Emissivity: Difference between revisions

From Glossary of Meteorology
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|Meaning=The ratio of the [[power]] emitted by a body at a [[temperature]] ''T'' to the power emitted if  the body obeyed [[Planck's radiation law]].
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|Explanation=Strictly, emissivity should be qualified by the [[frequency]], direction, and even [[polarization]] state  of the emitted [[radiation]]. This is recognized in qualifiers such as [[monochromatic]] (at a given  frequency) as opposed to total (over a broad [[range]] of frequencies), and directional (the ratio for  a particular direction) as opposed to hemispherical (the ratio for a hemisphere of directions).  Contrary to a widespread misconception, the upper limit of emissivity is not 1. This upper limit  is valid (approximately) only for bodies large compared with all relevant wavelengths. <br/>''See also''  [[blackbody]], [[emissive power]], [[emittance]].
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== emissivity ==
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<div class="definition"><div class="short_definition">The ratio of the [[power]] emitted by a body at a [[temperature]] ''T'' to the power emitted if  the body obeyed [[Planck's radiation law]].</div><br/> <div class="paragraph">Strictly, emissivity should be qualified by the [[frequency]], direction, and even [[polarization]] state  of the emitted [[radiation]]. This is recognized in qualifiers such as [[monochromatic]] (at a given  frequency) as opposed to total (over a broad [[range]] of frequencies), and directional (the ratio for  a particular direction) as opposed to hemispherical (the ratio for a hemisphere of directions).  Contrary to a widespread misconception, the upper limit of emissivity is not 1. This upper limit  is valid (approximately) only for bodies large compared with all relevant wavelengths. <br/>''See also''  [[blackbody]], [[emissive power]], [[emittance]].</div><br/> </div>
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Latest revision as of 07:54, 29 March 2024

The ratio of the power emitted by a body at a temperature T to the power emitted if the body obeyed Planck's radiation law.

Strictly, emissivity should be qualified by the frequency, direction, and even polarization state of the emitted radiation. This is recognized in qualifiers such as monochromatic (at a given frequency) as opposed to total (over a broad range of frequencies), and directional (the ratio for a particular direction) as opposed to hemispherical (the ratio for a hemisphere of directions). Contrary to a widespread misconception, the upper limit of emissivity is not 1. This upper limit is valid (approximately) only for bodies large compared with all relevant wavelengths.
See also blackbody, emissive power, emittance.

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