 Unequal-line visibility in a spectral doublet

Two narrow incoherent lines of powers $J_1,J_2$ and wave-number difference $\Delta k$ give visibility $\sqrt{J_1^2+J_2^2+2J_1{J}_2\cos (2\Delta k\delta )}/(J_1+J_2)$ in a Michelson scan of one-way arm difference $\delta$. The minimum is $|J_1-J_2|/(J_1+J_2)$, and revivals repeat in $\pi /|\Delta k|$. Finite linewidth damps this envelope.