Depressed Curve?

[Whitwoo]

Hi guys,

Calculate the radius of the depressed curve required to give a total add if +3.25D, if radii of r1 = +87.17mm.
r2= +348.67mm, are ground on a lens with n=1.523 and n=1.678.

What on earth is a depressed curve?
A little nudge please guys. Sorry to be sucking off your brains again, but I've got no one else~ :P

[Jacqui]

What on earth is a depressed curve?

Damn if I know :(

[chip anderson]

I suspect it's what would be on the back of a back surface progressive.

[HarryChiling]

Hi guys,

Calculate the radius of the depressed curve required to give a total add if +3.25D, if radii of r1 = +87.17mm.
r2= +348.67mm, are ground on a lens with n=1.523 and n=1.678.

What on earth is a depressed curve?
A little nudge please guys. Sorry to be sucking off your brains again, but I've got no one else~ :P

Sounds like an old fused glass bifocal, I thought the old heads would have guessed that one, what happens is you fuse a piece of higher index glass onto the front of a lens, that lens needs to have a depression in it that the higher index piece fits into. Ths piece must have specific curvature if it will meet the add of +3.25. There's your nudge let me know if you need pushing.;)

[tmorse]

Sounds like an old fused glass bifocal, I thought the old heads would have guessed that one, what happens is you fuse a piece of higher index glass onto the front of a lens, that lens needs to have a depression in it that the higher index piece fits into. Ths piece must have specific curvature if it will meet the add of +3.25. There's your nudge let me know if you need pushing.;)

They still sell/use fused glass bifocals in New Zealand??:D:D

[chip anderson]

Harry:


Don't know where you made fused glass bifocals but at Modern Optics this was called a countersink. The convex surface to be fused in it was called a "button." My first job in the optical business was polishing "buttons."


Chip

[Jacqui]

At AO it was called a "hollow" .

[HarryChiling]

Harry:


Don't know where you made fused glass bifocals but at Modern Optics this was called a countersink. The convex surface to be fused in it was called a "button." My first job in the optical business was polishing "buttons."


Chip

Never made them just thinking outside the box. I am sure they were called a million different things being that the question was posed by someone outside the country I thought it would be helpful.

[HarryChiling]

BTW, The answer to your question is in Mo Jalie's book Ophthalmic Lenses and Dispensing pages 228 and 229, get to reading cause I won't post the answer and my guess is that their will be few that know it.

[Fezz]

Are we talking about the Bummed Out Curve?

Or maybe, the Perpetual Downer Curve?

[Whitwoo]

Haha thanks Harry!
Looks like I need to invest in a text book.
Cheers guys.
LOL and no, we don't still use glass bifocals, but I'm studying and I suppose our examiners think we need to know.

[Darryl Meister]

Harry is correct; it is the rear curve of a fused bifocal segment. Chip is also correct though; this is typically referred to as a countersink curve.

The steps needed to calculate the countersink curve are somewhat involved, although you can simplify them to two basic calculations:

1. Determine the gain in power caused by the refractive index of the segment and the base (front) curve of the lens blank:



2. Determine the radius of the countersink curve by calculating the additional surface power needed at the back surface of the segment to obtain the desired addition power:



where n_SEG is the refractive index of the segment, n_LENS is the refractive index of the lens or major portion, F_BASE is the surface power of the base (front) curve of the lens blank, F_ADD is the specified addition power, and r_SEG is the radius of the countersink curve in meters.