Energy Flux
(1)
The mean energy
flux is
, also written as
and called
I, the
intensity. For a traveling wave,
(2)
The unit of energy flux is 1 J m
-2 s
-1 = 1 kg s
-3. For radiation, consider a half-plane filled with
energy density u, then
(3)
where
c is the
speed of light,
is the angle from the propagation axis,
a is the
radiation constant,
T is the
temperature, and
is the
Stefan-Boltzmann constant
Foot Pound
A unit of energy equal to 
