Many phosphors nowadays are lanthanide (Ln) activated semi-conductor and insulator materials due to their serviceable luminescent properties. Few years ago, an empirical model for semi-conductor and insulator materials with an Ln activator has been developed that not only explains the optical properties of phosphors but helps as well to find hitherto unknown materials with selected photoluminescence (PL), thermoluminescence (TL) or charge carrier trapping properties. Within the model, such properties are determined by the locations of the 4f and 5d energy levels of Ln dopant ions relative to the conduction band (CB) and the valence band (VB) of the host. One objective of this project was to localize the Ln 4f levels in different materials relative to the VB and the CB by means of temperature- (T-) dependent PL spectroscopy and (wavelength resolved) TL studies. Another objective was to study possible regularities when going from one type of compound to another one. In this context not only own measured data have been analyzed but also different models have been compared with each other which all aim at explaining the very same phenomenon. For the important class of compounds with formula ABO4:Ln3+ (A = rare earth; B = transition metal, P) it will be shown that the most important charge transfer (CT) energies can be approximated by using always the same approach.