 Fluence

For a beam at a specified surface, fluence is the time-integrated incident light intensity: $F=\int I(t)dt$, with units of energy per area. It records how much energy arrived during an exposure rather than the instantaneous rate of delivery.

For a constant rectangular pulse, $F=I_0\tau$. Two pulses can therefore have equal fluence and different peak intensities if their durations differ. This matters in nonlinear optics, where the response can depend strongly on instantaneous intensity, and in comparing exposure protocols. Photon fluence instead counts incident photons per area; converting it to energy fluence requires the photon energies.