The historical problem with step-scan FT-IR is undoubtedly caused by low-frequency variations in the refractive index of the air inside the spectrometer, particularly the interferometer. Other important multiplicative noise sources include, at least, temperature induced fluctuations of the pyroelectric detector and beamsplitter efficiencies, and steering of the source image on the detector.

The importance of refractive index variations to rapid-scan FT-IR spectrometer performance has been largely overlooked. It is likely that temperature fluctuations are the limitation that has kept the SNR of rapid-scan spectrometers from reaching theoretical (detector noise limited) performance. If step-scan is done correctly, its SNR will approach that of rapid-scan.