Lens system for optical beam scanner

Claim 2

A lens system for an optical beam scanner which scans a plurality of optical beams over a recording surface (RS), said lens system comprising a first lens group and a second lens group, the first lens group having an entrance pupil (EP), which lens system is characterised in that said first lens group (F L ) includes a first lens (L₁) in the form of meniscus having a positive power and a concave surface (S₁) directed towards said entrance pupil (EP), a second lens (L₂) in the form of meniscus having a negative power, a third lens (L₃) in the form of meniscus having a positive power, and a fourth lens (L₄) having a positive power, and said second lens group (F R ) includes a fifth lens (L₅) having a positive power, said first to fifth lenses (L₁ to L₅) being successively disposed in that order from said entrance pupil (EP).

Claim 3

A lens system according to claim 1, wherein said lens system satisfies the following inequalities: 0.6 < (r₂/r₃) ≦ 2.1 -0.43 < (f₂/f) < -0.21 1.3 < (f₅/f) ≦ 2.6 where r₂ is the radius of curvature of the surface (S₂) of said first lens (L₁) which is directed towards said recording surface (RS), r₃ is the radius of curvature of the surface (S₃) of said second lens (L₂) which is directed towards said entrance pupil (EP), f₂ is the local length of said second lens (L₂), f₅ is the focal length of said fifth lens (L₅) and f is the focal length of the lens system.

Claim 4

A lens system according to claim 1 or 2, wherein said fifth lens (L₅) is a plano-convex lens having a flat surface which is directed towards said entrance pupil (EP).

Claim 5

An optical beam scanner for scanning a plurality of optical beams over a recording surface (RS), said optical beam scanner comprising: a light source (11, 13) for generating the plurality of optical beams; a deflector (28) for deflecting the optical beams outgoing from said light source; and a lens system (29) for imaging the optical beams deflected by said deflector (28) on said recording surface (RS), said lens system comprising a first lens group (F L ) having an entrance pupil and a second lens group (R L ); which scanner is characterised in that: said first lens group (F₄) includes a first lens (L₁) in the form of meniscus having a positive power and a concave surface (S₁) which is directed towards said entrance pupil (EP), a second lens (L₂) having a negative power, a third lens (L₃) in the form of meniscus having a positive power, and a fourth lens (L₄) having a positive power; and said second lens group (R L ) includes a fifth lens (L₅) having a positive power, said first to fifth lenses (L₁ to L₅) being successively disposed in that order from said entrance pupil (RS).

Claim 6

An optical beam scanner for scanning a plurality of optical beams over a recording surface (RS), said optical beam scanner comprising: a light source (11, 13) for generating the plurality of optical beams; a deflector (28) for deflecting the optical beams outgoing from said light source; and a lens system (29) for imaging the optical beams deflected by said deflector (28) on said recording surface (RS), said lens system comprising a first lens group (F L ) having an entrance pupil and a second lens group (R L ); which scanner is characterised in that: said first lens group (F₄) includes a first lens (L₁) in the form of meniscus having a positive power and a concave surface (S₁) which is directed towards said entrance pupil (EP), a second lens (L₂) having a negative power, a third lens (L₃) in the form of meniscus having a positive power, and a fourth lens (L₄) having a positive power; and said second lens group (R L ) includes a fifth lens (L₅) having a positive power, said first to fifth lenses (L₁ to L₅) being successively disposed in that order from said entrance pupil (RS).

Claim 7

An optical beam scanner according to claim 4, wherein said lens system satisfies the following inequalities: 0.6 < (r₂/r₃) ≦ 2.1 -0.43 < (f₂/f) < -0.21 1.3 < (f₅/f) ≦ 2.6 where r₂ is the radius of curvature of the surface (S₁) of said first lens (L₁) which is directed towards said recording surface (RS), r₃ is the radius of curvature of the surface (S₃) of said second lens (L₂) which is directed towards said entrance pupil (EP), f₂ is the focal length of said second lens (L₂), f₅ is the focal length of said fifth lens (L₅); and f is the focal length of the lens system.