Fourier Transform instruments make use of
interference rather than
wavelength dispersion to collect spectral data. With a
wavelength dispersion instrument, the instrument spreads the source radiation out according to wavelength and then measures the response of the chemical system to narrow wavelength regions. With a Fourier transform instrument, the instrument measures the spectral response to all wavelengths simultaneously. This is done by collecting the data with respect to time rather than with respect to frequency (as is done in a dispersion instrument).
This manner of data collection is accomplished by passing the broadband source electromagnetic radiation through an interferometer. The interferometer is composed of two arms that are separated from each other by a beam-splitter. The beam splitter is 50% reflective and 50% transmissive; therefore, it divides the input radiation into two equal portions, and directs these portions of the source radiation through the different paths of the interferometer. At the end of one path is a fixed mirror that reflects that portion of the light back toward the beam-splitter. For the fixed-mirror path of the interferometer, the total path length is fixed and constant. At the end of the other path is a moving mirror that reflects the light back toward the beam-splitter.
The path-length of this arm varies during the course of the measurement. The light from the two arms of the interferometer are combined at the beam-splitter and directed through the sample and, ultimately to the detector.
For any given wavelength, when the light recombines at the beam splitter (after travelling down different arms of the interferometer), constructive or destructive interference will occur depending on the path length difference between the two arms of the interferometer. If the path-length differs by an integer multiple of the wavelength, then constructive interference occurs. If the path-length differs by a multiple of 1/2-wavelength, then destructive interference occurs. Because we are monitoring all wavelengths simultaneously, the output of the interferometer (the interferogram) is the superposition of all the wavelengths of the light that are emitted by the source.