Hohlraum effect

Examples

• In a cavity with optically thick walls, or Hohlraum, a material surrounded in this cavity will achieve the same temperature in time as the surroundings and will become indistinguishable from the Hohlraum’s surrounding walls. It becomes indistinguishable because everything in and surrounding the cavity is emitting at the same temperature and also reflecting the emissions of the cavity. Since by Kirchhoff’s law emissivity + reflectivity = 1, everything in the cavity appears the same. ... Another example of this Hohlraum effect can be seen in the process of ashing a chemical sample or firing clay in an oven. As everything in the oven comes to the same temperature, the individual objects inside the oven disappear and cannot be seen or at least are very hard to see. Radiatively, everything has to reach an equilibrium state where photons are continually absorbed or reflected and those absorbed are re-radiated, but all at the same blackbody temperature. Hence, everything optically blends together and contrast between the walls and the object effectively disappears. - Mitchell, Herbert J.; Salvaggio, Carl (2003); "The MWIR and LWIR Spectral Signatures of Water and Associated Materials" in Shen, Sylvia S.; and Lewis, Paul E. (ed.), Algorithms and Technologies for Multispectral, Hyperspectral, and Ultraspectral Imagery IX, vol. 5093, Proceedings of SPIE