Patent ID: 7121709

Claim:
A surface light source device comprising a point light source, a light guide plate for spreading light incident from the point light source in a plane shape and then emitting the light from a light emission face thereof and a prism sheet disposed so as to face the light emission face of the light guide plate, wherein said light guide plate has means for deflecting light in said light guide plate on only a plane which is vertical to the light emission face and contains the point light source and emitting the light substantially along the light emission face and in a direction opposite to said point light source, and said prism sheet has a function of deflecting the light emitted from the light emission face in a direction perpendicular to the light emission face, wherein said means for deflecting the light in said light guide plate is constructed by an uneven pattern having a light deflecting face, and an inclination angle of the light deflecting face on the plane which is vertical to the light emission face and contains the point light source is set to be substantially fixed in said light guide plate, wherein said light guide plate has means for emitting light from the light emission face substantially along the light emission face and in a direction opposite to the light source, and said prism sheet has a region in which the light emitted from the light emission face is deflected to a direction vertical to the light emission face, the area of the region occupying 50% or more of said prism sheet when viewing said prism sheet from the direction perpendicular to said prism sheet, wherein plural prisms each of which has an incident face and a reflection face crooked in a projecting shape are arranged on a surface of said prism sheet which faces said light guide plate, and after light emitted from said light guide plate is incident from the incident face into each prism, the light is reflected from the substantially whole area of each reflection face and deflected in the direction perpendicular to said prism sheet, and in the prism sheet, a slope surface (reflection face) more remote from the light emitting portion in the slope surfaces constituting the prism in section is bent so as to project from a midpoint.