| 摘要 |
PROBLEM TO BE SOLVED: To provide a scanning optical system capable of correcting power variation, without depending on coating which incurs cost. SOLUTION: A laser beam emitted from a semiconductor laser 10 is made incident on a polygon mirror 13 through a collimating lens 11 and a cylindrical lens 12, reflected, deflected and transmitted through a diffraction element 20 and is transmitted through an f-θlens 30 being an image formation optical system and converged so as to form a spot to scan in the main scanning direction on a surface to be scanned 40. The diffraction element 20 is a nearly planar parallel plate-like element, where a diffraction surface 21 is formed on its surface on the polygon mirror 13 side and has a function of compensating chromatic aberration caused by the dispersion of the wavelength of a light source and a function of counteracting a difference between the on-axis and the off-axis transmitted light quantities of the f-θlens 30 which is the image formation optical system. Therefore, the surface 21 is set such that the on-axis diffraction efficiency is lower than off-axis diffraction efficiency.
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