Fluence (source code)
= Fluence
{title2=$F=\int I\,dt$}
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>.
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