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| {{otheruses|Intensity (disambiguation)}}
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| In the field of [[heat transfer]], '''intensity of radiation''' <math>I</math> is a measure of the distribution of radiant heat flux per unit area and [[solid angle]], in a particular direction, defined according to
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| :<math>dq = I\, d\omega\, \cos \theta\, dA</math>
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| where
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| *<math>dA</math> is the infinitesimal source area
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| *<math>dq</math> is the ''outgoing'' heat transfer from the area <math>dA</math>
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| *<math>d\omega</math> is the [[solid angle]] subtended by the infinitesimal 'target' (or 'aperture') area <math>dA_a</math>
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| *<math>\theta</math> is the angle between the source area normal vector and the line-of-sight between the source and the target areas.
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| Typical units of intensity are W·m<sup>-2</sup>·sr<sup>-1</sup>.
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| Intensity can sometimes be called [[radiance]], especially in other fields of study.
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| The emissive power of a surface can be determined by integrating the intensity of emitted radiation over a hemisphere surrounding the surface:
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| :<math>q = \int_{\phi=0}^{2\pi} \int_{\theta=0}^{\pi/2} I \cos \theta \sin \theta d\theta d\phi</math> | |
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| For diffuse emitters, the emitted radiation intensity is the same in all directions, with the result that
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| :<math>E = \pi I</math>
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| The factor <math>\pi</math> (which really should have the units of [[steradian]]s) is a result of the fact that intensity is defined to exclude the effect of reduced [[view factor]] at large values <math>\theta</math>; note that the solid angle corresponding to a hemisphere is equal to <math>2\pi</math> steradians.
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| '''[[Specific radiative intensity|Spectral intensity]]''' <math>I_\lambda</math> is the corresponding spectral measurement of intensity; in other words, the intensity as a function of [[wavelength]].
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| == See also ==
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| * [[Non-ionising radiation]]
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| * [[Emissivity]]
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| == References ==
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| * Lienhard and Lienhard, ''[http://web.mit.edu/lienhard/www/ahtt.html A heat transfer textbook]'', 3rd Ed, 2008 (available for free online)
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| * J P Holman, '' Heat Transfer'' 9th Ed, McGraw Hill, 2002.
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| * [[F. P. Incropera]] and D. P. DeWitt, ''Fundamentals of Heat and Mass Transfer'', 4th Ed, Wiley, 1996.
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| [[Category:Heat transfer]]
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| [[Category:Radiation]]
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