Abstract:
A light guiding plate includes: an X axis direction light distribution control section; and a diffuse reflection section. The X axis direction light distribution control section performs light distribution control by which the incident light is deflected such that a tilt of an X axis direction with respect to a normal direction approaches to zero, and outputted from the light emitting surface. A direction of the incident light is the X axis direction, a direction orthogonal to the X axis direction and parallel to the light emitting surface is a Y axis direction, and a direction vertical to the X axis direction and the Y axis direction is the normal direction. The diffuse reflection section reflects the guided incident light from the light incident surface to diffuse in the Y axis direction inside of the light guiding plate and guides to the X axis direction light distribution control section.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS  
       [0001]     The present invention claims priority to its priority document No. 2003-372752 filed in the Japanese Patent Office on Oct. 31, 2003, the entire contents of which being incorporated by reference herein.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a light guiding plate used in a backlight apparatus for a liquid crystal display (LCD), and the like. Specifically, the present invention relates to a light guiding plate having improvements in front luminance of a light emitting surface, as well as a backlight apparatus provided with the aforesaid light guiding plate.  
         [0004]     2. Description of the Related Art  
         [0005]     Backlight units that serve as a backlight apparatus for liquid crystal display are in demand and produced in large quantities along with the spread of electronic equipment having a liquid crystal display thereon. The backlight units are mainly formed by a light source, and a light guiding plate for guiding and causing surface emission of light emitted from the light source.  
         [0006]     As the light source, fluorescent tubes, light emitting diodes (LED), or the like are used. Especially when mounted on electronic equipment with smaller and thinner form, the light emitting diode is often used.  
         [0007]     As shown in  FIG. 19 , a backlight unit  100 , which uses light emitting diodes as the light source, is generally provided with a light guiding plate  80 , a plurality of light emitting diodes such as three light emitting diodes  90   a,    90   b,  and  90   c,  a reflecting sheet  101 , a diffusing sheet  102 , a first lens sheet  103 , and a second lens sheet  104 .  
         [0008]     The backlight unit  100  is constructed as follows: the light emitting diodes  90   a,    90   b,  and  90   c  are disposed at a predetermine pitch on a light incident surface  81  of the light guiding plate  80  such that light emitting surfaces  91   a,    91   b,  and  91   c  are tightly attached thereto; the reflecting sheet  101  is disposed on the light reflecting surface  82  side of the light guiding plate  80 ; the diffusing sheet  102 , the first lens sheet  103  and the second lens sheet  104  are disposed by overlying them in order of mention on a light emitting surface  83  of the light guiding plate  80 ; and these are housed and assembled in a casing that is not shown. In the following description, the light emitting diodes  90   a,    90   b  and  90   c  are also generally referred to as a “light emitting diode  90 .” 
         [0009]     The light that is incident on the light incident surface  81  of the light guiding plate  80  from the light emitting diode  90  is guided while causing total reflection with the internal surfaces of the light reflecting surface  82  and the light emitting surface  83  in the light guiding plate  80 . On the light reflecting surface  82  of the light guiding plate  80 , there are formed a prism pattern for efficiently reflecting upward direction the incident light in the direction of the light emitting surface  83 , a dot pattern, and the like. The light incident in a critical angle onto the internal surface of the light emitting surface  83  by these patterns is then emitted from the light emitting surface  83 .  
         [0010]     The light emitted from the light emitting surface  83 , due to considerable variations in the distribution of in-plane light amount, is incident on the diffusing sheet  102  for purposes of homogenization.  
         [0011]     The light emitted from the diffusing sheet  102  is deflected so as to enter the first lens sheet  103  and the second lens sheet  104 , and condense in the normal direction of the light emitting surface  83 .  
         [0012]     The directional components of lights, which the first lens sheet  103  and the second lens sheet  104  condense the light passed through the diffusing sheet  102 , are different from each other.  
         [0013]     The first lens sheet  103  is also called Y direction lens sheet, which is a lens sheet to have the component of the light in the Y axis direction and condense in the normal direction of the light emitting surface  83 , when the light guiding plate  80  provided with the light emitting diode  90  is arranged on an X-Y coordinate plane such that the light emitting surface  83  becomes the front, as shown in  FIG. 20 . On the other hand, the second lens sheet  104  is also called an X direction lens sheet, which is a lens sheet to have the component of the light in the X axis direction shown in  FIG. 20  and condense in the normal direction of the light emitting surface  83 .  
         [0014]     Thus, the front luminance of the backlight unit  100  may be improved efficiently by having the light emitted from the light emitting surface  83  of the light guiding plate  80  and passed through the diffusing sheet  102  pass through the first lens sheet  103  and the second lens sheet  104 .  
