How does a scanning electron microscope build a surface image?

A scanning electron microscope, or SEM, reveals surface detail too small for an ordinary light microscope to show clearly. It sends a focused beam of electrons toward a specimen inside a chamber. Electron optics shape the beam, and scanning coils move its landing point across the surface in rows.

Electrons in the beam interact with the specimen. Some of those interactions release low-energy secondary electrons close to its surface. A detector collects a signal from these escaping electrons. As the beam moves, the instrument keeps track of both its position and the detector response.

The display assigns each sampled position a pixel. A stronger selected signal becomes a brighter pixel in the simple mapping shown here. Our invented nine-point scan has a center signal of 80 on a scale from zero to 100, so the center pixel is bright. The result is assembled position by position rather than taken as one visible-light photograph.

Surface slope and edges can affect the signal, but brightness is not a direct height ruler. Material, detector arrangement and settings also matter. Ordinary SEM images are often grayscale; added color may be a presentation choice. Other SEM detectors collect different signals. This explanation follows secondary electrons leaving the surface, rather than electrons transmitted through a thin specimen.