Patent ID: 8269916

Claim:
A surface light source device including plural light emitting device assembly units, each light emitting device assembly unit including plural light emitting device assemblies, each light emitting device assembly comprising: (A) a light emitting device; and (B) a light output member provided on the light emitting element and having an upper surface on which a curved part that outputs light from the light emitting device is provided, wherein, assuming that (i) an axis line passing through the center of the curved part and being in parallel with a normal line of a light exit surface of the light emitting device is a curved part center axis, (ii) an orthogonal coordinate system (x,y,z) in a three-dimensional space has the normal line of the light exit surface of the light emitting device passing through the center of the light emitting device as the Z-axis, a plane containing the curved part center axis and the Z-axis as the XZ-plane, a point at which the Z-axis and the light exit surface of the light emitting device intersects as the origin, (iii) when light output from the curved part travels in a positive direction of the X-axis within the XZ-plane, an angle formed by the traveling direction of the light and the Z-axis takes a positive value, (iv) the light output from the origin at an angle φ formed with the Z-axis within the XZ-plane is output from the curved part at an angle θ formed with the Z-axis within the XZ-plane, (v) the light output from the origin at an angle (φ+|Δφ|/2) formed with the Z-axis within the XZ-plane is output from the curved part at an angle (θ+Δθ + ) formed with the Z-axis within the XZ-plane, and (vi) the light output from the origin at an angle (φ−|Δφ|/2) formed with the Z-axis within the XZ-plane is output from the curved part at an angle (θ+Δθ − ) formed with the Z-axis within the XZ-plane, the value of the x coordinate of the curved part center axis is a positive value, a locus of an edge of the curved part forms a circle or oval around the curved part center axis, the light on the Z-axis output from the origin is output from the curved part at an angle θ 0 (θ 0 >0) formed with the Z-axis within the XZ-plane, function F(φ,Δφ) expressed by F (φ,Δφ)=(Δθ + −Δθ − )/Δφ (1) monotonously increases, takes the maximum value at φ=φ 0 (<0), and then, monotonously decreases as the value of φ increases from the minimum value, and in each light emitting device assembly unit, X-axes toward the positive directions of the respective plural light emitting device assemblies intersect at one point.