Two-element f(theta) lens with short focal distance for laser scanning unit

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, d1 is a distance from a reflecting surface of the polygon mirror on the optical axis to the first optical surface;
• d3 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;
• d5 is a distance from the fourth optical surface of the second lens to the target along the optical axis;
• fs is a composition focal length of the two-element fθ lens ;
• and β is a maximum effective window angle.