Solving Specific Intensity: A Beginner's Guide

[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/astrocourses/a185v3.pdf

 

[tacman]

1) A radiation field can be defined as an area in space where electromagnetic radiation is present. It can be created by sources such as stars, light bulbs, or even our own bodies, and can also be detected by instruments such as telescopes or radiation detectors.

2) The specific intensity of a radiation field is a measure of the amount of energy that passes through a unit area, in a specific direction, in a given amount of time. It is a property of the radiation field at a specific point, and is independent of the orientation of the differential surface area.

3) The differential solid angle element is measured with respect to the point of interest, where the specific intensity is being calculated. The area element is chosen to be perpendicular to the direction of the radiation, and the differential solid angle is measured from the center of this area element.

4) When we say "energy flowing through the unit solid angle", we are referring to the amount of energy passing through a unit area, in a specific direction, in a given amount of time. This unit solid angle is measured from the center of the area element, which is the point of interest. Therefore, the energy flowing through a solid angle makes sense in the context of specific intensity, as it is a measure of the energy passing through a specific direction at a given point.