| 摘要 |
PROBLEM TO BE SOLVED: To precisely detect the reference position or the absolute position of a scale in an optical encoder. SOLUTION: A third photodetector 24 is formed on an area neared to a second photodetector 22 for detecting the reference position or the absolute position of the scale 18 so that the peripheral part of a light beam 16 emitted from a second coherent light source 12 is incident. Voltage or a current value applied on the second coherent light source 12 is controlled so that incident intensity on the third photodetector 24 is constant on the basis of the intensity of the light beam from the second coherent light source 12 detected with the third photodetector 24 by a light beam intensity control means.
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