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Radiation heat transfer is concerned with the exchange of thermal radiation energy between two or more bodies. Thermal radiation is defined as electromagnetic radiation in the wavelength range of 0.1 to 100 microns (which encompasses the visible light regime), and arises as a result of a temperature difference between 2 bodies.

No medium need exist between the two bodies for heat transfer to take place (as is needed by conduction and convection). Rather, the intermediaries are photons which travel at the speed of light.

The heat transferred into or out of an object by thermal radiation is a function of several components. These include its surface reflectivity, emissivity, surface area, temperature, and geometric orientation with respect to other thermally participating objects. In turn, an object's surface reflectivity and emissivity is a function of its surface conditions (roughness, finish, etc.) and composition.

Absorption and Emissivity
Radiation heat transfer must account for both incoming and outgoing thermal radiation.

Incoming radiation can be either absorbed, reflected, or transmitted. This decomposition can be expressed by the relative fractions,

Since most solid bodies are opaque to thermal radiation, we can ignore the transmission component and write,
To account for a body's outgoing radiation (or its emissive power, defined as the heat flux per unit time), one makes a comparison to a perfect body who emits as much thermal radiation as possible. Such an object is known as a blackbody, and the ratio of the actual emissive power E to the emissive power of a blackbody is defined as the surface emissivity e,
By stating that a body's surface emissivity is equal to its absorption fraction, Kirchhoff's Identity binds incoming and outgoing radiation into a useful dependent relationship,