3D reconstruction with scanning electron microscope images. SEM is often the first analytical instrument used. An electron beam is directed onto a target in the SEM. When a beam interacts with a sample, it produces a variety of signals secondary electrons, back scattered electrons. These signals are extremely concentrated in the region immediately underneath the beam. An picture is created on the screen by using these signals to modulate the brightness of a cathode ray tube that is raster scanned in synchronism with the electron beam.