Measurement of the refractive index of air and comparison with modified Edl n's formulae

Abstract

An air refractometer has been developed for accurate measurements of the refractive index of gases under well-defined conditions. Measurements have been performed for dry and moist air at four wavelengths distributed over the visible part of the spectrum. The measured values have been compared with values determined by modified Edlén's formulae and measured parameters. A fitting procedure is described to adjust the humidity dependence and the constant term of the dispersion formula so that optimum agreement is obtained. A standard deviation of 3 × 10-9 for the residuals describes the reproducibility of the measurements and the success of fitting. For 50 % relative humidity, a standard uncertainty of 10-8 for the fitted formulae is claimed, arising mainly from the uncertainty contributions of the air parameters.