Diffusing member, illuminating device, image reading apparatus, and image forming apparatus to improve diffusivity of light with suppressing a decrease of incident light on an illuminated region of an original

Abstract

The diffusing member includes a diffusing surface with protruding portions arrayed in a first direction. The protruding portions include a first group of protruding portions in which tops thereof are arranged on a first line parallel to the first direction when they are projected on a first cross section parallel to the first direction. Frequencies of inclination angles of the first group of protruding portions α within a range of 0°±5°, β within a range of and γ within a range of at respective positions on a predetermined line parallel to the first direction of the first group when the first group is projected on a second cross section including each top thereof and perpendicular to the first cross section, a radiation angle of light incident on the diffusing surface ω, and a refractive index of the diffusing member n, are appropriately set.