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The natural line width of the emitted X-rays (e.g. Mn K_α line) in EMs (electron microscopes) is only ~ 1 to 10 eV, but the line widths obtained with EDS systems are normally larger than 100 eV, for instance, the EDS-detected FWHM (full width at half maximum) at the energy of Mn K_α is 135-165 eV. The natural energy distribution of the characteristic X-rays of a single line is well described by the Lorentzian probability distribution instead of Gaussian shape obtained by the energy-dispersive detectors. The electronic noise in the EDS system introduces the peak broadening that is a major source causing the difference between the EDS-detected and natural energy resolutions. The width of the electronic noise is also described as the ‘point-spread function’ of the detector.