A new type of aspheric surface calibrator has been developed at the National Research Council of Canada. It employs a nonmechanical contact technique for measuring rotationally symmetrical uncoated concave optical surfaces which depart from a sphere by as much as 0.5 mm with a precision of 0.3 µm. Using two precise air bearings for providing rotations about two mutually perpendicular axes, a focused laser beam is scanned across the surface under test. A fringe counting process measures the difference in sag between the surface shape and some reference sphere. Two surfaces, one spherical and the other ellipsoidal, were measured to illustrate the practicality of such an instrument.
Ian Powell, "Aspheric surface calibrator," Appl. Opt. 20, 3367-3377 (1981)