Patent Publication Number: US-2006002138-A1

Title: Light guide plate and method for munufacturing the same

Description:
BACKGROUND  
      The invention relates generally to light guide plates and methods for manufacturing them, and more particularly to a light guide plate used in back light systems of liquid crystal display devices and a method for manufacturing it.  
      Back light systems are used in liquid crystal display devices for converting linear light sources such as cold cathode ray tubes or point light sources such as light emitting diodes into area light sources having high uniformity and brightness.  
      Referring to  FIGS. 12 and 13 , a conventional back light system includes a light guide plate  2 , a cold cathode ray tube  1  disposed beside a light introduction surface of the light guide plate  2 , a reflector  1 A disposed essentially around three sides of the cold cathode ray tube  1 , a plurality of micro structures  3  formed on a light reflection surface of the light guide plate  2 , a lower diffusion plate  4  installed upon a light emitting surface of the light guide plate  2 , a lower prism sheet  5  and an upper prism sheet  6  installed upon the lower diffusion plate  4  in turn, and an upper diffusion plate  7  installed upon the upper prism sheet  6 .  
      As indicated in  FIG. 12 , when a light beam emitted from the cold cathode ray tube  1  is introduced into the light guide plate  2  and travels therein, the micro structures  3  can break up what would otherwise be a total reflection condition of the light beam. This ensures that most of the light beam can pass through the light emitting surface of the light guide plate  2 . Furthermore, the lower diffusion plate  4  and the upper diffusion plate  7  can diffuse the emitted light beam, and the upper prism sheet  6  and the lower prism sheet  5  can adjust a direction of the emitted light beam, thereby ensuring that the emitted light beam finally travels along a path normal to the light guide plate  2 . This ensures that the emitted light beam has good uniformity and brightness.  
      The micro structures  3  can be circular, dome-shaped, elliptic or semicircular. Sizes of the micro structures  3  are in the range from a scale of micrometers to a scale of millimeters. Conventional methods for forming the micro structures  3  include screen printing, mechanical treatment, and lithographic galvanoformung abformung (LIGA).  
      Screen printing is performed by printing ink or resin on a reflection surface of the light guide plate  2  through a screen. One shortcoming of screen printing is that a size of every micro structure  3  must be greater than 300 micrometers. In addition, the shapes of the micro structures  3  must in general be uniform.  
      A typical mechanical treatment is performed as follows. Firstly, a female die and a mold are formed. Secondly, the light guide plate  2  with the micro structures  3  is formed by injection molding or press molding using the female die and mold. Shapes of the micro structures  3  can vary. However, sizes of the micro structures  3  must be greater than several tens of micrometers.  
      LIGA is performed as follows. Firstly, a photo resist is coated on a base plate. Secondly, the photo resist is exposed and developed to form structures corresponding to the micro structures  3 . Thirdly, a mold is formed by means of galvanoformung abformung. Finally, the light guide plate  2  with the micro structures  3  is formed by injection molding or press molding using the mold. LIGA is a relatively complex process having a high cost.  
      In general, the micro structures  3  formed by the above-described conventional methods must have a large size and cannot have various different shapes. Therefore the conventional light guide plate  2  needs to be combined with the diffusion plates  4 ,  7  and the prism sheets  5 ,  6  in order to obtain emission of light beams with good uniformity and brightness. The assembly comprising the light guide plate  2  is complex. Furthermore, the cost of producing the assembly is high.  
      What is needed, therefore, is a light guide plate that has a relatively simple structure and that can provide emission of light beams with good uniformity and brightness.  
      What is also needed is a method for manufacturing the above-described light guide plate, the method having a low cost, and the micro structures formed by the method having a small size and varying in shape.  
     SUMMARY  
      In one embodiment, a light guide plate includes an incidence surface, an emission surface and a reflection surface opposite to the emission surface. A plurality of recesses are formed on at least one of the three surfaces according to a predetermined pattern. Some recesses have angular cross-sections and can determine a direction of emission of light beams, and the other recesses have arcuate cross-sections and can determine the degree of uniformity of the emitted light beams. Therefore, the light beams can be emitted from a predetermined region of the light guide plate, and this ensures the emission light beams with improved uniformity and brightness.  
