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

Published

June 3, 2026

Author(s)

Patrick Egan

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 0.6329908 um and 1.542383 um, and cover temperatures (20 < t_90 < 100) degC and pressures p < 0.5 MPa. For water vapor, the refractivity ratio was studied for six wavelengths (1.52 < \lambda < 1.57) um relative to a simultaneous measurement made at 0.6329908 um. The carbon dioxide and oxygen measurements establish the molar refractivities and second density virial coefficients more accurately than anything done before. A Sellmeier 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 group index.

Citation

Measurement Science and Technology

Volume

37

Issue

25

Pub Type

Journals

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Keywords

refractometry, Ciddor equation, length metrology, thermodynamic metrology

Citation

Egan, P. (2026), Two-color measurements supporting a revised formulation for the refractive index of air, Measurement Science and Technology, [online], https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=961981 (Accessed June 23, 2026)

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