A system of interferometric length measurements has been realized that supports wringing and phase change errors of ～1 nm. In this system, special reference blocks allowing reproducible wringing are used to find the relation between optical and mechanical lengths. The optical length of a reference block is found by the double-sided method on a quartz plate. The mechanical length is found by the combination of the reproducible wringing and slave-block techniques. A comparison of these two length values gives the phase change at the optical reflection, which is then used to find the mechanical length from the double-sided measurement results of a block of arbitrary length (up to 100 nm). The proposed system makes it possible to measure the phase change difference between the blocks with sub-nanometre accuracy. The system can be used for the practical realization of an alternative definition of the length of a material artefact, based on the perpendicular distance between its mechanical surfaces. Precise measurements of the excessive thickness of the wringing film, which is included in the present definition of the length of a block, are demonstrated. Standard single-sided measurements on quartz are shown to have some additional uncertainty relative to standard measurements on steel plates.
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