= Intensity interferometry
<Intensity interferometry> infers source coherence from correlations between measured intensities, rather than directly combining electric-field <amplitudes>. For stationary chaotic Gaussian light, the normalized correlations satisfy $g^{(2)}(\tau)=1+|g^{(1)}(\tau)|^2$, under the relevant statistical assumptions.
Separated detectors can therefore measure the magnitude of <spatial coherence>, which is related to the source's angular structure. Optical carrier phase need not be stabilized as in <amplitude> interferometry, but fast timing and sufficient <photon statistics> are crucial. The squared magnitude does not by itself retain every phase needed for unrestricted image reconstruction.
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