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.