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).

The equation of radiative transfer

The equation of radiative transfer simply says

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