It seems that many opticians think prism is a place for optical criminals, and they don’t want to go there. Why is there panic when they see a prescription with prescribed prism? After all, every lens we work with is constructed of prisms. Plus lenses with their thicker centers are base to base prisms to converge light for a shorter focal length that brings an image focused behind the retina to the front. Minus lenses with thicker edges are apex to apex prisms to diverge light for a longer focal length that brings an image focused in front of the retina back. Opticians have no problem checking lenses for vertical imbalance and decentration, but do they understand they are checking for induced prism? Prescribed prism, like cylinder, is simply another part of the prescription and shouldn’t be cause for alarm.

Problems with prism aren’t new. In “Transactions of the American Ophthalmological Society Meeting” in 1900, Dr. Lucien Howe notes, “… there still remains a difference in methods of numbering, and that prisms supplied by opticians do differ from each other so decidedly as to have this difference of real clinical importance…” Whether the problem was one of prescribing methods, instrumentation, equipment or both, I cannot speculate. Opticians took the blame again from Joseph E. Willetts, MD, in “The Use of Prisms in Ophthalmic Practice,” The Pennsylvania Medical Journal, April 1917. Dr. Willetts lamented, “Some opticians seem to have opinions of their own regarding the use of prisms and omit the low degrees.” Again, the problem could have been with the technology of the time or perhaps with communication from the prescribers.

Prism is prescribed when the brain cannot fuse the separate images from each eye, resulting in double vision (diplopia), as occurs with muscle imbalance and eye alignment issues. It is also prescribed as an aid to peripheral visual field loss (scotoma), such as can occur with a stroke or brain injury. Alleviating those conditions goes a long way to improving the quality of life. Considering prism in that context may reduce anxiety about filling the prescription.

As opticians, we’re here for the tough stuff, not just +1.00 readers. We can’t let numbers on a prescription pad intimidate us. Complex prescriptions should inspire us to dig into our knowledge and experience to bring the best vision to our patients. 20/20 is here to help build your confidence by expanding your knowledge and understanding of our field. Check out Part 1 of our three-part CE, “The Spectrum of Prism Optics” at, and step up to the challenge!

Linda Conlin
Pro to Pro Managing Editor
[email protected]