An Updated Edlén Equation for the Refractive Index of Air

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.