Introduction

So far we have looked at optical systems that we could call “direct vision” systems; that is the optical system is pointed directly at the object and, apart from the effect of image erecting prisms, the optical axis joining the object and image is a straight line. In contrast, in a number of situations, there is a need to bend an image forming beam around a “corner” or several “corners”. Usually this also requires the beam to be restricted to lie within a tube of a certain size and be transmitted over a distance that is long relative to the diameter of the beam. If the beam width is to be so constrained, it is often necessary to form intermediate images along the beam path, which in turn requires the use of what are known as relay lenses or relay systems. The main use of relay systems is to view normally inaccessible or hazardous places or simply to transmit luminous flux (light) to places remote from the source.

The earliest type of relay system was the periscope, which has been developed to a high degree for use in submarines. Much smaller systems have been developed to examine the inside of bodily organs or machines. The most recently developed type of relay system is based upon optical fibres. Optical fibres are usually circular in cross-section, much longer than their diameter and usually made from glass or plastic. They operate on the principle that light entering one end is constrained in the fibre by successive total internal reflections at the internal walls.