The g(2) correlation function of a light beam depends on its photon statistcs. In quantum optics, this effect can be used to determine the effective mode number of a type II parametric downconversion source.
However, the outcome is very sensitive against background light from other sources. We consider the experimental challenges of this approach and introduce a simple model to account for influence of the background. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)