Definitions
The fundamental quantity which describes a field of radiation in radiometric terms is nowadays called the spectral radiance, traditionally called the specific intensity. For a very small area element in the radiation field, there can be electromagnetic radiation passing in both senses in every spatial direction through it. In radiometric terms, the passage can be completely characterized by the amount of energy radiated in each of the two senses in each spatial direction, per unit time, per unit area of surface of sourcing passage, per unit solid angle of reception at a distance, per unit wavelength interval being considered (polarization will be ignored for the moment).
In terms of the spectral radiance, the energy flowing across an area element of area located at in time in the solid angle about the direction in the frequency interval to is
where is the angle that the unit direction vector makes with a normal to the area element. The units of the spectral radiance are seen to be energy/time/area/solid angle/frequency. In MKS units this would be W·m−2·sr−1·Hz−1 (watts per square-metre-steradian-hertz).
Read more about this topic: Radiative Transfer
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