QUANTITATIVE-ANALYSIS OF IMAGE-CONTRAST IN ELECTRON-MICROGRAPHS OF BEAM-SENSITIVE CRYSTALS

The contrast in high resolution electron micrographs of three different thin crystals has been compared quantitatively with that predicted theoretically from separate measurements of their electron diffraction patterns. The results, all at 4. 0 to 4. 5 A resolution, show that the absolute contrast in images of vermiculite is roughly 1/5th of that expected for a theoretically perfect microscope, whereas images of paraffin and purple membrane seldom reach more than 1/25th of theoretical contrast. Much of this loss of contrast can be explained on the basis of known microscope parameters in the case of the non-beam-sensitive specimens. However, for the images of paraffin and purple membrane, it is necessary to postulate that beam-induced specimen movement results in further substantial blurring of the image.