Visibility of a one-dimensional uniform source ($V/V(0)=\mathrm{sinc}(\pi b_p\Delta \alpha /\lambda )$)

A centred uniform source of angular width $\Delta \alpha$ has normalized visibility $V/V(0)=\mathrm{sinc}(\pi b_p\Delta \alpha /\lambda )$, with $b_p$ the projected baseline along that angular coordinate and $\mathrm{sinc}x=\sin x/x$. The result is the integral of the point-source complex phase over source brightness. Its first null occurs at width $\lambda /|b_p|$; negative values are phase reversals rather than negative brightness.