A collimated wave reflected by a plane mirror is introduced on a hexagonal transmission grating. Intensity of the diffracted wave is detected by an image sensor located at a small distance from the grating. The intensity of the grating image or grating shadow on the sensor displaces according to the tilt of the incident wave. Three components of the displacement are detected from phase shifts of three components of the grating. They are calculated by Fourier transform method. With the displacements we can calculate 2-D tilt of the plane wave. This method can be applied to a compact sensor for detecting 2-D tilt of the mirror or wave-front like an autocollimator.
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