| 摘要 |
PURPOSE:To keep the spot intensity of a light beam nearly constant by setting a distortion aberration coefficient of an image-formation optical system to >=2/3 and <=1.13, and by prolonging the time of a modulated signal from a modulation control means when a light beam travels at a position away from the optical axis of the image-formation optical system. CONSTITUTION:Image-forming lens 15 consists of three elements 20, 21 and 22 in three groups. In the scanning system of 200mm. in scanning width, concave lens 20 is located 40mm. away from surface S of rotary polygon mirror 13. Lenses 21 and 22 are both convex ones and the interval between convex lens 22 and photosensitive material 16 is 403.3mm.. The constitution of image-forming lens 15 is expressed by formula I (R is a radius of curvature, D is an air interval on the axis or thickness on the axis, and N is a refractive index). The focal length of lens 15 is 300mm. and its F number is 60. A light beam modulated usually by modulator 12 and deflected by rotary polygon mirror 13 passes through lens 15 to form an image, which scans on photosensitive material 16 as shown by arrow 17; the dot width on the scanning surface is equal and its spot intensity never lowers at the circumference of the surface to be scanned. |
