Why Does the RMS Value Represent Noise in a Photoconductor Signal?

[CassiopeiaA]

I am confused about very fundamental question.

Why does the rms value of number of electrons collected from a signal(like in photoconductor) gives you the noise in that signal.

 

[mfb]

That is the definition of noise.

More electrons without signal -> more noise. rms is chosen because it is a nice quantity to look at.

 

[Babadag]

See:
http://en.wikipedia.org/wiki/Root_mean_square

 

[sophiecentaur]

I am confused about very fundamental question.

Why does the rms value of number of electrons collected from a signal(like in photoconductor) gives you the noise in that signal.

To get a good idea of the 'amount' of Noise entering a system, you can't look at the 'peak' value because, in the short term, it could vary a lot. Doing an RMS calculation is effectively looking at the power arriving (V²/R) at every instant and adding it up over a relatively short period of time. RMS is an attempt to replace the randomly varying noise with one equivalent (average) voltage. It's the most basic measure of noise and assumes that the noise is of a Gaussian nature.
The actual effect of random variations in a signal (noise) is different from application to application and a simple RMS measurement may not be representative. It is common to use a 'weighted' noise measurement, where the noise signal is passed through a filter before the RMS value is calculated so that, for instance, the frequency sensitivity curve of the ear is included in audio noise measurement.
In the case of a photo detector, you can get a very 'grainy' sort of noise with very high peaks. RMS will iron these out and may give a far too optimistic assessment.