Signal-to-Noise (S/N) versus Time
Signal to Noise Measurement
from N Fairley’s website on CasaXPS
The Standard Report configuration file for Regions allows the inclusion of a quantity referred to by the heading Noise. The meaning of the Noise column depends on the manner in which the quantification regions are prepared. Both values reported for the Noise column require the data are made available in counts. To estimate the noise in an XPS spectrum, Poisson statistics associated with the pulse counting system dictates that the noise is proportional to the square root of the counts per bin.
- The default action is to compute a metric for the noise in the spectrum for which the region is defined. The method employed attempts to construct a smooth curve which passes through the data such that the residual between the spectral data and the smooth curve measures the distribution of the noise within the spectrum. Since XPS spectra generally include a substantial background signal and the noise is proportional to the square root of the counts per bin, the residual distribution must be normalised by dividing by the square root of the counts per bin for each bin. The standard deviation for the normalised residual distribution is computed and reported in the Noise column of the Standard Report. For a single channeltron detection system, the standard deviation is typically unity. Multiple detector systems effectively smooth the data and therefore the value for the standard deviation tends to be less than unity.
- As an alternative, the signal to noise for each region can be computed and reported in the Noise column of the Standard Report. To alter the default action from that described above, the Tag field in the region must be assigned the string S:N. The Tag field is case sensitive and there should be no additional spaces between the characters. Given that the Tag field is set to S:N, the Noise column report the value (peak-background) / SQRT (peak). Again this measurement is normalising the signal above background using the counts at the peak maximum and therefore should be a similar value for similarly intense peaks regardless of the background height.