Abstract

A Fourier transform spectrometer has been equipped with a liquid helium cryostat to measure transmittance over the wavelength range from 1 to 1000 μm. The infrared filters selected for measurement are of interest to infrared astronomy performed as part of NASA's Cosmic Background Explorer (COBE) program. The filters were constructed from individual substrates overcoated with multilayer thin films or from a combination of multilayer types and reststrahlen crystals plus mesh grid interference filters. Their bandwidths are quite broad, ranging from ~15% to almost 70% of the central wavelength in some cases. Filters consisting of multilayer stacks on individual substrates were used at wavelengths shorter than 10 μm. These exhibited bandshifts of the order of 2% when cooled from 300 to below 4 K. The temperature dependence of the composite filter transmittances was dominated by the narrowing of the various crystal reststrahlen reflectance bands as filter temperature dropped from 300 to 2 K. Filters with bandpasses below 10 μm also became highly transmitting at wavelengths beyond the reststrahlen band regions of their substrates where the multilayer band shaping films had absorptances too small to be of consequence.

© 1985 Optical Society of America