1. Much early work is discussed in the classic text by J.H. Schulman, W.D. Compton: Colour Centres in Solids (Pergamon, Oxford 1962)
2. W.B. Fowler (ed.): Physics of Colour Centres (Academic, New York 1968)
3. J.M. Spaeth: In Defects and Their Structure in Nonmetallic Solids, ed. by B. Henderson, A.E. Hughes (Plenum, New York 1976) and references therein
4. L.F. Mollenauer: In Quantum Electronics: Part B, ed. by C.L. Tang (Academic, New York 1979) Chap 6; and in Laser Handbook 4, ed. by M. Bass (North-Holland, Amsterdam 1985)
5. E. Sonder, W.A. Sibley: In Defects in Solids, ed. by J.H. Crawford, L.F. Slifkin (Plenum, New York 1972) p. 381