The computed atmospheric transmission is a number between 0 and 1, and the sampling here is over either the wave number or the wavelength. Intrinsically, the line-by-line computation is computed as a function of the wave number (the inverse of the vacuum wavelength). The unit of the default wavenumber wavnum is the cm-1, which results in the formula: wavnum (cm-1)= 10000/ (vacuum wavelength in µm).<br>
The user has the choice of three systems of units for the output: <br>
- Vacuum wavelength (nm).<br>
- Standard wavelength (nm).
The vacuum wavelength is transformed into air wavelength, at a pressure of 760 mm Hg (1 atm) and temperature +15°C. This is usually the standard atmospheric conditions used for the calibration of all spectrometers, since lines of elements in calibration lamps (example: Thorium-Argon lamps) are given in this standard reference system. We use the air index of refraction as a function of wavenumber of Edlen (B. Edlen, «The refractive Index of Air», Metrologia Vol. 2, No. 2, 1996).<br>
- Wavenumber (cm-1). As for the vacuum wavelength, there is no need for refraction correction.