> > > As I understand it, by providing a threshhold against which pulses are > judged, the idea of photon counting is to reject the thermal counts > (low energy) and accept genuine photon events (high energy). The > consequence of this is inevitably that some genuine photons are in > fact thrown away - total signal will be reduced - but even more noise > is thrown away so that S/N ratio is improved. > The only problem is that the PMT cannot distinguish between a photoelectron and a thermally generated electron which is emitted from the photocathode. Photon counting will eliminate noise from the electronics, provided that the total noise of this kind is less than the threshold for pulse counting. It can also eliminate most noise from electrons which are thermally generated off the dynode (or at least most of the dynodes) provided the PMT gain (ie electron amplification) is set correctly. When next at my scope, I will try to measure this by accumulating a dark image in the photon counting mode. --aryeh Aryeh Weiss | email: [log in to unmask] Department of Electronics | URL: http://optics.jct.ac.il/~aryeh Jerusalem College of Technology | phone: 972-2-6751146 POB 16031 | FAX: 972-2-6422075 Jerusalem, Israel | ham radio: 4X1PB/KA1PB