Erroneous findings in polarized testing caused by plastic prisms

Certain plastic prisms, when used with polarized tests (eg, stereopsis), can destroy the uniqueness of each eye's view. We investigated this effect by administering the Randot stereotest to 12 visually normal observers. The test was administered monocularly (simulating suppression), under habitual conditions, and through six representative prisms, both fused and monocularly. Whereas stereo thresholds with an anaglyphic random dot stereogram (TNO) were not significantly affected by the prism, fusing through some of the prisms resulted in significantly poorer stereo thresholds and a failure to detect random dot stereopsis (RDS) on the polarized stereotest. Furthermore, some subjects appreciated RDS viewing monocularly through each of the prisms. Thus, it is important to conduct polarized testing through prisms that are known not to interfere with polarization. This optical phenomenon is explained here.