An Updated Edlén Equation for the Refractive Index of Air

TLDR

The Edlén equation, published in 1966, remains widely used but newer data on air density, water‑vapour refractivity, the revised temperature scale, and rising CO₂ levels necessitate an update. The updated equation reduces the typical 1 × 10⁻⁷ discrepancy, achieving agreement with experiment within ±3 × 10⁻⁸.

Abstract

The Edlén equation for the refractive index of air, published in 1966, is still widely used. Subsequent to its formulation, however, improved data have become available on the density of air and the refractivity of water vapour. The practical temperature scale has also been revised and the carbon dioxide content of normal laboratory air has increased. These effects result in a discrepancy of typically 1 × 10-7. The consequent revision of the equation brings the agreement between calculation and experiment within an experimental uncertainty of ± 3 × 10-8.