Introduction

So far, all the visual optical instruments that have been discussed have been essentially describable in terms of geometrical or ray optics. In this and the following chapter, we will describe some important visual optical instruments and devices based upon physical or wave optics. The essential difference between geometrical and physical optics is the difference between the geometrical description of the propagation of light by rays and the physical optical description using waves. The wave nature of light gives rise to two very important phenomena, interference and diffraction. This chapter is concerned with interference and the next will look at diffraction.

The phenomenon of interference allows us to produce visual stimuli in which the light level varies sinusoidally across the pattern. Since early work by Campbell and Green (1965), the visual system is now regularly analysed in terms of its response to sinusoidally varying luminance profiles. This is often done by measuring the threshold contrast of sinusoidal patterns at a range of spatial frequencies. The resulting function is known as the contrast sensitivity function (see Chapter 35). Sinusoidal patterns can be produced in several ways. They may be produced in printed form, for example as photographs, or can be produced on television screens or by optical interference techniques. This chapter is concerned with the last mode of production.

There are two fundamental differences between the production of sinusoidal patterns by interference techniques and those in printed or television screen forms.