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Hello All, 
    Thanks for all replies!
    Here is a quick summary wrote by my senior colleague and I hope his summary 
will clarify some questions.  "I have been using the standard deviation of a dark 
area as the noise, the average of the dark area as a background and the average 
of a high signal area as the “max” (SNR=(Max-Min)/noise).  I have checked, the 
min and noise in min figures are the same in the completely dark region of my 
sample as they are in a truly dark image.  The gain on the EM is 255 and the 
amplifier is 5.  I am guessing a total gain of 550-750 which puts the photon count 
per pixel somewhere between 40-60." Under the condition of 40-60 photons, 
EMCCD SNR is 46, much higher than 12 on sCMOS. 
    I had a phone call with Hamamatsu technical support today, based on their 
experiment, the EMCCD should have a higher SNR once the signal is higher than 6-
15 photons. However, our results have big disagreement with them. Something 
must be wrong, either our measurement or either the camera. So we send our 
original data to Hamamatsu for further investigation. I will keep you updated  once 
I hear something from Hamamatsu. 
   Ps, the EMCCD we used is also from Hamamatsu (c9100-13).
   Thanks a lot, Zulin