US Patent 7419292 Illuminating device

An illuminating device includes a light source, an optical waveguide that emits light from the light source through an emission surface thereof, and a prism sheet that is provided on the emission surface side of the optical waveguide. In the illuminating device, the following relationship is established between a refractive index x of the prism sheet and an apex angle y of a prism: y=107.86x−140.43+M (−4≦M≦4).