The three groups of fluorescent line spectra of Tb3+ in CaF2 " spectrum I" between the crystal-field split levels of the 5D4 and the 7F3, 7F4, and 7F5 free-ion states have been combined into energy-level schemes which tend to support a cubic crystal field symmetry and which give information on the crystal-field split-leve representations and energies to 5 cm-1 for a thin crystal.
NORMAN RABBINER, "Fluorescence of Tb3+ in CaF2," J. Opt. Soc. Am. 55, 436-438 (1965)