The electron energy filter admits an electron beam with a spread of energies, and emits a beam of electrons only within a special energy band. The electron energy filter contains a pair of yokes (1a, 1b) set close together, such that multiple pole pieces ( e.g. 2b, 3b) on the yoke surfaces are set at a fixed separation. The filter also contains an electron beam channel in the yokes, and exciting coils (11, 12) to generate the magnetic field between the pole pieces. An incoming electron beam at one end of the electron beam channel is bent between the pole pieces and emerges at the other end of the channel (62).
