Cathodoluminescence (CL) occurs when an SEM beam causes photon emission in the visible and UV regions. The photon energy relates to the material, its purity and defect state.  CL is associated with low energy transitions resulting from factors such as physical chemistry, opto-electronic properties, temperature, crystal structure, strain etc. Images showing the spatial light intensity variations over a range of wavelengths characterise variations in trace element composition and defect densities. 

Trace element composition
Defect densities 

Trace element variation in geological materials
Identification of semiconductor and optoelectronic material defects and dislocations