= Transverse optical coherence length
{title2=$\ell_\perp\sim\lambda/\Theta$}
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.
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