Patent ID: 7357552

Claim:
A surface light source device comprising: a light guide plate having an emission face of a generally rectangular first shape and a primary light source supplying primary light into said light guide plate through an incidence face formed at at least one corner portion of said light guide plate; said incidence face has a configuration which is obtained by undulation-modification to an oblique plane provided by cutting off said corner portion obliquely as viewed from above said emission face; said undulation-modification giving said incidence face first and second regions; said first incidence region being provided with unevenness that produces an inner propagation light having a diverging angle greater than that which would be obtained under a case where primary light was refracted on incident to said oblique plane; said second incidence region being provided with a slope inclined with respect to said oblique plane, wherein, when an inner propagation light beam of a first direction is defined as an inner propagation light beam that is produced by being refracted on incidence of a light beam along an optical axis of said primary light source onto said slope and an inner propagation light beam of a second direction is defined as an inner propagation light beam that is produced by being refracted on incidence of a light beam along an optical axis of said primary light source onto said oblique plane, said slope is inclined with respect to said second direction as to be deflected toward one of both sides of said second direction so that an brightness unbalance arising in a case where said first direction accords with said second direction is reduced.