In Fourier Transform Infrared (FTIR) spectroscopy, light from a broad spectrum point-like source is collimated and input into a split beam interferometer, such as a Michelson. Displacing one of the mirrors sinusoidally using a motorized mount creates a time dependent optical path difference (OPD) between the two interferometer arms. This OPD corresponds to a different number of wave cycles for each spectral component in the source. Spectral components that undergo constructive interference will output the system and be focused onto a detector. Spectral composition of the source may be calculated by taking the Fourier transform of the detected light intensity vs time. In FTIR, the maximum spectral resolution depends on system étendue, which is often limited by the size and collection angle of the detector. Off axis parabolic mirrors allow large collection angles without chromatic aberrations, which helps FTIR systems achieve high spectral resolution over wide spectral ranges.

Compared with on-axis parabolic or ellipsoidal reflectors, and compared with lenses of similar light gathering capability, these off axis mirrors offer more convenient access to the focal point for placement of sources, detectors, and associated mounting hardware.