  The variance of difference of photocounts (VDPs) is an established measure of
quantum correlations for quantum states of light. It enables us to discriminate
between the classical correlation of a two-mode coherent state and the quantum
correlation of a twin-beam state. We study the effect of loss and saturation of
the photon-number-resolving detector on the measurement of the VDPs. An
analytic function is derived for this variance, both for the coherent and the
twin-beam states. It is found that the VDPs is no longer a reliable
entanglement measure in the nonlinear regime of the detector response but it
remains useful in some range of values of average photon numbers of the
incident light. We also quantify the linear regime of the detector with
saturation which will be useful for calibration of the detector quantum
efficiency.
