Collimation test by double grating shearing interferometer.

The double grating shearing interferometry method for determination of the degree of light collimation is described. High accuracy is obtained by performing the observation of fringes in the area of the size twice as big as the one usually assumed in shearing interferometry experiments. The conditions under which such a detection mode is feasible are derived. They represent at the same time a very strong argument proving the highly diffractive (wave optics) character of the classical Ronchi test.