The number of electronic states from which luminescent emission by rare earth ions can be observed is dependent on the extent to which the host lattice perturbs their f‐orbital electrons. Among others, perturbing influences may stem from the following sources: (a) coulombic affinities of the ions or molecules surrounding the rare earth ions for the electrons they share with the latter; (b) concentration effects, i.e., exchange coupling; (c) thermal effects, i.e., vibronic interactions.