A method for characterizing samples having units, by monitoring fluctuating intensities of radiation emitted, scattered and/or reflected by said units in at least one measurement volume, the monitoring being performed by at least one detection means, said method comprising the steps of: a) measuring in a repetitive mode a number of photon counts per time interval of defined length, b) determining a function of the number of photon counts per said time interval, c) determining a function of specific brightness of said units on basis of said function or the number of photon counts.