In recent years, for the purpose of energy saving and downsizing, light emitting diodes (hereinafter also referred to as “LEDs”) are used as light sources for lighting. Light emitting devices each having a combination of an LED and a light flux controlling member for controlling distribution of light emitted from the LED are used in place of, for example, fluorescent lighting and halogen lamps. Further, direct surface light source devices each incorporating the light emitting devices in a lattice shape are used as a backlight in transmission type image display apparatuses such as liquid crystal display apparatuses (refer to, for example, PTL 1).
FIGS. 1A to 1C illustrate a configuration of surface light source device 10 disclosed in PTL 1. FIG. 1A is a schematic plan view of surface light source device 10, FIG. 1B is a plan view of light emitting device 30 of surface light source device 10, and FIG. 1C is a cross-sectional view taken along line A-A of FIG. 1B. In FIG. 1A, a broken line schematically illustrates a range irradiated with light emitted from light emitting device 30.
As illustrated in FIGS. 1A to 1C, surface light source device 10 disclosed in PTL 1 includes printed wiring substrate 20, a plurality of light emitting devices 30 disposed on printed wiring substrate 20 in a rectangular lattice shape and a light diffusion plate 32 disposed over the plurality of light emitting devices 30. Each of light emitting devices 30 includes light emitting element 35 and a light guide member (light flux controlling member) 40 disposed so as to cover light emitting element 35.
Light guide member 40 includes substantially hemisphere-shaped lens 41 and flange 42 disposed to surround lens 41. Lens 41 includes incidence surface 44, namely the inner surface of recess 43 disposed on the rear side, and emission surface 45 disposed on the front side. Emission surface 45 includes two flat surfaces 46 parallel to central axis CA and to each other, and curved surface 47 protruding toward the surface and disposed between two flat surfaces 46. In surface light source device 10 disclosed in PTL 1, light guide member 40 controls light emitted from light emitting element 35 so that the light expands more in one direction, where the distance between light emitting devices 30 is longer (long side direction in the rectangular lattice; X-direction), than in another direction, where the distance between light emitting devices 30 is shorter (short side direction in the rectangular lattice; Y-direction). This configuration enables uniform irradiation of light diffusion plate 32 in the surface light source device disclosed in PTL 1 even in a case where light emitting devices 30 are disposed in a rectangular lattice shape.