The complex spectral degree of coherence of a general random, statistically stationary electromagnetic field is introduced in a manner similar to the way it is defined for a beamlike field, namely, by means of Young’s interference experiment. Both its modulus and its phase are measurable. We illustrate the definition by applying it to blackbody radiation emerging from a cavity. The results are of particular interest for near-field optics.
Korotkova, Olga and Wolf, Emil, "Spectral Degree of Coherence of a Random Three-Dimensional Electromagnetic Field" (2004). Physics Articles and Papers. 30.