 Half-plane shadow-boundary intensity

At the geometric shadow boundary of a straight opaque half-plane, the transmitted Fresnel integral is exactly half the unobstructed complex amplitude for ideal point illumination. The Cornu spiral chord is from its origin to one limiting endpoint, rather than between both limiting endpoints. Therefore $U/U_0=1/2$ and $I/I_0=1/4$. Half the unobstructed intensity would confuse a field fraction with its square. Edge thickness, source extent and alignment can modify the ideal result.