The invention describes a particle source in which energy selection occurs. The energy selection occurs by sending a beam of electrically charged particles 103 eccentrically through a lens 107. As a result of this, energy dispersion will occur in an image formed by the lens. By projecting this image onto a slit 109 in an energy selecting diaphragm 108, it is possible to allow only particles in a limited portion of the energy spectrum to pass. Consequently, the passed beam 113 will have a reduced energy spread. Deflection unit 112 deflects the beam to the optical axis 101. One can also elect to deflect a beam 105 going through the middle of the lens toward the optical axis and having, for example, greater current.
The energy dispersed spot is imaged on the slit by a deflector 111. When positioning the energy dispersed spot on the slit, central beam 105 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 112 is avoided.