  Rapid progress in electromagnetic black hole observation presents a
theoretical challenge: how can the universal signatures of extreme
gravitational lensing be distilled from stochastic astrophysical signals? With
this motivation, the two-point correlation function of specific intensity
fluctuations across image positions, times, and frequencies is here considered.
The contribution of strongly deflected light rays, those which make up the
photon ring, is analytically computed for a Kerr black hole illuminated by a
simple geometric-statistical emission model. We subsequently integrate over the
image to yield a spectro-temporal correlation function which is relevant for
unresolved sources. Finally, some observational aspects are discussed and a
preliminary assessment of detectability with current and upcoming missions is
provided.
