Babinet's principle
= Babinet's principle
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In linear <scalar diffraction theory>, complementary aperture transmissions $t$ and $1-t$ give complex fields $U_t+U_{1-t}=U_0$, where $U_0$ is the unobstructed field under identical illumination. Away from the direct beam in <Fraunhofer diffraction>, their diffracted fields differ by a sign and have equal intensity. Intensities do not generally add: in particular, complementary on-axis <Fresnel diffraction> patterns need not have equal intensity. The ideal scalar statement does not incorporate vector boundary details at a real conducting edge.