CL, in broad terms, is the process of light emission from a material as a result of excitation by electrons. It therefore provide a signature of the electronic excitation processes. PL is the process of light emission as a result of excitation by photons. Since electrons are massive and carry more momentum, the selection rules for CL emission are different from those for PL. The light sources, often lasers, used for PL excitation have a narrow energy range and therefore probe specific transitions. Excitation by electrons, on the other hand, is broad band, allowing inspection from the deep UV to the IR. Moreover, due to the high energy of the electrons, multiple excitation paths are available, resulting in different relaxation pathways with corresponding CL signatures. Materials can also show both CL and PL.