| 摘要 |
PROBLEM TO BE SOLVED: To provide a scanning optical system capable of avoiding degradation of plotting performance caused by fluctuation of wavelengths of light emitted from a light source by correcting chromatic aberration of magnification without increasing the number of scanning lenses. SOLUTION: A laser beam which is emitted from a light source 1 like semiconductor laser and made to be a parallel luminous flux is made incident on a polygon mirror (deflector) 4 driven to rotate around a rotary axis 4a via a cylindrical lens 3 having power only in the direction of sub-scanning, and scanned and deflected by the polygon mirror 4, to form an image on a plane to be scanned 5 via an fθlens 20 of three-piece composition for scanning. A Fresnel lens-type diffraction lens structure acting to correct the chromatic aberration of magnification is formed at least on one surface of the fθlens 20, and when the number of the periodical structures of the diffraction lens structure contained in the luminous flux made incident at a maximum angle is expressed by m and the maximum angle is expressed by W (degree), the diffraction lens structure is to be determined so as to satisfy the condition 0.3<=m/W<=4.0. |
