| 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. | 
