Abstract:
A light guide plate ( 30 ) includes two opposite incident surfaces ( 31 ) for receiving light beams, an emitting surface ( 32 ) joining the two incident surfaces and a bottom surface ( 33 ) opposite to the emitting surface. The emitting surface has a V-shaped profile defined by two symmetric oblique plates ( 321, 322 ) slanting down toward each other at a predetermined angle. The surface light source incorporating the light guide plate is inexpensive and has higher intensity of illumination at the center region thereof. Thus a liquid crystal display incorporating the surface light source satisfactorily illuminate a main visible range thereof.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a light guide plate and a surface light source device incorporating the light guide plate, especially to a light guide plate configure to provide high light intensity at a center thereof.  
           [0003]    2. Prior Art  
           [0004]    Liquid crystal displays are thin, lightweight, and have low power consumption. Owing to these characteristics, liquid crystal displays have been widely used in the fields of communications and consumer products, in devices such as personal computers, liquid crystal TVs, image telephones, PDAs (personal digital assistants), etc. The liquid crystal display is a passive device, which must usually incorporate a surface light source device in order to provide images.  
           [0005]    A conventional surface light source device  10  is illustrated in FIG. 9. The surface light source device  10  includes a light guide plate  1 , a reflector  2  arranged on a bottom surface  11  of the light guide plate  1 , a diffuser  3  arranged on an emitting surface  12  of the light guide plate  1 , a light source  6  positioned adjacent to an incident surface  13  of the light guide plate  1 , a reflector positioned behind the light source  6 , and two prism sheets  4 ,  5  arranged on the diffuser  3 . In operation, light beams coming from the light guide plate  1  are scattered by the diffuser  3 , and uniformly emit from the diffuser  3 . The light beams are then converged by the prism sheets  4 ,  5  to improve the intensity of illumination. However, the surface light source device  10  has numerous elements, which makes it unduly thick and costly. The light beams must pass through a plurality of interfaces before emitting from the prism sheet  5 , which reduces the intensity of the emitted light.  
           [0006]    [0006]FIG. 10 shows a surface light source device  20  as disclosed in China Patent No. CN99214211. The surface light source device  20  includes a light guide plate  21 , a reflector  22 , a light source  24 , and a diffusing film  23 . A plurality of terraced printing lines  261  is defined on an emitting surface  26  of the light guide plate  21 , to enhance the light intensity thereof. However, light beams from the light source  24  are converged only along a direction of the printing lines  261 . Therefore, the light beams cannot be utilized efficiently, and the light intensity of the surface light source device  20  is relatively poor.  
           [0007]    In order to solve these problems, China Patent No. CN98207946 provides a light guide plate which comprises a prism array with single axis on an emitting surface, in order to increase the intensity of emitted light. Alternatively, a prism array with two axes can be defined on the emitting surface. However, a liquid crystal display generally needs a higher intensity of illumination in a center region of a display screen thereof, in order to satisfactorily illuminate a main visible range of the liquid crystal display. The light guide plate does not provide the required illumination.  
         SUMMARY OF THE INVENTION  
         [0008]    An object of the present invention is to provide a light guide plate having a higher intensity of illumination at a center region thereof.  
           [0009]    Another object of the present invention is to provide a surface light source device having a light guide plate with higher intensity of illumination at a center region thereof, and which is inexpensive.  
           [0010]    In order to achieve the objects above, a light guide plate in accordance with one embodiment of the present invention comprises two opposite incident surfaces for receiving light beams, an emitting surface interconnecting the two incident surfaces, and a bottom surface opposite to the emitting surface. A thickness of a center portion of the light guide plate is less than thicknesses of two peripheral portions of the light guide plate that are at opposite sides of the center portion.  
           [0011]    Also in order to achieve the objects above, a surface light source device in accordance with another embodiment of the present invention comprising a light guide plate and two light sources. The light guide plate comprises two opposite incident surfaces for receiving light beams, an emitting surface between the incident surfaces, and a bottom surface opposite to the emitting surface. The two light sources are disposed adjacent to the incident surfaces respectively of the light guide plate. A thickness of a center portion of the light guide plate is less than thicknesses of two peripheral portions of the light guide plate that are at opposite sides of the center portion  
           [0012]    The surface light source incorporating the light guide plate is inexpensive and has higher intensity of illumination at the center region thereof. Thus a liquid crystal display incorporating the surface light source satisfactorily illuminate a main visible range thereof.  
