Patent Document

CROSS-REFERENCE OF RELATED APPLIICATION  
         [0001]    This application claims the priority benefit of Taiwan patent application serial No. 091120402, filed Sep. 2, 2002, the full disclosure of which is incorporated herein by reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention generally relates to a panel light device, and more particularly to a light guide of a back light module for a liquid crystal display.  
           [0004]    2. Description of the Related Art  
           [0005]    Referring to FIG. 1, it depicts the structure of a conventional liquid crystal display. Generally, the liquid crystal display device includes a liquid crystal panel  10  which has two substrates and a liquid crystal material sealed therebetween, a back light module  20  disposed, under the liquid crystal panel  10 , and cases  11  and  12 .  
           [0006]    The back light module  20  is utilized to distribute the light from a light source uniformly over the surface of the liquid crystal panel  10 . There are several kinds of back light modules  20  such as a direct back light type (or direct type) and an edge light type.  
           [0007]    Referring to FIG. 2, which is a cross-sectional view along line  2 - 2  of FIG. 1, it depicts a back light module  21  of the direct type. The direct type back light module  21  includes a housing  70  which has a reflective sheet  60  disposed on the bottom surface of the housing  70  and, lamps  50 , such as a fluorescence cathode tube, disposed at the bottom portion of the housing  70 , a diffusing sheet  40  disposed on the top surface of the housing  70  and a prism sheet  30  disposed on the diffusing sheet  40 . Since the lamps  50  are spacedly disposed inside the display area of the liquid crystal panel  10 , the brightness distribution of the liquid crystal panel may be not uniform. Besides, the thin film transistors or the switching elements of the liquid crystal panel  10  may be damaged by the heat of the back light source.  
           [0008]    Therefore, some distance or space between the lamp  50  and the diffusing sheet  40  is required. This distance or space is provided by means of supports  55  disposed in the housing  70 . Typically, the distance between the center of the lamp  50  and the diffusing sheet  40  is required to be longer than a half distance between the adjacent lamps  50 . Because of these structural requirements, it is very difficult to reduce the thickness of the direct type back light module  21 . Furthermore, the lamp  50  also absorbs the light emitted from other lamps such that the optical efficiency is lowered.  
           [0009]    Referring to FIG. 3, which is a cross-sectional view along line  2 - 2  of FIG. 1, it depicts a back light module  22  of the edge light type. The edge light type back light module  22  includes a light guide  80 , a lamp  50  which is attached to at least one edge of the light guide  80 , and a U-shaped reflector  61  which surrounds the lamp  50 . An open portion of the reflector  61  is disposed at the edge of the light guide  80 , a reflecting sheet  60  is disposed at the bottom of the light guide  80 , a diffusing sheet  40  is disposed on the light guide  80  and a prism sheet  30  is disposed on the diffusing sheet  40 . Because the lamp  50  is disposed at the edge of the light guide  80 , the thickness of the LCD can be relatively decreased.  
           [0010]    The light guide  80  includes a printed dot pattern or a V-shaped grooved pattern on one surface thereof and is typically made of PMMA by the process of molding. The diffusing sheet  40  is disposed on the light guide  80  and typically made of half-transparent polyethylene terephthalate (PET), polyethylene terephthalate, or polycarbonate for further evenly diffusing the light emitted from the light guide  80 . The prism sheet  30  is disposed on the diffusing sheet  40  for gathering the light from the diffusing sheet  40  in the direction perpendicular thereto.  
           [0011]    Since the liquid crystal panel  10  is illuminated by the light emitted from the light guide  80 , the light guide  80  is required to uniformly or evenly illuminate over the liquid crystal panel  10 . However, for the liquid crystal display of large dimensions in particular, the light guide  80  cannot provide even luminance. Furthermore, the manufacturing cost of the light guide  80  and the pattern thereon is relatively high.  
