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.
Corrosion increases the emissivity of the material.
Although, as reported in [27], two problems were identified in the VIIRS LST EDR: (1) inaccurate spectral emissivity values used in the VIIRS algorithm, which are derived from fixed values depending on a limited number of surface types; (2) a nonrepresentative algorithm coefficients under hot and very humid conditions, the data used in our study are still reliable because most of the surfaces in our study area were the typical ones and the water vapor contents of the corresponding atmospheric profiles were all less than 3 g/[cm.sup.2].
The NFL correction which appears in the phase space integral of the MFP [20] and gradually in the expression of the emissivity [19] is actually related to the unscreened magnetic/transverse interaction.
IR Emissivity. The relationship between IR emissivity of epoxy-lacquer based coatings and Al mass fraction is shown in Table 1.
(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.
For emissivity measurements, the infrared (IR) spectrometer Nicolet 380 FTIR was used.
This is how the emissivity of a real material is often measured in the laboratory.
THERMAL CONDUCTIVITY AND EMISSIVITY OF EACH MATERIAL Material Thermal Emissivity conductivity [W/m*K] Heat sink and transistors Silica glass 1.38 0.92 Aluminum alloy 200 0.1 6063 (shiny) Copper 400 0.2 Silicon 150 -- FR4 0.3 0.6 Inductor Core material (Carbonyl E 50.16 -- Iron powder) Copper 400 0.2 Kapton tape 0.12 0.08 Transformer Core material 3.5 0.78 3F3 (MnZn) Copper 400 0.2 FR4 0.3 0.2 Steel AISI 54 0.04 4340 TABLE III.
The focus of this paper is to study the effects of emissivity of the absorber tubes on the thermal performance of the solar cavity receiver.
Waveguide cavities can also be used to determine emissivity of high reflective low losses samples, by exciting, i.e., in circular cavities, the [TE.sub.01] mode (where one of the cavity walls is the sample under test), and comparing the measured Q-factor with the reference one.
The unit incorporates a 12:1 optic ratio (target distance/diameter ratio) and a fixed emissivity of 0.95 making it suitable for a wide range of applications.