         [0015]     The reflecting sheet  101  disposed on the light reflecting surface  82  side of the light guiding plate  80  functions to reflect the light escaped to the outside of the light reflecting surface  82  so as to bring back to the light guiding plate  80 , when the light guiding plate  80  guides the light incident from the light incident surface  81  while causing total reflection. For example, on of such related art is disclosed by Japanese Patent Application Publication No. 2003-35910.  
       SUMMARY OF THE INVENTION  
       [0016]     Although, the first lens sheet  103  and the second lens sheet  104 , which are disposed on the light guiding plate  80  shown in  FIG. 19 , have extremely strong condensing capability, and a dramatic improving effect of the front luminance may be anticipated, they are extremely expensive parts.  
         [0017]     Specifically, in cases where the first lens sheet  103  and the second lens sheet  104  are assembled to the backlight unit  100 , the first lens sheet  103  and the second lens sheet  104  occupy several tens of percentages of the total cost of the backlight unit  100 . There is a drawback that if a high front luminance is required, an increase in the cost is inevitable.  
         [0018]     Accordingly, the present invention is devised in view of such a drawback. It is desirable to provide a light guiding plate capable of causing a surface emission at a uniform luminance free from non-uniformity of luminance, without using any lens sheet, and also obtaining a high front luminance, as well as a backlight apparatus using the aforesaid light guiding plate.  
         [0019]     According to one embodiment of the present invention, there is provided a light guiding plate in which: one of side surfaces is taken as a light incident surface; an incident light from the light incident surface is totally reflected and guided with a light emitting surface that is one of main surfaces and a light reflecting surface that is the other main surface; and the light emitting surface provides surface emission. The light guiding plate includes an X axis direction light distribution control section and a diffuse reflection section. The X axis direction light distribution control section performs light distribution control by which the incident light from the light incident surface is deflected such that a tilt of an X axis direction with respect to a normal direction approaches to zero, and outputted from the light emitting surface, where a direction of the incident light is the X axis direction, a direction orthogonal to the X axis direction and parallel to the light emitting surface is a Y axis direction, and a direction vertical to the X axis direction and the Y axis direction is the normal direction. The diffuse reflection section reflects the guided incident light from the light incident surface so as to diffuse in the Y axis direction inside of the light guiding plate and guides to the X axis direction light distribution control section.  
         [0020]     According to another embodiment of the present invention, there is provided a backlight apparatus. The backlight apparatus includes: a light guiding plate in which: one of side surfaces is taken as a light incident surface, an incident light from the light incident surface is totally reflected and guided with a light emitting surface that is one of main surfaces and a light reflecting surface that is the other main surface, and the light emitting surface provides surface emission; a light source having a plurality of light emitting elements disposed in row arrangement with a predetermined interval in a lengthwise direction of the light incident surface of the light guiding plate; a reflecting sheet disposed on the light reflecting surface side of the light guiding plate; a diffusing sheet disposed on the light emitting surface side of the light guiding plate; a lens sheet disposed on the diffusing sheet; and a frame for holding the light source, the reflecting sheet, the diffusing sheet and the lens sheet in the light guiding plate. The light guiding plate includes an X axis direction light distribution control section and a diffuse reflection section. The X axis direction light distribution control section performs light distribution control by which the incident light from the light incident surface is deflected such that a tilt of an X axis direction with respect to a normal direction approaches to zero, and outputted from the light emitting surface, where a direction of the incident light is the X axis direction, a direction orthogonal to the X axis direction and parallel to the light emitting surface is a Y axis direction, and a direction vertical to the X axis direction and the Y axis direction is the normal direction. The diffuse reflection section reflects the guided incident light from the light incident surface so as to diffuse in the Y axis direction inside of the light guiding plate and guides to the X axis direction light distribution control section. The lens sheet performs light distribution control by which light emitted from the light emitting surface of the light guiding plate via the diffusing sheet is deflected such that a tilt of the Y axis direction with respected to the normal direction approaches zero, and outputted from the light emitting surface.  
         [0021]     The light guiding plate according to the embodiment of the present invention enables to perform a uniform surface emission free from non-uniformity of luminance because the diffuse reflection section reflects the light within the light guiding plate so as to diffuse in the Y axis direction inside of the light guiding plate and guide to the X axis direction light distribution control section, thereby the light emitted from the light emitting surface is diffused also in the Y axis direction.  
         [0022]     Further, by using the light guiding plate having the diffuse reflection section and the X axis direction light distribution control section, the backlight apparatus according to the embodiment of the present invention enables to perform a high luminance surface emission free from non-uniformity of luminance, by using, as the lens sheet, only a lens sheet that performs the Y direction light distribution control, because the X axis direction light distribution control, by which the light guided into the light guiding plate is emitted from the light emitting surface, may be performed without causing non-uniformity of luminance in the Y axis direction.  
