Abstract:
A light guide plate comprises: a plate ember having a light transmitting capability; and a light incidence face provided on a side surface of the plate member; wherein the plate member changes a path of light emitted from a light source arranged to face the light incidence face and emits planar illumination light from an upper surface of the plate member onto an object to be illuminated; wherein an undulated surface portion is formed on the light incidence face. Further, the undulated surface portion is formed by smoothly connecting curved recessed portions and curved raised portions. This arrangement prevents bright lines from being produced near the light source and also prevents brightness variations, thus making uniform an intensity distribution of light that has entered the light guide plate.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     The application claims the priority benefit of Japanese Patent Application No. 2004-39086, filed on Feb. 16, 2004, and No. 2004-39087, filed on Feb. 16, 2004, the entire descriptions of which are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a light guide plate used for a planar light source that illuminates a liquid crystal panel from its back and more particularly to a light guide plate for a planar light source suitably applied to small liquid crystal panels used on cell phones.  
         [0004]     2. Description of Related Art  
         [0005]     A growing number of thin, easy-to-see liquid crystal displays having a backlight mechanism are being used as displays on small, thin information devices, such as notebook word processors or computers, cell phones and portable TV sets. Such a backlight mechanism uses a planar light source to illuminate an entire surface or a liquid crystal panel from its back. The planar light source generally comprises a light source, such as a fluorescent lamp or a light emitting diode (LED), and a light guide plate that converts a light flux into a planar light flux for illuminating the liquid crystal panel. As for the light source, an increasing number of planar light sources are using LEDs as the light source for further reductions in size and thickness and for increased longevity. These planar light sources may be classified into a direct type, in which the light source is arranged directly below the light guide plate, and a side light type, in which the light source is arranged at the sides of the light guide plate. For devices that put importance on small size and small thickness, such as cell phones, the side light type is usually adopted.  
         [0006]     Now, a conventional side light type planar light source will be explained by referring to  FIG. 1  and  FIG. 2 .  FIG. 1  shows an example of a conventional planar light source of this kind. It basically consists of a rectangular prism-shaped light guide plate  1  made of a transparent material and a light source  2  having three LEDs arranged at the side of the light guide plate  1 .  
         [0007]     The light guide plate  1  is arranged on the back of a liquid crystal panel  7 . The light guide plate  1  is often provided on its underside with a light reflection sheet  8  that directs light from the light source  2  toward the liquid crystal panel  7 , as shown in  FIG. 2B . On the upper surface side of the light guide plate  1  there are provided a diffusion sheet  9  that uniformly scatters light from the light source  2  and a prism sheet  10  that focuses light toward the liquid crystal panel  7 .  
         [0008]     The light guide plate  1  is a rectangular prism-shaped plate member capable of transmitting light, which is formed of, for example, a colorless, transparent plastic material. The upper surface of the light guide plate  1  is used as a light emitting face  1   b  and one of side surfaces of the light guide plate  1  is used as a light incidence face  1   a . The light source  2  is arranged at a position facing the light incidence face  1   a . A light beam  3  radiated from the light source  2  enters the light incidence face  1   a  and then is repetitively reflected in the light guide plate toward the light emitting face  1   b  as it travels in the light guide plate until those components of light incident on the upper surface at smaller than the critical angle are extracted from the light emitting face  1   b  of the light guide plate  1  as illumination light  4 . The illumination light  4  extracted outside then illuminates the liquid crystal panel  7  from the back.  
         [0009]      FIG. 2A  and  FIG. 2B  show another example of a conventional side light type planar light source (see Japanese Patent Disclosure No. 2003-262734, page 2 and FIG. 3). In the planar light source of this kind, one corner portion  1   d  of the almost rectangular prism-shaped light guide plate  1  is cut off to form an additional side surface as the light incidence face  1   a . A light source  2  made up of one LED is arranged at a position facing the light incidence face  1   a . As shown in  FIG. 2B , the upper surface of the light guide plate  1  constitutes a light emitting face  1   b , and a bottom surface  1   c  opposite the light emitting face  1   b  is used as a light reflection surface that is formed with a fine texture or a plurality of hemispherical dots to reflect the incoming light  5  toward the light emitting face  1   b.    
