8 - Prisms

Summary

Introduction

In many visual optical systems, prisms play an important role in the formation of the final image. They are used to control or change the direction of the image forming beam and are often used to change the orientation of the image. In these operations, the prism may only refract the beam, may refract and reflect it or may only reflect it but one or more times. Therefore in discussing the structure and properties of prisms, it is convenient to classify prisms according to whether they (a) are purely refracting, (b) combine refraction with internal reflections or (c) are purely reflecting.

The optical construction of most prisms is relatively simple but the ray paths inside a prism, which reflect the beam one or more times, can be complex and some of the properties of the prism are not readily obvious from looking at the ray paths. However these properties are sometimes made clearer by examining the unfolded version. Unfolding a prism involves reflecting each reflecting surface in the next reflecting surface along the ray path. Ideally, this process will become clearer when examples are discussed later in this chapter.

Refracting-only prisms

Refracting prisms occur frequently in visual optical and ophthalmic instruments. They can be used to deviate beams or images, as aids to focussing to form double or split images and often to compensate for eye problems such as phorias and squint.

Just as lenses may be examined using finite (or real) ray tracing on one hand or paraxial theory on the other, so can refracting prisms, but before we examine the paraxial properties of prisms, we will examine their effects on finite rays.