Optical system for detecting motion of a body

The invention relates to a system (1) for detecting motion of a body (2), said body comprising a first diffraction pattern (3A) and a second diffraction pattern (3B) with a predetermined orientation relative to said first diffraction pattern. The system comprises optical means (4A, 4B) adapted to provide at least a first incident beam to said first diffraction pattern to obtain a first diffracted beam from said first diffraction pattern and at least a second incident beam, with a predetermined orientation relative to said first incident beam, to said second diffraction pattern to obtain a second diffracted beam from said second diffraction pattern. The system has means for detecting motion of said body on the basis of the phase difference between at least one of said first diffracted beam and said second diffracted beam. Accordingly a larger in-plane rotation range is obtained for detecting motion of the body (2). The invention also relates to a wafer (2) provided with two-dimensional diffraction patterns (3A,3B) and a method for detecting motion of a body.