[2.11] (at a point in space)
The omnidirectional radiant flux externally incident on an elementary sphere, per unit cross-sectional area of the sphere.

{ F }_{ t }=\mathrm{d}\Phi /\mathrm{d}a\left[=\int _{ 4\pi }^{ }{ L\,\mathrm{d}\omega } \right] ,

where da is the cross-sectional area of the sphere.

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