An electron microprobe operates under the principle that if a solid is bombarded by a high energy beam of electrons, the electrons will have sufficient energy to 'liberate' both orbital electrons and x-rays (photons) from the sample. These liberated or secondary signals can be used to evaluate sample properties. Their origins are shown below.
Secondary Electrons, used for imaging three dimensional objects. These electrons were part of the sample and ejected via a collision with a beam electron.
BackScattered Electrons, used for imaging multi-component objects. These electrons were part of the primary electron beam that bounce 'backwards' out of the sample.
X-rays, used for quantitative chemical analysis. These are created when inner orbital vacancies are filled by outer orbital electrons. Energy is released during the transition.