Illuminating System for High-Resolution Transmission Microscopy Using a Field-Emission Source and Prefield of the Condenser-Objective

Abstract

The characteristics of an illuminating system consisting of a field‐emission source, a condenser lens, and the prefield of a condenser‐objective were studied by computer experiments to determine its applicability for high‐resolution transmission microscopy. The effects of demagnification and spherical aberration by the prefield on the illumination were calculated. The high brightness of the field emission source gives this system illumination characteristics that are different from those attainable with a thermionic gun system and provides illumination suitable for high‐resolution phase‐contrast transmission microscopy. The adjustment required to produce the desired illuminated areas and angular divergences at the specimen can be achieved by focusing the condenser lens. Typically areas of 500–1000 Å diam can be illuminated with angular divergences of less than 1 × 10⁻³ rad without requiring critical axial positioning of the specimen. Defocusing the condenser makes it possible to illuminate areas large enough for survey of the specimen.