Abstract:
A backlight module capable of increasing light output efficiency is disclosed. The backlight module includes a light emitting element, a light guide plate, a circuit board, and an intermediate layer. The light guide plate is disposed at a side of the light emitting element. The circuit board is disposed above the light emitting element and the light guide plate. The intermediate layer is disposed between the light emitting element and the light guide plate, and its light transmittance is greater than that of air. The intermediate layer decreases optical energy loss between the light emitting element and the light guide plate, and a goal to increase the light output efficiency of the backlight module is achieved. The present invention also improves an alignment between the light emitting element and the light guide plate when the light emitting element and the light guide plate are assembled together.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention generally relates to a backlight module, and more particularly to a backlight module capable of increasing light output efficiency. 
         [0003]    2. Description of Prior Art 
         [0004]    Liquid crystal displays (LCDs) are being utilized in various products, for example, mobile phones, televisions, digital cameras, and computers. In the liquid crystal displays, liquid crystal alignment is controlled to change lights passing through the liquid crystal so as to display images. The liquid crystal cannot generate lights per se, and therefore a backlight module is utilized in the liquid crystal display as an external light source for providing lights that are required for displaying the images. 
         [0005]    Please refer to  FIG. 1A .  FIG. 1A  illustrates an exploded stereographic view of a conventional backlight module  10 . The backlight module  10  mainly comprises prism sheets  104 ,  106 , a diffusion sheet  108 , a light guide plate (LGP)  110 , a frame  112 , a reflecting sheet  114 , at least one light emitting element  116 , and a flexible printed circuit board (FPCB)  120 . The above elements are to fix the prism sheets  104 ,  106 , and the diffusion sheet  108  on the light guide plate  110  by tapes (not shown) first. Then, the light guide plate  110  is assembled with the frame  112  and the reflecting sheet  114  to complete the assembly of the backlight module  10 . A method of fixing the light emitting element  116  and the flexible printed circuit board  120  to the light guide plate  110  will be described later in detail. 
         [0006]    When the light emitting element  116  such as a light emitting diode or a lamp generates lights, the light guide plate  110  guides the lights upwardly to the diffusion sheet  108 . A function of the diffusion sheet  108  is to form uniform surface lights by refracting, reflecting, and scattering lights. A directionality of the lights passing through the diffusion sheet  108  is usually unsatisfactory, and therefore the prism sheets  104 ,  106  are utilized to modify light direction by refracting and reflecting the lights so as to increase light output efficiency after the lights pass through the diffusion sheet  108 . In general, the prism sheets  104 ,  106  are also called brightness enhancement films (BEFs) and they are utilized for concentrating lights. 
         [0007]    In addition, the reflecting sheet  114  is made of material of high reflectivity, and it is utilized for reflecting lights not received by the light guide plate  110  to the light guide plate  110  so as to increase the light output efficiency. 
         [0008]    Please refer to  FIG. 1B .  FIG. 1B  illustrates a cross-sectional view along a line  1 B- 1 B′ in  FIG. 1A . The flexible printed circuit board  120  is fixed on both the light emitting elements  116  and the light guide plate  110  by a tape  122 . In the conventional fixing method, there exists a gap  118  between the light emitting element  116  and the light guide plate  110 . As a result, some of the lights emitted by the light emitting element  116  are lost in air due to the gap  118 . 
         [0009]    When the light guide plate  110  and the light emitting element  116  are assembled together, a misalignment therebetween might occur and the gap  118  is generated in the assembling process accordingly. This makes the light output efficiency of the backlight module  10  worse. 
         [0010]    In conventional techniques, liquid crystal display panel manufacturers seek for a way of increasing the light output efficiency of the backlight module  10 . However, the objective is difficult to achieve. Therefore, there is a need to solve the above-mentioned problem of optical energy loss in the backlight module  10 . 
       SUMMARY OF THE INVENTION 
       [0011]    A primary objective of the present invention is to provide a backlight module capable of increasing light output efficiency so as to decrease optical energy loss in the backlight module. 
         [0012]    Another objective of the present invention is to provide a backlight module capable of increasing light output efficiency, and the backlight module improves an alignment when a light emitting element of the backlight module and a light guide plate of the backlight module are assembled together. 
