Mylar beam-splitter efficiency in far infrared interferometers: angle of incidence and absorption effects.

Absorption and complete multipath interference have been included in the calculation of the modulation efficiency of a far i.r. Michelson interferometer with 25-mum and 100-mum Mylar beam splitters over the range of wavenumbers between 20 cm(-1) and 125 cm(-1) for angles of incidence between 45 degrees and 80 degrees . These results show that optimum performance of an interferometer in terms of highest and most uniform modulation efficiency for unpolarized radiation will be obtained by selecting the beam-splitter thickness to cover the wavenumber range of interest within the first interference lobe and by utilizing higher angles of incidence than the mechanically convenient 45 degrees . Practical constraints will usually limit this angle to a value somewhat less than the optimum of about 75 degrees for Mylar beam splitters.