Some transmission type image display apparatuses such as liquid crystal display apparatuses and signboards use a direct surface light source device as a backlight. In recent years, direct surface light source devices having a plurality of light emitting elements as the light source have been used. In such a surface light source device, there is disposed, over a light emitting element, a light flux controlling member for controlling the distribution of emission light from the light emitting element (see, for example, PTL 1).
FIGS. 1A and 1B are drawings illustrating the configuration of surface light source device (surface light emitting unit) 10 set forth in PTL 1 (see FIGS. 8A and 8B of PTL 1). FIG 1A is a perspective view of surface light source device 10, and FIG. 1B is a sectional view of surface light source device 10. As illustrated in these drawings, surface light source device 10 has housing 20, substrate (mounting plate) 30 disposed inside the housing, and a plurality of light emitting devices (light emitting units) 40 disposed at constant intervals on substrate 30. Light emitting device 40 has a light emitting element 42, and a light flux controlling member (lens part) 44 disposed over light emitting element 42. An incidence surface and an emission surface of light flux controlling member 44 are formed so as to expand the emission light from light emitting element 42. Here, it is assumed that light diffusion plate 50 is disposed at an opening of housing 20. The upper surface of light diffusion plate 50 functions as a light emitting surface.
FIG. 2 is a graph illustrating the light distribution characteristics of light emitting device 40 (see FIG. 6 of PTL 1). As illustrated in this graph, a large amount of light is emitted from light flux controlling member 44 in a larger angular range relative to the optical axis of light emitting device 40, whereas almost no light is emitted from light flux controlling member 44 in a smaller angular range relative to the optical axis of light emitting device 40. Therefore, when only one light emitting device 40 is disposed on substrate 30, the luminance of an area of the light emitting surface directly above light emitting device 40 is decreased. However, when a plurality of light emitting devices 40 are disposed on substrate 30, emission light from one light emitting device 40 reaches an area directly above the other light emitting devices 40, as illustrated in FIG. 1B. As a result, the luminance unevenness on the light emitting surface becomes smaller in surface light source device 10.
As illustrated in FIG. 1B, the center-to-center distance (pitch) between light emitting devices 40 is set as P (mm), and the interval (height) between the upper surface of substrate 30 and the lower surface of light diffusion plate 50 is set as H (mm). Referring to FIG. 8B of PTL 1 (FIG. 1B of the present application), in surface light source device 10 set forth in PTL 1, H/P is about 0.77.