| 摘要 |
PURPOSE:To improve the manufacture accuracy and detection accuracy of a pattern to be detected. CONSTITUTION:A pattern ring 16 for which a shielding pin 16b having a radius of (r) is disposed at a P cycle is fixed on one end of a focus ring 12, and a photointerrupter 19 having one light emitting element 17 and two light receiving elements 18a and 18b in a unitized state is disposed in the rotational area. As for the photointerrupter and the pattern ring 16, the following conditions are satisfied; 0.5<(b/d).(2d-4delta)/P<2 and 0.5<(b/d).(d/2-delta)/{r.sq. rt. (1+[b/(2d)]<2>)}<2. Provided that (d) denotes a distance between the light emitting element and two light receiving elements, (b) denotes the distance between two light receiving elements and (delta) denotes the positional deviation from the center line F of the distance (d) to the center point (h) of the shielding plate 16b. |
