I think the problem with creating a new definition of signal to noise is
that this is an attempt to make something that corresponds to the
quality of the measurement, while the true signal to noise is only a
measure of the quality of the signal.  This can be used for estimating
the time needed for a measurement to obtain the desired statistics, but
should not be used as a way to judge the quality of a completed
measurement. 

As far as taking the sum of the squares, it is my understanding that the
signal to noise is intended to be applied to a single mode, so the true
signal-to-noise could vary widely for different modes.  As I mentioned,
it can be a very useful predictor of the time needed for the
measurement, but probably shouldn't be used in discussing the quality of
the measurement.  If you are interested in a peak other than the one for
the highest mode, you should use that peak when estimating the time
needed for a measurement.

Another link is http://en.wikipedia.org/wiki/Signal-to-noise_ratio

Tom Worlton

-----Original Message-----
From: Leonid Solovyov [mailto:[EMAIL PROTECTED] 
Sent: Tuesday, February 19, 2008 3:51 AM
To: rietveld_l@ill.fr
Subject: RE: advice on new powder diffractometer

> It seems to me that if there is no internationally accepted
definition
> of signal to noise ratio in powder diffraction, then let me suggest  
>that  this forum might be a way to define it once and for all. How 
>about  this?
>                       SNR = (P-B)/SQRT(B). 
>
> Of course this would only give a  value for a single peak. I suppose 
> one could also define a global SNR as a sum over all Global SNR = sum 
> over peaks (P-B)/SQRT(B).
>
> Does this make sense?
> Mike

A more overall value would be a normalized sum of the ratios
(Ip-Ib)/sqrt(Ib), where Ip and Ib are the profile intensities of the
pattern and the background for each datapoint.

Leonid



 
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