A very similar equation has been developed by Bonsch and Potulski 5.Ĭiddor 1 and Rüeger 6 have argued that the equation derived by Ciddor should give somewhat better results than Birch and Downs over a broader range of wavelengths and under extreme environmental conditions of temperature, pressure, and humidity. The Birch and Downs version of the Edlén equation is probably the most commonly used equation for calculating index of refraction of air for precision measurements at temperatures near 20 ☌ (68 ☏). We suggest that, unless you are an expert in the field with a specific reason to choose one equation over the other, there is no need to worry about the small differences between the two equations-you can choose either one and need not read any further.
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Two basic methods for calculating the refractive index are available here-an equation developed by Ciddor 1 and an updated version of the Edlén equation 2,3,4 as modified by Birch and Downs and with an additional modification of our own. Various points on the web page shown in blue hypertext are linked to relevant sections of this article.
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The following text discusses some issues relevant to the calculation of index of refraction, provides guidance relating to the input parameters on the web page, and discusses the uncertainty of the results. This page is intended to serve as an aid to people doing interferometry who wish to check their own calculations of the refractive index against this NIST calculation.Īt present the web page calculates only the standard index of refraction (the phase index) certain modulated-light, multi-frequency, or pulsed-laser applications require the group index to properly account for the effects of the atmosphere. It is the wavelength in air that determines the basic length scale of measurements performed with laser interferometers. The web page also calculates the wavelength in ambient air, λ, from the vacuum wavelength λ vac and the refractive index n, through the relationship This web page 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). Wavelength Calculator based on the Modified Edlén Equation Wavelength Calculator based on the Ciddor Equation Edlén Calculation of the Index of RefractionĪppendix B: Simple Shop-floor Formula for the Refractive Index of Air Ciddor Calculation of the Index of RefractionĪ-IV.
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Comparison of Ciddor and Modified Edlén EquationsĪppendix A: Summary of Computations Used on the Web PageĪ-III. Comments Regarding Inputs to the Equations Edlén or Ciddor? Guidance in Choosing the Equation Refractive Index of Air Calculator is a web-based tool for calculating the index of refraction of air and wavelength of light in air as a function of various input parameters, using the Ciddor Equation or a modified version of the Edlén Equation. Johansson to Director Burgess on the International Reference Temperature Uncertainties Related to Thermal Expansion in Dimensional Metrology Uncertainties Related to Thermal Expansion in Dimensional Metrology Thermal Effects in Dimensional Metrology International Status of Thermal Error Research 20 ☌-A Short History of the Standard Reference Temperature for Industrial Dimensional Measurements Letter from C.