Heat transfer by radiation

At sufficiently high temperatures, materials lose energy (or heat) primarily by radiation, which is the emission of electromagnetic energy. This electromagnetic energy is most readily observed as the emission of visible light, which occurs for materials above about 550C, with the tungsten wire filament of a conventional light bulb or tungsten-halogen lamp indicative of the very bright light emission for materials at much higher temperature (of order 3000C). In these cases, the power radiated as visible light is accompanied by infrared (lower energy, or longer wavelength light) radiation, which can be sensed as heat but not seen with the eye.

The power radiated from a body at temperature T is given by the Stefan-Boltzman law:

In addition, the power is distributed over a continuous spectrum (the Planck distribution) of wavelengths of light (or photon energies), with the distribution characterized by a wavelength of peak power (which shifts to shorter wavelength or higher photon energy at higher T), and a long tail of energy below the peak photon energy.

The Stefan-Boltzmann law demonstrates that bodies at high temperature radiate large amounts of power (the sun is a prime example). This is essentially because of the T^4 dependence of the radiated power: if the absolute temperature (degrees K) doubles, then the radiated power increases by 2^4 = 16-fold. When the body radiates power, it loses energy (power is just the rate of energy change), so that it cools. If radiative cooling is an important energy exchange mechanism, as when the temperature is high and radiated power large, then the rate of energy loss is large, and the cooling is fast.

Since the temperature a material body reaches (e.g., a wafer in lamp heating arrangements) is determined by a steady-state balance of power absorbed and power emitted (or radiated), raising a body to a high temperature requires large power input and absorption by the body, in order to balance the high radiation loss energy.

Finally, the emissivity or absorptivity of a material is a dimensionless constant between 0 and 1 (larger values are more efficient materials for radiating power). In reality the emissivity/absorptivity is wavelength-dependence which is determined by the specific electronic structure of the material at or near the surface.