Correlation of Sapphire Quality with Uniformity and Optical Properties, Chandra P. Khattak, Frederick Schmid and Maynard B. Smith, SPIE Proc. 3060 (1997), Window and Dome Technologies and Materials V, Orlando, FL 21-22 April 1997. Abstract: Large sapphire boules, up to 34cm diameter, 65kg, are being grown by the Heat Exchanger Method (HEM) and even larger sizes are sought to meet future requirements of advanced optical systems. These boules, especially in large sizes, exhibit lattice distortion and light scatter in a very narrow range. A qualitative grading system has been developed to characterize sapphire. Windows of five grades and different orientations were prepared and measured for refractive index homogeneity to evaluate transmitted wavefront distortion. The data showed that the refractive index homogeneity for all samples was in the 10^-7 (0.1 ppm) range. The fact that lattice distortion does not affect the transmitted wavefront allows fabrication of large sapphire windows in production mode at low cost.