A method of determining an energy spectrum or energy width of a charged particle beam (11) focused by a focusing lens (120) toward a sample plane (p.sub.S) in a charged particle beam imaging device is described. The method includes (a) introducing an energy-dependent deflection of the charged particle beam (11) that leads to a spot broadening along a dispersion axis in the sample plane (p.sub.S), and taking an image of a sample (10) arranged in the sample plane using the charged particle beam; (b) retrieving a beam profile of the charged particle beam from the image; and (c) determining the energy spectrum or energy width from the beam profile. Further embodiments described herein relate to a charged particle beam imaging device configured to determine the energy spectrum or energy width of a charged particle beam, particularly according to any of the methods described herein.