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

where

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 ${\displaystyle \pi }$ (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 ${\displaystyle \theta }$; note that the solid angle corresponding to a hemisphere is equal to ${\displaystyle 2\pi }$ steradians.

Spectral intensity ${\displaystyle I_{\lambda }}$ is the corresponding spectral measurement of intensity; in other words, the intensity as a function of wavelength.