Minimization of dose as a criterion for the selection of imaging modes in electron microscopy of amorphous specimens.

A fundamental limitation in electron microscopy of organic specimens is radiation damage by the electron beam. To minimize damage it is necessary to have maximum information collection for a given dose. Various modes of operation of conventional and scanning transmission microscopes are compared with respect to their ability to detect small changes in specimen thickness or density with a given signal to noise ratio. Incoherent imaging is assumed, and this is expected to apply to amorphous specimens under a variety of microscope conditions. For either very thin or very thick specimens, the scanning transmission microscope is found to require nearly 10 times less dose than a conventional microscope for the same signal to noise ratio in the image. For specimens of intermediate thickness, scanning and conventional transmission electron microscopes are roughly equivalent.