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 θ0 (θ0>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=φ0 (<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.