Two-photon correlations are discussed within the formalism of Hanbury--Brown and Twiss interferometry and Bose--Einstein correlations. The technique is presented as a universal tool to study the properties of any boson source --- light sources such as stars, or photon and meson sources in the early phase of heavy-ion collisions. The method is illustrated using experimental data on photon-pair production in nuclear reactions, available only in the intermediate (several tens of MeV/N) energy domain. The observed interference signal is interpreted in terms of source size and reaction dynamics.

PACS numbers: 25.70.--z, 24.10.--i, 95.90.+v