Ophthalmic prisms. Measurement errors and how to minimize them.

Variable results of strabismus surgery may be due in part to errors in prism measurement. The amount of deviation neutralized by an ophthalmic prism is variable depending on how the prism is held. For example, a 40 delta glass prism with the posterior face held in the frontal plane gives only 32 delta of effect. Glass prisms are calibrated for use in the Prentice position. Plastic prisms are calibrated for use in the frontal plane position. Surprisingly large errors in prism measurement are produced when adding a small prism to a large prism. For example, adding a 5 delta glass prism to a 40 delta glass prism gives not 45 delta of effect, but 59 delta. This error can be minimized but not eliminated by holding one prism in front of each eye. The error can also be calculated so that the appropriate correction can be made.