Abstract:
A backlight unit is disposed beneath a display panel. The backlight unit includes an illumination means, which is used to provide a light source, and a diffuser positioned between the illumination means and the display panel, which is used to scatter the light generated by the illumination means. The diffuser, which is composed of liquid crystal particles and polymer, has a plurality of regions. The region thatis closer to the illumination means has greater scattering effect. In addition, the region with the greatest scattering effect has a shape corresponding to the illumination means.

Description:
BACKGROUND OF INVENTION 
   1. Field of the Invention 
   The present invention relates to a back light unit, and more particularly, to a diffuser of a back light unit. 
   2. Description of the Prior Art 
   As manufacturing costs decrease and quality improves, liquid crystal displays are widely applied to different kinds of products such as notebooks, personal digital assistants (PDAs), mobile phones, clocks, and so on. Since the liquid crystal displays are passive luminous devices, a back light unit is required for the LCD devices to meet the requirements of night vision and full color imagery. 
   Basically, the back light unit, which is disposed under the display panel, comprises a luminous means and a diffuser for generating light beams and scattering the light beams uniformly to the display panel so as to produce image on the display panel in advance. According to the position of the luminous means, the back light units are divided into several types, such as the vertical back light unit, in which the light source is generated under the display panel, or the edge type back light unit, in which the light source is projected from the edges. Among all kinds of back light units, since the vertical back light unit generates light beams under the display panel, therefore, the vertical back light units are widely used in large size display panels or those display panels which need high brightness, such as TV sets. 
   Please refer to  FIG. 1  of a schematic diagram of a conventional vertical back light unit  10 . The back light unit  10  is disposed under a display panel  18  for providing a light source to the display panel  18 . The back light unit comprises a light generator  12  for producing light beams, and a reflective layer  14 , which is often composed of a metal layer, for reflecting the light beams generated from the light generator  12  upward to increase the luminous intensity. The back light unit further comprises a diffuser  16  disposed above the light generator  12  for scattering the light beams so that the uniformity of the luminous intensity is improved when the light beams are provided to the display panel  18 . 
   Please refer to  FIG. 2  of a top view of the light generator  12 . Basically, the light generator  12  is composed by one or a plurality of light tubes  13 . In order to increase efficiency, the light tubes  13  usually occupy only parts of the region of the light generator  12  and leave a lot of unoccupied space. Therefore, the light beams generated by the light generator  12  are scattered in various directions. The regions closer to the light tubes  13  have higher luminous intensities. Although the light beams are scattering through the diffuser  16  disposed on the light generator  12  so that the differences among the luminous intensity of each region are reduced, some specific regions, which are too close to the light tubes  13  such as the regions located on the tubes  13  directly, still have relative higher luminous intensities. Thus, when the display panel is used, a highlight region is shown on the specific regions of the display panel  18 , which corresponds to the shapes of the tubes  13 . The display performance of the display panel  18  is deteriorated. 
   In conventional back light unit technologies, some methods are used for solving the aforementioned problem. For example, an additional light guide is positioned between the diffuser  16  and the light generator  12  or a light enhancing film is disposed between the diffuser  16  and the display panel  18  for reducing the difference of the luminous intensities in advance. Those methods work successfully when the back light unit  10  outputs light beams with a high luminous intensity. However, when the back light unit  10  outputs light beams with a low luminous intensity or when the screen is almost dark, an obvious highlight region, which has a shape similar to the shape of the light tubes  13 , is always shown on the display panel  18  and therefore the display performance is deteriorated. 
   SUMMARY OF INVENTION 
   It is therefore a primary objective of the claimed invention to provide a back light unit with a diffuser which has diffusing effects corresponding to the shapes and positions of the light tubes so as to solve the aforementioned problem. 
   In a preferred embodiment, the claimed invention provides a back light unit disposed under a display panel. The back light unit comprises a light tube, which is used to provide a light source, a reflective layer disposed under the light tube, which reflects the light generated from the light tube upward to increase outputting brightness, and a diffuser positioned between the light tube and the display panel, which is used to scattering the light beams generated by the light tube. The diffuser composed of liquid crystal particles and polymer comprises a plurality of regions with different scattering effects. The region which is closer to the tube has greater scattering effects. In addition, the region with the greatest scattering effects has a shape corresponding to the light tube. 
   It is an advantage of the claimed invention that the back light unit comprises a diffuser composed of liquid crystal molecules and polymers. The diffuser has a greater scattering effect in the region closer to the tube so as to provide light beams with a better uniformity to the display panel. 
   These and other objectives of the claimed invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment which is illustrated in the various figures and drawings. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  is a cross-sectional diagram of a conventional back light unit. 
       FIG. 2  is a schematic diagram of a light generator shown in FIG.  1 . 
       FIG. 3  is a cross-sectional diagram of a back light unit according to the present invention. 
       FIG. 4  is a schematic diagram of the light generator shown in FIG.  3 . 
       FIG. 5  is a schematic diagram of the diffuser shown in FIG.  3 . 
       FIG. 6  is a schematic diagram of the electrode plates in an embodiment of the present invention. 
       FIG. 7  is a schematic diagram of the backlight unit with the luminous source and the diffuser having region with greater diffusion effect closer to the luminous means according to the present invention. 
