Investigation On Measurements Of Radiance In Far Infrared Wavelength Range At Cryogenic Temperature

The outer space is a vacuum,black and cold environment.The heat exchange between the spacecraft and the outer space is mainly carried out by means of heat radiation.In order to make sure the spacecraft work properly in space,thermal control materials are usually applied to the spacecraft surface to adjust the heat balance.The thermal materials are characterized by two main thermal properties: the solar absorption ratio ? and the emissivity ?.In deep space,due to the absence of solar radiation,the thermal materials emissivity plays a decisive role in regulating the thermal radiation characteristics of the system.Calculated by Wien's theorem,the wavelength of the thermal radiation wave is mainly in the far infrared wavelength range.The emissivity is defined as the ratio of the sample radiation to the black body radiation at the same temperature.To obtain the materials emissivity at low temperatures,it is important to measure a high-quality radiation of the thermal control materials in their application environment.The device is mainly composed of a low temperature sample chamber,a mechanical modulation system,and a Fourier transform infrared spectrometer.The temperature range of the sample chamber is 5~300 K and the temperature deviation at the set point is less than 0.2 K.By keeping the entire optical system at a high vacuum environment,the effect of water vapor is eliminated.A 4.2K Bolometer detector with a high sensitivity in the far infrared band is used to receive the radiation.The mechanical modulation of the chopper and the interferometer of Fourier transform infrared spectrometer on the step-scan mode modulation to extract the sample radiation effectively and eliminate the interference of the background radiation.The interferogram is converted into a radiance spectrum by computer software,thereby realizing the detection of small signals and obtaining a high signal-to-noise ratio far-infrared radiation.The measured far infrared radiance spectrums are used to calculate the emissivity of the materials in a reasonable calculation method.Through the measurement of materials,it is proved that the device can eliminate the interference of room temperature background radiation effectively,obtain the mid-far infrared radiation spectrum of the materials at low temperature.Uncertainty analysis of the emissivity reveals that the uncertainty is mainly caused by the sample temperature and the sample temperature deviation,the blackbody temperature and its temperature deviation,the blackbody emissivity and the temperature difference of the sample and the blackbody.The combined standard uncertainty of the emissivity measurement instrument at low temperatures was less than 3.302%.