Patent ID: 7753544

Claim:
A light control plate, which is disposed, relative to a surface light source, on a side of a predetermined direction in which the surface light source emits parallel light having a substantially uniform luminance distribution, and which is disposed so as to be substantially perpendicular to the predetermined direction, the light control plate comprising: a light incidence surface onto which the parallel light from the surface light source is incident; and a planar light exit surface, disposed opposite the light incidence surface, from which the parallel light that is incident on the light incidence surface exits as outgoing light, wherein an optical path control section extending in one direction and having a concave and/or convex cross-sectional shape is formed on the light incidence surface, a surface of the optical path control section comprises first to M-th planar portions (M is an integer equal to or greater than 2) respectively extending in the one direction, an inclination angle α m formed between a plane parallel to the light exit surface and an m-th planar portion (m is an integer from 1 to M) satisfies [ Equation ⁢ ⁢ 1 ] α m = sin - 1 ⁢ { n n 0 ⁢ sin ⁡ ( α m - ξ m ) } ⁢ ⁢ i ⁢ ⁢ n ⁢ ⁢ which ( 1 ) [ Equation ⁢ ⁢ 2 ] ξ m = sin - 1 ⁢ { n 1 n ⁢ sin ⁢ ⁢ θ m } ( 2 ) [ Equation ⁢ ⁢ 3 ] θ m = 2 ⁢ m - M - 1 M - 1 ⁢ θ max ( 3 ) where n is a refractive index of the light control plate, n 0 is a refractive index at the light incidence surface on the side of the surface light source, n 1 is a refractive index at the light exit surface on an opposite side to the light incidence surface, −θ max to θ max is an exit angle range of the outgoing light relative to the predetermined direction, θ m is an exit angle, relative to the predetermined direction, of light, within the parallel light, which is incident via the m-th planar portion and exits through the light exit surface, I(θ m ) is a luminance in a direction of the exit angle θm, and T is a transmittance of the light control plate to the parallel light, a pitch ratio L m of the m-th planar portion satisfies [ Equation ⁢ ⁢ 4 ] L m = l m ∑ k = 1 M ⁢ l k ⁢ ⁢ i ⁢ ⁢ n ⁢ ⁢ which ( 4 ) [ Equation ⁢ ⁢ 5 ] l m = I ⁡ ( θ m ) ⁢ cos ⁢ ⁢ θ m T ( 5 ) and the luminance I(θ m ) is substantially constant within the exit angle range.