           [0013]    These and other features, aspects and advantages of the present invention will become more apparent from the following detailed description and claims, and from the accompanying drawings in which: 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    [0014]FIG. 1 is an isometric view of a first embodiment of a light guide plate according to the present invention;  
         [0015]    [0015]FIG. 2 is a side elevation of a second embodiment of a light guide plate according to the present invention;  
         [0016]    [0016]FIG. 3 is an isometric view of a third embodiment of a light guide plate according to the present invention;  
         [0017]    [0017]FIG. 4 is a side elevation of a fourth embodiment of a light guide plate according to the present invention;  
         [0018]    [0018]FIG. 5 is an isometric view of a fifth embodiment of a light guide plate according to the present invention;  
         [0019]    [0019]FIG. 6 is a side elevation of a sixth embodiment of a light guide plate according to the present invention;  
         [0020]    [0020]FIG. 7 is an isometric view of a seventh embodiment of a light guide plate according to the present invention;  
         [0021]    [0021]FIG. 8 is a schematic, cross-sectional view of a surface light source device of the present invention incorporating the light guide plate of FIG. 3;  
         [0022]    [0022]FIG. 9 is an exploded, side elevation of a conventional surface light source device; and  
         [0023]    [0023]FIG. 10 is an exploded, isometric view of another conventional surface light source device. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]    Reference will now be made to the drawings to describe the present invention in detail.  
         [0025]    Referring to FIG. 1, a light guide plate  30  according to a first embodiment of the present invention is generally made from transparent material. The light guide plate  30  has two opposite incident surfaces  31  for receiving incident light beams, an emitting surface  32  located between the incident surfaces  31 , and a planar bottom surface  33  opposite to the emitting surface  32 . The emitting surface  32  has a V-shaped profile which is defined by two symmetric oblique surfaces  321 ,  322  slanting down toward each other at a predetermined angle. Therefore, light beams from opposite light sources (not shown) are reflected and emit from the emitting surface  32 , with increasing intensity proportional to increasing distance away from the light sources. That is, an intensity of illumination is highest at a junction of the two oblique surfaces  321 ,  322 . The light guide plate  30  further comprises a plurality of scattering dots  331  on the bottom surface  33 . Each scattering dot  331  is a micro-prism, which may be parallelepiped-shaped, hemispherical, cylindrical or frustum-shaped. The scattering dots  331  can be injection molded integrally with the light guide plate  30 , or formed by printing.  
         [0026]    Referring to FIG. 2, a light guide plate  40  according to a second embodiment of the present invention is similar to the light guide plate  30  of the first embodiment, except that a bottom surface  43  of the light guide plate  40  has a V-shaped profile and an emitting surface  42  is planar. Such structure of the light guide plate  40  can provide the same high optical performance attained by the light guide plate  30  as described above.  
         [0027]    Referring to FIG. 3, a light guide plate  50  according to a third embodiment of the present invention is similar to the light guide plate  30  of the first embodiment. The only difference is that an emitting surface  52  of the light guide plate  50  has a plurality of V-shaped grooves  521 . The V-shaped grooves  521  may have different angles, heights and widths, such that the angles, heights and widths all progressively increase in size from each of two incident surfaces  51  to a middle of the emitting surface  52 . The angles of the V-shaped grooves  521  vary in the range from 70 degrees to 110 degrees, and preferably in the range from 80 degrees to 100 degrees. The heights of the V-shaped grooves  521  vary in the range from 20 um to 100 um, and preferably in the range from 40 um to 60 um. Maximum widths of the V-shaped grooves  521  vary in the range from 50 um to 200 um, and preferably in the range from 75 um to 125 um. Alternatively, the V-shaped grooves  521  may have the same angle, height and width. Further, each groove  521  may alternatively be step-shaped, polygonal, or have another suitable configuration. The V-shaped grooves  521  can be injection molded integrally with the light guide plate  50 . Alternatively, the V-shaped grooves  521  can be formed by chemical etching, laser printing, mechanical processing, light etching, etc. The light guide plate  50  also comprises a bottom surface  53 , and a plurality of scattering dots  551  is provided on the bottom surface  53 .  
         [0028]    Referring to FIG. 4, a light guide plate  60  according to a fourth embodiment of the present invention is similar to the light guide plate  40  of the second embodiment. The only difference is that an emitting surface  62  of the light guide plate  60  has a plurality of V-shape grooves  621 . The V-shaped grooves  621  may have different angles, heights and widths, such that the angles, heights and widths all progressively increase in size from each of two opposite incident surfaces (not labeled) to a middle of the emitting surface  62 . The light guide plate  60  also has a bottom surface  63 .  
