Signal to Noise Ratio of EDS  Practical Electron Microscopy and Database   An Online Book  

Microanalysis  EM Book https://www.globalsino.com/EM/  
For the EDS data, the Signal to Noise Ratio (SNR) is largely limited by counting statistics. Therefore, noise is approximately equal to the square root of the signal for thin TEM samples, while it can be low for thick TEM samples or bulk SEM samples. For an elemental image (e.g. EDS and EELS) taken from an uniformly thick TEM sample, the SNR is approximately equal to the ratio of mean signal (e.g. counts)/standard deviation (e.g. counts). Here, both the mean signal and the standard deviation are from the same uniformly thick area. 

