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.

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For an absolute measurement of a density standard, mass and volume are measured separately. The absolute determination of the volume is still the quantity limiting the uncertainty. Lowest uncertainties are achieved by interferometric measurements of the diameter of spheres made from a silicon single crystal. So far, measurements have been based on a plane interferometer. At PTB, we demonstrate a new method of determining the diameter using a spherical interferometer, which yields more and particular information about the resulting volume.

The wavelengths of an Ar' laser and of a He-Ne laser at 515 nm and 612 nm, gespectively, have been compared with the wavelength of a He-Ne laser at 633 nm. All the lasers involved were stabilized by using saturated absorption in iodine-1 27. The wavelength ratios were related to the He-Ne laser at 633 nm stabilized to the hyperfine component i of the rotational and vibrational transition R(127) 11-5 of iodine. For the Ar' laser locked to the component a3 of the P(13) 43-0 line, the ratio determined was ha3/Xi = 0.813 081 2954 x (1 f 5 * lo-'') and for the 6 12 nm He-Ne laser locked to the component o of the R(47) 9-2 line, the measured ratio was &,/hi = 0.966 791 6050 x (1 i-3 * IO-'').

Phase errors that arise in phase-stepping interferometry are discussed. Investigations were performed by use of a Twyman-Green interferometer equipped with a compensation plate with a variable and servo-controlled tilt angle. With this instrument, phase-stepping errors can be reduced to a negligible level. There are, however, phase errors that are caused by camera nonlinearities. Two methods for minimizing these errors are presented. The first method is based on the simple idea that the interference intensity at the output of a two-beam interferometer has an exact cosine shape. The camera signals were monitored as a function of the tilt angle of the compensation plate, and the deviation from the cosine form was used to produce a correction. The second method is based on the idea that, under specific conditions, errors of an average of two phase measurements may compensate for each other. Numerical calculations were performed and give evidence of this hypothesis. Each method, the signal-correction and the averaging method, drastically reduces errors in evaluation of phases. The combination of both methods is a powerful tool that allows precise phase data to be obtained with an uncertainty, in the range lambda/2000 approximately 0.3 nm, that is caused mainly by signal noise.