1. T. M. Che, J. B. Caldwell and R. M. Mininni, “Sol-gel derived gradient indexoptical materials,” SPIE - Int. Soc. Opt. Eng., Vol. 1328, Sol-Gel Optics, 145–159 (1990).
2. T. M. Che, P. R. Soskey, M. A. Banash, J. B. Caldwell, M. Javidi, I. McCallum, R. M. Mininni, and V. N. Warden, “Optimization of a gel derived gradient index material,” SPIE - Int. Soc. Opt. Eng., Vol. 1758, Sol-Gel Optics II, 193–204 (1992).
3. J. B. Caldwell, T. M. Che, M. A. Banash, R. M. Mininni and V. N. Warden,“Gradient index glasses and sol-gel method for their preparation,” U.S. Patent No. 5,182,236 (January 26, 1993).
4. P. B. Dorain and R. G. Wheeler, Xx Phys. Rev. Lett. 15, 968 (1965).
5. P. Schulz, Xx J. Am. Ceramic Soc. 59, 214 (1976).