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. An efficient scheme for making such measurements is presented. Radiation from the emission source is directed into the interferometer by a 45 degree mirror. This mirror is positioned so that one edge is located at the center, J, of an image of the Jacquinot stop, which is typically found at the normal sample position. Viewed from the detector, this mirror covers one half of the Jacquinot stop image. The radiation reflected back from the interferometer is imaged at the other half of the Jacquinot stop image and so passes to the detector. Emission measurements are made with the instrument source switched off so that it cools to ambient temperature. It should be pointed out that black-body radiation from the detector which enters the interferometer is not reflected back onto the detector as it is intercepted by the mirror. 2 refs.
