Prism thinning

**nixg** · 07-12-2006, 10:10 AM

Can any body tell me formula for prism thinning in progressive
or readymade calculator on net for what prism value should apply while surfacing
Please help

Nixg

**HarryChiling** · 07-12-2006, 11:22 AM

A rough estimate is 2/3 the add power, but if you are looking for more exact you will need to determine the power at the 90 degree meridian for the distance (top) and the reading area (bottom). Then determine the thickness at the top and the bottom of the frame (vertical decentration should be figured out for the frame b measurement to use for thickness) Now the difference in the thickness (top from bottom) is the amount of prism you would use to compensate for the thickness difference. I will see if I could make you a calculator, or write out the equation step by step, later tonight.

**Jim Stone** · 07-15-2006, 07:24 PM

Originally Posted by nixg

Can any body tell me formula for prism thinning in progressive
or readymade calculator on net for what prism value should apply while surfacing
Please help

Nixg

What are you using for surface calculations? ANY program on the market today should do that for you. If yours don't, you need another.