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 (FT) 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 equation Δ E = f E hc λN or Δ E = f E E p N where λ is the resonant wavelength of the light coupled into the optical resonator , h is Planck's constant, c is the speed of light in vacuum, E p is the resonant photon energy of the light coupled into the optical resonator , N is the number of channels or data points along an energy axis of the at least one electron energy loss spectrum used in determining the Fourier transform (FT) of the at least one electron energy loss spectrum or a part thereof , and f E is the number of reciprocal energy space data points or coordinates between (i) a DC spectral component peak of the Fourier transform (FT) of the at least one electron energy loss spectrum or a part thereof and (ii) a sideband peak or a center of a sideband peak of the Fourier transform (FT) of the at least one electron energy loss spectrum or a part thereof , the sideband peak originating from electron energy exchange with the at least one resonant optical mode of the optical resonator . - 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 (FT) 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 equation Δ E = f E hc λN or Δ E = f E E p N where λ is the resonant wavelength of the light coupled into the optical resonator , h is Planck's constant, c is the speed of light in vacuum, E p is the resonant photon energy of the light coupled into the optical resonator , N is the number of channels or data points along an energy axis of the at least one electron energy loss spectrum used in determining the Fourier transform (FT) of the at least one electron energy loss spectrum or a part thereof , and f E is the number of reciprocal energy space data points or coordinates between (i) a DC spectral component peak of the Fourier transform (FT) of the at least one electron energy loss spectrum or a part thereof and (ii) a sideband peak or a center of a sideband peak of the Fourier transform (FT) of the at least one electron energy loss spectrum or a part thereof , the sideband peak originating from electron energy exchange with the at least one resonant optical mode of the optical resonator . <EOS>