Poorly conducting specimens can be examined without coating by using a variable pressure SEM. However, many labs may only have a high vacuum SEM, or for other reasons, choose a high vacuum mode. In order to examine insulating specimens in a high vacuum SEM (using operating conditions conducive to BSE and X-ray analysis) specimens must be coated with a conductive thin film.

The perspective of this article is from a materials point of view, but the principles remain the same for biological examinations requiring similar information.

Back Scattered Electron (BSE) image contrast is primarily a function of the average atomic number of an imaged area. This is particularly true for polished specimens where there is no topography to contribute to contrast. The BSE coefficient is the ratio of back scattered electrons to incident (beam) electrons.