Liquid crystal per se cannot emit light. Therefore, in a liquid crystal display device, it is necessary to use a back light module to provide a light source having even and high brightness for a liquid crystal cell.
The back light module generally includes a light source, an optical film, a light guide plate and a back plate, for example. In particular, depending on distributed location of the light source in the back light module, the back light module can be classified into an edge-lit back light module and a direct-lit back light module, wherein the light source in the edge-lit back light module is located on a side of a display panel and usually suitable for a liquid crystal display device having relative small size; and the light source in the direct-lit back light module is located on a bottom of the display panel and usually suitable for a liquid crystal display device having relative large size. The light guide plate is an essential component in the back light module which can transform a point light source and/or linear light source into an area light source, to provide an area light source output having even brightness. The optical film is arranged on a light exiting surface of the light guide plate, to further process light output from the light guide plate, such that the light can be emitted evenly and perpendicularly to the light exiting surface of the light guide plate. The back plate is used to support and fix components such as the light source, the light guide plate and the optical film.
Usually, in order to position the light guide plate, a positioning slot is provided in a middle position of a non-light incident surface on the edge of the light guide plate, and a positioning pin is mounted in the positioning slot to position the light guide plate. In practice, since the light guide plate is fragile and has the thermal expansion and contraction characteristics. For this reason, it is necessary to leave an expansion gap (or an expansion size) in advance between the positioning slot and the positioning pin of the light guide plate and between the light guide plate and the light source. Also, machining errors in sizes of the positioning slot, the positioning pin and the light guide plate may further influence a gap between the light incident surface of the light guide plate and the light exiting surface of the light source. Therefore, it may be easy to cause that the gap between the light incident surface of the light guide plate and the light exiting surface of the light source has relatively large amplitude of variation. Not only may display images be uneven, but also the gap between the light guide plate and the light source may have significant differences for different display devices, such that the display quality is different according to the display devices.