Weak phase object
= Weak phase object
{title2=$t\simeq1+i\phi$}
A <phase object> for which the relevant relative <phase> variations are much smaller than one radian. The <Taylor expansion> $e^{i\phi}=1+i\phi+O(\phi^2)$ separates a direct reference term from a small specimen term. A constant overall <phase> can be removed before making this approximation: <phase-contrast microscopy> measures spatial variations, not an absolute common <phase>.