Intensity (heat transfer)

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In the field of heat transfer, intensity of radiation is a measure of the distribution of radiant heat flux per unit area and solid angle, in a particular direction, defined according to


  • is the infinitesimal source area
  • is the outgoing heat transfer from the area
  • is the solid angle subtended by the infinitesimal 'target' (or 'aperture') area
  • 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:

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

The factor (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 ; note that the solid angle corresponding to a hemisphere is equal to steradians.

Spectral intensity is the corresponding spectral measurement of intensity; in other words, the intensity as a function of wavelength.

See also[edit]


  • 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.