apparatus and means for calibrating optical gap - measuring instruments , including a preferred calibration standard comprised of a substantially flat , transparent element held in contact with the convex spherical surface of a substantially opaque element to provide an obvious and unambiguous region over which the gap is zero . the optical gap measuring tool that is to be calibrated is oriented so as to illuminate the interface . the calibration standard is translated in position with respect the gap measuring tool in such a way as to vary the measured gap , while at the same time data storage means record the gap measurement as a function of relative position . this data is then compared to the predicted variation from the geometry of the calibration standard . provided that the elements are of high optical quality , deviations of the measured data from an ideal parabolic curve are indicative of calibration errors .