7. Appendix: advantages of Fourier transform spectroscopy

7.1 Basic equation for Fourier transform spectroscopy

Fourier transform spectroscopy makes use of the properties of two-wave interference: when two coherent, quasi-monochromatic waves interfere, the resulting intensity is a sinusoidal function of the step difference between the two waves. The amplitude of this sinusoid is proportional to the flux received, and its frequency is proportional to the wavenumber (the inverse of the wavelength). In Fourier transform spectroscopy, coherence is ensured by the fact that these two waves originate from the same primary wave, after passing through an interferometer. If this wave is not monochromatic, the detector's output current, measured as a function of the step difference and referred to as the interferogram, becomes a sum of sinusoids, whose...