Patent Publication Number: US-2005140258-A1

Title: Backlight assembly with diffusion sheet having diffraction gratings

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a backlight assembly for liquid crystal display (LCD) devices, and more particularly to a direct type backlight assembly employing a diffusion sheet with a plurality of diffraction gratings.  
      2. Description of the Prior Art  
      A typical LCD device comprises an LCD panel, and a backlight system mounted under the LCD panel for supplying light beams thereto. There are two types of backlight systems: the edge type and the direct type. The edge type backlight system mainly comprises a light guide plate, and a light source disposed adjacent to a thin side of the light guide plate. The light guide plate is used for guiding the light beams emitted by the light source to uniformly illuminate the LCD panel.  
      In contrast, the direct type backlight system employs light sources placed in an air-filled cavity under the LCD panel, and a diffuser disposed between the LCD panel and the light sources.  
      Referring to  FIG. 5 , U.S. Pat. No. 5,479,328 discloses a direct type backlight  10  that includes a reflector  14 , a serpentine fluorescent tube  12 , a glass substrate  15 , a diffuser  17 , a brightness enhancement film (BEF)  18 , an air gap  19 , and an LCD panel  20 . The glass substrate  15 , the diffuser  17 , and the BEF  18  cooperatively combine to form an image plate. Many of the light beams emitted by the tube  12  are directly emitted to the image plate, and the remainder of the light beams emitted by the tube  12  are reflected to the image plate by the reflector  14 . In this manner, a bright and uniform image is formed at the image plate.  
      However, the configuration of the direct type backlight is unduly complicated. The tube  12  is a fluorescent tube having a circular cross section. The tube  12  is one hundred inches long, and has ten straight, parallel segments with nine 180-degree U-bends. Correspondingly, the reflector  14  has ten channels receiving the straight segments. In addition, the manufacturing process of the reflector  14  is complicated and laborious, because of the need for the channels and the need for attaining high reflectivity. The steps involved in the manufacturing process comprise: cutting a workpiece to form the channels; polishing the workpiece; and silver coating the workpiece.  
      Therefore it is desired to provide a new kind of direct type backlight which can introduce bright and uniform light beams to illuminate an LCD panel, and which can overcome the above-described disadvantages of a conventional direct type backlight.  
     SUMMARY OF THE INVENTION  
      An object of the present invention is to provide a backlight assembly having a simple configuration, and which can introduce bright and uniform light beams to illuminate an LCD panel.  
      In order to achieve the above-described object, a backlight assembly in accordance with the present invention includes a light source, a reflector, and a diffusion sheet. The light source is positioned between the reflector and the diffusion sheet, and the diffusion sheet has at least one diffraction grating formed on a surface thereof facing the light source. The backlight assembly has a simple configuration for providing bright and uniform light beams to illuminate an LCD panel. Furthermore, the diffusion sheet is made of a transparent piezoelectric material. When an electric field is applied on the diffusion sheet by a controlling circuit, the diffusion sheet is induced to deform. Thus the distribution of light intensity can be modulated by the controlling circuit according to need.  
      Other objects, advantages, and novel features of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is an exploded, isometric view of a backlight assembly in accordance with the present invention.  
       FIG. 2  is a bottom plan view of a diffusion sheet of the backlight assembly of  FIG. 1 .  
       FIG. 3  is an enlarged, side plan view of part of the diffusion sheet of the backlight assembly of  FIG. 1 , showing essential optical paths of light beams transmitting through a diffraction grating of the diffusion sheet.  
       FIG. 4  is essentially an assembled, side plan view of the backlight assembly of  FIG. 1 , showing essential optical paths thereof.  
       FIG. 5  is an exploded, cross-sectional view of a conventional backlight assembly. 
    
