Patent ID: 7817320

Claim:
A two-element fθ lens with short focal distance, applied to a laser scanning unit, and said laser scanning unit including a light source for emitting a laser beam, a polygon mirror for scanning said laser beam to form a scanning light, and a target for sensing a light; and said two-element fθ lens with short focal distance along an optical axis from a polygon mirror side to a target side comprising, a first lens, in a meniscus shape, and having a concave surface toward said polygon mirror; and a second lens, in a meniscus shape, and having a concave surface toward said polygon mirror; wherein, said first lens includes a first optical surface and a second optical surface, having positive refractive power in main scanning direction on optical axis of said laser scanning unit; wherein, said second lens includes a third optical surface and a fourth optical surface, having negative refractive power in main scanning direction on optical axis of said laser scanning unit; said fourth optical surface having an inflection point in SAG counted from the optical axis to peripheral portion and its paraxial portion is convex toward the polygon mirror side; said first optical surface, said second optical surface, said third optical surface and said fourth optical surface are aspherical surfaces in the main scanning direction; satisfies the conditions of: 0.6 ≤ d 1 + d 3 + d 5 f s ≤ 2.0 ; ⁢ and 0.4557≦tan(β)≦0.7265; wherein d 1 is a distance from a reflecting surface of the polygon mirror to said first optical surface of said first lens along the optical axis; d 3 is a distance from said second optical surface of said first lens to said third optical surface of said second lens along the optical axis; d 5 is a distance from said fourth optical surface of said second lens to said target along the optical axis; f s is a composition focal length of said two-element fθ lens; and β is a maximum effective window angle.