Emissivity

The emissivity of the surface of a material is its effectiveness in emitting energy as <a href="/facts/Thermal_radiation/1PD9jtCv">thermal radiation</a>. Thermal radiation is <a href="/facts/Electromagnetic_radiation/Ynx7ujCW">electromagnetic radiation</a> that most commonly includes both visible radiation (light) and <a href="/facts/Infrared/44XdJNcc">infrared</a> radiation, which is not visible to human eyes. A portion of the thermal radiation from very hot objects (see photograph) is easily visible to the eye.
The emissivity of a surface depends on its chemical composition and geometrical structure. Quantitatively, it is the ratio of the thermal radiation from a surface to the radiation from an <a href="/facts/Black_body/f0fHghDy">ideal black surface</a> at the same temperature as given by the <a href="/facts/Stefan%25E2%2580%2593Boltzmann_law/vA4tbwKC">Stefan–Boltzmann law</a>. (A comparison with <a href="/facts/Planck%2527s_law/7Umkdb9m">Planck's law</a> is used if one is concerned with particular wavelengths of thermal radiation.) The ratio varies from 0 to 1.
The surface of a perfect black body (with an emissivity of 1) emits thermal radiation at the rate of approximately 448 watts per square metre (W/m2) at a room temperature of 25 °C (298 K; 77 °F).
Objects have emissivities less than 1.0, and emit radiation at correspondingly lower rates.
However, wavelength- and subwavelength-scale particles, <a href="/facts/Metamaterial/3DeLU6ME">metamaterials</a>, and other nanostructures may have an emissivity greater than 1.

Emissivity open-in-new

Emissivity