1. R. Reiff, Fundamentals of Statistical and Thermal Physics, (McGraw-Hill, New York, 1965), Chap. 9
2. W. Heitler, The Quantum Theory of Radiation, 3rd edn.(Oxford University Press, London, 1953), Sect. II.9
3. J.A. Stratton, Electromagnetic Theory, 1st edn. (McGraw-Hill, New York, 1941), pp. 431–438
4. R.H. Pantell and H.E. Puthoff, Fundamentals of Quantum Electronics, (Wiley, New York, 1964), Chap. 6
5. W. Louisell, Radiation and Noise in Quantum Electronics, (McGraw-Hill, New York, 1964), Chap. 6