What is an interferogram in Fourier-transform spectroscopy?

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In Fourier-transform spectroscopy, an interferogram is the recorded signal of light intensity produced by an interferometer as the path length of one of its beams is varied.1

In a Michelson interferometer, light is split into two paths and then recombined. By moving one of the mirrors, the instrument creates a modulation in the light intensity at the detector based on the interference pattern of the recombined waves. This resulting signal, which contains information about many wavelengths simultaneously, is known as the interferogram. To obtain the final spectrum, a mathematical operation called a Fourier transform is applied to this recorded modulation, converting it into the frequency domain.1