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

Abstract 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 0.632 9908 μ m and 1.542 383 μ m , and cover temperatures ( 20 < t 90 < 100 ) ∘ C and pressures p < 0.5 MPa − 1 . For water vapor, the refractivity ratio was studied for six wavelengths ( 1.52 < λ < 1.57 ) μ m relative to a simultaneous measurement made at 0.632 9908 μ 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 10 3 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.