Equipment for precise measurements of the complex refractive index of gases as a function of wavenumber, temperature and pressure

The apparatus in precise scanning-wavelength interferometry with gases is described. The dispersion of the refractive index is measured with the aid of an evacuated Michelson interferometer in the wavelength range 400<or= lambda <or=700 nm with precision mod Delta (n-1) mod =2*10 -8 and an uncertainty in the absolute values of mod Delta (n-1)/(n-1) mod =2*10 -4 . The gas pressure is determined absolutely using a repeatedly calibrated pressure transducer to about mod Delta p mod =5 Pa or mod Delta p/p mod =10 -4 in the pressure range 0-3*10 5 Pa. The temperature is measured with precision mod Delta T mod =1 mK and uncertainty mod Delta T mod =10 mK using platinum resistance thermometers and voltage ratio measurements with the aid of a digital voltmeter. The system of automatic data acquisition is also described.