Patent ID: 8100549

Claim:
A surface light source, comprising: a plurality of lighting devices arranged in a plane, each lighting device, comprising: a light emitting diode for emitting light; and an illuminating lens for spreading light emitted from the light emitting diode so that a surface to be irradiated is irradiated with the spread light, the illuminating lens including: a light entrance surface through which the light emitted from the light emitting diode enters the lens; and a light exit surface through which the light that has entered the lens exits the lens, the light exit surface being rotationally symmetric with respect to an optical axis, having a concave portion that intersects the optical axis and a convex portion provided around the concave portion to extend continuously from the concave portion, and being formed in a shape such that when the light exit surface is divided into micro-segments in a cross section that includes the optical axis, wherein the micro-segments have respective curvatures C, a maximum curvature Cm among curvatures C is positioned outward from a midpoint of the convex portion, wherein the lighting device satisfies the following inequality: 0.3 <D/t< 0.9 where D is a maximum width of a light emitting surface of the light emitting diode, and t is a center thickness of the illuminating lens; and a diffusing plate disposed to cover the plurality of lighting devices, the diffusing plate configured to receive on one surface thereof light emitted from the plurality of lighting devices to emit the light from an other surface thereof in a diffused manner, wherein the surface light source satisfies the following inequality: 0.04 <D/h< 0.15 where D is the maximum width of the light emitting surface of the light emitting diode, and h is a distance from a light emitting surface of the light emitting diode in each of the lighting devices to the one surface of the diffusing plate.