Response of fourier-transform spectrometers to absorption and emission in a homogeneous single-layered beam splitter

Thermal self-emission contributes to interferograms measured with Fourier-transform infrared spectrometers. If the beam-splitter is almost transparent, the complex spectral amplitude that is due to the detector port emission is opposite that of the input port, whereas the amplitude that is due to the beam-splitter emission is in quadrature. The situation of an absorbing beam splitter is examined here. The volume beam-splitter emission is modeled by a superposition of dipole sources spread in an absorbing film. Angular polarization correlations are taken into account. It is found that the phase relations between the complex spectral amplitudes are affected. Numerical data are given for experimental conditions adapted to those of the airborne limb sounder MIPAS-FT.