A primary-level comparator, with a reproducibility of 0.2 nm and intended for realization of a Systeme International length unit in the range of 1-100 mm, is reported. High-precision differential measurements of phase change on reflection from blocks and end plates are demonstrated. A set of experiments has been developed to measure systematic error associated with nonideal interferometer optics and deviations from flatness of an auxiliary plate. For specially selected high-grade 6-mm blocks, reproducible wringing has been achieved with a random uncertainty in length measurements of 0.1-0.2 nm. Subnanometer wear-off of the blocks as a result of the cleaning has been detected. Under the conditions of reproducible wringing, the accuracy of the length measurements is evaluated to be in the 2-3-nm range for 6-mm blocks.

A modern fringe-pattern-analyzing interferometer with a resolution of 1 x 10(-9) and without exclusion of systematic uncertainties owing to optic effects of less than 1 nm was used to test a new method of interferometric length measurement based on a combination of the reproducible wringing and slave-block techniques. Measurements without excessive wringing film error are demonstrated for blocks with nominal lengths of 2-6 mm and with high surface flatness. The uncertainty achieved for these blocks is less than 1 nm. Deformations of steel gauge blocks and reference platens, caused by wringing forces, are investigated, and the necessary conditions for reproducible wringing are outlined. A subnanometer uncertainty level in phase-change-correction measurements has been achieved for gauge blocks as long as 100 mm. Limitations on the accuracy standard method of interferometric length measurements and shortcomings of the present definition of the length of the material artifact are emphasized.

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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