| 摘要 |
PURPOSE:To make an optical part usable for detecting tracking errors in both 1-spot system and 3-spot system by making the optical part slide. CONSTITUTION:With respect to the optical part 20, a diffraction grating 21 is formed in the left half of one of the surfaces of a transluscent base plate 22 composed, for instance, of glass plate, but in the right half is no grating formed. In case of the 3-spot system, the optical part 20 is so positioned that the luminous flux from a light source 1 is made incident on the diffraction grating 21. On the other hand, in cases of the 1-spot system, the optical part 20 is made slid in the longitudinal direction so that the light from the source 1 is made incident on the area where the diffraction grating 21 is not formed.
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