| 摘要 |
<p>A readhead (10) is movable relative to a scale (11) in a direction defined as the X direction. The lines (11A) on the scale extend in the Y direction, and the scale extends in an XY plane. A light source (S) provides a non-coherent beam of light which is incident upon the scale (11) via an index grating (12). Light from the index grating (12) passes via an aperture (A3) onto the scale, and the scale cooperates with the light pattern generated by the index grating (12) to produce a set of diffraction orders (B0, B1, and B2). Primary diffraction orders (B0, B1, and B2) interfere with each other to produce a light modulation at an analyser grating (13). The index grating and analyser gratings are provided adjacent each other on a single glass plate. An auxiliary grating (14) is provided on the glass plate on the opposite side (2), and in register with the analyser grating (13). The lines (14A) of the auxiliary grating (14) extend substantially in the X direction, but are skewed relative to the X direction by approximately 1°. The auxiliary grating (14) splits the set of primary orders (B0, B1, and B2) into three such sets of primary orders, each corresponding to a secondary order of diffraction (D0, D1, and D2) produced at the auxiliary grating (14). The skewing of the lines (14A) on the grating (14) causes each other light modulations corresponding to one of the plurality offsets of primary diffraction orders to have a phase difference with respect to each other light modulation. Each light modulation is detected at a respective light sensitive transducer (T1, T2, and T3) each of which are offset from the analyser grating (13) in the Z direction to enable spatial separation of the three phase shifted light modulations.</p> |
