Fourier transform spectrometry is a technique in which the spectrum of a light source is estimated by taking the following steps:

An interferometer that can generate a large optical path difference will allow a high physical resolution in the spectrum domain. Since a Michelson interferometer cannot produce an infinite optical path difference, the autocorrelation signal produced will always be truncated, that is, multiplied by a rectangle (or boxcar) function. Since the interferogram is Fourier-transformed, this means that all the spectrum gets convoluted with a sinc function. The greater the optical path difference, the larger is the rectangle and the narrower the sinc is.

The Jacquinot (or throughput, or Ă©tendue) advantage of the Fourier transform spectrometers (over dispersive spectrometers) is the fact that information is multiplexed in a FTS so that no spatial dimension is lost to retrieve it. For a given measure time and physical resolution, a FTS will always get more photons on the detector.