         [0010]     In the above conventional side light type planar light sources when the incident light  3  from the light source  2  enters the light guide plate  1  at an incidence angle a, as shown in  FIG. 3A  and  FIG. 3B , the light is refracted and travels in the light guide plate  1  at an angle b with respect to a normal, as indicated at  5 . Since the material of the light guide plate  1 , such as acrylic resin and polycarbonate resin, has a higher refractive index than that of air, the angle b with respect to the normal is smaller than the incidence angle a. At this time, the incident light  3  from the light source  2  has a directivity of the LED itself, so the directivity or the light  5 , which is refracted after it has entered the light guide plate  1 , is narrower than that of the incident light  3 .  
         [0011]      FIG. 4  shows directivities of the light  3  from the light source  2  and of the light  5  after it enters the light guide plate  1 .  FIG. 4A  shows a directivity of the incident light  3  from an LED as the light source  2  and  FIG. 4B  shows a directivity of the incoming light S that has entered the light guide plate  1  from the light incidence face  1   a . The light beam  3  of the LED as a point light source has a directivity indicated by a curve  101  in  FIG. 4A . The light  5 , which has resulted from the light  3  entering the light guide plate  1 , has a directivity indicated by a curve  102  in  FIG. 4B . As described above, the directivity of the light  5  after it has entered the light guide plate  1  is narrower than that of the incident light  3  of the LED itself. Thus, in the conventional light guide plate  1  which has the light incidence face  1   a  formed as a flat surface, there is a problem that a distribution of intensity of the light  5  after it enters the light guide plate  1  is ununiform.  
         [0012]     To solve this problem, a light guide plate has been proposed in which a light incidence face of the light guide plate is provided with undulations made by a plurality of prisms of similar shape (see Japanese Patent Disclosure No. 2002-196151, pages 3-5 and FIG. 2). A light guide plate  11  shown in  FIG. 5  and  FIG. 6  has the similar construction to that of the conventional light guide plate  1  except that a light incidence face  11   a  differs in shape from the counterpart of the conventional light guide plate  1 . So, only the construction of the light incidence face  11   a  will be explained and descriptions of other constructions omitted. This construction similarly applies also to other conventional light guide plates whose corners are cut off, so in the following explanation we take the light guide plate  11  of  FIG. 5  and  FIG. 6  as a representative conventional light guide plate.  
         [0013]     As shown in  FIG. 5A  and  FIG. 5B , the light guide plate  11  has a light incidence face  11   a  on one side which forms into undulations. The undulated surface portion has a uniform distribution of prismlike protrusions  12 . The prismlike protrusions  12  each have a triangular shape in cross section defined by a pair of inclined surfaces  12   a ,  12   b . Between the adjacent protrusions  12  there is a flat portion  13 .  
         [0014]     When the light incidence face  11   a  of the light guide plate  11  is taken as a virtual plane, the angle that light beams  15 ,  16  make with the normal after the beams have entered into the light guide plate  11  can be made larger than the angle they make in the case of the light guide plate  1 , by the effect of the inclined surfaces  12   a ,  12   b  of the prismlike protrusions  12  on the light incidence face  11   a , as shown in  FIG. 6 . This is true even for light beams whose incidence angle on the light incidence face  11   a  is large. Thus, when the light incidence face  11   a  is seen as a whole, a range of angle of the light beams  15 ,  17  that have entered into the light guide plate  11  from the prismlike protrusions  12  can be increased. Further, the light beam  16  that has entered the light guide plate  11  from the flat portion  13  enters straight into the light guide plate  11 , as in the case of the planar light incidence face  11   a  of the light guide plate  1 .  
