emissivity

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Emissivity

The ratio of the radiation intensity of a nonblack body to the radiation intensity of a blackbody. This ratio, which is usually designated by the Greek letter ε, is always less than or just equal to one. The emissivity characterizes the radiation or absorption quality of nonblack bodies. Published values are readily available for most substances. Emissivities vary with temperature and also vary throughout the spectrum. For an extended discussion of blackbody radiation and related information See Heat radiation.

A spectral emissivity of zero means that the heat radiator emits no radiation at this wavelength. Strongly selective radiators, such as insulators or ceramics, have spectral emissivities close to 1 in some parts of the spectrum, and close to zero in other parts. Carbon has a high spectral emissivity throughout the visible and infrared spectrum, exceeding 0.90 in certain portions; thus carbon is a good blackbody radiator. Tantalum is the only metal with a spectral emissivity greater than 0.5 in the visible spectrum. All other metals have a lower spectral emissivity. Tungsten is a relatively good emitter, with a spectral emissivity of 0.43–0.47 within the visible region of the spectrum. See Blackbody

emissivity

(em-ă-siv -ă-tee) Symbol: ∊. A measure of a body's ability to radiate electromagnetic radiation as compared to that of a perfect radiator – a black body – at the same temperature.

Emissivity

Ratio of radiation a surface gives off. Using products with low emissivity—for example, white roofs rather than black roofs—reduces the heat island effect, whereby urban landscapes become significantly warmer than surrounding rural landscapes.

emissivity

[‚ē·mə′siv·əd·ē]
(thermodynamics)
The ratio of the radiation emitted by a surface to the radiation emitted by a perfect blackbody radiator at the same temperature. Also known as thermal emissivity.

thermal emissivity

The ratio of the rate of radiant heat energy emitted by a body at a given temperature to the rate of radiant heat energy emitted by a blackbody, 1 at the same temperature, in the same surroundings.