Abstract: The method relates to determining the autocollimation angle of a grating coupler comprising a thin waveguiding film and a diffraction grating. A laser beam is approximately normally directed to the diffraction grating which is mounted at a turntable. The turntable is movable in positive and negative rotational directions through small angles of rotation by a lever arm which is engaged by a micrometer screw driven by a stepping motor until the incoupling condition is satisfied for excitation of a waveguiding mode in the forward and rearward propagation directions in the waveguiding film. The autocollimation angle is determined as the mean value of the two angles of incidence for resonance excitation of the waveguiding modes in the waveguiding film under the incoupling condition.