         [0029]    Referring to FIG. 5, a light guide plate  70  according to a fifth embodiment of the present invention is similar to the light guide plate  30  of the first embodiment. The only difference is that an emitting surface  72  has three surfaces  721 ,  722 ,  723 . The surfaces  721 ,  723  are symmetric oblique surfaces that slant down toward each other at a predetermined angle. The surface  722  is an intermediate surface interconnecting the oblique surfaces  721 ,  723 . The intermediate surface  722  is parallel to a bottom surface  73  of the light guide plate  70 . In addition, a plurality of scattering dots  731  is provided on the bottom surface  73 .  
         [0030]    Referring to FIG. 6, a light guide plate  80  according to a sixth embodiment of the present invention is similar to the light guide plate  70  of the fifth embodiment. The only difference is that a bottom surface  83  has three surfaces  831 ,  832 ,  833 . The surfaces  831 ,  833  are symmetric oblique surfaces that slant up toward each other at a predetermined angle. The surface  832  is an intermediate surface interconnecting the oblique surfaces  831 ,  833 . The intermediate surface  832  is parallel to an emitting surface  82 .  
         [0031]    Referring also to FIG. 7, a light guide plate  90  according to a seventh embodiment of the present invention is similar to the light guide plate  70  of the fifth embodiment. The only difference is that an emitting surface  92  of the light guide plate  90  has a plurality of V-shaped grooves  921 . The V-shape grooves  921  may have different angles, heights and widths, such that the angles, heights and widths progressively increase in size from each of two opposite incident surfaces  91  to a middle of the emitting surface  92 . The angles of the V-shaped grooves  921  vary in the range from 70 degrees to 110 degrees, and preferably in the range from 80 degrees to 100 degrees. The heights of the V-shaped grooves  921  vary in the range from 20 um to 100 um, and preferably in the range from 40 um to 60 um. Maximum widths of the V-shaped grooves  921  vary in the range from  50  urn to 200 um, and preferably in the range from 75 um to 125 um. Preferably, the V-shaped grooves  921  at a generally horizontal middle portion of the emitting surface  92  have the same angle, height and width. Alternatively, all the V-shaped grooves  921  may have the same angle, height and width. Further, each groove  921  may alternatively be step-shaped, polygonal, or have another suitable configuration. The V-shaped grooves  921  can be injection molded integrally with the light guide plate  90 . Alternatively, the V-shaped grooves  921  can be formed by chemical etching, laser printing, mechanical processing, light etching, etc.  
         [0032]    Further alternative embodiments of the light guide plates  30 ,  40 ,  50 ,  60 ,  70 ,  80 ,  90  of the first through seventh embodiments can be configured. In particular, the emitting surfaces  32 ,  42 ,  52 ,  62 ,  72 ,  82  may have essentially concave profiles instead of essentially linear profiles. Further or alternatively, the bottom surfaces  33 ,  43 ,  53 ,  63 ,  73 ,  83  may have essentially concave profiles instead of essentially linear profiles.  
         [0033]    Referring to FIG. 8, a surface light source device  100  of the present invention includes the light guide plate  50 , two light sources  104  respectively adjacent to the incident surfaces  51  of the light guide plate  50 , and a shell-shaped reflector  105 . The reflector  105  substantially encloses the light sources  104 , the bottom surface  53  and opposite side surfaces of the light guide plate  50 , in order to reflect light beams from the light sources  104  to the light guide plate  110 . In particular, two opposite first parts (not labeled) of the reflector  105  reflect light beams from the light sources  104  to the incident surfaces  51 , and an intermediate second part (not labeled) of the reflector  105  reflects light beams to the bottom surface  53 . An inner surface  1051  of the reflector  105  which is opposite to the bottom surface  53  and the incident surfaces  51  has high reflectivity, between 90% and 99%. This is obtained by plating the inner surface  1051  with a highly reflective metal or oxide material by way of ion plating, sputtering or another suitable method. The metal or oxide material can be gold (Au), silver (Ag), or titanium oxide (TiO 2 ). Each light source  104  is a cold cathode fluorescent lamp or a light emitting diode.  
         [0034]    Similarly, any of the light guide plates  30 ,  40 ,  60 ,  70 ,  80 ,  90  of the first, second and fourth through seventh embodiments can be combined with the light sources  104  and the reflector  105  to provide a surface light source device similar to the surface light source device  100  described above.  
         [0035]    While the present invention has been described with reference to particular embodiments, the description is illustrative of the invention and is not to be construed as limiting the invention. Therefore, various modifications of the described embodiments can be made by those skilled in the art without departing from the true spirit and scope of the invention as defined by the appended claims.