Correlations between intensity fluctuations in stochastic electromagnetic beams of any state of coherence and polarization

A formula is derived for the correlation between intensity fluctuations at two points in the cross-section of a wide class of stochastic electromagnetic beams of any state of coherence and polarization. The formula is of particular interest in connection with the Hanbury Brown-Twiss effect. The analysis leads to the introduction of a generalization of the degree of polarization which, unlike the usual degree of polarization, depends on position of two field points and reduces to the usual degree of polarization when the two points coincide.