Signed coherence and fringe visibility
= Signed coherence and fringe visibility
{title2=$V=|g^{(1)}|$}
Combining equal-intensity beams with complex degree of first-order coherence $g$ gives a fringe modulation proportional to $\operatorname{Re}(e^{i\varphi}g)$. Maximizing and minimizing over observation <phase> gives visibility $V=|g|$. A symmetric source can make $g$ real, but it may still be negative. Its sign changes fringe <phase> by $\pi$; it does not represent negative physical contrast.