         [0015]     However, since in the light guide plate  11  the light incidence face  11   a  formed by the prismlike protrusions  12  and the flat portion  12  is discontinuous in shape, as shown in  FIG. 6 , the directivities of light beams  15 ,  16 ,  17  that have entered into the light guide plate  11  from the light incidence face  11   a  are distorted as shown in  FIG. 7 . The light beam  16  that enters the light guide plate  11  from the flat portion  13  of the light guide plate  11  in  FIG. 6  has a directivity indicated by a curve  104  in  FIG. 7 . The light beam  17  that enters the light guide plate  11  from one  12   a  of the inclined surfaces  12  of the light guide plate  11  in  FIG. 6  has a directivity indicated by a curve  105  in  FIG. 7 . The light beam  15  that enters the light guide plate  11  from the other  12   b  of the prismlike protrusions  12  of the light guide plate  11  in  FIG. 6  has a directivity indicated by a curve  103  in  FIG. 7 .  
         [0016]     In the conventional light guide plate  11  as described above, the light beams  15 ,  16 ,  17  that enters the light guide plate  11  from the light incidence face  11   a  have distorted directivities, giving rise to a problem that the light intensity distribution becomes non-uniform because of the distorted characteristics of the directivities If the light guide plate  11  with an ununiform light intensity distribution is used in a side light type planar light source for a liquid crystal display, there is a problem that bright lines are produced from the light source or the brightness on the display screen varies, significantly degrading an image quality.  
       SUMMARY OF THE INVENTION  
       [0017]     An object of this invention is to provide a light guide plate for a planar light source that can solve the problems associated with the conventional light guide plates described above, prevent bright lines near the light source or brightness variations and make uniform an intensity distribution of light that has entered into the light guide plate, thereby improving a quality of display image.  
         [0018]     To achieve the above objective, the light guide plate of this invention comprises: a plate member having a light transmitting capability; and a light incidence face provided on a side surface of the plate member; wherein the plate member changes a path or light emitted from a light source arranged to face the light incidence face and emits planar illumination light from an upper surface of the plate member onto an object to be illuminated; wherein the light incidence face forms into an undulated surface portion.  
         [0019]     In one example, the plate member is quadrangular-shaped and has the light incidence face on one of side surfaces thereof. In another example, at least one of corner portions of the plate member is cut off to form an additional side surface, which is provided as the light incidence face.  
         [0020]     In another example, the undulated surface portion is formed by repetitively alternating curved recessed portions and curved raised portions in a longitudinal direction of the light incidence face. In particular, the undulated surface portion in formed by smoothly connecting the curved recessed portions and the curved raised portions.  
         [0021]     As described above, since one of its side surfaces is made the light incidence face which then forms into the undulated surface portion, the directivity of light that has entered the light guide plate can be corrected, providing a well-balanced characteristic without deviations. As a result, the intensity of light inside the light guide plate can be made uniform, preventing bright lines from being produced near the light source or eliminating brightness variations. This in turn allows a high quality side light type planar light source to be realized by the light guide plate of this invention and LEDs. Further, where the light guide plate of this invention is used as a side light type planar light source for a liquid crystal display, the liquid crystal display can produce a high quality image with reduced brightness variations. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]      FIG. 1  is a perspective view showing an essential portion of a conventional side light type planar light source.  
         [0023]      FIG. 2A  is a plan view showing an essential portion of another conventional side light type planar light source.  
         [0024]      FIG. 2B  is a cross-sectional view taken along the line C-C of  FIG. 2A .  
         [0025]      FIG. 3A  is a partly enlarged plan view showing a light incidence face and its surrounding area of the conventional light source.  
         [0026]      FIG. 3B  is a partly enlarged plan view showing a light incidence face and its surrounding area of another conventional light source.  
         [0027]      FIG. 4A  is a diagram showing a directivity of a light beam emitted from a light source according to a conventional art.  
         [0028]      FIG. 4B  is a diagram showing a directivity of a light beam that has entered into the light guide plate according to a conventional art.  
         [0029]      FIG. 5A  is a plan view of a conventional light guide plate.  
