The most common mode of detection is by secondary
electrons emitted by atoms excited by the electron beam. The
number of secondary electrons is a function of the angle
between the surface and the beam.
On a flat surface, the plume of secondary electrons is
mostly contained by the sample, but on a tilted surface, the
plume is partially exposed and more electrons are emitted. By
scanning the sample and detecting the secondary electrons, an
image displaying the tilt of the surface is created.