Michelson Interferometer: Relationship between fringe visibility, alignment, and ILS

In single-passed FTIR spectrometers, spectra can be affected by shear misalignment, for example, due to pad wear gradually lowering the moving hollow cube-corner retro-reflector (HCCRR) over a period of months or years. This slowly reduces the modulation depth of the measured igram, reducing spectral signal and hence SNR, and distorting the ILS. The GFIT algorithm currently assumes a theoretical ILS (no misalignment) and so produces a bias in the retrieved columns for misaligned instruments. This bias doesn’t completely cancel when ratioing against retrieved O2 due to line depth and wavenumber differences. Current ILS retrieval algorithms (e.g., LINEFIT) produces an empirical ILS from low-P gas cell measurements in the HCl overtone band at 5670 cm-1 (TCCON) or the HBr fundamental at 2559 cm-1 (IRWG), and extrapolate to other wavenumbers by assuming a known functional forms for the ME and Phase. A physics-based ILS approach would require fewer parameters. It might make alignment easier by expressing the current alignment status in terms of tangible physical parameters.