In a normal Michelson interferometer, modulated radiation is transmitted and also reflected back toward the source of the radiation. The back-reflected radiation is generally ignored but it can be exploited to provide a very simple means of modifying an FT-IR spectrometer to measure emission spectra, without the necessity of access to the source or source optics. Radiation from the emission source is directed into the interferometer from the direction of the normal sample location. Modulated radiation is reflected back from the interferometer through the sample area to be measured by the normal detector. It has been noted previously that radiation originating from the sample area can contribute to the observed spectrum. However, this has been regarded as a problem rather than as a means of obtaining useful measurements.
M. A. Ford and R. A. Spragg, "A Simple Method of Measuring Emission Spectra with an FT-IR Spectrometer," Appl. Spectrosc. 40, 715-716 (1986)