Base problem on Free Form PAL`s.

**ADD** · 03-19-2007, 01:21 PM

One independent Lab told me that they couldn´t make a FF PAL on CR 39 following a RX -4.50 add 2.00 on a 5.25 base. The frame I was using had a sporty look and 5.25 is the demo lens base. The Lab told me that they could do it with a 3 base or if not they suggested to change it to polycarbonate.
Does anyone know about the equation between base an minus powers on free form pal`s.
Thanks.

**Jacqui** · 03-19-2007, 02:16 PM

The problem may be polishing. Free from tech. uses a soft lap and it may not conform to that high of a curve.

**ADD** · 03-19-2007, 02:41 PM

I see... But why they do can with Polycarbonate? Any idea? Has your Lab Free Form tech.? Can you input this data to see what you get? Thanks a lot.

**Jacqui** · 03-19-2007, 03:18 PM

We don't have free form ...yet, but I'm trying to get it. I don't know why they think they can do it in poly, that makes no sense to me.

**gemstone** · 03-19-2007, 03:38 PM

Originally Posted by Jacqui

The problem may be polishing. Free from tech. uses a soft lap and it may not conform to that high of a curve.

Wow! Man I am totally disillusioned. That be somewhere around a 10.50 curve. I had no idea those things were that limited.

**shanbaum** · 03-19-2007, 10:51 PM

Maybe they think they could make it in poly because the back curves would be flatter (all other things being equal)?

**AWTECH** · 03-19-2007, 11:55 PM

ADD said:

Does anyone know about the equation between base an minus powers on free form pal`s.
Thanks.

Poly is much harder to process with freeform equipment than CR-39. The steeper the curve usually makes processing more difficult.

ICE-TECH produces specialized freeform lenses using software designs that with different base curves possible. Some other freeform designs automaticly select the front base curve depending on the prescription and add power. If the lab you are talking to has freeform software limitations they may not be able to process the job using CR-39 due to lens blank thickness or software calculation ability.

**bt5050** · 03-22-2007, 01:33 PM

Is the BC of the demo lens the only reason why you are requesting to only use that BC ??

With a 10-50 back curve - i would be more concerned with how they are going to look - and also how that steep of a back curve will effect the optics -