Patent ID: 8167474

Claim:
A surface light source device, comprising: a light source; and a light guide plate introducing light of the light source from a light incidence surface and letting the light exit to outside from the light exit surface, wherein the light source is provided at a position where a light-emitting surface thereof faces the light incidence surface of the light guide plate, and wherein the light guide plate comprises: a light introducing part for confining the light from the light source entering from the light incidence surface; and a light guide plate body having a thickness smaller than a maximum thickness of the light introducing part, being provided so as to continue to the light introducing part, and letting the confined light exit to outside from the light exit surface by light emitting means, wherein a ratio of the thickness of the light guide plate body with respect to the maximum thickness of the light introducing part is equal to or lower than 0.8, wherein the light introducing part has an inclined surface inclined from a surface of a part having a thickness larger than the light guide plate body toward an end of a surface of the light guide plate body, the inclined surface is on a surface on a light exit side of the light guide plate or a surface opposite thereto, wherein an inclination angle of the inclined surface is larger than an inclination angle formed with a line segment connecting an upper end of the inclined surface and an end of a surface opposite to the inclined surface of the light guide plate body on a side away from the light source in a vertical section of the inclined surface along a maximum inclination direction, and the inclination angle is smaller than 90°, wherein the light guide plate has a directivity conversion pattern on the surface on the light exit side or the surface opposite thereto, the directivity conversion pattern for converting a directivity spread of the light entering the light introducing part in a thickness direction of the light guide plate to a directivity characteristic inclined toward a direction parallel to a surface direction of the light guide plate, wherein the inclined surface and the directivity conversion pattern are disposed so that a part of the light entering the light introducing part is reflected simultaneously or alternately from the inclined surface and the directivity conversion pattern to be guided in the light guide plate, and wherein a length of the inclined surface in an inclined direction is defined so that a part of the light entering the light introducing part enters the inclined surface at least twice.