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What is emissivity and why is it important?

All objects at temperatures above absolute zero emit thermal radiation

However, for any particular wavelength and temperature, the amount of thermal radiation emitted depends on the emissivity of the object's surface.

Emissivity is defined as the ratio of the energy radiated from a material's surface to that radiated from a a perfect emitter, known as a blackbody, at the same temperature and wavelength and under the same viewing conditions. It is a dimensionless number between 0 (for a perfect reflector) and 1 (for a perfect emitter).

The emissivity of a surface depends not only on the material but also on the nature of the surface. For example, a clean and polished metal surface will have a low emissivity, whereas a roughened and oxidised metal surface will have a high emissivity. The emissivity also depends on the temperature of the surface as well as wavelength and angle.

Knowledge of surface emissivity is important both for accurate non-contact temperature measurement and for heat transfer calculations. Radiation thermometers detect the thermal radiation emitted by a surface. They are generally calibrated using blackbody reference sources that have an emissivity as close to 1 as makes no practical difference.

When viewing ‘real’, more reflective surfaces, with a lower emissivity, less radiation will be received by the thermometer than from a blackbody at the same temperature and so the surface will appear colder than it is unless the thermometer reading is adjusted to take into account the material surface emissivity.

Unfortunately, because the emissivity of a material surface depends on many chemical and physical properties, it is often difficult to estimate. It must either be measured or modified in some way, for example by coating the surface with high emissivity black paint, to provide a known emissivity value.

NPL provides a service for measuring the emissivity of some types of material at temperatures close to ambient.

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