           [0012]    For the back light module of the liquid crystal display in the prior art, there are many attempts made to increase the screen brightness, to provide uniform brightness, to lower power consumption and cost, as well as to decrease the dimension. Prior art attempts, for example, can be seen in U.S. Pat. No. 6,164,790 issued to Lee on Dec. 26, 2000, U.S. Pat. No. 5,477,422 issued to Hooker et al. on Dec. 19, 1995, and U.S. Pat. No. 5,485,354 issued to Ciupke et al. on Jan. 16, 1996. However, the back light modules in these patents and the other prior art still cannot meet the above-mentioned requirements.  
           [0013]    Referring to FIG. 4 a  and FIG. 4 b , they depict a back light module  23  having light sources of light emitting diodes in the prior art. The back light module  23  includes a light guide  80 , a plurality of light emitting diodes  51  disposed beside the light guide  80 , and reflectors  60 ,  62  respectively disposed on the bottom surface and on the side surface of the back light module  23 . The back light module  23  cannot meet above-mentioned requirements, either, and particularly for the liquid crystal display of large dimensions, further cannot provide even light because the light emitting diode  51  is substantially a point light source instead of a linear light source, and located beside the back light module  23 .  
           [0014]    Accordingly, there exists a need for a back light module of a liquid crystal display capable of meeting the above-mentioned requirements.  
         SUMMARY OF THE INVENTION  
         [0015]    It is an object of the present invention to provide a panel light source device capable of providing uniform luminance, being compact, and being manufactured at a lowered cost.  
           [0016]    It is an object of the present invention to provide a back light module of a liquid crystal display capable of providing uniform luminance, being compact, and being manufactured at a lowered cost.  
           [0017]    In order to achieve the above objects, the present invention provides a panel light source device comprising light sources, a light guide, and a reflector. The light guide defines a bottom surface and a top surface, and comprises grooves disposed on the bottom surface for receiving the light sources and pairs of first inclined surfaces disposed on the top surface and corresponding to the groove. The grooves and the first inclined surfaces cooperate with each other to transmit the light from the light source into the light guide. The reflector is disposed on the bottom surface of the light guide and covers the linear light source.  
           [0018]    Accordingly, the panel light source can be free from the loss of the light energy which results from the inter-emitting and inter-absorption of the lamps, so the brightness thereof can be increased and the thickness thereof can be reduced. Since the brightness of the back light module is increased, the amount of prism sheets can be decreased or eliminated such that the cost of the back light module is lowered without decreasing the angle of view of the back light module. Furthermore, the light source in the back light module according to the present invention is embedded in the light guide, so the dimension of the back light module can be further reduced. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    Other objects, novel features, and advantages of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawing.  
         [0020]    [0020]FIG. 1 is a schematic perspective exploded view of a liquid crystal display in the prior art.  
         [0021]    [0021]FIG. 2 is a schematic cross sectional view of a direct type back light module in the prior art along line  2 - 2  in FIG. 1.  
         [0022]    [0022]FIG. 3 is a cross sectional schematic view of an edge light type back light module in the prior art along line  2 - 2  in FIG. 1.  
         [0023]    [0023]FIG. 4 a  is a schematic plan view of a back light module with light emitting diodes in the prior art.  
         [0024]    [0024]FIG. 4 b  is a schematic cross sectional view of the back light module with the light emitting diodes shown along line  4   b - 4   b  in FIG. 4 a.    
         [0025]    [0025]FIG. 5 is a schematic cross sectional view of a liquid crystal display according to an embodiment of the present invention.  
         [0026]    [0026]FIG. 6 is a schematic partial enlarged cross sectional view of a light guide in a back light module according to another embodiment of the present invention.  
         [0027]    [0027]FIG. 7 is a schematic view of the light guide in the back light module according to the embodiment of the present invention for showing the dimensions thereof.  
         [0028]    [0028]FIG. 8 a  is a schematic plan view of a back light module with light emitting diodes according to a further embodiment of the present invention.  
