SEM generally has a lower resolving power than the TEM. It is very useful for providing three-dimensional images of the surface of microscopic objects.
In this electrons are focused by means of lenses in to a very fine point. The interaction of electrons with the specimen results in the release of different forms of radiation (eg secondary electrons) from the surface of the specimen. These radiations are then captured by an appropriate detector, amplified and then imaged on a television screen.
Other important techniques in EM include the use of ultra thin sections of embedded material; a method of freeze- drying specimen, which prevents the distortion caused by conventional drying procedure; and the use of negative staining with an electro dense material such as phosphotungstic acid or Uranyl salts. These heavy metal salts provide enough contrast to detect the details of the specimen.