   

   DETAILED DESCRIPTION 
   The present invention focuses on a diffuser structure of a back light unit. The remaining structure of the back light unit of the present invention is similar to the conventional back light unit  10 , and will not be explained again fully. Please refer to  FIG. 3  of a cross=sectional diagram of a back light unit  110  according to the present invention. The back light unit  110  is disposed under a display panel  118  for providing light source to the display panel  118 . The back light unit  110  comprises a light source generator  112  for generating light beams and a diffuser  116  disposed on the light source generator  112 . The diffuser  116  is composed of liquid crystal molecules and polymers. Since the refraction of the liquid crystal molecules and polymers is anisotropic, when the light beams generated from the light source generator  112  pass through the diffuser  116 , the light beams will be refracted by the diffuser  116  so that light beams with a better uniformity can be provided to the display panel  118 . Moreover, in order to increase the output brightness of the back light unit  110 , an additional reflective layer  114  is often disposed under the light generator  112 . The reflective layer  114 , which may be composed of a metal layer, is used to reflect the light beams generated form the light source generator  112  upward so as to increase the output brightness. 
   Please refer to  FIG. 4  of a top view of the light source generator  112  shown in FIG.  3 . Generally speaking, the light source generator  112  is composed of one or a plurality of light tubes  113 . The light tubes  113  may have specific shapes as shown in  FIG. 3  or any other shapes. In order to increase efficiency, the light tubes  113  usually occupy only parts of the region of the light generator  112  and leave a lot of unoccupied space. Therefore, the light output from the light generator  112  is not uniform, and varies according to locations. A region closer to the light tubes  113  has a higher luminous intensity. Please refer to  FIG. 5  of the diffuser  116  shown in FIG.  3 . As shown in  FIG. 5 , the diffuser  116  comprises a first region  116 A and a second region  116 B. An interval between the light tubes  113  and the first region  116 A is smaller than that between the tubes  113  and the second region  116 B. The diffusing effect of the first region  116 A is better than that of the second region  116 B. Therefore, the outputting light of the back light unit  110  is more uniform after passing through the diffuser  116 . In addition, according to requirements of display panel  118 , the diffuser  116  of the present invention may comprise a plurality of regions with different scattering effects by using photo masks with different designed patterns or any other exposure methods. 
   In the preferred embodiment of the present invention the first region  116 A has a shape corresponding to the shapes of the light tubes  113 . For example, the area overlapping with the light tubes  113  is defined as the first region  116 A. Thus, the light beams with high luminous intensity surrounding the light tubes  113  are scattered by the first region  116 A, which has a higher scattering effect. Therefore, a more uniform light output can be obtained via the diffuser  116 . 
   In the preferred embodiment of the present invention as shown in  FIG. 7 , the diffuser  116  is composed of liquid crystal molecules and polymers. The manufacturing method thereof includes first filling the mixed monomer and liquid crystal molecules. Then, a curing process is performed by irradiation under UV light. When UV light with higher intensity is used, the liquid crystal molecules form smaller liquid crystal droplets, which have a higher refraction index. A first region  116 A and a second region  116 B are defined by using photo masks during the curing process. Furthermore, the two regions  116 A and  116 B can be irradiated under the UV light with different intensities so that liquid droplets with different sizes are formed in the two regions. In the embodiment of the present invention, the first region  116 A, which is composed of liquid crystal droplets with smaller sizes, has a higher refraction index than the second region  116 B. Because the refraction index distribution in the diffuser  116  corresponds to the shape of light tubes  113 , the uniformity of the light through the diffuser  116  can be improved. In addition, when the diffuser  116  is fabricated, the sizes of liquid crystal droplets or the diffusing effect of the diffuser  116  can be adjusted or set properly to an optimized condition according to the distribution of the brightness of the light tubes  113  so as to improve the uniformity of light output in advance. In the preferred embodiment of the present invention, the size of the liquid crystal droplets is about 0.1 to 1 μm. 
   However, the luminous intensity of the light tubes  113  may be varied due to aging of materials. In addition, sometimes the brightness of the light tubes  113  is also adjusted for display performance according to the change of the screen or environment. Therefore, once the relationship between the diffusing effect of the diffuser  116  and the luminous intensity of the light tubes  113  changes from the original optimized design, the display performance may be deteriorated. Thus, in the preferred embodiment of the present invention, the diffuser  116  is an electrically variable diffuser composed of polymer dispersed liquid crystals (PDLC). As shown in  FIG. 6 , at least one pair of electrode plates  120  is used, with one electrode plate  120  being disposed on each side of the diffuser  116 . The electrode plates  120  are electrically connected to a power supply so as to provide an external electric field to the diffuser  116 . The polymer dispersed liquid crystal is a kind of nematic liquid crystal and the refraction index of the liquid crystal molecules is variable according to the electric field applied thereon. The pattern of the electrode plates  120  can be designed properly to fit the requirement of the back light unit  110 , for example the pattern of the electrode plates  120  may correspond to the shapes of the tubes  113 . Thus, when the brightness of the light tubes  113  varies due to aging materials or some other reasons, the external electric field can be adjusted so as to modify the diffusing effect of the diffuser  116 . Therefore, a new optimized 
   relationship between the diffusing effect of the diffuser  116  and the brightness of the light tubes  113  can be made. 
   In contrast with the conventional back light unit, the diffuser in the back light unit of the present invention can be adjusted properly according to the shapes or positionsof the tubes. Since the regions closer to the tubes have greater diffusion effect, the uniformity of the output light distribution can be improved. In addition, the present invention also provides an electrically variable diffuser which is composed of dispersed polymer liquid crystal molecules. Thus, the diffusing effect of the diffuser can be further adjusted by the electric field applied thereon according to the brightness of the tubes so as to solve the aforementioned problem of light tube brightness variation caused by aging. 
   Those skilled in the art will readily observe that numerous modifications and alterations of the device may be made while retaining the teaching of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.