         [0029]    [0029]FIG. 8 b  is a schematic cross sectional view of the back light module with the light emitting diodes shown in FIG. 8 a.   
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0030]    Referring to FIG. 5, it depicts a liquid crystal display device  100  according to the present invention. The liquid crystal display device  100  includes a liquid crystal panel  110  and a back light module  120 . The liquid crystal panel  110  has two transparent substrates  112 ,  114  and liquid crystal material  116  sealed therebetween. The two transparent substrates  112 ,  114  of the liquid crystal panel  110  are provided with switching elements for changing the arrangements of the molecular of the liquid crystal material  116  and thus generating images.  
         [0031]    The back light module  120  is served as a panel light source device for uniformly emitting the light over the entire display area and illuminating the liquid crystal panel  110 . The back light module  120  includes a plurality of linear light sources, such as lamps  150 , a light guide  180 , and a plurality of optical films, such as a diffusing sheet  140  and a prism sheet  130 . The lamps  150  can be a cold cathode fluorescent lamp (CCFL).  
         [0032]    The light guide  180  has a plurality of grooves  210  disposed on the bottom surface thereof for receiving the lamps  150  and has a plurality of notches  220  disposed on the top surface thereof. The location of the notch  220  is corresponding to that of the groove  210  and the notch  220  is utilized for cooperating with the groove  210  such that the light emitted from the lamp  150  can be transmitted into the light guide  180  and then diffused in the light guide  180  by the total internal reflection. Therefore, the light emitted from the lamp  150  can be transmitted through the light guide  180  and uniformly emitted from the top surface (the outgoing surface)  226  of the light guide  180  to uniformly illuminate the liquid crystal panel  110 . The back light module  120  further has four side reflectors  182  and a bottom reflector  184 . The side reflectors  182  are disposed around the light guide  180  for reflecting the light emitted to the sides of the light guide  180  back into the light guide  180 . The reflector  184  is disposed on the bottom surface of the light guide  180  for reflecting the light emitted to the bottom surface of the light guide  180  back to the light guide. Besides, the reflector  184  is further provided with a plurality of ridges  186  respectively disposed under the lamps  150  for further transmitting the light from the lamps  150  into the light guide  180 .  
         [0033]    Generally, the light guide  180  is made of transparent material, such as Acrylic Resin (PMMA). The light guide  180  can be integrally formed, as shown in FIG. 5. However, according to another embodiment of the present invention, as shown in FIG. 6, the light guide  180  can also be formed by means of inter-engaging a plurality of modularized light guide elements  180 ′ in series. The light guide element  180 ′ has a snap portion  192  and a snap pin  190  engaged with the snap portion  192  so as to form the whole light guide  180 .  
         [0034]    Referring to FIG. 7, it depicts the dimensions of the light guide  180  according to the embodiment of the present invention. As shown in FIG. 7, under the critical condition, the light emitted from the lamp  150  (as a phantom line shown in FIG. 7) is transmitted into the light guide  180  with a direction perpendicular to a horizontal surface  212  of a groove  210  and then to a pair of first inclined surfaces  222  of the notch  220  such that the angle ? 1  has to be larger than 2*(90-?c) for the total internal reflection. That is, for Acrylic Resin (index of refraction=1.49), the critical angle θc=42.9 degrees and thus θ 1  has to be larger than 94.2 degrees. The width W 1  of a horizontal surface  212  of the groove  210  has to be substantially equal to the width W 2  of the two first inclined surfaces  222  of the notch  220 .  
         [0035]    According to the embodiment of the present invention, the groove  210  is further provided with a pair of slants  214  and the notch  220  is further provided with a pair of second inclined surfaces  224 . The summation of the angle θ 3  between slant  214  and the horizontal surface  212  of the groove  210  and the angle θ 2  between the second inclined surface  224  of the notch  220  and the top surface  226  of the light guide  180  is less than 90 degrees. The width W 3  of the second inclined surface  224  of the notch  220  is larger than the height H 1  between the lamp  150  and the top surface  226  of the light guide  180 , and preferably twice larger than the height H 1 .  
