  We use the covariant current formalism to derive the general form of the
2-photon correlation function for fully chaotic sources. Motivated by the
recent discussion in the literature we concentrate on the effects from the
photon spin on the correlator. We show that for locally thermalized expanding
sources, like those expected to be created in relativistic heavy ion
collisions, the only change relative to 2-pion interferometry is a statistical
factor 1/2 for the overall strength of the correlation which results from the
experimental averaging over the photon spin.
