If we wished to record a trace in the time domain, it could be possible to do so by allowing radiation to fall on a detector and recording its response over time. This problem can be solved by using interference to modulate the i.r. signal at a detectable frequency. The Michelson interferometer is used to produce a new signal of a much lower frequency which contains the same information as the original IR signal. The output from the interferometer is an interferogram . The Michelson interferometer Radiation leaves the source and is split.
Half is reflected to a stationary mirror and then back to the splitter.
This radiation has travelled a fixed distance. The other half of the radiation from the source passes through the splitter and is reflected back by a movable mirror. Therefore, the path length of this beam is variable. If the movable mirror moves away from the beam splitter at a constant speed, radiation reaching the detector goes through a steady sequence of maxima and minima as the interference alternates between constructive and destructive phases. Because all wavelengths emitted by the source are present, the interferogram is extremely complicated.