Bi-directional fringe counting interference refractometer

A bi-directional fringe counting interference refractometer has been developed primarily to measure the refractive index of air so that the accuracy of laser length measuring interferometer systems used in the free atmosphere can be maintained. Two signals suitable for electronic counting in phase-quadrature and sinusoidally related to path difference are produced by a technique that does not require any form of modulation and yet has the advantages of a modulated system. This refractometer is cheap to produce and easy to set up and align. Measurement sensitivity of 2.5 parts in 10⁷ for the refractive index of air can be obtained directly from the counting signals and this has been further improved by fringe sub-division. Comparisons with calculated values indicate that the refractometer can measure ambient refractive index in a laboratory without temperature control with an uncertainty approaching 1 part in 10⁷