A rotary prism (Risley prism), mounted on the front of most refractors, consists of 2 prisms of equal power that are counter-rotated with respect to one another to produce prismatic powers, varying from zero (prisms neutralizing each other) to the sum of the 2 powers (prisms aligned in the same direction). The Risley prism is particularly useful for measuring phorias (often in conjunction with the Maddox rod) and fusional vergence amplitudes.

Prism Aberrations

In addition to chromatic aberration (which produces colored fringes at the edges of objects viewed through the prism), prisms have other aberrations, such as asymmetric magnification and curvature of field. These aberrations can occasionally produce symptoms even with low-power ophthalmic prisms.

Fresnel Prisms

To avoid the weight and some of the aberrations of conventional prisms, clinicians may use Fresnel (pronounced fre·nell′) prisms, which are composed of plastic sheets of side-by-side long, narrow, thin prisms (Fig 1-64).

Figure 1-64 Comparison of a Fresnel prism with a conventional prism. (Redrawn from Duane TD, ed. Clinical Ophthalmology. Vol

1. Hagerstown, MD: Harper & Row; 1976:chap 52, fig 52-2.)

One form of Fresnel prism, the Press-On membrane prism, is applied with water to the back surface of a spectacle lens. Visual acuity suffers somewhat from the chromatic aberration and the presence of many edges.