The spot size is smallest at the optimum plane of focus and as long as the beam doesn t spread out and cause spot size overlapping, the image will be in focus. This results in an almost 3-D image in the SEM and is an obvious advantage of the SEM over an optical microscope. In focus Beam has spread out too much. This ejected electron is a secondary electron. The electrons are converted to photons with a scintillator and photomultiplier.
Backscattered electrons have high energies, up to the beam energy, due to the elastic collisions.
When the electron moves to an inner shell from an outer shell, energy is emitted in the form of x-rays or Auger electrons. The nature of the specimen will influence the detected x-ray intensity. This includes the amount of x-ray absorption and fluorescence in the specimen as well as the fraction of backscattered electrons. An iterative procedure must be employed to determine the apparent concentration. The atomic number effect.