can repeatedly reconstruct themselves during free propagation. The phenomena is known as the Talbot effect that was discovered by Henry F. Talbot [@TalbotOrigin] in 1836. A demonstration of the Talbot effect usually starts with a collimated monochromatic light with wavelength $\lambda$. The light passes through a transmission grating with a grating constant $d$ to generate a periodic wavefront. Full or partial re-constructions of the transmission wavefront at integer or fractional multiples of the Talbot distance $z_T=d^2/\lambda$ are observed downstream. Many intricate and elegant features of the Talbot phenomena have been discussed in the literature [@berrytalbot]. Instead of using a collimated or spatially coherent input, in 1948 Ernst Lau [@LauOrigin] found that spatially incoherent light, propagating through two gratings with same grating constant $d$ separated by a distance $z_{12}$, can generate an interference pattern at the focal plane of a lens (Fraunhofer diffraction) placed downstream from the second grating. The contrast of the interference is maximized if $z_{12}$ takes integer multiples of $z_T/2$. More general than Fraunhofer diffraction, in the Lau effect the interference pattern appears at a distance $z_{23}=p/q z_{12}$ downstream from the second grating where $p,q$ are two integers. Lau’s original discovery corresponds to $p/q=N \rightarrow
\infty$ [@LauOrigin; @laueffect79].

