The types of signals produced by a SEM to create an image include secondary electrons, back-scattered electrons, X-rays and specimen current. A two-dimensional map of a signal represents the specimen's image.

In a scanning electron microscope (see Fig. а), the electron beam with initial energy ranging from 1 keV to 10 keV is focused by a system of lenses to a spot 1-10 nm in diameter on the specimen surface. A system of deflector coils scans the focused electron beam across the surface synchronously with an electron beam in a video tube that is used as an optical display. Both of these electron beams are controlled by one scanning generator, which is why the magnification scale equals the relation of the size of display to the size of the area scanned on the specimen surface. The types of signals produced by a SEM include secondary electrons, back-scattered electrons, X-rays and specimen current (see Fig. b). Fig. c presents an energy spectrum structure of electrons emitted from the surface scanned by a beam of electrons with energy E0. The spectrum shows which electron energy ranges are used in the imaging process.