The College of Optometrists

I know this sounds like a silly question!, but please can someone tell me how to split a prism?

If a px requires 5BOut prism for the right eye, why can we not give it to them in full?

What is the best way to split this prism?

Thank you in advance!

'cos the lens would be thick and not accepted by px. prism splitting is to equalise thickness. Also - you can't get more than 3 prisms in any lens blank unless the Rx is very high.

to split a prism - chop it in half and be careful with the bases to add up. ie 2.5 out EE.

However, on vertical prisms - it is cosmetically better to put more base up than base down - the thick edge is then hidden by the brow bar. IE: 3 dn lE = 2 up RE 1DN LE.

hope it helps - palf

don't think it matters about amount of prism required as a semifinished lens is maybe 8 or 10 mm thick so low rx can have a lot of prism (depending on required blank size). Contrarily, prism may be restricted on a higher power due to inadequate semi thickness. (it is a long time since i did any surfacing - but i think that's right).

otherwise split for cosmesis.

Some manufactures will charge for prism over 3PD in a lens so it is good business sense to split prism as well.