Patent ID: 8253767

Claim:
A line head comprising: a first light-emitting element, a second light-emitting element, and a third light-emitting element that are arranged in a first direction; and an optical system that forms an image by light emitted from the first light-emitting element, an image by light emitted from the second light-emitting element, and an image by light emitted from the third light-emitting element on an imaging surface to form an image by light emitted from the first light-emitting element, an image by light emitted from the second light-emitting element, and an image by light emitted from the third light-emitting element, wherein the first light-emitting element is arranged between the second light-emitting element and the third light-emitting element in the first direction; the optical system has a first lens surface that has refractive power and is arranged so as to satisfy the relationship below; light emitted from the first light-emitting element and light emitted from the second light-emitting element do not overlap with each other on a cross section of the first lens surface taken along the first direction so as to include an optical axis of the optical system; H>0.5D, where H is a distance in the first direction between the geometrical center of the image of the second light-emitting element and the geometrical center of the image of the third light-emitting element, imaged by the optical system; and D is the maximum width in the first direction of a region of the first lens surface through which light emitted from the second light-emitting element and the third light-emitting element pass; an aperture diaphragm is provided between the second light-emitting element and the optical system; and the lens surface is arranged so as to satisfy a relation of L 1 ·tan ω>2·L 2 ·tan μ, where ω is an angle in the first direction between a principal ray of light emitted from the second light-emitting element and the optical axis; μ is an image-side aperture angle (half-angle) of the optical system; L 1 is a distance between the aperture diaphragm and the first lens surface; and L 2 is a distance between the first lens surface and the imaging surface.