  We experimentally investigate a method of directly characterizing the photon
number distribution of nonclassical light beams that is tolerant to losses and
makes use only of standard binary detectors. This is achieved in a single
measurement by calibrating the detector using some small amount of prior
information about the source. We demonstrate the technique on a freely
propagating heralded two-photon number state created by conditional detection
of a two-mode squeezed state generated by a parametric downconverter.
