The invention describes a particle source in which energy selection occurs. The energy selection occurs by sending a beam of electrically charged particles 13 eccentrically through a lens 6. As a result of this, energy dispersion will occur in an image 15 formed by the lens 6. By projecting this image 15 onto a diaphragm 7, it is possible to only allow particles in a limited portion of the energy spectrum to pass. Consequently, the passed beam 16 will have a reduced energy spread. By adding a deflection unit 10, this particle beam 16 can be deflected toward the optical axis 2. One can also elect to deflect a beam 12 going through the middle of the lens 6—and having, for example, greater current—toward the optical axis.