Index of Refraction of Air
Jack A. Stone and Jay H. Zimmerman

Abstract:  These web pages are intended primarily as a computational tool that can be used to calculate the refractive index of air for a given wavelength of light and given atmospheric conditions (air temperature, pressure, and humidity). The calculations are performed using either the Ciddor¹ equation or an updated version of the Edlén equation^2,3,4 with a modification of the water vapor term so as to improve accuracy for shop-floor use at temperatures departing substantially from 20 °C. The text accompanying the calculations explains our modification of the Edlén water vapor term, provides a tutorial for users of the web page, and reviews and discusses the relative merits of the two equations, the range of validity, and the estimated uncertainty of the calculations. In addition, the text gives a complete description of all calculations employed by the web page so that others desiring to reproduce the calculation can do so. Finally, we propose a simplified formula that can be used for shop-floor computations of the refractive index when only a hand calculator is available.

1.  Phillip E. Ciddor, "Refractive index of air: new equations for the visible and near infrared," Appl. Optics 35, 1566-1573 (1996).
2.  B. Edlén, "The refractive index of air," Metrologia 2, 71-80 (1966).
3.  K.P. Birch and M.J. Downs, "An updated Edlén equation for the refractive index of air," Metrologia 30, 155-162 (1993).
4.  K.P. Birch and M.J. Downs, "Correction to the updated Edlén equation for the refractive index of air," Metrologia 31, 315-316 (1994).

Key words:  air refractive index, Ciddor equation, Edlén equation, index of refraction, interferometry

Place of Publication:  electronic, NIST web page