  We test ideas put forward e.g in arXiv:1508.02368, which suggest that using
rates in quantum optics can lead to better indicators of non-classicality for
states of quantum optical fields with undefined photon numbers. By rate we mean
the ratio of registered photons in a given detector to the total number of
detected photons in all detectors in the experiment. For the Hong-Ou-Mandel
effect for parametric down conversion fields, we show that by using two
detector correlation functions which are defined in terms of averages of
products of measured rates, rather than usual intensities, one can observe non
-- classical visibilities beyond $1/2$ for significantly higher pump rates. At
such rates we already have a partially stimulated emission which leads to
significant amplitudes for multiple pairs production, still the new approach
allows to clearly see the non-classical dip.
