= Electron beam brightness
{title2=$\beta$}
The <electric current> delivered per transverse probe area per <solid angle>. For a uniform circular probe of diameter $d$ and a small convergence semiangle $\alpha$, one convention is $\beta=4I/(\pi^2d^2\alpha^2)$. State the accelerating <voltage>, angular convention and whether brightness is reduced by a <voltage> factor before comparing sources. A smaller aperture changes current as well as convergence; finite <electron beam brightness> prevents arbitrary current in an arbitrarily small probe.
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