AN UPDATED EDLEN EQUATION FOR THE REFRACTIVE-INDEX OF AIR

The Edlen equation for the refractive index of air, published in 1966, is still widely used. Subsequent to its formulation, however, improve d data have become available on the density of air and the refractivit y of water vapour. The practical temperature scale has also been revis ed and the carbon dioxide content of normal laboratory air has increas ed. These effects result in a discrepancy of typically 1 x 10(-7). The consequent revision of the equation brings the agreement between calc ulation and experiment within an experimental uncertainty of +/- 3 x 1 0(-8).