1. P. Pringsheim: Fluorescence and Phosphorescence (Interscience, New York 1949)
2. H.W. Leverenz: An Introduction to Luminescence in Solids (Wiley, New York 1950)
3. T. Förster: Naturwissenschaften 33, 166 (1946); Ann. Phys. 2, 55 (1948)
4. D.L. Dexter: J. Chem. Phys. 21, 836 (1953)
5. M.J. Weber: In Handbook on the Physics and Chemistry of Rare Earths, Vol. 4, ed. by K.A. Gschneider, L.E. Young (North-Holland, New York 1979) Chap. 35