Patent ID: 7199932

Claim:
A surface emitting device comprising: a light guiding plate; a light source arranged in the vicinity of one edge of the light guiding plate; and a prism sheet composed of a transparent sheet and arranged on one surface of the light guiding plate orthogonal to the one edge, wherein at least one of the one surface and an opposing surface of the light guiding plate has a plurality of rib-shaped prism portions each having a slight inclined plane and a steep inclined plane arranged closer to the one edge than the slight inclined plane, both the slight inclined plane and the steep inclined plane consecutively provided on at least one of the one surface and the opposing surface of the light guiding plate opposing the one surface, wherein a plurality of rib-shaped light refracting portions each having a refractive surface and a reflecting surface are consecutively provided on an incident surface of the prism sheet facing the light guiding plate, an emission surface opposite to the incident surface is a flat surface, and the prism sheet is arranged on the light guiding plate with the refractive surfaces facing the light source, wherein light introduced into the light guiding plate from the light source through the one edge is emitted from the slight inclined plane and is then incident on the light refracting portion, and the light incident on the light refracting portion is introduced into the transparent sheet by the refractive surface and is then refracted from the reflective surface in the transparent sheet to be emitted from the emission surface, and wherein, when an incident angle of the incident light with respect to the normal direction of the emission surface of the prism sheet is α, an emission angle with respect to the emission surface is ψ, an inclination angle of the refractive surface with respect to the emission surface is θ 1 , an inclination angle of the reflective surface with respect to the emission surface is θ 2 , and a refractive index of the transparent sheet is N, the inclination angles θ 1 and θ 2 satisfy the following Expression 3: θ 2 =½[180−θ 1 −sin −1 {sin(α−θ 1 )/ N }−sin −1 (sin ψ/ N )]. [Expression 3]