Ring interferometer for two-sided measurement of the absolute lengths of end standards.

Special features of a two-sided interferometer based on a ring optical scheme previously proposed [V. M. Khavinson and L. F. Khavinson, in Investigations in the Field of Length and Angle Measurements, N. P. Gerasimov, ed., Proceedings of the D. I. Mendeleyev Institute for Metrology (Energoatomizdat, Leningrad, 1983), pp. 14-18], which exploits an alternative measurement method to the conventional method for measuring absolutely the lengths of precision gauge blocks. Both measuring surfaces of a gauge are viewed directly without an auxiliary platen wrung onto one of them. The two-sided method results in improved consistency of measurement results because the elimination of wringing avoids the contact error that can occur in the gauge length obtained when the conventional method is used. A variety of means to control the optical phase differences in the interferometer are considered.