Advantages of Koehler illumination for selected area diffraction

Abstract

The advantages of Koehler illumination for imaging are well known. This new mode of illumination in the Transmission Electron Microscope (TEM), however, provides advantageous results for all modes of operation of a TEM, in particular for Selected Area (SA) diffraction.In a conventional TEM, the last condenser lens is strongly excited (coherent illumination) to obtain selected area diffraction. The size of the diffraction spots is determined by the demagnified image of the electron source (cross-over) in the back focal plane of the objective lens. Since a large area of the specimen is illuminated, the diffracting area must be limited in the intermediate image plane by the selected area aperture (Fig. 1a).Due to the spherical aberration of the objective lens, the higher orders of the diffraction pattern of the axial points are eliminated by the selected area apertures and are thus lost in the photograph of the diffraction pattern (dashed rays in Fig. 2).