Some aspects of the scalar coherence theory of radiation employing two-point correlation functions are considered. In particular, it is shown that “real” waveforms in polychromatic fields need not appear similar to be considered coherent under the present definition. The decomposition of the mutual coherence function for such fields is discussed. Under certain assumptions a general form for coherent fields is derived and these fields are shown to satisfy the equations of geometric optics. For homogeneous media the surfaces of constant phase are the so-called cyclides of Dupin. The significance of these results in relating coherent sources and fields is mentioned as is the decomposition of incoherent sources.