In the field of heat transfer, intensity of radiation I is a measure of the distribution of radiant heat flux per unit area and solid angle, in a particular direction, defined according to
d q = I d ω cos θ d A where
d A is the infinitesimal source area d q is the outgoing heat transfer from the area d A d ω is the solid angle subtended by the infinitesimal 'target' (or 'aperture') area d A a θ 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:
q = ∫ ϕ = 0 2 π ∫ θ = 0 π / 2 I cos θ sin θ d θ d ϕ For diffuse emitters, the emitted radiation intensity is the same in all directions, with the result that
E = π I The factor π (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 θ ; note that the solid angle corresponding to a hemisphere is equal to 2 π steradians.
Spectral intensity I λ is the corresponding spectral measurement of intensity; in other words, the intensity as a function of wavelength.