         [0023]     Accordingly, the backlight apparatus of the embodiments enables to lessen the use of lens sheets, thereby allowing a considerable cost reduction and production of thinner apparatuses. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]     The above and other objects, features and advantages of the present invention will become more apparent from the following description of the presently exemplary embodiment of the present invention taken in conjunction with the accompanying drawings, in which:  
         [0025]      FIG. 1  is a diagram to explain the configuration of a light guiding plate, which is shown as one embodiment of the present invention;  
         [0026]      FIG. 2  is a diagram to explain an XY coordinate axis that is set to a light guiding plate;  
         [0027]      FIG. 3  is a diagram to explain a plurality of prisms formed on a light reflecting surface of a light guiding plate;  
         [0028]      FIG. 4  is a diagram to explain light emitted when only prisms are formed on a light reflecting surface in a light guiding plate;  
         [0029]      FIG. 5  is a diagram to explain disadvantages when only prisms are formed on a light reflecting surface in a light guiding plate;  
         [0030]      FIG. 6  is a diagram to explain a plurality of grooves formed on a light emitting surface in a light guiding plate;  
         [0031]      FIG. 7  is a diagram to explain operation of a plurality of grooves formed on a light emitting surface in a light guiding plate;  
         [0032]      FIG. 8  is a diagram schematically showing that incident light is diffused by a plurality of grooves formed on a light emitting surface in a light guiding plate;  
         [0033]      FIG. 9  is a diagram to explain a configuration of a backlight unit using a light guiding plate;  
         [0034]      FIG. 10  is a diagram showing one example of a cross-sectional shape of prisms formed on a light emitting surface of a light guiding plate;  
         [0035]      FIG. 11  is a diagram showing other example of a cross-sectional shape of prisms formed on a light reflecting surface of a light guiding plate;  
         [0036]      FIG. 12  is a diagram showing a still other example of a cross-sectional shape of prisms formed on a light reflecting surface of a light guiding plate;  
         [0037]      FIG. 13  is a diagram to explain a light guiding plate in which only a plurality of prisms are formed on a light reflecting surface, which was prepared as one embodiment;  
         [0038]      FIG. 14 ( a ) is a diagram showing an emission angle distribution of the luminance in the X axis direction of the light guiding plate shown in  FIG. 13 , and  FIG. 14 ( b ) is a diagram showing an emission angle distribution of the luminance in Y axis direction;  
         [0039]      FIG. 15  is a diagram to explain a light guiding plate prepared as one embodiment, in which a plurality of prisms are formed on a light reflecting surface, and a plurality of prisms are formed on a light emitting surface;  
         [0040]      FIG. 16 ( a ) is a diagram showing an emission angle distribution of the luminance in the X axis direction of the light guiding plate shown in  FIG. 15 , and  FIG. 16 ( b ) is a diagram showing an emission angle distribution of the luminance in Y axis direction;  
         [0041]      FIG. 17  is a diagram to explain a light guiding plate prepared as one embodiment, in which a plurality of prisms are formed on a light reflecting surface, and a plurality of grooves, whose pitch is different from that in  FIG. 15 , are formed on a light emitting surface;  
         [0042]      FIG. 18 ( a ) is a diagram showing an emission angle distribution of the luminance in X axis direction of a light guiding plate shown in  FIG. 17 , and  FIG. 18 ( b ) is a diagram showing an emission angle distribution of the luminance in Y axis direction;  
         [0043]      FIG. 19  is a diagram to explain a configuration of a backlight unit, which is shown as technique of related art; and  
         [0044]      FIG. 20  is a diagram to explain an XY coordinate axis set to a light guiding plate provided with a backlight unit. 
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0045]     A light guiding plate and a backlight apparatus in accordance with one embodiment of the present invention will be described in detail with reference to the drawings.  
         [0046]      FIG. 1  shows a light guiding plate  10 . For example, the light guiding plate  10  may be used for a backlight unit that is a backlight apparatus of a liquid crystal display.  
         [0047]     The light guiding plate  10  is obtained by injection molding with highly transparent material such as acrylic resin. Besides the acrylic resin, transparent thermoplastic resins such as methacrylic resin, styrene resin, polycarbonate resin may be employed as the material for the light guiding plate  10 .  
         [0048]     In the light guiding plate  10 , a light reflecting surface  12  that is one of the main surfaces of the light guiding plate  10 , and a light emitting surface  13  that is the other main surface parallel to and opposed to the light reflecting surface  12  perform guiding by use of total reflection of the light incident from a light incident surface  11 , and emit it as homogeneous light from the light emitting surface so as to cause surface emission. As described above, the light guiding plate  10  may be of such a shape that the light reflecting surface  12  and the light emitting surface  13  are parallel, or such a wedge shape that the thickness in the guiding direction of the guided light is gradually reduced.  