         [0013]    The backlight module capable of increasing the light output efficiency according to the present invention comprises a light emitting element, a light guide plate, a circuit board, and an intermediate layer. The light guide plate is disposed at a side of the light emitting element to receive lights emitted by the light emitting element. The circuit board is disposed above the light emitting element and the light guide plate to provide circuit arrangements of the backlight module. The intermediate layer is disposed between the light emitting element and the light guide plate. A light transmittance of the intermediate layer is greater than a light transmittance of air, and a refractive index of the intermediate layer is substantially equal to a refractive index of air. 
         [0014]    The present invention is capable of decreasing the optical energy loss between the light emitting element and the light guide plate, and therefore the objective of increasing the light output efficiency of the backlight module is achieved. In addition, the alignment can be improved when the light emitting element and the light guide plate are assembled together. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1A  illustrates an exploded stereographic view of a conventional backlight module; 
           [0016]      FIG. 1B  illustrates a cross-sectional view along a line  1 B- 1 B′ in  FIG. 1A ; 
           [0017]      FIG. 2A  illustrates a top view of a backlight module according to a preferred embodiment of the present invention; 
           [0018]      FIG. 2B  illustrates a cross-sectional view along a line  2 B- 2 B′ in  FIG. 2A ; and 
           [0019]      FIG. 3  illustrates a diagram of an electronic apparatus comprising a liquid crystal display. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0020]    Please refer to  FIG. 2A  and  FIG. 2B .  FIG. 2A  illustrates a top view of a backlight module  300  according to a preferred embodiment of the present invention.  FIG. 2B  illustrates a cross-sectional view along a line  2 B- 2 B′ in  FIG. 2A . The backlight module  300  mainly comprises light emitting elements  302 , a light guide plate  304 , an intermediate layer  306 , and a flexible printed circuit board  320 . The light emitting elements  302  can be light sources generally used in the backlight module  300 , for example, light emitting diodes, lamps, or other usable light sources. The light guide plate  304  is disposed at a side of the light emitting elements  302  to receive lights emitted by the light emitting elements  302 . The flexible printed circuit board  320  adheres to the tops of the both the light emitting elements  302  and the light guide plate  304  by using a tape  322 . The flexible printed circuit board  320  provides circuit arrangements of the backlight module  300  for transmitting and controlling electrical signals. 
         [0021]    As mentioned above, an air gap exists between the light emitting elements  302  and the light guide plate  304  in the backlight module  300  of prior arts. That is, the lights would be transmitted through air. Because the air is the medium through which the lights are scattered, this results in optical energy loss. According to the present invention, an intermediate layer  306  is disposed between the light emitting elements  302  and the light guide plate  304 . The intermediate layer  306  is made of material having a light transmittance greater than a light transmittance of air, i.e. material of low haze, so as to achieve an object of increasing light output efficiency. Preferably, a refractive index of the intermediate layer  306  is substantially equal to a refractive index of the air, which is about 1.5 to 1.52. 
         [0022]    Preferably, the intermediate layer  306  is made of soft and adhesive material. In an embodiment, the intermediate layer  306  is implemented by a film made of optical adhesive. The optical adhesive is filled between the light emitting elements  302  and the light guide plate  304  as the intermediate layer  306  to guide the lights of the light emitting elements  302  toward the light guide plate  304 . As a result, the light output efficiency is increased. The optical adhesive can be a non-based adhesive material or a based adhesive material. One example of the based adhesive material is 3M™ Optically Clear Adhesives (OCAs), e.g. product numbers 8141, 8142, 8161, 8171, 8172, 9483. A light transmittance of the 3M™ OCAs is greater than 99% and a haze thereof is less than 1%. One example of the non-based adhesive material is CS3623 Transparent Double-coated adhesive tape manufactured by NITTO DENKO. A light transmittance of CS3623 is about 91.4%. In another embodiment, Super View Resin (SVR), which is a pasty resin, manufactured by Sony Chemical &amp; Information Device Corporation is another choice for the intermediate layer  306 . A light transmittance of SVR is greater than 95%. Furthermore, a tape having a light transmittance greater than air can also be a choice of the intermediate layer  306 . 
         [0023]    TABLE 1 shows specifications of elements of a conventional backlight module and the backlight module  300  of the present embodiment for testing the light output efficiency. The intermediate layer  306  is formed by 3M™ OCAs mentioned above. 
         [0000]    
       
