Two-color measurements supporting a revised formulation for the refractive index of air

Two-color refractivity measurements are reported for carbon dioxide, oxygen, binary mixtures of nitrogen-oxygen, natural dry air, and water vapor. For the 'dry' gases, the measurements were simultaneously performed at $0.6329908\mu \text{m}$ and $1.542383\mu \text{m}$ , and cover temperatures $(20<\left(t_{90}<100\right)$ °C and pressures $p<0.5{\text{MPa}}^{-1}$ . For water vapor, the refractivity ratio was studied for six wavelengths $\left(1.52<\lambda <1.57\right)\mu \text{m}$ relative to a simultaneous measurement made at $0.6329908\mu \text{m}$ . The carbon dioxide and oxygen measurements establish the molar refractivities and second density virial coefficients more accurately than anything done before. A Sellmeier equation is formulated for oxygen which is about 10³ times more accurate than extant knowledge dating from 1932. The nitrogen-oxygen measurements are used to derive the second density cross virial coefficient. The natural dry air measurements provide a targeted refinement to the Ciddor formulation for the refractive index of air, because they establish both a reference value for refractivity and the second density virial coefficient. The water vapor measurements ensure an accurate Ciddor formulation at telecom wavelengths, and guide the implementation of the group index.