Surface light source device, diffusion plate and liquid crystal display device

1. 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.

2. A surface light source device comprising

a light source,

a light guide plate for spreading light introduced from the light source in a plane shape and emitting the light thus spread from a light emission face thereof, and

a diffusion plate disposed so as to face the light emission face of the light guide plate,

wherein the directivity of light transmitted through said diffusion plate with respect to vertical incidence has at least one local maximum value within the angular range of 20 degrees at each of both the sides of the direction vertical to said diffusion plate,

wherein said diffusion plate comprises plural substantially-conical uneven portions which are arranged,

a prism sheet disposed so as to face the light emission face of the light guide plate, and

a diffusion plate is arranged on the far side of the side of a prism sheet disposed so as to face the light emission face of the light guide plate.

3. A diffusion plate for spreading light from a light emission face of a light guide plate, the diffusion plate including plural uneven portions which are long in one direction,

wherein a direction of a long side of each of said uneven portions is oriented toward a predetermined point, and

a direction of a short side of each of said uneven portions is oriented toward a concentric direction with respect to said predetermined point, and

wherein light from the light emission face passing through said uneven portions is substantially perpendicular to the diffusion plate and is spread more greatly in the direction of the short side than in the direction of the long side.

4. A surface light source device comprising:

a light source;

a light guide plate for spreading light introduced from said light source in a planar shape and then emitting the light from a light emission face; and

said diffusion plate as claimed in claim 3 which is disposed so as to face the light emission face of said light guide plate.