# Why higher FAT level implies more signal in XPS analysis?

In XPS analysis, the FAT (Fixed Analyzer Transmission) parameter controls the electrostatic field of an emispheric analyzer, called Pass Energy $E_P$.

The FWHM of a peak of signal is given by $$\Delta E = E_P \cdot \text{const.}$$ and the Pass energy is proportional to the FAT number (e.g. a peak of signal acquired at FAT10 is thinner than a peak acquired at FAT90).

Now the question is: as one raises the FAT parameter, the peak becomes larger, but why does the total signal increase?

This plot is a measure made by my team and me varying the FAT parameter on peaks of a sample of Silver.