Thibault and coworkers tell that the "spectral lines were individually fitted to the Voigt profile and the hard collision model profile of Rautian and Sobel'man. Since the collisional width determined from both models was within the error bars of the fitted parameter, [Thibault and coworkers] used only the Voigt profile for all measured lines. Figure [in shows, as an example, the Q(5) line at 195 K and total pressure of 123.8 mbar for a 3% CO/Ar mixture spectrum. From the observed fitted residuals, it can be seen that the Voigt model fits the observed profile to within the noise". Being within the noise, the Voigt line is proper for fitting.