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Emissivity
 
 
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The emissivity of a material (usually written e or e) is the ratio of energy radiated
Radiation

In physics, radiation describes any process in which energy emitted by one body travels through a medium or through space, ultimately to be absorbed by another body....
 by a particular material to energy radiated by a black body
Black body

In physics, a black body is an Physical body that absorbs all electromagnetic radiation that falls on it. No electromagnetic radiation passes through it and none is Reflection ....
 at the same temperature. It is a measure of a material's ability to radiate absorbed energy. A true black body
Black body

In physics, a black body is an Physical body that absorbs all electromagnetic radiation that falls on it. No electromagnetic radiation passes through it and none is Reflection ....
 would have an while any real object would have . Emissivity is a dimensionless quantity
Dimensionless quantity

In dimensional analysis, a dimensionless quantity is a quantity without any physical units and thus a pure number. Such a number is typically defined as a product or ratio of quantity which do have units, in such a way that all the units cancel out....
 (does not have units).

In general, the duller and blacker a material is, the closer its emissivity is to 1.






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Encyclopedia


The emissivity of a material (usually written e or e) is the ratio of energy radiated
Radiation

In physics, radiation describes any process in which energy emitted by one body travels through a medium or through space, ultimately to be absorbed by another body....
 by a particular material to energy radiated by a black body
Black body

In physics, a black body is an Physical body that absorbs all electromagnetic radiation that falls on it. No electromagnetic radiation passes through it and none is Reflection ....
 at the same temperature. It is a measure of a material's ability to radiate absorbed energy. A true black body
Black body

In physics, a black body is an Physical body that absorbs all electromagnetic radiation that falls on it. No electromagnetic radiation passes through it and none is Reflection ....
 would have an while any real object would have . Emissivity is a dimensionless quantity
Dimensionless quantity

In dimensional analysis, a dimensionless quantity is a quantity without any physical units and thus a pure number. Such a number is typically defined as a product or ratio of quantity which do have units, in such a way that all the units cancel out....
 (does not have units).

In general, the duller and blacker a material is, the closer its emissivity is to 1. The more reflective
Reflectivity

In photometry and heat transfer, reflectivity is the fraction of incident radiation Reflection by a surface. In general it must be treated as a directional property that is a function of the reflected direction, the incident direction, and the incident wavelength....
 a material is, the lower its emissivity. Highly polished silver
Silver

Silver is a chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal....
 has an emissivity of about 0.02.

Explanation


Emissivity depends on factors such as temperature
Temperature

In physics, temperature is a physical property of a Physical system that underlies the common notions of hot and cold; something that feels hotter generally has the greater temperature....
, emission angle
Angle

In geometry and trigonometry, an angle is the figure formed by two Ray sharing a common endpoint, called the vertex of the angle . The magnitude of the angle is the "amount of rotation" that separates the two rays, and can be measured by considering the length of circular arc swept out when one ray is rotated about the vertex to coincide...
, and wavelength
Wavelength

In physics, wavelength is the distance between repeating units of a propagating wave of a given frequency. It is commonly designated by the Greek language letter lambda ....
. A typical engineering
Engineering

Engineering is the discipline and profession of applying Technology and science knowledge and utilizing natural laws and physical resources in order to design and implement materials, structures, machines, devices, systems, and process that safely realize a desired objective and meet specified criteria....
 assumption is to assume that a surface's spectral emissivity and absorptivity do not depend on wavelength, so that the emissivity is a constant. This is known as the grey body assumption.

When dealing with non-black surfaces, the deviations from ideal black body behavior are determined by both the geometrical
Geometry

Geometry arose as the field of knowledge dealing with spatial relationships. Geometry was one of the two fields of pre-modern mathematics, the other being the study of numbers....
 structure and the chemical composition
Chemical compound

A chemical compound is a Chemical substance consisting of two or more different chemical element Chemical bond together in a fixed mass ratio that can be split into simpler substances....
, and follow Kirchhoff's law of thermal radiation
Kirchhoff's law of thermal radiation

In thermodynamics, Kirchhoff's law of thermal radiation, or Kirchhoff's law for short, is a general statement equating emission and absorption in heated objects, proposed by Gustav Kirchhoff in 1859, following from general considerations of thermodynamic equilibrium....
: emissivity equals absorptivity (for an object in thermal equilibrium), so that an object that does not absorb all incident light will also emit less radiation than an ideal black body.

Emissivity of earth's atmosphere
Atmosphere

An atmosphere is a layer of gases that may surround a material body of sufficient mass, by the gravity of the body, and are retained for a longer duration if gravity is high and the atmosphere's temperature is low....
 

The emissivity of Earth's atmosphere
Atmosphere

An atmosphere is a layer of gases that may surround a material body of sufficient mass, by the gravity of the body, and are retained for a longer duration if gravity is high and the atmosphere's temperature is low....
 varies according to cloud cover and the concentration of gases that absorb and emit energy in the thermal infrared (i.e., wavelengths around 8 to 14 micrometre
Micrometre

A micrometre or micron is one Micro- of a metre, or equivalently one thousandth of a millimetre. It is also commonly known as a micron....
s). These gases are often called greenhouse gases, from their role in the greenhouse effect
Greenhouse effect

The greenhouse effect refers to the change in the steady state temperature of a planet or moon by the presence of an atmosphere containing gas that absorbs and emits infrared....
. The main naturally-occurring greenhouse gases are water vapor, carbon dioxide, methane, and ozone. The major constituents of the atmosphere, N2 and O2, do not absorb or emit in the thermal infrared.

Astrophysical graybody

The monochromatic flux density radiated by a greybody at frequency through solid angle is given by where is the Planck function
Planck's law of black body radiation

For a general introduction, see black body.In physics, Planck's law describes the radiance of electromagnetic radiation at all wavelengths from a black body at temperature ....
 for a blackbody at temperature T and emissivity .

For a uniform medium of optical depth
Optical depth

Optical depth, or optical thickness is a measure of transparency , and is defined as the negative logarithm of the fraction of radiation ...
  radiative transfer
Radiative transfer

Radiative transfer is the physical phenomenon of energy transfer in the form of electromagnetic radiation. The propagation of radiation through a medium is affected by Absorption , Emission and scattering processes....
 means that the radiation will be reduced by a factor giving . The optical depth is often approximated by the ratio of the emitting frequency to the frequency where all raised to an exponent ß. For cold dust clouds in the interstellar medium ß is approximately two. Therefore Q becomes,


Emissivity between 2 walls

See also

  • Radiant barrier
    Radiant barrier

    Radiant barriers or reflective barriers inhibit heat transfer by thermal radiation. Thermal energy may also be transferred via conduction or convection, however, radiant barriers do not necessarily protect against heat transfer via conduction or convection....
  • Reflectivity
    Reflectivity

    In photometry and heat transfer, reflectivity is the fraction of incident radiation Reflection by a surface. In general it must be treated as a directional property that is a function of the reflected direction, the incident direction, and the incident wavelength....


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