Английская Википедия:Intensity (heat transfer)

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Шаблон:Other uses 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

<math>dq = I\, d\omega\, \cos \theta\, dA</math>

where

• <math>dA</math> is the infinitesimal source area
• <math>dq</math> is the outgoing heat transfer from the area <math>dA</math>
• <math>d\omega</math> is the solid angle subtended by the infinitesimal 'target' (or 'aperture') area <math>dA_a</math>
• <math>\theta</math> is the angle between the source area normal vector and the line-of-sight between the source and the target areas.

Typical units of intensity are W·m⁻²·sr⁻¹.

Intensity can sometimes be called radiance, especially in other fields of study.

The emissive power of a surface can be determined by integrating the intensity of emitted radiation over a hemisphere surrounding the surface:

<math>q = \int_{\phi=0}^{2\pi} \int_{\theta=0}^{\pi/2} I \cos \theta \sin \theta d\theta d\phi</math>

For diffuse emitters, the emitted radiation intensity is the same in all directions, with the result that

<math>E = \pi I</math>

The factor <math>\pi</math> (which really should have the units of steradians) 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.

Spectral intensity <math>I_\lambda</math> is the corresponding spectral measurement of intensity; in other words, the intensity as a function of wavelength.

See also

• Non-ionising radiation
• Emissivity
• Radiant intensity

References

• Lienhard and Lienhard, A heat transfer textbook, 5th Ed, 2019 (available for free online)
• J P Holman, Heat Transfer 9th Ed, McGraw Hill, 2002.
• F. P. Incropera and D. P. DeWitt, Fundamentals of Heat and Mass Transfer, 4th Ed, Wiley, 1996.