Hohlraum
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In radiation thermodynamics, a hohlraum (German: [ˈhoːlˌʁaʊ̯m] ⓘ; a non-specific German word for a "hollow space", "empty room", or "cavity") is a cavity whose walls are in radiative equilibrium with the radiant energy within the cavity. First proposed by Gustav Kirchhoff in 1860 and used in the study of black-body radiation (hohlraumstrahlung),[1] this idealized cavity can be approximated in practice by a hollow container of any opaque material. The radiation escaping through a small perforation in the wall of such a container will be a good approximation of black-body radiation at the temperature of the interior of the container.[2] Indeed, a hohlraum can even be constructed from carboard, as shown by Purcell's Black Body Box, a hohlraum demonstrator.[3]
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This black-body-ness gives rise to the Hohlraum effect: 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 of thermal radiation 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.[4]