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| | Title of the author is probably Gabrielle Lattimer. For years she's been working because a library assistant. For a while she's only been in Massachusetts. As a woman what your woman really likes is mah jongg but she have not made a dime by using it. She has become running and maintaining the best blog here: http://prometeu.net/<br><br>Feel free to surf to my blog; [http://prometeu.net/ clash of clans cheat] |
| '''Radiance''' and '''spectral radiance''' are measures of the quantity of [[radiation]] that passes through or is emitted from a surface and falls within a given [[solid angle]] in a specified direction. They are used in [[radiometry]] to characterize diffuse emission and [[diffuse reflection|reflection]] of [[electromagnetic radiation]]. In [[astrophysics]], radiance is also used to quantify emission of [[neutrino]]s and other particles. The [[SI]] unit of radiance is [[watt]]s per [[steradian]] per [[square metre]] (W·sr<sup>−1</sup>·m<sup>−2</sup>), while that of [[Electromagnetic spectrum|spectral]] radiance is {{nobreak|W·sr<sup>−1</sup>·m<sup>−2</sup>·Hz<sup>−1</sup>}} or {{nobreak|W·sr<sup>−1</sup>·m<sup>−3</sup>}} depending on whether the spectrum is a function of [[frequency]] or of [[wavelength]].
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| ==Description==
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| Radiance characterizes total emission or reflection. Radiance is useful because it indicates how much of the power emitted by an emitting or reflecting surface will be received by an optical system looking at the surface from some angle of view. In this case, the [[solid angle]] of interest is the solid angle subtended by the optical system's [[entrance pupil]]. Since the [[human eye|eye]] is an optical system, radiance and its cousin [[luminance]] are good indicators of how bright an object will appear. For this reason, radiance and luminance are both sometimes called "brightness". This usage is now discouraged – see [[Brightness]] for a discussion. The nonstandard usage of "brightness" for "radiance" persists in some fields, notably [[laser physics]].
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| The radiance divided by the index of refraction squared is [[Invariant (physics)|invariant]] in [[geometric optics]]. This means that for an ideal optical system in air, the radiance at the output is the same as the input radiance. This is sometimes called '''conservation of radiance'''. For real, passive, optical systems, the output radiance is ''at most'' equal to the input, unless the index of refraction changes. As an example, if you form a demagnified image with a lens, the optical power is concentrated into a smaller area, so the [[irradiance]] is higher at the image. The light at the image plane, however, fills a larger solid angle so the radiance comes out to be the same assuming there is no loss at the lens.
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| ''Spectral'' radiance expresses radiance as a function of frequency (Hz) with SI units {{nobreak|W·sr<sup>−1</sup>·m<sup>−2</sup>·Hz<sup>−1</sup>}} or wavelength (nm) with units of {{nobreak|W·sr<sup>−1</sup>·m<sup>−2</sup>·nm<sup>−1</sup>}} (more common than {{nobreak|W·sr<sup>−1</sup>·m<sup>-3<!--YES THIS IS CORRECT. ONE LENGTH DIMENSION FOR WAVELENGTH--></sup>}}). In some fields spectral radiance is also measured in [[microflick]]s.<ref>{{cite web|last=Palmer|first=James M.|title=The SI system and SI units for Radiometry and photometry|url=http://www.optics.arizona.edu/palmer/opti400/suppdocs/bkappndx.pdf}}</ref><ref>{{cite web|last=Rowlett|first=Russ|title=How Many? A Dictionary of Units of Measurement|url=http://www.unc.edu/~rowlett/units/dictF.html#flick|accessdate=10 August 2012}}</ref> Radiance is the integral of the spectral radiance over all wavelengths or frequencies.
