Patent ID: 7564517

Claim:
A surface light source comprising: a prism sheet including an entrance surface at which a plurality of triangular elongated prism portions are arrayed parallel to each other, each of the triangular elongated prism portions having a first ridge surface and a second ridge surface which meet at an angle to define an angular ridge, and an exit surface from which light entering from the entrance surface exits; and at least one light source unit which is placed outside of a region on an entrance surface side of the prism sheet, said region corresponding in position to the entrance surface of the prism sheet, so as to provide a space, in said region, in which no solid optical member is provided, wherein the at least one light source unit emits light through the space toward at least one of the first ridge surfaces of the triangular elongated prism portions, and the second ridge surfaces of the triangular elongated prism portions; wherein the prism sheet includes at least one entrance end surface aligned with at least one boundary of the space, which is a boundary between the space and outside of said region, and wherein the at least one boundary faces the at least one light source unit; wherein the surface light source further comprises a reflecting plate which has dimensions corresponding to the prism sheet, and is placed at the entrance surface side of the prism sheet to face the entrance surface of the prism sheet through the space through which the light from the light source unit passes, and which reflects the light from the light source unit; wherein no solid optical member is provided between the prism sheet and the reflecting plate, such that the space in which no solid optical member is provided extends between the prism sheet and the reflecting plate; and wherein each said light source unit comprises: at least one solid-state light-emitting element, and a rectangular light guide plate which is formed from a light scattering transparent material and has a length substantially equal to a length of said plurality of triangular elongated prism portions; wherein the light guide plate includes an entrance surface through which light emitted from the solid-state light-emitting element enters as a first one of two side surfaces of the light guide plate that extend along a longitudinal direction of the light guide plate, and an exit surface as a second one of the two side surfaces of the light guide plate that extend along the longitudinal direction of the light guide plate; and wherein the exit surface of the light guide plate is Placed-at a side of and faces the space between the reflecting plate and the prism sheet, and the light guide plate diffuses the light emitted from the at least one solid-state light-emitting element entering from the entrance surface, and causes the light to exit from the exit surface.