Patent ID: 6921178

Claim:
A surface light source device comprising: a light guide plate having a side face and an emission surface; and a point light source, arranged on said side face of said light guide plate, for emitting light which enters said light guide plate to be propagated through said light guide and which leaves said emission surface of said light guide plate when an angle of incidence of the light on said emission surface is a critical angle or less while the light is propagated, wherein said light guide plate has a large number of first prism protrusions which are formed on one of said emission surface of said light guide plate and a surface opposite to said emission surface of said light guide plate to extend in parallel to such a direction that a distance from said side face increases and which are arranged along the side face, part of said first prism protrusions on the side of said side face have a height having a tendency to gradually decrease as a distance from said side face decreases, and said first prism protrusions have a height which is minimum on an optical axis of said point light source and which gradually increases as a distance from said optical axis of said point light source increases in directions in parallel to said side face, in at least the vicinity of said side face.