| 摘要 |
PURPOSE:To change the diameter of a picture element in a sub-scanning direction, to perform tone expression without reducing resolution and to form a high-quality image by providing a polygonal rotor consisting of deflection surfaces forming an angle with the center axis of the rotor and deflection surfaces parallel with the center axis and an optical system for setting the diameter of a light beam. CONSTITUTION:The polygonal rotor 7 is formed of the deflection surfaces 71, 73 and 75 parallel with the center axis 70 and the deflection surface 72, 74 and 76 forming the angle theta with the center axis 70. The light beam 16-1 deflected by the deflection surfaces 71, 73 and 75 is condensed on a photosensitive body 5 through a cylindrical lens 10 to become a beam spot (1). The light beam 16-2 deflected by the deflection surfaces 72, 74 and 76 is made incident on beam diameter changing lenses 14 and 14' and reduced to be small in the subscanning direction (y), then it is condensed on the photosensitive body 5 through a cylindrical lens 10' to become the beam spot (2). The picture element is formed based on the diameter of the beam spot in accordance with image information, so that the desired diameter of the picture element is obtained. Then, the tone expression is performed without reducing the resolution and the high-quality image is obtained. |
