A theoretical study of the fractional Lau effect is performed for a Lau set-up equipped with binary gratings. The separations between the two gratings are chosen equal to an elementary distance divided by a positive integer. Fourier components of the observed pattern function are calculated for these distances. Also, this study shows that it is sufficient to study the Lau patterns in a restricted interval of the gratings separation. Experimental and simulation results are consistent with the theoretical predictions.
