Two-grating diffraction and Lau effect under laser illumination

We first describe an unconventional Lau experiment that is performed with a collimated He-Ne laser beam. We then describe an elegant experimental arrangement in which a collimated He-Ne laser beam is gradually rendered spatially incoherent; this demonstrates the evolution of the Lau fringes from the Fraunhofer diffraction pattern of the two gratings in the Lau setup.