| 摘要 |
PROBLEM TO BE SOLVED: To facilitate adjustment and to increase photodetection sensitivity. SOLUTION: A graduation line edge image of a scale 1 is enlarged by an objective 2 and impinges on a beam splitter 3; and its transmitted image is photodetected by a 1st photodetecting element 4 and its reflected image is photodetected by a 2nd photodetecting element 5. A slit plate 7 having 1st and 2nd slits 6a and 6c formed is arranged in front of the photodetection surface of the 1st photodetecting element. A slit plate 8 in which a 2nd slit 6b is formed is arranged in front of the photodetection surface of the 2nd photodetecting element 5. The slits 6a and 6c are arranged across the slit 6b at equal distances. The slits 6a, 6b, and 6c are arranged in parallel to graduation line edges 21 so that when the scale 1 is moved at right angles to its graduation lines, an image of the graduation line edges is photodetected by the 1st photodetecting element 4, 2nd photodetecting element 5, and 1st photodetecting element 4 in order through the slits 6a, 6b, and 6c in order.
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