Interferogram of an equally spaced spectral multiplet ($|g^{(1)}|=|\sin (N\Delta \omega \tau /2)/[N\sin (\Delta \omega \tau /2)]|$)

For $N$ equally intense spectral lines at ${\omega }_n={\omega }_0+n\Delta \omega$, sum their delayed self-correlations as a geometric series. The result isThe interferometric visibility revives every $T=2\pi /\Delta \omega$. There are $N-1$ zeros between adjacent revivals, at $\tau =jT/N$ for $j=1,\dots ,N-1$. The removable singularity at a revival has unit modulus. Revival spacing fixes line spacing; the internal zero count fixes $N$ under the equal-intensity model. The interferogram alone does not establish a fixed relative phase between the source lines or prove the emission is a coherent pulse train.