Patent ID: 8830542

Claim:
An illumination apparatus for use in an image reading apparatus that forms a reduction image of a read area of an original on an image pickup element by an imaging optical system, the illumination apparatus comprising: a light source illuminating the original; and a light guide guiding light from the light source to the original; wherein the light guide has a first optical surface on which light from the light source is incident or that diffuses light from the light source, a second optical surface from which light exits toward the read area, a third optical surface, and a fourth optical surface, the third optical surface and the fourth optical surface being arranged between the first optical surface and the second optical surface in a sub-scanning section perpendicular to a longitudinal direction of the light guide, the third optical surface being arranged on a side closer to a reading optical axis of the imaging optical system, and the fourth optical surface being arranged on a side farther from the reading optical axis of the imaging optical system, wherein the third optical surface and the fourth optical surface are reflective surfaces having paraxial powers equal to each other, wherein the following condition is satisfied: 1.5≦φ≦2.8, where φ in mm −1 is the paraxial power of the third optical surface and the fourth optical surface.