On the validity of the weak-phase and other approximations in the analysis of electron microscope images

Abstract

The application of approximate methods such as the weak-phase and the weak-phase/weak-amplitude approximations, to the analysis of conventional transmission electron microscope images is examined. The specimen thickness must be respectively less than 5 and 15 nm for stained and unstained biological specimens for the valid application of the weak-phase approximation. These thickness limits can be extended to 10 and 25 nm respectively using the weak-phase/weak-amplitude approximation. The effect of specimen thickness is to place severe restrictions on the interpretation of an image in terms of the specimen structure. The thickness effect becomes increasingly severe as the image resolution improves. Suggestions are made for minimizing experimentally both the effects of specimen thickness and defocus error.