Abstract
No general theory exists that can predict the efficiency of phosphors with electronic transitions in localized centers. In this paper a survey and discussion are given of the correlation between fluorescent properties and chemical properties, like composition and structure. Points of discussion are (a) choice of the activator together with concentration quenching and critical concentration for this phenomenon, (b) influence of the host lattice on the photoluminescence efficiency for direct excitation of the activator, and (c) energy transfer between fluorescent centers. These considerations are illustrated by a large number of experimental examples.