scanning electron microscope

Electron microscope that produces three-dimensional images, magnified 10–200,000 times. A fine beam of electrons, focused by electromagnets, is moved, or scanned, across the specimen. Secondary radiation reflected from the specimen is collected by a detector, giving rise to an electrical signal, which is then used to generate a point of brightness on a television-like screen. As the point moves rapidly over the screen, in phase with the scanning electron beam, an image of the specimen is built up.

The resolving power of an SEM depends on the size of the electron beam – the finer the beam, the greater the resolution. Present-day instruments typically have a resolution of 7–10 nm.

The first scanning electron picture was produced in 1935 by Max Knoll of the German company Telefunken, although the first commercial SEM (produced by the Cambridge Instrument Company in the UK) did not go on sale until 1965.