Consider the propagation of light in a homogeneous medium as sketched in Figure 4.2. For the interference pattern to be stable in time, we require and to be independent of time. These conditions are called spatial and temporal coherence respectively. Recall that conventional light sources such as a tungsten filament emit sporadically and the phase is a random variable. Phase quality is a special feature of lasers. A stable interference pattern in which the fringes are ordered is a pre-requisite in all interferometry measurements. Since fringes improve signal-to-noise ratio in LDV measurements, coherence is an essential requirement here as well.

Figure 4.2: Definition of Temporal (between points 3 and 4) and Spatial Coherence (between points 1 and 2).

However, no light source is perfectly coherent and the quality of a light source is judged by its coherent length CL, i.e., the maximum distance between points 1 and 2 in Figure 4.2, over which is time independent. If is the time duration over which the source emits continuously without interruption then where is the speed of light Helium-Neon lasers have a coherence length of 100-200 mm and are considered most suitable for interferometry.