2. Basis of the luminescence process

Depending on the nature of the excitation responsible for light emission, there are several types of luminescence: photoluminescence when excited by photons, radioluminescence when excited by ionizing radiation (X or γ rays), cathodoluminescence when bombarded by particles (electrons, positrons), electroluminescence from electrical excitation, bioluminescence from chemical reaction, thermoluminescence from temperature rise, and even triboluminescence from mechanical action. In all cases, the luminescent material absorbs the excitation energy and releases it in the form of a light emission whose energy lies between the ultraviolet (UV) and near infrared (IR) wavelengths.

In this article, we will only describe photoluminescence processes, and confine ourselves to the case of crystalline or polycrystalline materials, which make up the vast majority of solid luminophore...