specific intensity

astronut

I am having fundamental difficulty in feeling the definition of specific intensity of a radiation field...
questions:
1) Define a radiation field as precisely as possible.
2) Specific intensity defined at a point should remain the same whichever way we choose for the differential surface area..
3) With respect to which point and which area element is the differential solid angle element measured?
4) What do we mean by dE(energy flowing through the unit solid angle)?
(because if the differential solid angle is defined keeping the centre of the area element as centre of the sphere then dE does not make any sense).
i.e. 'energy flowing through a surface' makes sense but
'energy flowing through a solid angle' makes sense only if the source is at the centre.

Astronuc

This may help

http://www.drake.edu/artsci/physics/...ses/a185v3.pdf