[3.6] F(Δt)
The omnidirectional luminous energy externally incident on an elementary sphere per unit cross-sectional area, in time Δt.
F\left ( \Delta t \right )= dQ/da\left [ =\int _{\Delta t} \int _{4\pi } L d\omega dt\right ]
where da is the cross-sectional area of the sphere.
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