  The main argument against the reality of the electromagnetic vacuum
fluctuations is that they do not activate photon detectors. In order to meet
this objection we propose a classical model of a photodetector which, in the
simple case of a light signal with constant intensity, gives a counting rate
which is a non-linear function of the intensity. For sufficiently large signal
intensity, the counting rate is proportional to the intensity, in agreement
with the standard quantum results, but there is a dark rate when the signal
intensity is low.
