| 摘要 |
A surface light source device is provided with a primary light source and light guide plate. The light guide plate has an incidence side face opposite to which the primary light source employing LEDs or the like is disposed, an emission face for emitting light, which has been incident to the incidence side face, during the propagation process, and a back face oppositely located with respect to the emission face. The back face may be provided with a plurality of prism projections running generally vertically to the incidence side face. The primary light source has an effective light emitting surface provided with length L in a thickness direction of the light guide plate, which is not smaller than thickness t of the light guide plate at the incidence side face. This causes primary light to be incident to the incidence side face as to cover from an upper edge to lower edge thereof, preventing dark parts from being generated. As a result, the emission face comes to avoid stripe-like uneven brightness from appearing thereon.
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