Diffraction envelope (${\mathrm{sinc}}^2(\pi b\sin \theta /\lambda )$)

A diffraction envelope is the angular factor imposed by the shape and width of each opening in an interference pattern. For two equally illuminated rectangular slits of width $b$ and separation $s$, Fraunhofer diffraction gives $I(\theta )=4I_s{\mathrm{sinc}}^2(\pi b\sin \theta /\lambda ){\cos }^2(\pi s\sin \theta /\lambda )$, where $\mathrm{sinc}u=\sin u/u$ and $I_s$ is one slit's on-axis light intensity. The first factor limits the observable angular range even when the rapid interference fringes remain perfectly coherent.