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.