| 摘要 |
PURPOSE:To accurately search for the data in an optical card by recording the position of the least significant bit among the data showing the positions of individual data recording regions at the position near the center of the laser beam within a laser beam irradiation range from the position of the most significant bit. CONSTITUTION:A track number showing pattern 3 is arranged so that the LSB (least significant bit) is located at the center of illumination light 8, that is, the position max. in quantity of light and having high resolving power and goes to a position of low resolving power where the end of the illumination light, that is, the quantity of light successively becomes min. as a bit transfers from the LSB to the MSB (most significant bit). The unchanged part on a ring where luminous intensity does not almost change even when the distance between a condensing lens 19 and an optical card 23 is changed is generated in an illumination region and the change of luminous intensity becomes reverse on the inside and outside of the unchanged part by the change of the focus state of the condensing lens 19. That is, when the optical card 23 approaches the condensing lens 19 from the focal position of the condensing lens 19, luminous intensity increases as compared with that at the time of focusing on the inside across the unchanged part but decreases on the outside. |
