Simple Emissivity Test Methods for Spacecraft Thermal Materials

Instrumentation and test methods have been developed for simple medium-resolution measurements of the emissivity of materials used as thermal control coatings for spacecraft. At typical operating temperatures of spacecraft, ranging from ~150 K to ~400 K, blackbody spectra are dominated by infrared emission from ~2 µm to ~40 µm that has peak emission from ~7 µm to ~19 µm. Measurements of the IR radiant power emitted from a sample taken using a standard noncontact infrared pyrometer to determine the effective temperature are compared to sample temperature measurements using a direct contact thermocouple. Estimates of the emitted power using the Stephan-Boltzmann T⁴ law—or more accurately from the Planck distribution integrated over the ~8 µm to ~14 µm wavelength window of the IR detector—allow determination of the sample emissivity to <0.02. Improvements to the energy balance model, instrumentation enhancements, and calibrations to improve the accuracy of the method are discussed. Measurements of emissivity are presented for several spacecraft materials from room temperature to ~400 K to investigate the accuracy and precision of the method.