A particle source in which energy selection occurs by sending a beam of electrically charged particles eccentrically through a lens so that energy dispersion will occur in an image formed by the lens. By projecting this image onto a slit in an energy selecting diaphragm, it is possible to allow only particles in a limited portion of the energy spectrum to pass. Consequently, the passed beam will have a reduced energy spread. The energy dispersed spot is imaged on the slit by a deflector. When positioning the energy dispersed spot on the slit, central beam is deflected from the axis to such an extent that it is stopped by the energy selecting diaphragm. Hereby reflections and contamination resulting from this beam in the region after the diaphragm are avoided. Also electron-electron interaction resulting from the electrons from the central beam interacting with the energy filtered beam in the area of deflector is avoided.