What is the minimum recommended beam power level?
There is none. In practice, the minimum, power level that can be measured is a function of the photodiode and the electronics used to read the signal from the photodiode. In other words, the quality of your equipment determines your ability to make low level measurements. However, if you are using your photodiodes for photon counting, please provide us with more details about your measurement and the actual levels you are trying to measure. Photon counting is an entirely different subject.
