Patent ID: 6927883

Claim:
A light-scanning optical apparatus comprising: an optical deflector; an incidence optical system adapted to cause a light beam emitted from a light source to strike a deflection plane of said optical deflector with a predetermined angle in a sub-scanning section; and a focusing optical system for focusing the light beam reflected by the deflection plane of the optical deflector on a surface to be scanned, wherein said focusing optical system includes an first optical system having a spherical lens and a first cylindrical lens showing power in the main-scanning direction and a second optical system showing power in the sub-scanning direction, wherein said focusing optical system satisfies the requirements of conditional formulas (1) and (2)  ( N1 - 1 ) R2 · F  < 0.15 ⁢ ⁢ and ( 1 )  ( N2 - 1 ) R3 · F  < 0.15 ; and ( 2 ) wherein F is the focal length of the first optical system in the main-scanning direction, R 2 is the radius of curvature of the surface of the spherical lens facing the surface to be scanned, R 3 is the radius of curvature of the surface of the first cylindrical lens facing the optical deflector as viewed in the main-scanning direction, N 1 is the refractive index of the material of the spherical lens at the operating wavelength, and N 2 is the refractive index of the material of the first cylindrical lens at the operating wavelength.