1. Field of the Invention
The present invention relates to a surface light source apparatus. For example, the surface light source apparatus according to the present invention is used as a backlight for a liquid crystal display device.
2. Description of the Related Art
FIG. 1 shows an enlarged cross-sectional view, partially broken away, of schematic structure near a light source of a conventional surface light source apparatus. In a light source 12 used in a surface light source apparatus 11, vertical and horizontal surfaces and a rear surface of a transparent resin 14 sealing a LED 13 are covered with a case 15 made of a white resin, and only a front face of the transparent resin 14 is exposed from the case 15. The light source 12 is mounted on a flexible printed board 16. The light source 12 is vertically inverted such that the flexible printed board 16 is faced up, and the front face of the light source 12 faces an end face (light incident plane 18) of a light guide plate 17. A diffusing sheet 20 is disposed while facing a light outgoing plane 19 of the light guide plate 17, and a reflector plate 21 is disposed while facing a back side of the light guide plate 17. For example, Japanese Patent Application Laid-Open No. 2003-215584 discloses the surface light source apparatus having the above configuration.
In order to achieve a low profile of the light guide plate 17 or enhancement of light use efficiency, as shown in FIG. 1, a height of the front face of the transparent resin 14 in the light source 12 is equal to a thickness of the light guide plate 17, and the light source 12 is disposed such that a center in a height direction of the transparent resin 14 coincides with a center in a thickness direction of the light guide plate 17. Therefore, a gap is generated between the light outgoing plane 19 of the light guide plate 17 and the flexible printed board 16, and light L leaking from a gap between the light source 12 and the light incident plane 18 of the light guide plate 17 passes to the outside through the gap between the light outgoing plane 19 and the flexible printed board 16 without passing through the light guide plate 17 as shown by an arrow of FIG. 1. As a result, although the gap between the light source 12 and the light incident plane 18 of the light guide plate 17 is covered with the flexible printed board 16, an eye-shaped high-brightness region is generated near (portion designated by the letter P of FIG. 1) an edge of the flexible printed board 16 in front of the light source 12, and the region glitters like an eye when the surface light source apparatus 11 is obliquely observed, which remarkably impairs evenness in a light emission surface of the surface light source apparatus 11.
Because a rim sheet (frame material) is overlapped on the flexible printed board 16, when an opening of the rim sheet is decreased to extend an edge on an inner peripheral side of the opening of the rim sheet to the inside from an end on a light guide plate side of the flexible printed board 16, the eye-shaped high-brightness region is hardly viewed. However, when the opening of the rim sheet is excessively decreased, because an effective region of the surface light source apparatus becomes small compared with an outer shape size of the surface light source apparatus, from a practical standpoint, it is difficult to excessively decrease the opening of the rim sheet.
In a backlight disclosed in Japanese Patent Application Laid-Open No. 2002-357823, gap between the light source and the light guide plate is covered with a light blocking tape. At this point, when the whole surface of the light blocking tape is bonded to the light outgoing plane of the light guide plate, an adhesive agent constitutes a light guide, the light leaks from between the light guide plate and the light blocking tape, and the edge of the light blocking tape also glitters like an eye. Therefore, in the backlight disclosed in Japanese Patent Application Laid-Open No. 2002-357823, only both ends of the light blocking tape are bonded to the light outgoing plane with an adhesive agent, and the light blocking tape is not bonded in at least the light source portion.
However, in the backlight disclosed in Japanese Patent Application Laid-Open No. 2002-357823, because the light blocking tape is not bonded to the light guide plate in the light source portion, the gap between the light blocking tape and the light guide plate is generated in that portion. Therefore, as with the surface light source apparatus shown in FIG. 1, the light leaks from the gap between the light guide plate and the light blocking tape, and the phenomenon in which the edge of the light blocking tape glitters like an eye cannot sufficiently be solved.
In a surface light source apparatus disclosed in Japanese Patent Application Laid-Open No. 2005-321586, the flexible printed board adheres to the light guide plate with a double-sided adhesive tape. In the double-sided adhesive tape, a reflecting layer is formed on one of surfaces of a tape substrate, and a tackiness agent having a high light transmission property is provided on a surface of the reflecting layer. A colored layer is formed on the other surface of the tape substrate, and the tackiness agent having a high light transmission property is provided on a surface of the colored layer. The tackiness agent on the reflecting surface side of the double-sided adhesive tape adheres to the light guide plate, and the tackiness agent on the colored layer side adheres to the flexible printed board. Therefore, the light leaking from between the flexible printed board and the light guide plate is reflected by the reflecting layer, whereby the light is incident on the light guide plate.
However, in the surface light source apparatus disclosed in Japanese Patent Application Laid-Open No. 2005-321586, the light passes through the tackiness agent of the double-sided adhesive tape adhering to the light guide plate, and the light leaks from the gap between the flexible printed board and the light guide plate. Even if the leaking light is reflected by the reflecting layer of the double-sided adhesive tape, at least part of the reflected light is transmitted through the light guide plate and reflected by the reflector plate on the back side, and the light is transmitted through the light guide plate again and outputted from the light outgoing plane. Therefore, even though the gap between the flexible printed board and the light guide plate is closed by the double-sided adhesive tape, there is no effect of eliminating the eye-shaped high-brightness region.
In a surface light source apparatus disclosed in Japanese Patent No. 3371052, end portions of the light guide plate and diffusing sheet are inserted into a recess provided in a front face of the light source, an end portion of the light outgoing plane of the light guide plate is covered with a case of the light source, and the end face of the diffusing sheet is covered with a light blocking wall provided in the case of the light source, a reflecting sheet in the light source and the like, thereby preventing the leakage of the light from the gap between the light guide plate and the case of the light source.
However, because a cold-cathode tube is used in the surface light source apparatus disclosed in Japanese Patent No. 3371052, the surface light source apparatus disclosed in Japanese Patent No. 3371052 cannot be applied to the surface light source apparatus having the light source in which a LED is used. That is, because the light source in which a LED is used is a micro component having a depth of several millimeters, a width of several millimeters, and a thickness not more than 1 mm, it is actually difficult that the end portions of the light guide plate and diffusing sheet are inserted into the recess of the light source, and the it is difficult to realize the practical application of the surface light source apparatus disclosed in Japanese Patent No. 3371052 for the light source in which a LED is used.