         [0030]      FIG. 5B  is a partly enlarged perspective view of part A in  FIG. 5A .  
         [0031]      FIG. 6  is a partly enlarged plan view showing a light incidence face and its surrounding area of the conventional light guide plate.  
         [0032]      FIG. 7  is a diagram showing a directivity of light that has entered into the conventional light guide plate.  
         [0033]      FIG. 8A  is a plan view showing a side light type planar light source using a light guide plate according to a first embodiment of this invention.  
         [0034]      FIG. 8B  is a cross-sectional view taken along the line A-A of  FIG. 8A .  
         [0035]      FIG. 9A  is an enlarged view of part B of  FIG. 8A .  
         [0036]      FIG. 9B  is a partly enlarged plan view showing a light incidence face of  FIG. 8A .  
         [0037]      FIG. 10  is a partly enlarged plan view showing a light incidence face of the light guide plate in the first embodiment of this invention.  
         [0038]      FIG. 11A  is a diagram showing a characteristic curve  106  representing a directivity of light that passes through the light guide plate in the first embodiment of this invention.  
         [0039]      FIG. 11B  is a diagram showing a characteristic curve  107  representing a directivity of light that has entered into the light guide plate in the first embodiment of this invention.  
         [0040]      FIG. 11C  is a diagram showing a characteristic curve  108  representing a directivity of light that has entered into the light guide plate in the first embodiment of this invention.  
         [0041]      FIG. 12A  is a plan view showing a side light type planar light source using a light guide plate according to a second embodiment of this invention.  
         [0042]      FIG. 12B  is a cross-sectional view taken along the line D-D of  FIG. 12A .  
         [0043]      FIG. 13A  is an enlarged view of part B in  FIG. 12A .  
         [0044]      FIG. 13B  is a partly enlarged perspective view of a light incidence face in  FIG. 12A . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0045]     Now, preferred embodiments of the light guide plate according to this invention will be described in detail by referring to the accompanying drawings.  FIG. 8A  and  FIG. 8B  show a side light type planar light source using the light guide plate according to a first embodiment of this invention. In this embodiment, constitutional elements identical with the corresponding elements in the conventional light guide plate are given like reference numbers and their detailed descriptions omitted.  
         [0046]     As shown in  FIG. 8A  and  FIG. 8B , the planar light source in this embodiment has a light guide plate  21  and a light source  2  made up of three light emitting diodes (LEDs). The light guide plate  21  is formed as an almost rectangular prism-shaped plate member that is made by injection-molding a light-transmitting plastic material, such as acrylic resin. The light guide plate  21  has its upper surface formed as a light emitting face  21   b . A bottom surface  21   c  of the light guide plate  21  opposite the light emitting face  21   b  is formed with a fine texture or a plurality of hemispherical dots to reflect the incoming light  26 , which has entered into the light guide plate  21 , toward the light emitting face  21   b . Thus, the bottom surface  21   c  constitutes a light reflection surface.  
         [0047]     One of side surfaces of the light guide plate  21  is a light incidence face  21   a . At a position facing the light incidence face  21   a  is arranged the light source  2 . As shown in  FIG. 8B , light emitted from the light source  2  enters the light guide plate  21  through the light incidence face  21   a  and is then repetitively reflected inside the light guide plate  21  toward the light emitting face  21   b  until those components of light incident on the upper surface at smaller than the critical angle are emitted from the light emitting face  21   b  of the light guide plate  21  as illumination light  25 . The illumination light  25  extracted outside then illuminates the liquid crystal panel  7  from the back. With the light emitting face  21   b  serving as a light illuminating surface (for the illumination light  25 ), the light guide plate  21  together with the light source  2  constitutes the side light type planar light source of this embodiment.  
