F.theta. lens system in a laser scanning unit

An f.theta. lens system in a laser scanning unit having a laser source for emitting a laser beam, a rotary deflector for deflecting said laser beam in a horizontal main scan direction, a scanned face on which said laser beam deflected from said rotary deflector is scanned, said scanned face being rotated such that said laser beam is scanned thereon in a cross scan direction perpendicular to said main scan direction, and a group of two f.theta. lenses, which are positioned between said rotary deflector and said scanned face, for focusing uniformly said laser beam deflected from said rotary deflector on said scanned face, wherein said group of two f.theta. lenses comprises a first f.theta. lens and a second f.theta. lens sequentially arranged from said deflector towards said scanned face, wherein a refractive index of said first f.theta. lens is lower than that of said second f.theta. lens, and incident and emergent faces of said first f.theta. lens and an incident face of said second f.theta. lens is shaped into a cylindrical face, and an emergent face of said second f.theta. lens is shaped into a toric face, wherein the radii of curvature in the cross scan direction of said incident and emergent faces of said first f.theta. lens are of infinite curvature, and wherein the radii of curvature in the cross scan direction of said incident and emergent faces of said second f.theta. lens are of negative curvature, wherein the ratio of the radius of curvature in the main direction of said incident face of said first f.theta. lens versus that of said emergent face of said first f.theta. lens satisfies the following condition,

• 1:0.153 to 0.187
• and the ratio of the radius of curvature in the main direction of said incident face of said first f.theta. lens versus that of said emergent face of said second f.theta. lens satisfies the following condition,
• 1:0.373 to 0.456.