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The van Cittert-Zernike Theorem
Our goal is to present a simple derivation of the van Cittert-Zernike theorem without invoking the theories of probability and stochastic processes. This is possible because most sources of practical interest are ergodic, meaning that time-averaging over a typical waveform emanated by the source yields statistical information about the source's inherently random radiation processes. We shall make exclusive use of time-averaging to derive the degree of coherence of a pair of points within the field of an extended, quasi-monochromatic, incoherent source.
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