“Radiant fluence rate, or spherical irradiance, is equal to the total radiant flux incident on a small sphere divided by the area of the diametrical cross-section of the sphere.”
\(E_0 = \int{L}{d\Omega}\), where \(d\Omega\) is the solid angle of each elementary beam passing through the given point and \(L\) its radiance at that point in the direction of the beam.
“Radiant fluence rate, or spherical irradiance, is equal to the total radiant flux incident on a small sphere divided by the area of the diametrical cross-section of the sphere.”