* Graphic OC, GC, PRP Tutorial *

**brucekrymow** · 02-09-2007, 07:18 PM

Is anyone aware of graphic representations explaining decentering & blocking on OC, GC, PRP w/ the differences & why different methods deployed under different process lines?

**lensgrinder** · 02-09-2007, 09:38 PM

Attached you find a plus lens. The GC is the center of the lens, the OC is where no prism is present in the lens. The PRP is where you will reference prism.

For finishing labs you will block on the OC if no prescribed prism is in the lens. If prescribed prism is in the lens you will block on the PRP.

For sufacing you generally block on GC and grind prism in the lens to "move" the OC to the correct location.

**HarryChiling** · 02-10-2007, 02:49 PM

The advantage of blocking on GC rather than OC is that the lens is less prone to flexing while being generated, fined and polished.

**gemstone** · 02-10-2007, 03:03 PM

We started grinding lenses on geo center when started needing 65 mm lenses, then huge, for the then huge frames. . If you tried to grind a 65 mm flat top on optical, the surfacing process created all kinds of unwanted prism due to lens overhang. Semi tech even invented a blocker where you could generate on oc ans surface on geo center. I talked to a lab last week that was still using one.