| 摘要 |
A laser beam scanner converges a laser beam by using a first ftheta lens and a second ftheta lens in a main-scanning direction and in a sub-scanning direction, respectively, onto a photosensitive medium, thereby forming a scanning line on the photosensitive medium along the main-scanning direction. Before scanning the photosensitive medium, the laser beam that has passed through and been converged by the first ftheta lens, but not passed through the second ftheta lens, is guided to a beam detector and converged by a cylinder lens in the sub-scanning direction so that the laser beam falls within a detection area of the beam detector. The beam detector determines a scan start time by detecting the laser beam.
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