Reflectance Measurements on Sub-millimetre Absorbing Coatings for HIFI

摘要

Specular and diffuse reflectance (BRDF) of black absorbing coatings, meant to be used for the HIFI instrument aboard the FIRST satellite, has been studied in the sub-millimetre region (0.1< lambda <0.9mm). These coatings have to meet space qualification requirements and must be usable for at least the overall wavelength band (0.1-0.6mm) covered by the HIFI spectrometer. Existing materials, coatings obtained from other research groups and home made samples ahve been studied. Optical characterisation fo these coatings has been performed at waveengths of 96.5 mu m, 118.8 mu m, 184.3 mu m, 496 mu m and 889 mu m, for a large range of directiosn of incident and reflected light and for different polarisation directions. A limited number of reflectance measurements at cryogenic temperatures have been carried out too. A simple experimental set up to study the effect of double scattering has been constructed to investigate the accuracy of numerical simulations based on experimental BRDF values. Data show that the best samples (home made) have BRDF values below about 2.10~(-2) Sr~(-1) throughout the wavelength range of interest, quite independent of directiosn of incidence, reflection and polarisation. The Total Hemispherical Reflection of such a coating will then be 0.06.