Intensity Fluctuations of Partially Polarized Light

The ensemble distribution is derived for the number of photons n counted in a short time interval by a photodetector which is illuminated by a partially polarized beam of thermal light. It is shown to be of the form expected for n bosons distributed between 2/(1 + P) cells of phase space, where P is the degree of polarization. This number also has a simple classical analogue as the statistical degrees of freedom of light waves with a Gaussian amplitude distribution, and it is completely determined by the ratio of the eigenvalues of the usual 2 × 2 coherence matrix. It is suggested that no comparable simple conclusions will normally hold for non-thermal light, as, for example, from an optical maser.