Dual wavelength F-theta scan lens

An F-theta scan lens for use in a dual beam raster output scanning (ROS) system, said F-theta scan lens being located in the optical paths of a first beam having a first wavelength and a second beam having a second wavelength between a deflector and a scanning surface, said deflector deflecting said first beam and said second beam to scan said scanning surface, said F-theta scan lens in order from said deflector to said scanning surface comprising:

• a first lens,
• a second lens,
• a third lens, and
• a fourth lens, wherein said third lens and said fourth lens form a doublet having a negative refractive power which further fulfills the following conditions:
• vd1<vd2,
• vd3<vd4,
• R11<0,
• R12<0,
• 0.9<R11/R12<1.1,
• R31<0,
• R32<0,
• 0.1<R31/R32<0.5,
• f1<0,
• f2>0,
• f3<0,
• f4>0,
• fG1>0,
• fG2<0,
• where vd1, vd2, vd3 and vd4 denote the Abbe dispersion numbers of said first, second, third and fourth lenses respectively, R11 and R12 denote the radii of curvature of said first and second surfaces of the first lens respectively in order from said deflector to said scanning surface, R31 and R32 denote the radii of curvature of said first and second surfaces of the third lens respectively in order from said deflector to said scanning surface, f1, f2, f3 and f4 denote the focal lengths of said first, second, third and fourth lenses respectively, fG1 denotes the focal length for the combination of said first and second lenses and fG2 denotes the focal length for the combination of said third and fourth lenses.