Patent ID: 7397620

Claim:
An image reading apparatus, comprising: a photoelectric conversion element having picture elements arrayed in a main-scan direction; and an image reading lens for imaging imagewise information on a surface of an original, upon a light receiving surface of said photoelectric conversion element, wherein said image reading lens includes at least one anamorphic lens having anamorphic surfaces formed at a light entrance surface and a light exist surface thereof, respectively, wherein, when a sectional shape of the anamorphic surface in a main-scan section which contains an optical axis of said image reading lens and the main-scan direction, is taken as a meridional, a curvature of a sagittal of the anamorphic surface defined within a plane being perpendicular to the main-scan section and containing a normal to the anamorphic surface at an arbitrary position in the meridional direction, changes continuously along the meridional direction from the optical axis, wherein, when N is a refractive index of a material of the anamorphic lens, r in is a curvature radius of the sagittal at a position where a principal ray passes through the light entrance surface of the anamorphic lens, r out is a curvature radius of the sagittal in a direction perpendicular to the main-scan section and at a position where the principal ray passes through the light exit surface of the anamorphic lens, d is a distance in the optical axis direction between the positions where the principal ray passes the light entrance surface and the light exit surface of the anamorphic lens, respectively, ω is a largest half field angle of said image reading lens, and when constants A and B expressed by A = N - 1 r i ⁢ ⁢ n ⁢ ( 1 - 1 r out ⁢ ( r i ⁢ ⁢ n - d ⁡ ( N - 1 ) N ) ) B = 1 - d ⁡ ( N - 1 ) N ⁢ ⁢ r i ⁢ ⁢ n are used, then conditions expressed by 0.8 < A off A on ⁢ cos ⁢ ⁢ ω < 1.2  1 - A off A on ⁢ cos ⁢ ⁢ ω  +  1 - B off B on ⁢ cos ⁢ ⁢ ω  < 0.2 are satisfied, where A on and B on are values of the constants A and B concerning a principal ray of abaxial light, respectively, and A off and B off are values of the constants A and B concerning a principal ray of most abaxial light, respectively.