Köhler illumination in the TEM: Fundamentals and advantages

Abstract

Summary Köhler illumination is the most favourable design for the illumination path of an electron microscope with a condenser objective lens. The new illumination system of the EM 910 and EM 912 OMEGA allows both wide area (Köhler) illumination for TEM operation and spot illumination for analytical investigations. Compared to conventional systems and objective lenses with a condenser mini lens, this system offers many advantages. In addition to the homogeneous, highly coherent and parallel illumination of every point in the specimen, it offers advantages for selected area diffraction and spot scan mode. Combined with the electron optical selection of a condenser aperture, this illumination system provides the flexibility necessary to achieve optimum illumination for the specimen.