scanning electron microscope (SEM) A microscope that operates by scanning a finely focused beam of electrons across the specimen. The reflected electron intensity is measured and displayed on a cathode-ray screen to produce an image. The SEM enables magnifications of up to 100 000 times to be made and provides a much better depth of field than a conventional light microscope (which suffers from focus limitations), making the three-dimensional structure of small objects (e.g. Foraminiferida) spectacularly visible. In geology it is used extensively for micropalaeontology, diagenetic studies (see DIAGENESIS), and grain textural examination. When coupled with an electron probe, semi-quantitative determinations of grain chemistry can be made.

scanning electron microscope See electron microscope.