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In some cases, emissive power and absorptivity may be defined to depend on angle, as described below.
Calculating emissive power and irradiation in heat transfer.
The emissive power of a surface can be determined by integrating the intensity of emitted radiation over a hemisphere surrounding the surface:
The characteristics of thermal radiation depend on various properties of the surface it is emanating from, including its temperature, its spectral absorptivity and spectral emissive power, as expressed by Kirchhoff's law.
Prior to Kirchhoff's studies, it was known that for total heat radiation, the ratio of emissive power to absorptive ratio was the same for all bodies emitting and absorbing thermal radiation in thermodynamic equilibrium.
For a prescribed temperature, T and the spectral interval from 0 to λ, is the ratio of the total emissive power of a black body from 0 to λ to the total emissive power over the entire spectrum.
Specifically, the Stefan-Boltzmann law states that the total energy radiated per unit surface area of a black body across all wavelengths per unit time (also known as the black-body irradiance or emissive power), , is directly proportional to the fourth power of the black body's thermodynamic temperature T:
For a body of any arbitrary material, emitting and absorbing thermal electromagnetic radiation at every wavelength in thermodynamic equilibrium, the ratio of its emissive power to its dimensionless coefficient of absorption is equal to a universal function only of radiative wavelength and temperature, the perfect black-body emissive power.
The more reflective a material is, the lower its emissivity.
The emissivity of a surface varies between zero and one.
The average emissivity of the earth is readily estimated from available data.
The intensity depends on the emissivity value of the material.
Human skin has an emissivity of very close to 1.0 .
The emissivity in the visible spectrum is closely related to color.
The thermal emissivity of various surfaces is listed in the following table.
Temperature and emissivity of the source are the determining parameters here.
That will be where is the emissivity at wavelength .
There are times, however, when an emissivity test is not possible due to dangerous or inaccessible conditions.
A source with lower emissivity independent of frequency often is referred to as a gray body.
Highly polished silver has an emissivity of about 0.02.
The use of effective emissivity and albedo account for the greenhouse effect.
Emissivity is expressed as a number between zero (0) and one (1) at a given wavelength.
The higher the emissivity, the greater the emitted radiation at that wavelength.
For these materials, the emissivity does not cancel out and the temperature measurement is in error.
A painted surface will have a greater emissivity than a bright, unpainted one.
This would be more accurate than attempting to determine the emissivity of the object via a table.
Though, it is possible to create visibly dark colored surfaces with low thermal emissivity.
With the average emissivity set to unity, the effective temperature of the Earth is:
The microwave emissivity of sea ice is found to vary quite significantly with thickness.
The ability of both objects to emit or absorb this radiation is called emissivity.
The net emissivity may be low due to surface or atmospheric properties, including greenhouse effect.
Radiation is also minimized by low emissivity (highly reflective) surfaces.
We use the following model for emissivity in U:
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