= Solution
By linear <scalar diffraction theory>, complementary <aperture> transmissions $t$ and $1-t$ give $U_t+U_{1-t}=U_{\rm open}$ under identical illumination. In <Fraunhofer diffraction>, away from the direct forward beam $U_{\rm open}=0$, so $U_{1-t}=-U_t$: \b[the complementary screens have identical off-axis diffraction <intensities>]. This is <Babinet's principle>. Their <complex amplitudes>, rather than their <intensities>, add. The direct beam is the exception; real conducting-edge vector effects are outside this ideal scalar statement.
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