[2.10] F(Δt) (for a time interval Δt, at a point in space)
The omnidirectional radiant energy externally incident on an elementary sphere about the point, per unit cross-sectional area of the sphere.
F(\Delta t)= dQ/da ={\int}_{\Delta t}{ {\int}_{ 4\pi} } Ld\omega dt
where da is the cross-sectional area of the sphere.