Accurate measurement of the refractive index of air

An interference refractometer for absolute measurement of the refractive index of air has been developed. It is essentially based on components of a commercial Zeeman type laser interferometer system and uses a differential interferometer and a vacuum chamber with four measurement cells. The resolution and accuracy of interference phase interpolation is improved by an electronic phase meter and a detection technique which compensates for interferometer non-linearity. The system has been designed for an accuracy of a few parts in 10⁸. Experimental investigations of individual components and comparison measurements with pure gases, with Edlén's formula and with the BCR interference refractometer confirm this goal.