  In this article we present an evaluation of the uncertainty in the average
number of photoelectrons, which is important for the calibration of
photodetectors. We show that the statistical uncertainty depends on light
intensity, and on the method of evaluation. For some cases there is optimal
light intensity where the accuracy reaches its optimal value with fixed
statistics. A method of photoelectron evaluation based on the extraction of
pedestal's (zero) probability gives the best accuracy at approximately 1.6
photoelectrons for a noiseless photodetector and shifts out to higher values
with the presence of noise. In the general case, estimation of the average
number of photoelectrons is biased and might need special consideration.