         
               
               
               
             
               
               
               
               
               
             
               
               
               
             
               
               
               
               
               
             
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 element 
                 prior art 
                 present embodiment 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 upper prism sheet 
                 0.065 
                 millimeter (mm) 
                 0.065 
                 millimeter (mm) 
               
               
                 lower prism sheet 
                 0.062 
                 millimeter (mm) 
                 0.062 
                 millimeter (mm) 
               
               
                 diffusion sheet 
                 0.053 
                 millimeter (mm) 
                 0.053 
                 millimeter (mm) 
               
             
          
           
               
                 light guide plate 
                 0.4 millimeter (mm)/ 
                 0.4 millimeter (mm)/ 
               
               
                   
                 0.5 millimeter (mm) 
                 0.5 millimeter (mm) 
               
             
          
           
               
                 reflecting sheet 
                 0.065 
                 millimeter (mm) 
                 0.065 
                 millimeter (mm) 
               
             
          
           
               
                 light emitting 
                 LT15056 5 sets 
                 LT15056 5 sets 
               
               
                 elements(light 
               
               
                 emitting diodes) 
               
               
                 intermediate layer 
                 none 
                 3M ™ Optically 
               
               
                   
                   
                 Clear Adhesives 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 practical 
                 theoretical 
                   
               
               
                   
                 output 
                 output 
                 light output 
               
               
                   
                 (milli candles 
                 (milli candles 
                 efficiency 
               
               
                   
                 (mcd)) 
                 (mcd)) 
                 (%) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 prior art 
                 backlight module 
                 3165 
                 8450 
                 37 
               
               
                   
                 backlight 
                 250 
                 668 
                 37 
               
               
                   
                 module + panel 
               
               
                 present 
                 backlight module 
                 3399 
                 8450 
                 40 
               
               
                 embodiment 
                 backlight 
                 264 
                 668 
                 40 
               
               
                   
                 module + panel 
               
               
                   
               
             
          
         
       
     
         [0024]    Each set of the light emitting elements outputs 1690 milli candles (mcd). Both the conventional backlight module and the backlight module  300  of the present embodiment utilize 5 sets of the light emitting elements. Therefore, a theoretical output is 1690*5=8450 mcd. A comparison of the conventional backlight module and the backlight module  300  of the present embodiment can be seen in TABLE 2 that the light output efficiency is increased from 37% to 40%. In addition, the conventional backlight module assembled with a panel is also compared with the backlight module  300  of the present embodiment assembled with a panel. A theoretical transmittance of a panel is 7.9%. Therefore, a theoretical output is 8450*7.9%=668 mcd. The comparison of the conventional backlight module assembled with the panel and the backlight module  300  of the present embodiment assembled with the panel can be seen in TABLE 2 that the light output efficiency is also increased from 37% to 40%. Since the light output efficiency of the backlight module is not easily increasable, an increase of 3% is a considerably great improvement. 
         [0025]    In addition, brightness and color in the conventional backlight module and in the backlight module  300  of the present invention are measured based on the specifications listed in TABLE 1. TABLE 3 shows comparisons of the brightness and the color between the conventional backlight module and the backlight module  300  of the present invention. TABLE 4 shows comparisons of the brightness and the color between the conventional backlight module assembled with the panel and the backlight module  300  of the present invention assembled with the panel. 
         [0000]    
       
         
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 3 
               
             
             
               
                   
                   
               
               
                   
                 backlight module 
                 backlight module 
               
               
                   
