emissivity

The emissivity of a material (usually written ε or e) is the relative power of its surface to emit heat by radiation. It is the ratio of energy radiated by a particular material to energy radiated by a black body at the same temperature. It is a measure of a material's ability to radiate absorbed energy. A true black body would have an while any real object would have . Emissivity is a dimensionless quantity, so it does not have units.

In general, the duller and blacker a material is, the closer its emissivity is to 1. The more reflective a material is, the lower its emissivity. Highly polished silver has an emissivity of about 0.02.^[1]

Explanation

Emissivity depends on factors such as temperature, emission angle, and wavelength. A typical engineering assumption is to assume that a surface's spectral emissivity and absorptivity do not depend on wavelength, so that the emissivity is a constant. This is known as the "grey body assumption".

Although it is common to discuss the "emissivity of a material" (such as the emissivity of highly polished silver), the emissivity of a material does in general depend on its thickness. The emissivities quoted for materials are for samples of infinite thickness (which, in practice, means samples which are optically thick) — thinner samples of material will have reduced emissivity.

When dealing with non-black surfaces, the deviations from ideal black body behavior are determined by both the geometrical structure and the chemical composition, and follow Kirchhoff's law of thermal radiation: emissivity equals absorptivity (for an object in thermal equilibrium), so that an object that does not absorb all incident light will also emit less radiation than an ideal black body.

Emissivity of earth's atmosphere

The emissivity of Earth's atmosphere varies according to cloud cover and the concentration of gases that absorb and emit energy in the thermal infrared (i.e., wavelengths around 8 to 14 micrometres). These gases are often called greenhouse gases, from their role in the greenhouse effect. The main naturally-occurring greenhouse gases are water vapor, carbon dioxide, methane, and ozone. The major constituents of the atmosphere, N₂ and O₂, do not absorb or emit in the thermal infrared.

Astrophysical graybody

The monochromatic flux density radiated by a greybody at frequency ν through solid angle dΩ is given by F_ν = B_ν(T)Q_νdΩ where B_ν is the Planck function for a blackbody at temperature T and emissivity Q_ν.

For a uniform medium of optical depth τ_ν radiative transfer means that the radiation will be reduced by a factor e ^{− τ} giving . The optical depth is often approximated by the ratio of the emitting frequency to the frequency where τ = 1 all raised to an exponent β. For cold dust clouds in the interstellar medium β is approximately two. Therefore Q becomes,

Emissivity between 2 walls

See also

• Radiant barrier
• Reflectivity
• Thermal radiation
• Form factor

References

External links

• Emissivity of some common materials