In the Black Body/Cavity Radiation Experiment, if one measures the
total radiated power per unit area, 
[energy/area/time], at
a temperature T, one will find
Stefan's Law
where 
is Stefan's constant, which has the value 
.
This doesn't provide that much basic information about the system
(radiation) that interests us. We then do more detailed experiments
and break the total radiation down into its spectrum. We measure the
radiated power per unit area per unit frequency interval 
at
temperature T. I always find it easier to think in terms of a
finite chunk of energy in the small frequency interval from 
to
, 
and to define
as the radiated power/area in the interval
. Every time I see 
, I imagine I
were looking at the experiment wearing ``filter glasses'' that allow
me to see and concentrate only on energy in the frequency interval
.
