design parameters to the same size segment and same vertical placement of the segment height you can ignore the segments prism contribution altogether and treat the lenses like a SV blank when calving off axis prism.

[Darryl Meister]

MakeOptics has done a nice job describing the technical nuances involved. The bifocal segment acts like a small plus lens affixed to the distance lens so, above the segment center, this prismatic effect is base down while, below the segment center, the prismatic effect is base up. The segment center is frequently, although not always, located at the widest point of the segment.

Best regards,
Darryl

[Robert Martellaro]

Strongly related to the above.

http://www.visioncareproducts.com/VC...2014-4447.aspx

[tmorse]

FTs and round exhibit base down.

This statement can be very misleading.
Whether you exhibit base down (BD^) on FT and round top depends entirely on the reading level... the amount of downward gaze to read form straight ahead distance. You can begin to read once you enter the segment, in this case say 4+ mm below distance optical center at just below the segment line. Since at the seg line you would also be located 4-5mm above the plus seg optical center, a base down (BD^) effect at that point would combine with the distance prism produced 4 mm below distance optical center. However, no one is required to read at this particular
reading level. You can choose lower your reading level all the way to the bottom of the seg is you do desire adn still be able to read.
If you lowered your gaze for reading to 10mm from straight ahead, your FT seg at that point would start to produce a small base up effect contribution to the net combined prismatic effect of distance prism and seg prism. Thus a FT does not always produce a base down effect...
it all depends on your reading level.
Draw your own side view picture. Your 'net combined prismatic effect' for the bifocal lens will change with each change in reading level.

[MakeOptics]

This statement can be very misleading.
Whether you exhibit base down (BD^) on FT and round top depends entirely on the reading level... the amount of downward gaze to read form straight ahead distance. You can begin to read once you enter the segment, in this case say 4+ mm below distance optical center at just below the segment line. Since at the seg line you would also be located 4-5mm above the plus seg optical center, a base down (BD^) effect at that point would combine with the distance prism produced 4 mm below distance optical center. However, no one is required to read at this particular
reading level. You can choose lower your reading level all the way to the bottom of the seg is you do desire adn still be able to read.
If you lowered your gaze for reading to 10mm from straight ahead, your FT seg at that point would start to produce a small base up effect contribution to the net combined prismatic effect of distance prism and seg prism. Thus a FT does not always produce a base down effect...
it all depends on your reading level.
Draw your own side view picture. Your 'net combined prismatic effect' for the bifocal lens will change with each change in reading level.

Just one question, what happens when you read the next sentence in a post? ; )

[tmorse]

Merely makes me wonder why you made such an absolute statement in the first sentence.
We are obviously dealing with someone who has no grounding in optics, and such a seemingly absolute statement can be misleading.

[standarduck]

Just one question, what happens when you read the next sentence in a post? ; )

Reading the whole post? What do you want from us? Blood?

[MakeOptics]

Reading the whole post? What do you want from us? Blood?

I'd be happy with a side of "hold the crap".