 Signed coherence and fringe visibility

Combining equal-intensity beams with complex degree of first-order coherence $g$ gives a fringe modulation proportional to $\mathrm{Re}(e^{i\phi }g)$. 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.