= Diffraction envelope
{title2=$\operatorname{sinc}^2(\pi b\sin\theta/\lambda)$}
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 $I(\theta)=4I_s\operatorname{sinc}^2(\pi b\sin\theta/\lambda)\cos^2(\pi s\sin\theta/\lambda)$, where $\operatorname{sinc}u=\sin u/u$ 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.
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