         [0048]      FIG. 9A  and  FIG. 9B  show an enlarged view of part B in  FIG. 8A . In the figure, the light incidence face  21   a  of the light guide plate  21  forms into an undulated surface portion  20 . The undulated surface portion  20  in this example comprises raised portions  22  with a smooth curved surface and recessed portions  23  with a smooth curved surface, with the raised portions  22  and the recessed portions  23  alternately formed in a longitudinal direction of the light incidence face  21   a  of the light guide plate  21 . Because the raised portions  22  and the recessed portions  23  are smoothly connected or merged, the undulated surface portion  20  when viewed as a whole provides a smooth, continuous undulated surface. Further, since the raised portions  22  and the recessed portions  23  have the same curved geometry and extend in the thickness direction of the light guide plate  21 , they together form a smooth quadratic surface and therefore their transverse or horizontal cross section is shaped like a quadratic curve.  
         [0049]     Although there are no particular limiting conditions on this quadratic curve, a geometry in horizontal cross section of the raised portions  22  and the recessed portions  23  in this embodiment is defined by a quadratic curve formed by an elliptic arc of an ellipse  27  which is described by an expression x 2 /C 2 +y 2 /D 2 =1, where C is a radius of the ellipse in an x-axis direction, D is a radius in an y-axis direction and a center of the ellipse is located at an intersecting point between the x-axis and the y-axis, as shown in  FIG. 10 . That is, the ellipse  27  is divided by the y-axis passing through the center  24  thereof. A quadratic curve formed by one elliptic arc is used to define the geometry of the horizontal cross section of the raised portions  22 , and a quadratic curve formed by the other elliptic arc is used to define the geometry of the horizontal cross section of the recessed portions  23 . If the light incidence face  21   a  is macroscopically viewed as a plane, the raised portions  22  and the recessed portions  23  are formed such that the y-axis direction of the ellipse  27  is parallel to the plane. It is preferred that the radii C, D of the ellipse  27  be set in a range of 30-500 μm.  
         [0050]     As described above, since the light guide plate  21  of this embodiment has its light incidence face  21   a  formed in a continuous, undulated, smooth quadratic curve in cross section, the throwing of light from the light source  2  into the light guide plate  21  through the light incidence face  21   a  allows light  26  to be diffused inside the light guide plate  21 , making the intensity distribution of light  26  uniform. As a result, the directivity of light  26  is corrected, as indicated by a characteristic curve  106  of a smooth circle in  FIG. 11A , so that the directivity smoothly changes between portions of strong light intensity and weak light intensity. This arrangement can prevent bright lines or spots from being produced near the light source  2 . Characteristic curves  107 ,  108  in  FIG. 11B  and  FIG. 11C  represent a directivity of light  26  inside the light guide plate  21  when the geometries of the raised portions  22  and the recessed portions  23  are changed by changing a ratio of radii C, D of the ellipse  27 , or D/C. By changing the shape of the raised portions  22  and the recessed portions  23  as described above, the directivity of light  26  inside the light guide plate  21  can be controlled.  
         [0051]     Therefore, by changing the value of D/C, a ratio between the radii C, D of the ellipse  27 , according to the positional relation between the light guide plate  21  and the light source  2 , it is possible to optimize the geometry of the light incidence face  21   a  and thereby correct the directivity of light  26  that has entered the light guide plate  21  through the light incidence face  21   a , thus providing a well balanced directivity characteristic without undue deviations.  
         [0052]     As shown in  FIG. 8B , the light  26  that has entered the light guide plate  21  is repetitively reflected between the light emitting face  21   b  and the bottom surface  21   c  as it travels in the light guide plate  21  until those components of light incident on the upper surface at smaller than the critical angle are emitted from the light emitting face  21   b  as illumination light  25  to illuminate the liquid crystal panel  7  from the back. This arrangement makes it possible to reduce brightness variations on the screen of the liquid crystal panel  7  and thereby display high quality images.  
         [0053]      FIG. 12  and  FIG. 13  show a second embodiment of the light guide plate according to this invention. The light guide plate  21  of this embodiment is similar in construction to the light guide plate of the first embodiment except that one corner portion  21   d  of the almost rectangular prism-shaped plate member is cut off to form an additional side surface as the light incidence face  21   a . So, constitutional elements identical with the corresponding elements in the first embodiment are given like reference numbers and detailed descriptions of the light guide plate omitted.  
