When a plurality of tips are used with the reference flat, it is especially desirable to fabricate the flat from a transparent material with a low thermal expansion coefficient such as fused silica, which has an expansion of 0.57 parts per million per degree C. (Assuming a ???500 micron path length from the tip to the ISPI marks, fused silica for the reference flat material (0.57 ppm/??? C.), yiel