Top: interferograms measured with an Hg discharge lamp and a room-temperature blackbody, band-pass filtered at 375 GHz. In the continuous interferogram, the Hg lamp is placed in the center of port A, while the center of port B is filled with the room-temperature blackbody. In the dashed interferogram the positions of the two sources are switched. Bottom: spectra obtained from the two interferograms above; the second spectrum has been multiplied by –1 to facilitate the comparison with the first one.