1. Field of the Invention
The invention relates to a back light module for flat display device, more particularly to a back light module for flat display device by putting the LED light source above one end of the light guide plate.
2. Description of the Prior Art
Among all kinds of flat display devices, A liquid crystal display (LCD) device with low electrical power consumption, low voltage operation, thinner thickness and lighter weight, is widely used in nowadays. A liquid crystal display device typically includes a first substrate having common electrodes and a color filter, and a second substrate having thin film transistors and pixel electrodes. The first substrate and the second substrate are provided substantially in parallel with a predetermined gap therebetween, and liquid crystal is injected between the two opposing substrates. An electric field is formed between the substrates by applying different voltages to the pixel electrodes and common electrodes. Accordingly, the alignment of liquid crystal molecules of the liquid crystal material is varied to thereby control the transmittance of incident light. The visions of liquid crystal display devices (LCD) are extremely excellent since the displayed figures or pictures are not offensive to the eyes of human being unlike light emitting diode (LED). But it is one of the drawbacks of LCD that an additional light module is needed in order to show figures or pictures on the screen under the dark circumstance because LCD does not have the character of light-emitting itself.
LED is a lighting component with low electricity and high illumination, and it has the steady lighting matter and long life cycle, too. It is very suitable as the light source of LCD device. Please refer to FIG. 1, a conventional structure 100 of back light module with LED light source 102 for LCD display device is shown. The light rays emitted from the LED light source 102 enter into the light guide plate (LGP) 104, then are well distributed and emitted out from the light-emitting surface through the reflector 106. The material of light guide plate 104 is usually PMMA or plastic division with TiO2 diffusion grains whose total volume is between about 50%˜80% of light guide plate. The light rays are well distributed when they collide with the TiO2 diffusion grains. The reflector 106 is placed below the light guide plate 104 to reflect the downward light rays into the light guide plate 104 for increasing the illumination of back light module 100. There are several light diffusion elements on the bottom surface of the light guide plate 104 for distributing the light rays. The light diffusion elements can be convex and/or concave dots formed by the print method or V-cuts.
Besides, there are sequentially a first diffusor sheet 108, a prism sheet 110, and a second diffusor sheet 112 on the light guide plate 104 for the light rays emitted from the light-emitting surface of light guide plate 104 can be further well distributed into the panel of liquid crystal device (not shown). The numbers of LED are decided on the illumination of the LCD device. Similarly, the numbers of diffusor sheet and prism sheet are decided on the demand of the back light module 100. A flexible printed circuit board (FPC) is put above the LED light source 102 for connecting electrically the pins of LCD device (not shown) and system board (not shown).
However, the aforementioned structure of back light module has many drawbacks. First, the light rays emitted into the light guide plate 104 are not well distributed. As shown in FIG. 2, the illumination of the section A of light guide plate 104 is darker than the section over LED 102 because the illuminating area of each LED is finite, in other words, the serious “firefly” phenomenon is observed because the LED light source 102 is in fact a point light source, not a plane light source. A method that shortens the distance between two LEDs to decrease the area of the section A of light guide plate 104 can solve this problem, but it will need the additional LEDs in the condition of same illuminated area, this method is helpless to reduce manufacturing cost. Next, using the thinner light guide plate can solve the demand for decreasing the thickness of notebook, but the thickness of light guide plate 104 must fit the LED light source 102 for considering the illumination of back light module when the LED light source 102 located at lateral end of light guide plate. So the thickness of back light module and notebook and the manufacturing cost of LCD panel and notebook are not reduced.