Introduction

A luminescence center is a construct aimed at understanding similarities in the absorption and emission performances found among luminescent materials. The word “center” implies that the most important observed properties can be understood while restricting one’s attention to a particular lattice site and its immediate surroundings. Free excitonic absorption and emission, for example, are beyond the scope of a model of a luminescence “center”, because the nature of these optical processes involves the whole lattice. But for the large majority of observed luminescent materials, the “center” is a useful concept.