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A radiation pattern specifies emitted power per solid angle as a function of direction. For a slowly moving point electric charge with acceleration $\mathbf{a}$, $dP/d\Omega =q^2|\mathbf{n}\times \mathbf{a}{|}^2/(16{\pi }^2{ϵ}_0{c}^3)$. Its integral over directions is the Larmor formula; its fore-aft symmetry implies zero net radiated three-momentum in the instantaneous rest inertial frame. Directional detector power includes the distinction between source emission time and arrival time.

A free nonrelativistic charge driven by a weak plane electromagnetic wave has peak acceleration $|q|E_0/m$. Its time-averaged Larmor formula power is $q^4{E}_0^2/(12\pi {ϵ}_0{m}^2{c}^3)$. Divide by incident light intensity ${ϵ}_{0}c{E}_0^2/2$ to obtain the displayed cross section, independent of frequency. For an electron, it is $8\pi r_e^2/3$. The approximation assumes negligible Compton recoil kinematics and small driven velocity compared with the speed of light.