Transverse optical coherence length (${\ell }_{\perp }\sim \lambda /\Theta$)

The transverse optical coherence length summarizes the separation over which equal-time spatial optical coherence remains appreciable. It depends on a contrast criterion and on the source brightness profile. For a distant incoherent source of angular diameter $\Theta$, its characteristic scale is $\lambda /\Theta$. A uniform circular source gives uniform-disk stellar visibility $2J_1(x)/x$, with $x=\pi b\Theta /\lambda$ and baseline $b$. Its first zero is $b=1.21967\lambda /\Theta$. A uniformly bright one-dimensional strip instead has its first zero at $\lambda /\Theta$; the numerical factor is a source-model choice, not a universal constant.