| 摘要 |
PURPOSE:To control the diameter of a beam on the surface of a photosensitive body by simple constitution corresponding to slits of different width and to control it automatically to the best beam diameter corresponding to each print density value by allowing plural slits of width to enter and exit from a laser light path corresponding to print density. CONSTITUTION:A laser beam projected by a semiconductor laser 1 has a constant spread and is converted into elliptic parallel light P1 by a collimator lens 2. A slit plate 11 is interposed between lenses 2 and 3, and the slit plate 11 has slits 11a-11c of different width formed at its peripheral edge part and is controlled by a pulse motor 12 so that the respective slits 11a-11c are positioned selectively on the optical axis. Thus, the slits 11a-11c are interposed and then the stopping of an ftheta lens 7 decreases by a decrease in beam diameter on a polygon mirror 4, so that the beam diameter on a photosensitive drum 20 increases on the photosensitive drum 20 as shown by P2'. Consequently, beam diameters are switched corresponding to print density.
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