It is pointed out that photoelectric counting correlations in an electromagnetic field that are associated with induced and spontaneous emission at the source, and the so-called Hanbury Brown-Twiss effect, are indistinguishable effects. It is shown by examples that interference effects involving the stimulated emission may be regarded as a source of the counting correlations. The results reported in a recent experiment of Scarl and Smith are therefore readily understandable.

• Received 7 April 1976

DOI:https://doi.org/10.1103/PhysRevA.14.2351