         [0054]     At a position facing the light incidence face  21   a  which is the cut-off corner portion of the light guide plate  21  is installed a light source  2  made up of one LED. As in the preceding embodiment, light emitted from the light source  2  enters the light guide plate  21  through the light incidence race  21   a  to become light  26 . The light  26  in the light guide plate  21  is then repetitively reflected inside the light guide plate  21  toward the light emitting face  21   b  until those components of light incident on the upper surface at smaller than the critical angle are emitted from the light emitting face  21   b  of the light guide plate  21  as illumination light  25 . The illumination light  25  emitted outside then illuminates the liquid crystal panel  7  from the back.  
         [0055]     The light incidence face  21   a  on the cut-off corner portion of the light guide plate is formed with an undulated surface portion  20  as in the preceding embodiment, as shown in  FIG. 13A  and  FIG. 13B . The undulated surface portion  20  comprises raised portions  22  with a smooth curved surface and recessed portions  23  with a smooth curved surface, with the raised portions  22  and the recessed portions  23  alternately formed in a longitudinal direction of the light incidence face  21   a  of the light guide plate  21 . Because the raised portions  22  and the recessed portions  23  are smoothly connected or merged, the undulated surface portion  20  when viewed as a whole provides a smooth, continuous undulated surface. Further, since the raised portions  22  and the recessed portions  23  have the same curved geometry and extend in the thickness direction of the light guide plate  21 , they together form a smooth quadratic surface and therefore their transverse or horizontal cross section is shaped like a quadratic curve. This construction is also similar to the preceding embodiment. Thus, the quadratic curve produces the similar effect to that of the preceding embodiment explained in  FIG. 10  and  FIG. 11  and therefore their explanations are omitted here.  
         [0056]     While in the first and second embodiment, our descriptions concern the almost rectangular prism-shaped light guide plate  1 , the present invention is not limited to this construction. The invention can also be applied to other than the rectangular prism-shaped light guide plate, such as polygonal light guide plates.  
         [0057]     Further, in the second embodiment the construction has been described in which one corner of the light guide plate is cut off to form a light incidence face and in which a light source made up of one LED is installed to face the light incidence face. This invention can also be applied to side light type planar light sources in which a plurality of light incidence faces are formed on two or more cut-off corner portions or in which a plurality of LEDs are arranged on the light incidence faces.  
         [0058]     In the first and second embodiment, the construction has been described in which the light incidence face, whose cross section is defined by a quadratic curve, forms into a smoothly connected or merged, undulated surface. This invention is not limited to this construction and may, for example, employ a construction in which the light incidence face forms into an undulated surface whose cross section is defined by a third- or fourth-degree curve.  
         [0059]     Further, in the above embodiments an example case has been described in which the raised portions and the recessed portions are directly connected. This invention is not limited to this construction and may be applied to a construction in which the raised portions and the recessed portions may be connected together with a smooth curved portion or a planar portion interposed in between.  
         [0060]     Further, in the above embodiments it is assumed that no surface treatment is performed on the light incidence face of the light guide plate. This invention may be applied to other construction in which the light incidence face forms into a rough surface to scatter light. This can further reduce brightness variations.  
         [0061]     Further, although in the above embodiments the light incidence face has been described to be formed by alternating the raised portions and the recessed portions of the same shape, it is also possible to form the raised portions and the recessed portions by combining a plurality of curved surfaces or to progressively change the geometry of the raised portions and the recessed portions as a point of interest moves away from the light source, according to the directivity of light emitted from the light source. This allows for a smoother change in directivity.  
         [0062]     Further, in the above embodiments, the construction has been described in which the light guide plate of this invention is applied to a planar light source of a liquid crystal display. This invention is not limited to this construction but may be applied to a wide range of side light type planar light sources used on a variety of illumination devices and displays.