System for determining fluctuations in refractive index of gas or air

摘要

A procedure and apparatus for determining and compensating for fluctuations in the refractive index of a gas or air in the optical measurement path (66) of a multiple interferometer (60) is used to establish the displacement of an object (67) independently of these fluctuations. The coherent light sources (1, 4) radiate two beams (11, 12) of different but harmonically related wavelength along the optical path (66). The beams (11, 12) are electronically processed (98) in order to generate modified heterodyne phase shifts which are based on the source wavelengths and which in similar fashion are harmonically related to the source harmonics to produce a superheterodyne modulation phase which is essentially insensitive to movements along the optical path.