    
     DETAILED DESCRIPTION OF THE PRESENT INVENTION  
      Reference now will be made to the drawings to describe the present invention in detail.  
      Referring to  FIG. 1 , an exemplary backlight assembly  100  of the present invention comprises a housing  110 , a reflector  130  received in the housing  110 , a plurality of light source tubes  120  located in the housing  110  above the reflector  130 , and a diffusion sheet  150  located above the housing  110 . The tubes  120  are normal, linear cold cathode fluorescent tubes (CCFLs)  120 . The reflector  130  has several parallel, contiguous arcuate portions (not labeled), each arcuate portion corresponding to a respective tube  120 .  
      Referring to  FIG. 2 , a bottom plan view of the diffusion sheet  150  is shown. The diffusion sheet  150  has several separate transmission diffraction gratings  151 . In the exemplary embodiment, the diffraction gratings  151  are integrally formed as part of the diffusion sheet  150 . Each diffraction grating  151  is opposite to a respective tube  120 . The diffusion sheet  150  is made of a transparent piezoelectric material, such as a transparent piezoelectric ceramic or polyvinylidene fluoride (PVDF). The diffusion sheet  150  is connected to an outer controlling circuit (not shown), to electrically control deformation of the diffusion sheet  150 . Because the diffusion sheet  150  is made of the piezoelectric material, which has the characteristic of electromechanical coupling, the diffusion sheet  150  is induced to mechanically deform when an electric field is applied.  
       FIG. 3  is an essential optical paths diagram of light beams transmitting through the diffusion sheet  150  at any one of the diffraction gratings  151 . A pitch between each two adjacent lines of the diffraction grating  151  is defined as A, and a width of each line of the diffraction grating  151  is defined as B. A is in the range from 10 um to 30 um, and B is in the range from 1 um to 10 um.  
      When light beams pass through the diffraction grating  151 , a multiple diffraction effect is produced. The following describes a characteristic of the distribution of light intensity due to the diffraction effect. One light beam is split into many light beams by passing through the diffraction grating  151 . Said many light beams comprise ±1st order beams, ±2nd order beams (not shown), ±3rd order beams (not shown) through to ±nth order beams (not shown), with these beams being respectively distributed at opposite sides of the zeroth order beam. The combination of the zeroth order beam, the ±1st order beams, the ±2nd order beams through to the ±nth order beams enlarges the radiation angle of the light beam passing through the diffraction grating  151 . This improves the uniformity of illumination provided by the tubes  120 .  
      Referring to  FIG. 4 , in operation, many of the light beams emitted by the tubes  120  are directly emitted to the diffusion sheet  150 , and substantially the entire remainder of the light beams emitted from the tubes  120  are reflected to the diffusion sheet  150  by the reflector  130 . Therefore almost all the light beams emitted by the tubes  120  are transmitted to the diffusion sheet  150 . Accordingly, an irradiance of light is higher at the diffraction gratings  151  than at other portions of the diffusion sheet  150  between the diffraction gratings  151 . The diffraction gratings  151  diffuse the light beams received thereat, thereby counteracting the disparity of intensity of light beams irradiating the diffusion sheet  150 . This improves the uniformity of irradiance provided by the backlight assembly to illuminate an LCD panel (not shown).  
      Furthermore, the distribution of light intensity due to the diffraction effect can be modulated. When an electric field (not shown) is applied on the diffusion sheet  150  by the controlling circuit, the diffusion sheet  150  is induced to deform. The pitch A and the width B of the diffraction gratings  151  vary according to the voltage used to generate the electric field. In particular, the width B increases with increasing voltage. In such case, more light beams can pass through the lines of the diffraction gratings  151 .  
      In summary, the present invention has a simple configuration, comprising the normal linear tubes  120  and the diffusion sheet  150  with the diffraction gratings  151 . The configuration provides bright and uniform light beams to illuminate an LCD panel. In particular, the diffraction gratings  151  are disposed corresponding to the tubes  120 . The diffraction gratings  151  diffuse light beams received from the tubes  120 , and counteract the disparity of intensity of light beams that reach the diffusion sheet  150 . Furthermore, the distribution of light intensity can be modulated by the controlling circuit according to need.  
      Various modifications and alterations are possible within the ambit of the invention herein. For example, the diffraction gratings  151  can be provided by way of an optical film being adhered to a surface of the diffusion sheet  150 , with the optical film having diffraction grating structures therein. Moreover, the reflector  130  can have a reflective film (not shown) coated on a surface thereof that is opposite to the diffusion sheet  150 . The reflective film preferably has a reflective ratio greater than 98%.  
      It is to be further understood that even though numerous characteristics and advantages of the present invention have been set forth in the foregoing description, together with details of the structure and function of the invention, the disclosure is illustrative only, and changes may be made in detail, especially in matters of shape, size, and arrangement of parts within the principles of the invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.