         [0036]    The width W 4  of the groove  210  (the distance between two vertical walls  216 ) is slightly larger than the diameter of the lamp  150  for providing a sufficient space for the heat dissipation of the lamp  150 , avoiding the thermal conduction, and lowering the risk of the thermal deformation of the light guide  180 , and the clearance between the vertical wall  216  and the lamp  150  is typically about 0.5 mm. Therefore, the width W 4  is larger than the diameter of the lamp  150  plus 1.0 mm. The angle θ 4  of the ridge  186  is less than 180 degrees. The height H 2  between the lamp  150  and the bottom surface of the light guide  180  is larger than that of the ridge  186 , i.e., H 2 &gt;tan(θ 4 /2). According to a geometric shape of the present invention, the groove  210  can meet the requirements of the total internal reflection and diffuse the light to avoid the uneven luminance above the lamp  150 .  
         [0037]    Furthermore, there are fillets or rounds at the junctions between the top surface  226  and the second inclined surface  224  of the light guide  180  and between the second inclined surface  224  and the first inclined surface  222  of the light guide  180  to avoid the uneven brightness at the junction. However, there are sharp edges (without a chamfer or fillet) at the junctions between the two first inclined surfaces  222 , between the slant  214  and the horizontal surface  212 , and between the slant  214  and the vertical wall  216  of the groove  210 .  
         [0038]    Referring to FIG. 8 a  and FIG. 8 b , they depict a back light module  520  according to a further embodiment of the present invention. The back light module  520  is substantially similar to the back light module  120 . The back light module  520  has a light guide  580  and a plurality of point light sources, such as light emitting diodes (LED)  550 . The light guide  580  has a plurality of depressions  610  for receiving the light source  550  and has a plurality of recesses  620  respectively corresponding to the depressions  610 . It will be apparent to those skilled in the art that the cross-sectional shapes of the depression  610  and the recess  620  is identical with that of the groove  210  and the notch  220  described hereinabove, and that as the foregoing description, the light guide  580  can be used for transmitting the light from the light source  550  into the light guide  580 . Specifically, the depression  610  is substantially frusta-conical in shape and defines a horizontal surface  612 , a frusta-conical surface  614  and a vertical wall  616 , and the recess  620  defines a first conical surface  622  and a second conical surface  624 .  
         [0039]    The light source  550  of the light emitting diode is embedded in the depression  610  of the light guide  580  and the light emitted from the light source  550  is uniformly diffused by the light guide  580 . Therefore, the back light module  520  according to the present invention can be served as a panel light source of a large area and adapted to the application of large dimension by means of only increasing the amount of the light emitting diodes.  
         [0040]    As the foregoing description, the back light module or the panel light source according to the present invention has a light guide which has groove or depression portions for receiving linear light sources (lamps) or point light sources (light emitting diodes), and notches or recesses cooperating with the groove portions for transmitting the light emitting from the light source into the light guide.  
         [0041]    Due to the light guide, the back light module or the panel light source can be free from the loss of the light energy which results from the inter-emitting and inter-absorption of the lamps, the brightness thereof can be increased, and the thickness thereof can be reduced. Since the brightness of the back light module is increased, the amount of prism sheets can be decreased or eliminated such that the cost of the back light module is lowered without decreasing the angle of view of the back light module. Furthermore, the light source in the back light module according to the present invention is embedded in the light guide, so the dimension of the back light module can be further reduced.  
         [0042]    Although the invention has been explained in relation to its preferred embodiments, it is not used to limit the invention. It is to be understood that many other possible modifications and variations can be made by those skilled in the art without departing from the spirit and scope of the invention as hereinafter claimed.

Technology Category: g