Dear all,   
      
   It's a practical as well as a theoretical question.   
   There are some conditions where light signal that we need to detect is below   
   the detector noise. I was guessing if it is possible in some conditions, to   
   add a controlled constant background light (which would act as an offset), so   
   that the total signal    
   gets higher than the detector threshold detection limit.   
   Linked to this question, is it possible that a photodetector (including the   
   sensor and the attached electronics)can't detect, let's say 0.1 mWatt light   
   out of the dark (because of it's dark current or other kind of noise), but can   
   resolve between 1 mW    
   and 1.1 mW.   
   Of course I'm aware that background light would carry an associated photon   
   shot noise, and the trick would be to add a minimum amount of background just   
   to get above the detector noise level.   
      
   Thanks,   
      
   Eric   
      
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