concentric surfaces...

[Pete Hanlin]

I downloaded and read the article on Prentice's Rule and its applications to plano and low powered lenses... interesting stuff.

It has me wondering, though... if adjustments need to be made to the ocular surface of a plano lens based on the thickness of the lens (to avoid a lens with a small amount of plus power), shouldn't an adjustment be made on Rx lenses as well?

I imagine, however, that the impact is probably miniscule on lenses with any substantial power.

Pete

[harry a saake]

Pete, i would imagine that would be true, but only from a purist standpoint. If you really think about it, very few people, especially in the high rx,s, wind up getting the exact effective power they need, because of vertex depth being different from refaction to fitting, glasses moving down the nose during the course of wearing them and other factors. Also taking into consideration, the human eye,s ability to accomodate for these small differences.

[shanbaum]

Originally posted by Pete Hanlin:

It has me wondering, though... if adjustments need to be made to the ocular surface of a plano lens based on the thickness of the lens (to avoid a lens with a small amount of plus power), shouldn't an adjustment be made on Rx lenses as well?

It should, and is. That's what makes the thick lens formula, thick; in it, the power of the front surface at the back surface is determined.

[Darryl Meister]

Mr. Shanbaum is entirely correct. The point of the article is the fact that lenses with plano power actually have different front and back surfaces (i.e., not a +6.00 and -6.00). And, since the back surfaces are not entirely concentric (even for plano lenses), there is a difference in thickness away from the center of the lens. This difference in thickness, albeit small, induces prism during certain measurements.

Best regards,
Darryl

[JRS]

A fairly easy was to demonstrate this is to take (or fabricate) and industrial thick PLANO. Off-center it about 10mm's or so in a lensometer and look at the prism created. A humphrey LA will work too.
There once was a computer system that would advise the lab to grind prism, in a plano, if they entered a PD that would off-set the OC from frame center. If no PD was entered it behaved as the labs guys expected....

(sorry for the edit - had a few mis-keys)



[This message has been edited by JRS (edited 07-28-2000).]

[chip anderson]

Many years ago when I was working on the line in contact lens lab. We had to calculate powers by changing the index of the base at 1.3375 index adding or subtracting the power desired converting to 1.49 index (PMMA lenses) and then adding 1/3 of the thickness in mm to the radius in mm of the front surface. I was told the calculation book we used was "an detention of a Shuron generator book".

Which all boils down to some compensation must be made to the front curve to allow for the detention in radius x pi from the posterior surface. This is much more critical in surfaces of short radii.

Chip (aberrated) Anderson