      In another embodiment, a method for manufacturing the light guide plate includes the steps of: 
          (a) providing an imprinter having a plurality of press heads with a plurality of shapes;     (b) providing a light guide plate preform;     (c) driving the imprinter with a first press head and forming a plurality of first recesses with a first shape in a first predetermined area of the light guide plate preform;     (d) driving the imprinter with another press head and forming a plurality of other recesses with another shape in another predetermined area of the light guide plate preform; and     (e) repeating step (d) a desired number of times to form micro-structures on the preform so as to complete the light guide plate.        

      Compared with a conventional light guide plate, the inventive light guide plate does not need diffusion plates or prism sheets, and thus has a simple structure. Furthermore, the recesses enable the light guide plate to achieve emission of light beams with improved uniformity and brightness. Therefore, the light guide plate can be advantageously applied in back light systems of liquid crystal display devices.  
      The method for manufacturing the light guide plate is simple, and yields plural recesses with desired plural shapes and small sizes. The method has a relatively low cost, and can be executed with high speed and precision.  
      Other advantages and novel features will become more apparent from the following detailed description of preferred embodiments when taken in conjunction with the accompanying drawings, in which: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is an isometric view of a press head that can be used in a preferred method according to one embodiment of the present invention;  
       FIG. 2  is an isometric, cut-away view of a light guide plate preform during manufacturing according to the preferred method, showing the light guide plate preform with a recess formed therein by using the press head of  FIG. 1 ;  
       FIG. 3  is an isometric view of another press head that can be used in the preferred method;  
       FIG. 4  is an isometric view of still another press head that can be used in the preferred method;  
       FIG. 5  is an isometric view of yet another press head that can be used in the preferred method;  
       FIG. 6  is similar to  FIG. 2 , but showing the whole light guide plate preform, and a plurality of recesses formed in the light guide plate preform, the recesses cooperatively defining a substantially linear groove;  
       FIG. 7  is a cross-sectional view taken along line VII-VII of  FIG. 6 ;  
       FIG. 8  is an isometric view of a backlight module including a light guide plate made according to the preferred method, the light guide plate having a plurality of recesses formed therein and being arranged in a pattern;  
       FIG. 9  is similar to  FIG. 8 , but showing a light guide plate having recesses arranged in a different pattern;  
       FIG. 10  is a schematic, isometric view of a backlight module including a light guide plate made according to the preferred method, the light guide plate having recesses arranged in another different pattern;  
       FIG. 11  is a flow chart of the preferred method for manufacturing the light guide plate;  
       FIG. 12  is a schematic, exploded side view of a conventional back light system, showing essential optical paths thereof; and  
       FIG. 13  is a simplified, exploded, isometric view of a light guide plate assembly of the back light system of  FIG. 12 . 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
      Reference will now be made to the drawings to describe embodiments of the present invention in detail.  
      Referring to  FIG. 11 , a preferred method for manufacturing a plate-like light guide member in accordance with one embodiment of the present invention comprises the steps of: 
          ( 10 ) providing an imprinter having a plurality of press heads with a plurality of shapes;     ( 20 ) providing a light guide plate preform;     ( 30 ) driving the imprinter with a first press head and forming a plurality of first recesses with a first shape in a first predetermined area of the light guide plate preform;     ( 40 ) driving the imprinter with another press head and forming a plurality of other recesses with another shape in another predetermined area of the light guide plate preform; and     ( 50 ) repeating step ( 40 ) a desired number of times, thereby forming a light guide plate having micro-structures thereon including different kinds of recesses with different shapes in the predetermined areas.        

      In step ( 10 ), the press heads are generally made of a wearable material with high rigidity, such as diamond. The press heads can have a variety of different tapered forms, such as being pyramidal, substantially pyramidal or conical, as shown in  FIGS. 1 ,  3 ,  4  and  5 . Furthermore, the press heads can for example be hemispherical, rounded or columnar. Recesses defining V-shaped or angular cross-sections can be formed by tapered press heads, recesses defining generally rectangular cross-sections can be formed by columnar press heads, and recesses defining arcuate cross-sections can be formed by hemispherical or rounded press heads. The arcuate cross-sections may for example be semicircular, elliptical, or bowl-shaped. In a duly formed light guide plate, recesses defining angular cross-sections can determine a direction of emission of light beams from the light guide plate. This helps to ensure that light beams are emitted from a predetermined region of the light guide plate. In contrast, recesses defining arcuate cross-sections can determine the degree of uniformity of light beams emitted from the light guide plate.  
