Time-domain spectroscopy
Spectroscopy done with visible or infrared radiation involves wavelengths of 1012 to 1015 Hz.  Phototransducers (instruments that convert light to information readable by a computer) cannot make measurements that quickly.  What is needed, then, is a way to make these high-frequency signals into low-frequency signals.  For optical spectroscopy (such as infrared spectroscopy) the most common way to do this is by using a Michelson interferometer.