We propose a study of the second-order coherence properties of radiation, such as inelastically scattered laser light, by performing an excess photon coincidence rate experiment a la Hanbury Brown-Twiss. The calculation evaluates the second-order correlation function of the electromagnetic field for an arbitrary number of coherent modes overlapped in phase space by a chaotic radiation field. We discuss the application of the general result to a specific case, namely the case of a single mode laser, and the Stokes radiation produced in a Brillouin-active medium, and a suitable part of the spectrum produced by causing dielectric breakdown in air. © 1966, Springer-Verlag. All rights reserved.