Coherence is theorized as the normalized correlation between two superposed optical signals, usually replicated by a two-beam interferometer or by a double-slit system. Prevailing coherence theory is presented as field–field correlation, as it does not go into the technical details of how the measurable transformations are physically generated and registered. Teaching coherence theory using a semiclassical model has been very successful as is evidenced in literature from the extensive reference to the text by M. Born and E. Wolf. This has been further strengthened by the work of Sudarshan, who demonstrated the optical equivalence between Wolf’s classical theory and Glauber’s quantum coherence theories. The discussion about the classical concept of coherence remains incomplete without presenting the van Cittert–Zernike theorem on spatial coherence, because the concept of coherence evolved with historic attempts to understand spatial coherence.