1. See, e.g., W.B. Fowler:Physics of Color Centers (Academic, New York 1968)
2. P.D. Townsend, J.C. Kelly:Color Centers and Imperfections in Insulators and Semiconductors (Sussex U. Press, Edinburgh 1973)
3. G. Castro, D. Haarer, R.M. Macfarlane, H.P Trommsdorff: U.S. Patent No. 4,101,976 (1978)
4. G.C. Bjorklund W. Lenth, C. Ortiz: SPIE,298, 107 (1981)
5. R.M. Macfarlane, R.M. Shelby: Phys. Rev. Lett.42, 788 (1979)