| 摘要 |
PURPOSE:To inhibit the number of revolution of a polygon mirror at a low value, to prevent the increase of cost of the polygon mirror and a polygon motor and to miniaturize the title device by providing a plurality of laser emission sources capable of changing over the number of laser emission sources used when the density of scanning is changed over. CONSTITUTION:In a laser beam printer, each laser unit 1, 2 is driven respectively at the same time by separate driver circuit in exposure scanning, and scanned onto a sensitizing drum 7 at every two line, and the point A of laser beams 1a proceeds from the point B of laser beams 2a regarding the position of a laser spot for the same time. When the density of scanning is changed over, a hexagonal mirror is used as a polygon mirror, and the LBP of speed at eight/ min is brought. Accordingly, the number of revolution of the polygon mirror is brought to 4500, 5600 and 7400 in the laser unit 1 regarding 240, 300 and 400 DPI, and the number of revolution of the polygon mirror does not exceed the revolution of ten thousands even in 600DPI in the laser unit 2. |
