| 摘要 |
In an optical scanner, a light beam emitted from a semiconductor laser is changed to a parallel light beam by a collimator lens and the parallel light beam is formed by a first optical system as a linear image extending in a main scan-corresponding direction. The image-formed parallel light beam is deflected at an equal angular velocity by a rotary polygon mirror having a deflection reflecting face in the vicinity of the linear image. The deflected light beam is formed as a light spot on a scanned face by a second optical system to perform an optical scanning operation. An f theta lens in the optical scanner is used as the second optical system. The f theta lens has an f theta function in the main scan-corresponding direction and a function for setting the scanned face and a deflecting start point on the deflection reflecting face in an approximately conjugate relation in geometrical optics in a cross scan-corresponding direction. The f theta lens has a first lens having a concave spherical face on an object side of the f theta lens and a cylindrical face on an image side of the f theta lens; a second lens having a concave spherical face on the object side and a convex spherical face on the image side; and a third lens having a flat face on the object side and a toric face on the image side. The f theta lens may be constructed by two lenses.
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