ABSTRACT
Most phosphors are composed of a transparent microcrystalline host (or a matrix) and an activator, i.e., a small amount of intentionally added impurity atoms distributed in the host crystal. Therefore, the luminescence processes of a phosphor can be divided into two parts: the processes mainly related to the host, and those that occur around and within the activator. Since a large number of phosphor host materials are transparent and nonmagnetic, their optical properties can be represented by the optical constants or by a complex dielectric constant. The emission of light from a material originates from two types of mechanisms: thermal emission and luminescence. The chapter discusses absorption and emission of light by the impurity atoms or local defects. The absorption and emission of light by an atom can be described in the most simplified scheme by a linear harmonic oscillator.