                 (conventional) 
                 (present embodiment) 
               
             
          
           
               
                   
                 brightness 
                 color 
                 brightness 
                 color 
               
             
          
           
               
                   
                 cd/m 2   
                 X 
                 Y 
                 cd/m 2   
                 X 
                 Y 
               
               
                   
                   
               
             
          
           
               
                 1 
                 2642 
                 0.346 
                 0.351 
                 2833 
                 0.349 
                 0.354 
               
               
                 2 
                 2638 
                 0.345 
                 0.351 
                 2842 
                 0.348 
                 0.354 
               
               
                 3 
                 2682 
                 0.346 
                 0.351 
                 2894 
                 0.348 
                 0.355 
               
               
                 4 
                 3056 
                 0.333 
                 0.339 
                 3222 
                 0.336 
                 0.342 
               
               
                 5 
                 3168 
                 0.332 
                 0.338 
                 3399 
                 0.334 
                 0.342 
               
               
                 6 
                 2820 
                 0.332 
                 0.337 
                 2993 
                 0.335 
                 0.341 
               
               
                 7 
                 3272 
                 0.322 
                 0.326 
                 3537 
                 0.324 
                 0.329 
               
               
                 8 
                 3240 
                 0.321 
                 0.327 
                 3350 
                 0.325 
                 0.333 
               
               
                 9 
                 3457 
                 0.320 
                 0.327 
                 3600 
                 0.323 
                 0.332 
               
               
                 average 
                 2997 
                   
                   
                 3186 
               
               
                 brightness 
               
               
                 light 
                 76.3% 
                   
                   
                 78.7% 
               
               
                 uniformity 
               
             
          
           
               
                 compared with the center point of the 
                 107.3% 
                 0.0025 
                 0.0037 
               
               
                 conventional backlight module 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 4 
               
             
             
               
                   
                   
               
               
                   
                 backlight module + panel 
                 backlight module + panel 
               
               
                   
                 (conventional) 
                 (present embodiment) 
               
             
          
           
               
                   
                 brightness 
                 color 
                 brightness 
                 color 
               
             
          
           
               
                   
                 cd/m 2   
                 X 
                 Y 
                 cd/m 2   
                 X 
                 Y 
               
               
                   
                   
               
             
          
           
               
                 1 
                 209 
                 0.344 
                 0.370 
                 218 
                 0.349 
                 0.354 
               
               
                 2 
                 207 
                 0.344 
                 0.371 
                 218 
                 0.348 
                 0.354 
               
               
                 3 
                 210 
                 0.344 
                 0.370 
                 221 
                 0.348 
                 0.355 
               
               
                 4 
                 239 
                 0.331 
                 0.357 
                 248 
                 0.336 
                 0.342 
               
               
                 5 
                 250 
                 0.330 
                 0.356 
                 264 
                 0.334 
                 0.342 
               
               
                 6 
                 223 
                 0.330 
                 0.356 
                 234 
                 0.335 
                 0.341 
               
               
                 7 
                 256 
                 0.319 
                 0.343 
                 272 
                 0.324 
                 0.329 
               
               
                 8 
                 256 
                 0.318 
                 0.344 
                 262 
                 0.325 
                 0.333 
               
               
                 9 
                 277 
                 0.319 
                 0.345 
                 283 
                 0.323 
                 0.332 
               
               
                 average 
                 236 
                   
                   
                 247 
               
               
                 brightness 
               
               
                 light 
                 74.8% 
                   
                   
                 76.9% 
               
               
                 uniformity 
               
             
          
           
               
                 compared with the center point of the 
                 105.4% 
                 0.0023 
                 0.0028 
               
               
                 conventional backlight module assembled 
               
               
                 with the panel 
               
               
                   
               
             
          
         
       
     
