Linking the optical and the mechanical measurements of dimension by a Newton's rings method

Optical (e.g. interferometric or laser focus probe) measurement of dimensions must be corrected to take into account phase change on reflection and the influence of surface roughness in order to be compatible with mechanical dimension measurement methods (e.g. tactile probes). One typical example is interferometric measurement of a gauge block; while a correction of only a few nanometres is needed for standard interferometric measurement of a gauge block wrung on a platen made of the same material, a correction of tens of nanometres is needed when different materials are used (e.g. a steel gauge block on a glass platen) and even a total correction of up to a hundred nanometres is needed for some gauges when a double-ended interferometer is used. Here we describe and evaluate a Newton's rings method that enables direct estimation of such correction. The implementation of this method is described, the sensitivities to experimental adjustments are discussed, and the results are compared with standard measurements within EURAMET project No. 1272. The resulting central length measured with a double-ended interferometer and corrected using the Newton's rings method agrees well with the standard measurement results within a total uncertainty of ±20 nm for both steel and ceramic gauges. Unlike the stack method, the Newton's rings method enables measurement of the correction for an individual sample (e.g. gauge block) and can easily be done for both short and long gauges with equal speed and uncertainty.