= Knife-edge diffraction
{title2=$\mathcal T(v)=\{[1/2-C(v)]^2+[1/2-S(v)]^2\}/2$}
For an opaque half-plane whose edge lies height $h$ above the direct source–receiver ray, the paraxial parameter is $v=h\sqrt{2(d_1+d_2)/(\lambda d_1d_2)}$. The unobstructed half-aperture supplies the <Cornu spiral> chord from $v$ to infinity. Division by the full-aperture chord gives the displayed <intensity> transmission. It is $1/4$ at grazing incidence, approaches one well into the illuminated region and tends to zero deep in shadow.<Propagation by diffraction (ITU-R P.526-13) by International Telecommunication Union>[Annex 1, Sections 2.7 and 4.1]{ref}
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