Patent ID: 8152320

Claim:
A surface light source device comprising: a light source unit structured by arranging a plurality of light-emitting sources that are substantially point light sources in a two-dimensional direction; and a lenticular lens sheet structured by arranging a plurality of unit lenses of one type or of various types, each of which is a part of a substantially elliptic cylinder, such that lens surfaces of the unit lenses face a light emergent side, wherein adjacent unit lenses are separated by flat troughs; wherein, when an interval at which the light-emitting sources are arranged in the same direction as an arrangement direction of the unit lenses of the lenticular lens sheet is defined as L, an interval between the light emitting sources and a rear surface of the lenticular lens sheet is defined as d, a unit-lens end-portion angle, which is an angle between (i) a tangent line that intersects a boundary between the unit lens and an adjacent flat trough and extends along a portion of the lens surface of the unit lens and (ii) a normal line of the lenticular lens sheet that intersects said tangent line, is defined as θ, a refractive index of a material forming the unit lenses of the lenticular lens sheet is defined as n, and a refractive angle at which a light ray emitted from a light-emitting source is refracted by a rear surface of the lenticular lens sheet, relative to a line normal to the rear surface of the lenticular lens sheet is defined as φ, and wherein relationships of: cos −1 ( n ×cos(φ+θ))≦θ; and φ=sin −1 (sin(tan −1 ( L/ 2 d ))/ n ); or relationships of: n ×cos(φ+θ)>1; and φ=sin −1 (sin(tan −1 ( L/ 2 d ))/ n ); are satisfied.