TWO-ELEMENT F(THETA) LENS WITH SHORT FOCAL DISTANCE FOR LASER SCANNING UNIT

摘要

A two-element f&thetas; lens with short focal distance for a laser scanning unit comprises a first lens and a second lens. The first lens has first and second optical surfaces, the second lens has third and fourth optical surfaces, and all the optical surfaces in a main scanning direction on the optical axis are aspherical surfaces. The fourth optical surface has an inflection point in SAG counted from the optical axis to peripheral portion and its paraxial portion is convex that is disposed on the polygon mirror side. The two-element f&thetas; lens satisfies an optical condition of: 0.5429≦̸tan(&bgr;)≦̸1.2799, wherein &bgr; is a maximum effective window angle. The first and second lenses of the two-element f&thetas; lens with short focal distance of the invention effectively reduces the distance from the polygon mirror to an imaging surface to achieve the purpose for reducing the volume of the laser scanning unit.