FT-IR photothermal beam deflection (PBD) signals are several orders of magnitude smaller than PBD signals generated in other spectral regions and by other means, so that vibrational rather than electronic noise is important to the production of FT-IR/PBD) spectra at present. The interferometer of the PBD spectrometer produces mechanical vibrations, but the most serious disturbances arise in the environment and are of unknown origin and uncontrollable. The mechanical vibrations imposed on the optics lead to random, broad-band noise which can be decreased by multiple scanning, as well as to noise at discrete frequencies which can be nonrandom and consequently not be diminished by multiple scanning. Some types of such noise and their effects are described and discussed.
M. J. D. Low, "Some Practical Aspects of FT-IR/PBDS Part I: Vibrational Noise," Appl. Spectrosc. 40, 1011-1019 (1986)