      As shown in  FIG. 1 , a press head  11  has a width b and a height h. The width b and height h are both less than 5 micrometers, which ensures that recesses formed by the press head  11  are small. Therefore, a liquid crystal display device incorporating the duly formed light guide plate can have good imaging quality.  
      In step ( 20 ), the light guide plate preform is generally made of plastic material, and is generally rectangular. Preferably, a guiding pattern is preformed on at least one major surface of the light guide plate perform, so that subsequently recesses are formed according to the guiding pattern. In step ( 30 ), a linear motor or piezoelectric material is used for driving the imprinter. As shown in  FIG. 2 , a recess  12  is formed in the light guide plate preform by impressing with the press head  11 . Preferably, an electronic controlling device is installed in the imprinter. The electronic controlling device controls a direction and depth of impression, and a pitch or distance between successive impressions. If a pitch is relatively small, a plurality of recesses  12  can cooperatively define a substantially linear groove or a substantially curvilinear groove. Either of such grooves can be either continuous or (in effect) discontinuous.  FIGS. 6 and 7  show a continuous-linear groove  13 . In step ( 40 ), selection of each of the press heads is controlled by the electronic controlling device. Upon completion of step ( 50 ), the different kinds of recesses are located at the surface of the light guide plate according to a predetermined desired pattern.  
      Various different light guide plates that can be manufactured by the preferred method are shown in  FIGS. 8, 9  and  10 . As shown in  FIG. 8 , a light guide plate  100  includes a light introduction surface  101 , a light emitting surface  102 , and a light reflection surface  103  opposite to the light emitting surface  102 . A pair of light sources  110 ,  120  are disposed beside the light introduction surface  101 . A plurality of recesses defining arcuate cross-sections and a plurality of recesses defining angular cross-sections are formed at the light reflection surface  103 . The recesses communicate with one another to define a plurality of substantially linear grooves  105 . Some of the grooves  105  are continuous, and some are discontinuous.  
      As shown in  FIG. 9 , a light guide plate  200  includes a light introduction surface  201 , a light emitting surface  202 , and a light reflection surface  203  opposite to the light emitting surface  202 . A pair of light sources  210 ,  220  are disposed beside the light introduction surface  201 . A plurality of recesses defining arcuate cross-sections and a plurality of recesses defining angular cross-sections are formed at the light reflection surface  203 . The recesses communicate with one another to define a plurality of substantially curvilinear grooves  205  and a plurality of substantially linear grooves (not labeled). All of the grooves are continuous.  
      As shown in  FIG. 10 , a light guide plate  300  includes a pair of light introduction surfaces  301 , a light emitting surface  302 , and a light reflection surface  303  opposite to the light emitting surface  302 . A light source  310  is disposed beside a corner of the light guide plate  300  where the two light introduction surfaces  301  meet. A plurality of recesses defining arcuate cross-sections and a plurality of recesses defining angular cross-sections are formed at the light reflection surface  303 . The recesses communicate with one another to define a plurality of substantially curvilinear grooves  305 . All of the grooves are discontinuous.  
      Compared with a conventional light guide plate, the light guide plate manufactured in accordance with the above-described preferred embodiment does not need any diffusion plates or prism sheets to be added thereto. That is, the light guide plate has a simple structure. Furthermore, the recesses enable the light guide plate to achieve emission of light beams with improved uniformity and brightness. Therefore, the fight guide plate can be advantageously applied in back light systems of liquid crystal display devices.  
      In addition, the preferred method for manufacturing the light guide plate is simple, and yields plural recesses with desired plural shapes and small sizes. The method has a relatively low cost, and can be executed with high speed and precision.  
      It is noted that the recesses can be formed at least one of the light reflection surface, the light introduction surface and the light emitting surface of a light guide plate. In addition, recesses defining arcuate cross-sections and recesses defining angular cross-sections can be configured to communicate with one another and cooperatively form any of various other desired patterns of grooves.  
      Finally, it is to be understood that the above-described embodiments are intended to illustrate rather than limit the invention. Variations may be made to the embodiments without departing from the spirit of the invention as claimed. The above-described embodiments illustrate the scope of the invention but do not restrict the scope of the invention.