A robust method is described for calculating temperature, mole fraction, and pressure from measured absorption spectra (absorption coefficients versus optical frequency). The key components to the method are smoothing, differentiation, spectral axis warping, and linear least-squares fitting. The method works best when spectra span a full rotational branch of the target molecule, but in principle it works for any spectral span. The examples presented assume a measured spectrum over the 7246.4–7518.8{\text{cm}}^{-1} range, which encompasses the R branch of the v_1+v_3 band of {\text{H 2}}_{}\text{O} ; however, the techniques should work for most measured spectra.

© 2007 Optical Society of America

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