Some methods and facilities for the measurement of thermo-optical properties as total hemispherical and normal emissivity as well as directional emissivity and directional solar absorptivity are presented.
Total hemispherical and normal emissivity result from the measurement of the heat flux radiated from the surface of a sample, which rotates in a isothermal heating zone of controllable temperature. This method is realized for emissivity measurements on e. g. spacecraft and thermal protection materials in the temperature range 150 K ≤ T ≤ 2000 K under vacuum conditions and at adjustable air pressures simulating typical exploitation conditions. This method and the corresponding facility have been modified for the evaluation of directional emis-sivities of materials and coatings. Inserting diaphragms of adapted apertures between radiating surface and detector enables the measurement of the axis-symmetric radiation heat fluxes under certain angles. At present, the facility permits investigations on test samples at temperatures up to 1100 K under vacuum conditions.
Directional absorptivities are determined by means of a calorimetric method. A collimated flux irradiates a test sample at adjustable temperature. The method is realized for the evaluation of directional solar absorptivities under simulated space environment.
The methods and facilities described in the following enable the investigations practically on any type of spacecraft and thermal protection materials, components as tiles, honeycomb and other flat samples as well as of thermal control coatings. The test results of different materials and coatings are presented including the emis-sivities of coated and non-coated Flexible External Insulation (FEI) blankets which have been developed in the frame of the ESA funded HERMES and MSTP programs.