Contrast of electron micrographs

The variations of contrast in electron micrographs of amorphous films, with film thickness, electron accelerating voltage and objective aperture angle, are discussed; elastic and inelastic electron scattering and the possibility of plural scattering are taken into account. Experimental results, obtained with vacuum-evaporated iron films, confirm a formula of the type first proposed by Hall. However, the contrast is about four times smaller than predicted, using scattering cross-sections based on Lenz's data. Results for other materials, obtained by previous workers, indicate similar discrepancies. At present, the likeliest explanation appears to be a theoretical underestimation of the scattering into angles smaller than 10 -3 rad. The limitations of contrast measurements for assessing specimen thickness or constitution are discussed, and a new criterion for replica and shadowing materials suggested.