Terbium is the key element for highly efficient green phosphors and visible-near IR Faraday isolators. We have recently shown the potential of LiTbF4 and Tb0.81Ca0.19F2.81 as visible Faraday rotators. In this work, we present a detail spectroscopic analysis of Tb3+ (4f8) in these two compounds with different crystal structures. By means of the Judd-Ofelt theory, the emission branching ratios and lifetimes of the Tb3+ excited states have been estimated. These results are compared with experimental values obtained for the emitting 5D4 level, as well as with the absolute light yield measurements. Tb3+ in LiTbF4 exhibits a high quantum efficiency, and its radiative lifetime is confirmed to be 7 ms. Instead, the ionic conductor Tb0.81Ca0.19F2.81, which presents a high concentration of vacant sites, shows a lower quantum efficiency and a radiative lifetime about three times larger than estimated. Absorption and emission spectra of Tb0.81Ca0.19F2.81 are broad, so that any fine structure of energy levels can be resolved. In contrast, a detailed study of the splitting of Tb3+ multiplets in Stark energy levels is carried out for LiTbF4.