Review of measurement systems for evaluating thermal expansion homogeneity of Corning Code 7971 ULE®

Early on in the development of glass Code 7971 ULE^TM, Corning recognized that applications demanded thermal expansion measurements with higher precision and accuracy than that available. Hence a program was launched starting with 10cm path laser Fizeau interferometry to establish an absolute base. Differential methods followed with large sandwich seals and a high precision vitreous silica superdilatometer. Destructive in nature, these tests led to a nondestructive ultrasonic procedure that permits absolute and differential measurements with an uncertainty of +/- 2 ppb/ degree(s)C and reliably performed at the manufacturing plant. Details of these measurement systems are described along with optional experimental variations. A principle, established with seal testing, is reviewed showing that expansion differentials in ULE^TM are independent of temperature. Finally, recent plant measurements of both axial and radial thermal expansion variations in ULE^TM boules are presented.