| 摘要 |
A position detecting apparatus projects a light beam to form a light spot within a certain range on an object under test by means of a light source, collimator lens, pinhole plate, half-mirror, and objective lens included in it. The apparatus further includes a deflecting prism which deflects the light spot relative to the object continuously so that the light spot has a circular or ellipsoidal locus of movement on the object, and a first and second light sensors which receive a reflected light beam from the object at positions short of and ahead of the convergent position of the reflected light beam. The apparatus determines that the object is located at the reference position if the outputs of the first and second light sensors thin a certain period of spot scanning are substantially equal.
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