The present invention concerns an energy dispersion calibration method for calibrating an electron spectrometer of an electron spectrometer system including at least one electron emission source, electron optics and the electron spectrometer. The method comprising:obtaining, providing or receiving at least one electron energy loss spectrum produced by electrons of the electron spectrometer system exchanging energy with at least one resonant optical mode of an optical resonator into which light at a resonant wavelength is coupled;calculating or providing a Fourier transform of the at least one electron energy loss spectrum or a part of the at least one electron energy loss spectrum;determining or providing an energy dispersion (ΔE) of the electron spectrometer according to the equationΔ⁢E=fE⁢hcλ⁢N⁢or⁢Δ⁢E=fE⁢EpN.