Fig. 1. Specular reflectance measurement curve of carbon nanotubes absorbing black materials at different incident angles

Fig. 2. Schematic diagram of ray tracing in absorption cavity

Fig. 3. Simulation diagram of irradiance distribution at the outlet of absorption cavity

Fig. 4. Schematic diagram of absorption cavity area division

Fig. 5. Schematic diagram of lateral expansion of absorption cavity

Fig. 6. Irradiance distribution of bottom surface of absorption cavity

Fig. 7. Irradiance distribution of the lower side of the absorption cavity （180°-360°）

Fig. 8. Irradiance distribution of the upper side of the absorption cavity （0°-180°）

Fig. 9. (a)Thermal link of cryogenic radiometer; (b) High absorption inclined bottom cavity of cryogenic radiometer; (c) Change of temperature of absorption cavity with time under different heating modes