Correction for stress-induced optical path length changes in a refractometer cell at variable external pressure

In the field of interferometry for high-precision length measurements the influence of the refractive index of air can either be eliminated by completely measuring in a vacuum or by the application of a refractometer cell which enables the precise in situ determination of the refractivity. In the latter case, systematic effects can occur in connection with the operation of the refractometer cell due to mechanical stress induced at different pressures. Operation at a pressure range beyond the usual atmospheric pressure variation is needed when, for instance, the compressibility of a material measure is investigated or the pressure dependence of the refractive index of gases is aimed at. In this work an approach is shown for the correction of pressure-induced effects for the type of interference refractometer being used at PTB when starting from a reference state in vacuum. The required correction of the refractivity's value was found to be of the order of for a refractometer cell of 420 mm length which corresponds to a relative contribution of when determining the air refractivity at standard laboratory conditions. For instance, in the application of high-precision length measurements (up to 1000 mm) the effect amounts to a length correction of several nanometers.