Instrument concept: Michelson interferometer
IASI is a spectrometer, that is an instrument that measures the spectral distribution of radiation.
The instrument concept is based on the Michelson interferometer. Incident radiation is divided into two beams by a beamsplitter (A): the first beam follows a path of constant length (B); the other is reflected by a moving mirror and follows a path of variable length (C).
The difference between both paths is called the optical path difference. The energy of the beams, when they recombine on the detector (D), varies with the path difference. It is maximum when the path difference is a multiple of the wavelength (beams in phase). It is zero when the path difference is an odd multiple of half the wavelength (beams in phase opposition).
Energy on the detector thus varies with the movement of the corner cube mirror. This variation produces what is called an interferogram, which represents the Fourier transform of the spectral distribution of analysed radiation.
The electrical signal from the detector is then digitized before performing a mathematical inverse Fourier transform to restitute the incident radiation spectrum.