Patent ID: 7800806

Claim:
A two-element fθ lens with short focal distance for a laser scanning unit, the laser scanning unit including a light source for emitting a laser beam, a polygon mirror for reflecting and converting the laser beam into a scanning light, and a target for sensing the scanning light; and the two-element fθ lens with short focal distance along an optical axis from a polygon mirror side to the target side comprising, a first lens, included a first optical surface and a second optical surface; and a second lens, included a third optical surface and a fourth optical surface; wherein, a concave surface of the first optical surface, the second optical surface and the third optical surface toward the polygon mirror on optical axis in a main scanning direction; and the fourth optical surface having an inflection point in SAG counted from the optical axis to a peripheral portion and a paraxial portion is convex toward the polygon mirror side; and the first optical surface, the second optical surface, the third optical surface, and the fourth optical surface are aspherical surfaces in the main scanning direction; satisfies the conditions of: 2.5 ≤ d 1 + d 3 + d 5 f s ≤ 5.2 ; 0.5429 ≤ tan ⁡ ( β ) ≤ 1.2799 ; wherein, d 1 is a distance from a reflecting surface of the polygon mirror on the optical axis to the first optical surface; d 3 is a distance from the second optical surface of the first lens to the third optical surface of the second lens along the optical axis; d 5 is a distance from the fourth optical surface of the second lens to the target along the optical axis; f s is a composition focal length of the two-element fθ lens; and β is a maximum effective window angle.