1. Allen, C.W.: 1976, “Astrophysical Quantities”, William Clowes & Sons, London
2. NATO Advanced Science Institutes Series;LS Anderson,1985
3. Athay, R.G.: 1972, “Radiation Transport in Spectral Lines”, Reidel Publ. Co., Dordrecht, The Netherlands
4. Athay, R.G., Lites, B.W.: 1972, Astrophys. J. 176, 809
5. Avrett, E.H.: 1985, in “Chromospheric Diagnostics and Modeling”, B.W. Lites (ed.), National Solar Observatory Conference, Sacramento Peak, Sunspot, p. 67