Hohlraum

In <a href="/facts/Radiation/RwMocyeq">radiation</a> <a href="/facts/Thermodynamics/q4hIx3yP">thermodynamics</a>, a hohlraum is a cavity whose walls are in <a href="/facts/Radiative_equilibrium/bDE3ryJB">radiative equilibrium</a> with the <a href="/facts/Radiant_energy/8QHpm6KY">radiant energy</a> within the cavity. First proposed by <a href="/facts/Gustav_Kirchhoff/Ygfzlywy">Gustav Kirchhoff</a> in 1860 and used in the study of <a href="/facts/Black-body_radiation/K4YKPKXL">black-body radiation</a> (hohlraumstrahlung), this idealized cavity can be approximated in practice by a hollow container of any <a href="/facts/Opacity_(optics)/l90q0sAV">opaque</a> 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. Indeed, a hohlraum can even be constructed from cardboard, as shown by Purcell's Black Body Box, a hohlraum demonstrator.
In spectroscopy, the Hohlraum effect occurs when an object achieves thermodynamic equilibrium with an enclosing hohlraum. As a consequence of <a href="/facts/Kirchhoff%2527s_law_of_thermal_radiation/JCLTqWeF">Kirchhoff’s law</a>, everything optically blends together and contrast between the walls and the object effectively disappears.

Hohlraum open-in-new

Hohlraum