| 摘要 |
PURPOSE:To improves S/N and a resolution by using a laser for a light source and by setting a grid space of a scale to be a prescribed pitch or less. CONSTITUTION:A semiconductor laser is used for a light source 20, and grid pitches of an index scale 22 and a main scale 24 are set to be 10mum or less, for instance. By this constitution, a laser beam emitted from the light source 20 is turned into parallel beams by a collimate lens 16 and then reaches the scale 22. Although moire fringes are formed by the respective grids by the scales 22 and 24, diffraction angles of primary and secondary diffracted beams are large since the grid pitches are small, and therefore these beams do not appear on the photodetector 18 side. Accordingly, only the moire fringes due to a zero-order diffracted beam is detected by the photodetector 18, and length and detection of displacement are measured by a signal processing means by using an output signal of the element 18. By making the grid pitches of the scales small so that only the zero-order diffracted beam be transmitted, in this way, a resolution can be improved, and thus an excellent contrast can be obtained.
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