  We theoretically demonstrate that detectors endowed with internal gain and
operated in regimes in which they do not necessarily behave as photon-counters,
but still ensure linear input/output responses, can allow a self-consistent
characterization of the statistics of the number of detected photons without
need of knowing their gain. We present experiments performed with a
photo-emissive hybrid detector on a number of classical fields endowed with
non-trivial statistics and show that the method works for both microscopic and
mesoscopic photon numbers. The obtained detected-photon probability
distributions agree with those expected for the photon numbers, which are also
reconstructed by an independent method.
