Objective  Blackbody radiation source is a standard instrument used to calibrate radiation thermometer and infrared thermal imager, etc. Weather satellites are equipped with blackbody radiation source for real-time calibration in orbit. As one of the most important indicators of black body, infrared spectral emissivity is defined as the ratio of the radiation capacity of an object to that of an ideal blackbody at the same temperature and wavelength. The actual emissivity of the blackbody is closely related to the surface microstructure of the blackbody and the emissivity of the coating material, that is, the emissivity of the blackbody is affected by the radiation characteristics of the surface coating under a specific structure. Therefore, it is necessary to explore the infrared radiation characteristics of blackbody coating, which is of great significance to achieve high-precision infrared remote sensing calibration.

  Methods  Based on the emissivity measuring device of controlling environmental radiation emission ratio method established by National Institute of Metrology, China (Fig.1), three kinds of high-emissivity blackbody coatings of different materials were prepared by manual spraying (Fig.3), and factors affecting the spectral emissivity of the coatings were explored, including the measuring environment, thickness and angle. The spectral emissivity of the blackbody coating before and after irradiation was compared by simulating the space radiation environment with ground equipment.

  Results and Discussions   There are a lot of gases such as carbon dioxide and water vapor in the air, and the spectral emissivity of the three coatings all show a large jitter at 5-7 µm in the atmosphere, while the emissivity results are relatively smooth under vacuum (Fig.4). When the blackbody coating thickness is relatively thick, the emissivity decreases by 0.01 at most (Tab.1). The reason is that when the coating thickness is relatively thick, the pores will decrease, which is not conducive to the multiple reflection and absorption of electromagnetic wave (Fig.6). The emissivity of GR and Nextel 811-21 coatings is almost constant while that of Nextel 811-21+MWCNT coatings is slightly reduced when the measurement angle is in the 30° range. Space irradiation has little influence on the emissivity of the coating. The maximum emissivity variation of GR coating is 0.000 9, the maximum emissivity variation of Nextel 811-21 coating is 0.001 7, and the maximum emissivity variation of Nextel 811-21+MWCNT coating is 0.002 1 (Tab.4).

  Conclusions  The emissivity of blackbody coating will change under the influence of measurement environment, measurement angle, coating thickness and space irradiation. The emissivity under vacuum coating is similar to that under atmosphere, but the measured results are better than those under atmosphere. When the measured angle changes within 30°, the spectral emissivity of GR and Nextel 811-21 coating is basically unchanged and still high, while the Nextel 811-21+MWCNT coating decreases with the increase of the measured angle. When the coating of GR and Nextel 811-21 is thick, the porosity of the coating will be reduced, resulting in lower emissivity, which is not conducive to high-precision infrared remote sensing calibration. The surface color of the coating changed from black to gray black after irradiation by ground simulation equipment, but the coating did not fall, crack and other phenomena did not occur, which proved that the coating has a certain spatial stability. At the maximum irradiation dose, the emissivity of GR coating decreases by 0.000 9, that of Nextel 811-21+MWCNT coating decreases by 0.002 1, and that of Nextel 811-21 coating increases by 0.001 7.