Tool Curve Formula for thick lenses

**LarryLane** · 10-22-2008, 08:48 PM

Hello, I am trying to find the formula to determine what tools to use for plus lens depending on the index of refraction. I know the thin lens formula but I do not know how to compensate for thickness.

**lensgrinder** · 10-22-2008, 09:50 PM

First use the back vertex formula to find the needed back curve.
+5.00 on a 7.00 base in a 1.6 index with a thickness of 4.0 mm

$Back_v = \frac{D1}{1-(\frac{t}{n})D_1} + D_2\\ 5.00 = \frac{7.00}{1-(\frac{0.004}{1.6})7.00} + D_2\\ 5.00 = 7.12 + D_2\\ D_2 = -2.12 $
Now find the lap needed
$\frac{D_{marked}}{D_{refractive}} = \frac{0.53}{1.6-1}\\ D_{marked} = \frac{-2.12\times0.53}{0.6}\\ D_{marked} = 1.87$

**LarryLane** · 10-23-2008, 08:43 AM

I used the following formula to determine D1 used in your formula.

Basecurve marked on box = 4.51 poly

.586/.530 X 4.51 = 4.98(I then used this number for D1 stated in your forumula along with the other parameters needed). My guess is that the values marked on the box is not the real front curve am I correct?

Thanks for your help

**shanbaum** · 10-23-2008, 08:49 AM

Originally Posted by LarryLane

I used the following formula to determine D1 used in your formula.

Basecurve marked on box = 4.51 poly

.586/.530 X 4.51 = 4.98(I then used this number for D1 stated in your forumula along with the other parameters needed). My guess is that the values marked on the box is not the real front curve am I correct?

Thanks for your help

The convention in the U.S. is to express front curves in a 1.530 ("shop") index, as opposed to the actual surface power, which would be expressed in the index of the material.