Modeling the responsivity and self-emission of a double-beam Fourier-transform infrared interferometer

A modeling study aimed at characterizing the radiometric properties of a double-beam Fourier-transform infrared interferometer is presented. Measurements showed that the two responsivities associated with each interferometer channel are different in certain spectral regions. This anomaly was attributed to a dissymmetry between the optical transmissions of the two plates that form the beam splitter. This dissymmetry is primarily responsible for the instrument residual emission. A secondary cause of residual emission is attributed to the relative alignment of the two input optics. Both effects were taken into account in a model that gives the instrument residual emission in terms of the beam splitter temperature. Actual results indicate that in the 7-14-microm window the instrument residual emission can be modeled with an absolute radiometric error smaller than 0.5 K (blackbody at 290 K). The model was used to develop an automatic calibration procedure that yields radiance errors smaller than 0.05 microW/cm(2) sr cm(-1) in the 7-14-microm band. The radiometric stability of the interferometer was analyzed.