Kirchhoff's law of thermal radiation

At thermal equilibrium, a body's emissivity equals its absorptivity at the same wavelength, direction, and polarization. The law links how well a surface emits thermal radiation to how well it absorbs it.

Thermal equilibrium: A state in which macroscopic flows of energy and matter cease and temperature is uniform. The law applies when matter and radiation share a common equilibrium temperature.
Thermal radiation: Electromagnetic radiation emitted by matter because of its temperature. The law concerns the emission and absorption of this radiation.
Gustav Kirchhoff: A German physicist whose work included circuit laws, spectroscopy, and thermal radiation. He formulated the relation between emission and absorption that bears his name.
Thermal imaging: Imaging that maps temperature-related infrared radiation from objects. Reliable temperature estimates require accounting for surfaces' wavelength-dependent emissivity.
Wien's displacement law: A relation giving the wavelength of maximum blackbody emission as inversely proportional to temperature. It predicts a spectral peak, whereas Kirchhoff's law equates emission and absorption.
Emissivity: A surface's emitted thermal radiation relative to that of a blackbody at the same temperature and conditions. It is one side of the equality, defined for matching wavelength and direction.
Spectral radiance: Radiant power per projected area, solid angle, and frequency or wavelength interval. It expresses radiation directionally and spectrally, matching the law's conditions.
Balfour Stewart: A Scottish physicist who studied the emission and absorption of radiant heat in the 1850s. His experiments and reasoning anticipated the connection Kirchhoff later generalized.
Infrared thermography: A measurement technique that infers surface temperatures from detected infrared radiation. The law connects a surface's measured infrared emission to its absorptive properties.
Stefan–Boltzmann law: A law stating that blackbody radiant exitance is proportional to the fourth power of absolute temperature. It governs total emitted power rather than matching emissivity to absorptivity by mode.

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