SOME REMARKS ON THE IMAGE CONTRAST IN ELECTRON MICROSCOPY AND THE TWO-COMPONENT OBJECTIVE

Abstract

An image produced by a microscope, whether through light or electrons, can convey information only if both of two requirements are satisfied. The first of these is the production of a sharp geometrical image. The second is the pro-duction of adequate contrast. Since the electron microscope is essentially a monochromatic instrument, contrast can be provided only through variations in intensity. Such variations are produced mainly by variations in the electron-scattering power of the different object points. There is negligible absorption of electrons in those parts of the specimen that are sufficiently thin for satisfactory electron microscope examination. Variations in electronic scattering power in the different object points can arise either as a result of differences in thickness or as a result of differences in density or both. Differences in electronic scattering power at the object are translated into intensity differences in the image by the action of the objective lens aperture. This is arranged such that only those electrons that are scattered by less than some predetermined angle (a in figure 1A) are accepted and trans-