Testing of surlace shapes by automatic interferometry is exact, fast, and comfortable, and therefore popular. It however suffers from drawbacks: the equipment is often quite voluminous and expensive, and the attainable accuracy is limited by the precision of the reference surface (relative testing). Using the phase-shift software of BIFO, which has been improved over the years, Fisba Optik AG recently developed a matchboxsized, cost-reduced interferometric surface shape sensor to overcome the first drawback. Furthermore, this sensor was combined with methods for absolute calibration to solve the reference-surface problem.
"High-precision surface shape testing by means of an interferometric sensor," Optical Engineering 34(9), (1 September 1995). http://dx.doi.org/10.1117/12.205674