Parallelism error characterization with mechanical and interferometric methods

Two methods for measuring the parallelism error of gauge blocks are discussed. The first method makes use of contact probes, sampling the gauge thickness at fixed locations; the parallelism error is given by the difference between maximum and minimum thickness over the sampled area. The second method is based on interferometry, producing the height maps of the opposing surfaces with respect to one another; the parallelism error is obtained after processing of the interferometric data. The two methods have been implemented in different laboratories, and compared by measuring five different gauge blocks. The results are reported, showing that the first method is more straightforward and simple, but requires prior calibration and provides information only on a limited number of sample points. The second method is more demanding in terms of equipment and measuring procedure, but in principle it is an absolute method, and provides information over a large number of data points.