From: pcdhobbs@gmail.com   
      
   >When calculating the signal-to-noise ratio in a (linear) digital image,    
   >should one define the decibel as 10*log10 or 20*log10 ?    
      
   >The case for 10*log10 is that the values represent luminous intensity, which   
   is power.    
      
   >The case for 20*log10 is that the values represent voltages in the CCD cells.    
      
   >I see some people doing it either way.  Which is best?  Is there a   
   standard?    
      
   I always use electrical dB, i.e. 10 log(I**2 R). That's what spectrum   
   analyzers and so on use, for a start. For another thing, there are electrical   
   contributions to the SNR, and it's unphysical to convert them all into optical   
   dB. (What would you do    
   about 1/f noise in optical dB?)    
      
   Of course that doubles the quoted SNR numbers. Some folks may suspect   
   specsmanship, so in talks and papers I mention early and often that I'm using   
   electrical SNR.     
      
   Cheers   
      
   Phil Hobbs   
      
   --- SoupGate-Win32 v1.05   
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