Introduction

A wide variety of visual optical instruments are designed for binocular viewing, but probably the most well known are binoculars. Binocular viewing seems to offer a number of advantages over monocular viewing. Monocular viewing usually requires one eye to be closed, which may often lead to fatigue or discomfort, especially over extended viewing periods, although it may be possible for some people to keep both eyes open when viewing monocularly. This is possible if the image of the other eye is suppressed. In comparison, binocular viewing uses both eyes and hence is probably far less fatiguing. Binocular instruments also have the potential for stereoscopic viewing. Thus superficially, binocular viewing seems to be superior to monocular viewing. However a badly designed, badly manufactured or damaged binocular system can lead to significant problems in binocular viewing. Therefore we should be aware of the need for design and constructional tolerances for binocular instruments.

Stereoscopic and non-stereoscopic constructions

Not all binocular systems provide a stereoscopic image. The production of stereoscopic images requires that the two optical axes at the eyepieces be separated in object space. Let us look at several constructions and see how this requirement may be achieved.

Non-stereoscopic systems

Figure 37.1 shows a possible schematic construction of a binocular instrument, in which the two optical axes are identical in object space and hence the eyes will not see a stereoscopic image. In practical systems, the beam-splitting and deviations are often done with prisms rather than with mirrors as depicted in the diagram.