A spectroscopic study is carried out in which the effects of added Ca 2 + and Ru 4 + transition metal ions on some characteristics of the emission of Gd 2 O 2 S:Tb 3 + phosphors (energy levels, intensities, lifetimes) are examined and compared. In order to distinguish the Tb 3 + emissions from impurity ones, the electronic energy levels of trivalent terbium are determined and the energy level scheme is completed by a crystal field analysis. The optical spectra reveal no terbium doped impurity phase; however, other rare earth ions present as impurities in the starting materials are detected. They are identified, and the influence of the added Ca 2 + and Ru 4 + on their emission lines is also examined.