Fθ lens and lens for forming linear image

Claim 1

An fθ lens in an optical scanner in which a light beam emitted from a semiconductor laser is changed to a parallel light beam by a collimator lens, the parallel light beam is formed by a first image forming optical system as a linear image extending in a main scan-corresponding direction, and the image-formed parallel light beam is then deflected by a rotary polygon mirror having a deflection reflecting face in the vicinity of said linear image so that the deflected light beam is formed as a light spot on a scanned face by a second image forming optical system to perform an optical scanning operation, said fθ lens being used as said second image forming optical system; said fθ lens having an fθ function with respect to the main scan-corresponding direction and a function for setting the scanned face and a deflecting start point on the deflection reflecting face in an approximately conjugate relation in geometrical optics with respect to a cross scan-corresponding direction; said fθ lens comprising a first lens group having negative refracting power in a main scan-corresponding direction, and a second lens group having positive refracting powers in both the main and cross scan-corresponding directions, said second lens group comprising a lens having different refracting powers in the main scan-corresponding direction and the cross scan-corresponding direction, said first lens group and said second lens group being sequentially arranged from an object side of said fθ lens to an image side of said fθ lens, said first lens group comprising only one lens made of glass and said second lens group comprising only one lens made of glass; an Abbe number of said one lens belonging to said first lens group and having said negative refracting power in the main scan-corresponding direction being less than an Abbe number of said one lens belonging to said second lens group and having said positive refracting powers in both the main and cross scan-corresponding directions.