         [0026]    It is noted that in a left column of TABLE 3, numbers 1-9 respectively represent nine points of different positions measured in the conventional backlight module and in the backlight module  300  of the present embodiment. In a left column of TABLE 4, numbers 1-9 respectively represent nine points of different positions measured in the conventional backlight module assembled with the panel and in the backlight module  300  of the present embodiment assembled with the panel. The number 5 in TABLEs 3, 4 represents the center point. In addition, X and Y in TABLEs 3, 4 respectively represent X-coordinate and Y-coordinate. The comparison of the conventional backlight module and the backlight module  300  of the present embodiment can be seen in TABLE 3 that an average brightness is increased from 2997 candles/meter 2  to 3186 candles/meter 2 . Compared with the brightness of the center point (number 5) of the conventional backlight module, the brightness of the center point (number 5) of the backlight module  300  of the present embodiment is 107.3%. This means that the present embodiment promotes not only the light output efficiency, but also the brightness. 
         [0027]    Furthermore, light uniformities in TABLEs 3, 4 are computed by the following equation: the minimum brightness/the maximum brightness. The comparison of the conventional backlight module and the backlight module  300  of the present embodiment can be seen in TABLE 3 that the light uniformity is increased from 76.3% to 78.7%, and this means that the light uniformity is increased about 2.4%. In addition, it can also be seen that color purity of the backlight module  300  of the present embodiment is not affected while the light output efficiency is increased. 
         [0028]    Compared with the prior arts, the average brightness of the present embodiment shown in TABLE 4 is increased from 236 candles/meter 2  to 247 candles/meter 2  after the backlight module  300  is assembled with the panel. Compared with the brightness of the center point (number 5) of the conventional backlight module assembled with the panel, the brightness of the center point (number 5) of the backlight module  300  of the present embodiment assembled with the panel is 105.4%. In addition, the comparison of the conventional backlight module assembled with the panel and the backlight module  300  of the present embodiment assembled with the panel can be seen in TABLE 4 that the light uniformity is increased from 74.8% to 76.9%, and this means that the light uniformity is increased about 2.1%. 
         [0029]    Please refer to  FIG. 2B . Because the intermediate layer  306  is made of soft and adhesive material, the gap between the light emitting elements  302  and the light guide plate  304  can be effectively filled with the intermediate layer  306 . Accordingly, the light emitting elements  302  and the light guide plate  304  can be assembled more firmly due to the adhesivity of the intermediate layer  306 . As mentioned above, a misalignment often occurs when the light emitting elements  302  and the light guide plate  304  are assembled together. The gap between the light emitting elements  302  and the light guide plate  304  can be effectively filled with the intermediate layer  306 , and therefore the problem of the misalignment of the light emitting elements  302  can be solved. As a result, the alignment is improved when the light emitting elements  302  and the light guide plate  304  are assembled together. 
         [0030]    The backlight module capable of increasing the light output efficiency according to the present invention substitutes the intermediate layer having the light transmittance greater than the light transmittance of air for the air gap, so as to decrease the optical energy loss between the light emitting elements and the light guide plate. Therefore, the light output efficiency and the light uniformity can be increased. The present invention also can improve the alignment between the light emitting elements and the light guide plate when they are assembled together. 
         [0031]    Please refer to  FIG. 3 .  FIG. 3  illustrates a diagram of an electronic apparatus  500  comprising a liquid crystal display  502 . The liquid crystal display  502  comprises the backlight module  300  capable of increasing the light output efficiency. The liquid crystal display  502  having the backlight module  300  shown in  FIG. 3  can be a part of the electronic apparatus  500 . The electronic apparatus  500  comprises the liquid crystal display  502  and a power supply  504 . The power supply  504  is coupled to the liquid crystal display  502  for providing power for the liquid crystal display  502 . The electronic apparatus  500  is a mobile phone, a digital camera, a Personal Digital Assistant, a notebook, a desktop computer, a television, a Global Positioning System, a vehicle display, an aeronautical display, or a portable digital video disc (DVD) player. 
         [0032]    As is understood by a person skilled in the art, the foregoing preferred embodiments of the present invention are illustrative rather than limiting of the present invention. It is intended that they cover various modifications and similar arrangements be included within the spirit and scope of the appended claims, the scope of which should be accorded the broadest interpretation so as to encompass all such modifications and similar structure.