Patent ID: 7672051

Claim:
A surface light source comprising a light guide and an optical film arranged on an exit surface of the light guide, wherein the optical film comprises: an incident surface having a shape to bend incident light; and an exit surface opposing the incident surface and having a diffuser to diffuse light only into a specific spatial range of angles, wherein the incident surface is a transmissive diffraction grating that bends while light having an incident angle of 60°±15° into a direction perpendicular to the exit surface of the optical film and a relational expression h=m×d/ ( n− 1) with 0.5< m< 1.0 is satisfied, wherein d is an average interval of the grating and 0.6 μm≦d≦10 μm, h is an average depth of the grating and n is a refractive index of the grating.