Fine annealing of optical glass for low residual stress and refractive index homogeneity.

A discussion is presented on both the theoretical and practical aspects of fine annealing. The role of the glass transformation range in determining refractive index through thermal history is discussed. Necessary conditions of thermal history during annealing which lead to low residual thermal stress and high homogeneity of refractive index are outlined. Two types of annealing, rate and soak, are described and the advantages of rate annealing in achieving better homogeneity and economy of time are emphasized. A practical scheme for rate annealing of large optical glass disks is described. The use of high conductivity metal plates and peripheral heaters reduces troublesome temperature gradients. For mixed alkali glasses, a low temperature ionic structural reorganization suggests the use of an intermediate cooling rate between the glass transformation range and 150 degrees C.