 Cornu chord power normalization

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.