Up until this point, we have assumed that the fields we were dealing with were purely monochromatic. IN other words, they were ideal complex exponentials with time dependence ejωt, and they have zero bandwidth. This assumption is not realistic, as all physical optical fields will have some finite bandwidth that is related to randomness in the physical processes that generate the radiation. There are two classes of coherence that we are concerned with in general. The first class is temporal coherence and the second is spatial coherence. Temporal coherence describes the ability of light to interfere with a delayed version of itself. Spatial coherence describes the ability of a beam of light to interfere with a shifted version of itself. To test the former, we have to make two copies of the field, delay one, and recombine them. This is referred to as amplitude splitting. To test the latter, we sample the wavefront at two different locations, bring those fields into conjunction, and allow them to interfere. The second strategy is referred to wavefront splitting interferometry. These concepts are depicted schematically in Fig. 1.