 Knife-edge diffraction

For an opaque half-plane whose edge lies height $h$ above the direct source–receiver ray, the paraxial parameter is $v=h\sqrt{2(d_1+d_2)/(\lambda d_1{d}_2)}$. 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.^[ref]