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.
References in periodicals archive ?
The experimental setup provides a simple method to determine the emissivity factor.
2004) developed another nighttime emissivity correlation based on a short period of time (August 10-October 25, 2002) for the climate of Negev Highlands, Israel.
Influence on the emissivity was more pronounced for the decrease in coatings thickness from 11 to 2 [micro]m.
This is how the emissivity of a real material is often measured in the laboratory.
When corrections of atmospheric and reflected temperatures, distance, relative humidity and object emissivity are made, the temperature field on the MCM surface can be monitored in real-time.
Kirchoff's Law is valid so that the emissivity [epsilon] and absorptivity ct of the surface are the same.
The generalized emission equation given in (4) can be used to derive a method for characterizing the emissions of mobile sources of an urban area, by describing the emissivity R of its roadway segments; the size of the roadway segments can be selected based on the characteristics of the district area and the spatial scale of the study.
To obtain a more accurate LST, roughness properties of the land surface, the amount and nature of vegetation cover, and the thermal properties and moisture content of the soil must be accounted for by correcting brightness temperatures using spectral emissivity values before computation occurs [14].
Heat Mirror has low emissivity and solar reflective film inside of an insulating glass unit.
Accordingly, the emissivity of the target object you are measuring is vital to achieving accurate temperatures.
Organized alphabetically, each entry summarizes the crystal system, natural production and artificial growth methods, and electrical properties before commenting on transmittance and reflectance spectra, optical constants, the dispersion relation, emissivity, and spectroscopy applications.