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| For radiation emitted by an ideal [[black body]] at temperature ''T'', spectral radiance is governed by [[Planck's law]], while the integral of radiance over the hemisphere into which it radiates, in W/m<sup>2</sup>, is governed by the [[Stefan-Boltzmann law]]. There is no need for a separate law for radiance normal to the surface of a black body, in W/m<sup>2</sup>/sr, since this is simply the Stefan-Boltzmann law divided by π. This factor is obtained from the solid angle 2π steradians of a hemisphere decreased by [[Stefan%E2%80%93Boltzmann_law#Integration_of_intensity_derivation|integration over the cosine of the zenith angle]]. More generally the radiance at an angle ''θ'' to the normal (the zenith angle) is given by the Stefan-Boltzmann law times cos(''θ'')/π.
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| ==Definition==
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| Radiance is defined by
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| :<math>L = \frac{\mathrm{d}^2 \Phi}{\mathrm{d}A\,\mathrm{d}{\Omega} \cos \theta} \approx \frac{\Phi}{\Omega A \cos \theta}</math> | |
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| where
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| :''L'' is the observed or measured radiance ({{nobreak|[[Watt|W]]·[[square metre|m<sup>−2</sup>]]·[[steradian|sr<sup>−1</sup>]]}}), in the direction ''θ'', | |
| :d is the [[differential operator]],
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| :''Φ'' is the total [[radiant flux]] or power ([[Watt|W]]) emitted
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| :''θ'' is the [[angle]] between the [[surface normal]] and the specified direction,
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| :''A'' is the [[area]] of the surface ([[square metre|m<sup>2</sup>]]), and
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| :<math>{\Omega}</math> is the [[solid angle]] ([[Steradian|sr]]) subtended by the observation or measurement.
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| :The approximation only holds for small ''A'' and ''Ω'' where cos ''θ'' is approximately constant.
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| In general, ''L'' is a function of viewing angle through the cos ''θ'' term in the denominator as well as the ''θ'', and potentially azimuth angle, dependence of <math>{\mathrm{d} \Phi}/{\mathrm{d}{\Omega}}</math>. For the special case of a [[Lambertian reflectance|Lambertian]] source, ''L'' is constant such that <math>\mathrm{d}^2 \Phi \over \mathrm{d}A\ \mathrm{d}{\Omega}</math> is proportional to cos ''θ''.
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| When calculating the radiance emitted by a source, ''A'' refers to an area on the surface of the source, and Ω to the solid angle into which the light is emitted. When calculating radiance at a detector, ''A'' refers to an area on the surface of the detector and Ω to the solid angle subtended by the source as viewed from that detector. When radiance is conserved, as discussed above, the radiance emitted by a source is the same as that received by a detector observing it.
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| The spectral radiance (radiance per unit wavelength) is written ''L''<sub>λ</sub> and the radiance per unit frequency is written ''L''<sub>ν</sub>.
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| ==Intensity==
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| {{See also|Intensity (disambiguation)|Intensity (heat transfer)}}
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| Radiance is often, confusingly, called ''intensity'' in other areas of study, especially [[heat transfer]], [[astrophysics]] and [[astronomy]]. ''Intensity'' has many other meanings in [[physics]], with the most common being [[intensity (physics)|power per unit area]]. The distinction lies in the area rather than the subtended angle of the observer, and relative area of the source.
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| ==See also==
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| *[[Etendue]]
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| *[[Light field]]
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| *[[Sakuma–Hattori equation]]
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| *[[Wien displacement law]]
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| ==References==
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| {{Reflist}}
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| == External links ==
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| * [http://ncr101.montana.edu/Light1994Conf/4_2_Sliney/Sliney%20Text.htm International Lighting in Controlled Environments Workshop]
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| {{SI radiometry units}}
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| [[Category:Physical quantities]]
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| [[Category:Radiometry]]
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| [[Category:Heat transfer]]
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Title of the author is probably Gabrielle Lattimer. For years she's been working because a library assistant. For a while she's only been in Massachusetts. As a woman what your woman really likes is mah jongg but she have not made a dime by using it. She has become running and maintaining the best blog here: http://prometeu.net/
Feel free to surf to my blog; clash of clans cheat