| 摘要 |
PURPOSE:To elevate the detection accuracy checking adverse effect from dust or the like with a simple construction by arranging an optical system for converging lights from a plurality of light sources, light transmitting/shielding means and a light receiving means. CONSTITUTION:Lights from light sources 7a and 7b form a converged light and converges at convergence points at the center of a slit plate 8. A light transmitting section F and a light shielding section G are formed alternately on the slit plate 8 and as the spots of the converged light on the transparent plate 9 are larger enough that a slit pitch P, possible effect can be reduced remarkably even when dust or the like exists on the transparent plate 9 than when dust attaches directly on the slit plate 8. On the other hand, as the slit plate 8 moves in the direction M, pulses are counted from variations in the quantity of light cylically incident into photo detectors 10a and 10b thereby enabling detection of the position of the slit plate 8. Thus, the lowering of the detection accuracy due to dust or the like can be prevented with a simple construction thereby maintaining a stable performance for a long time. |
