Luminescence is a general term given to the phenomenon that a substance releases extra energy in the form of electromagnetic radiation either immediately or over a long period, in excess of thermal radiation, after it is excited by a certain type of external energy, such as electromagnetic radiation (light), an electric ”eld, a high-energy electron, or an electron beam. Obviously, the luminescence process basically involves the excitation, energy transfer, and subsequent relaxation of the valence electrons from their excited states ( emission). According to different excitation sources, luminescence can be classi”ed into photoluminescence, electroluminescence, cathodoluminescence, radioluminescence, chemiluminescence, thermoluminescence, triboluminescence, etc. (García Solé et al. 2005; Yen et al. 2006), as seen in Table 2.1. In this book, we pay attention to the photoluminescence of a material.