= Cornu chord power normalization
{c}
{title2=$P/P_{\rm open}=|F(w_+)-F(w_-)|^2/2$}
For normalized Fresnel integrals $F(w)=C(w)+iS(w)$, an interval aperture contributes the chord between its two limiting points on the <Cornu spiral>. The full unobstructed field is $F(+\infty)-F(-\infty)=1+i$, so its squared magnitude is2. Comparing to only the half-plane limit instead of this full chord introduces a factor-of-four normalization error. The formula compares power at one observation point under the same illumination and propagation factors.
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