Dimensional Stability of Materials Useful in Optical Engineering

Current requirements for increasingly stable structures have motivated the precision measurements described here. We survey the thermal expansivity versus temperature characteristics of some popular low-expansivity materials, including metals, for which the situation is especially unsatisfactory (for example, Invar and Super Invar) and point out the unusual expansion behaviour upon thermal cycling of Zerodur. A new differential technique is described which makes possible measurements of the uniformity of expansivity with an uncertainty as small as ±0·1 x 10-−10 K−1. Finally, we discuss ongoing measurements of temporal stability at constant temperature and call attention to the problems with Invars.