LEO's field emission scanning electron microscopes are all based an the “GEMINI” principle as shown in figure 1. In order to reduce aberrations and sensitivity to interfering stray-fields the electron optical column possesses a positively biased booster that shifts the energy of the primary electrons. The incident beam is focussed by a combination of a magnetic lens with an axial gap that avoids field leakage to the specimen and an electrostatic retarding lens formed by the beam booster together with the grounded pole piece cap. Shortly before the electrons hit the specimen they are decelerated down to the desired primary energy.