Total power calibration in FTIR emission spectroscopy for finite interferograms

Abstract. The commonly used total power calibration procedure in FTIR (Fourier transform infrared) emission spectroscopy first described by Revercomb et al. (1988) is strictly speaking only valid for double-sided, infinite interferograms. Here we investigate the effect of interferogram truncation on calibrated mid-resolution emission spectra, describe the underlying theory, and quantify the errors in a case study. This quantification is important as the demands on precision in atmospheric measurements increase. While application of the Revercomb formula might lead to large errors in the unequal-sided case, we show that one can obtain the same spectral estimate as from equal-sided interferograms. This is achieved by modifying Revercomb's method incorporating a phase correction procedure using low-resolution phase spectra from the black body measurements. We include a case study simulating atmospheric and black body spectra with a line-by-line radiative transfer model. For a mid-resolution use case, we find that the effect of truncation on the spectral radiance measurement is generally well below 0.05 %. Only in spectral regions where strong absorption lines in the black body spectra are present can the errors be larger. However, those spectral regions are usually saturated in atmospheric spectra for observations from the ground and thus contain little information about the atmosphere except for the lowermost layer and can and usually should be excluded for most retrieval procedures.