         [0049]      FIG. 2  is a diagram in which the light guiding direction is an X-axis direction, and the light guiding plate  10  is disposed on an XY coordinate plane such that the light emitting surface  13  becomes the front. Thus, when the light guiding plate  10  is disposed on the XY coordinate plane, the emission direction of the light from the light emitting surface  13  becomes a Z-axis direction.  
         [0050]     As a light source for emitting light incident from the light incident surface  13  of the light guiding plate  10 , there is used a light source provided with a plurality of light emitting diodes, such as a light source section  20  provided with light emitting diodes (LEDs)  21   a,    21   b  and  21   c  disposed in row at a predetermined interval, as shown in  FIG. 2 . The light emitting diodes  21   a,    21   b  and  21   c  provided in the light source section  20  are disposed as described above on a substrate that is not shown in the figure, and are arranged such that light emitting surfaces  22   a,    22   b  and  22   c  for emitting the light of the respective light emitting diodes are tightly attached to the light incident surface  11  of the light guiding plate  10 . In the following description, the light emitting diodes  21   a,    21   b  and  21   c  are also generally referred to as a “light emitting diode  21 ,” and their respective light emitting surfaces  22   a,    22   b  and  22   c  are also generally referred to as a “light emitting surface  22 .” The light emitting diode  21  is, for example, a chip type light emitting diode.  
         [0051]     As shown in  FIG. 1 , on the light reflecting surface  12 , a prism pattern composed of n (n is a natural number) prisms  12 An is formed in such a way that the prism can efficiently guides the light incident on and guided into the light guiding plate  10  upward in the direction of the light emitting surface  13 . The prism pattern of the prisms  12 An formed on the light reflecting surface  12  is structured in an injection molding die and formed at the time of injection molding of the light guiding plate  10 . Alternatively, the prisms  12 An may be formed by so-called direct cutting, by which the light guiding plate  10  is directly machined.  
         [0052]      FIG. 3  shows a sectional view when the light guiding plate  10  is cut along the line A-A shown in  FIG. 1 . As shown in  FIG. 3 , the prisms  12 An, whose cross-sectional shape is a triangle with a base at a side of the light reflecting surface  12 , is formed by making, on the light reflecting surface  12 , grooves whose base angle on the light incident surface  11  side of the aforesaid triangle is in the range of angle □=50° to 60°, and the other base angle is in an angle □=70° to 90°.  
         [0053]     For example, when performing injection molding of the light guiding plate  10 , a plurality of convex portions, whose cross-sectional shape is a triangle having base angles of the angle □=50° to 60°, and the angle □=70° to 90°, may be formed on the internal surface of a cavity of an injection molding die for forming the light reflecting surface  12 .  
         [0054]     Although in  FIG. 1  and  FIG. 3 , the pitch of the prisms  12 An formed on the light reflecting surface  12  is fixed, and there are shown regularly arranged prisms  12 An, they may be arranged so as to vary irregularly. Alternatively, it is also possible to vary the pitch for each adjacent prisms  12 An. Further, in  FIG. 3 , the height of the prisms  12 An, namely, the depth of grooves made on the light reflecting surface  12  is made constant for illustration. Without limiting to these examples, the present invention may also be applicable to various designs in which the pitch of the prisms  12 An and the height of the prisms  12 An that is determined by the depth of a groove made on the light reflecting surface  12  are arbitrary set, in order to effectively cause upward-reflection in the normal direction of the light emitting surface  13 .  
         [0055]     For example, when forming the prisms  12 An at a fixed pitch, the depth of a groove may be changed to vary the height of the prisms  12 An. When forming the prisms  12 An while changing pitches, the depth of a groove may be fixed to have a constant height of the prisms  12 An.  
         [0056]     As shown in  FIG. 3 , an incident light L 1  incident from the light incident surface  11  of the light guiding plate  10  enters the prism  12 An and is divided by a slope S of the prism  12 An into a deflected light L 2 , whose X axis direction component is deflected to the normal direction of the light emitting surface  13 , a transmitted light L 3  traveling in the X axis direction that is the light guiding direction, and a deflected light L 4  that emits to the outside of the light guiding plate  10 .  
         [0057]     The angle of deflection of the X axis direction component of the deflected light L 2  is determined by the angle □ and the angle □ of a plurality of grooves formed on the light reflecting surface  12  at the time of injection molding. As shown in  FIG. 3 , by setting the angel □=50° to 60°, and the angle □=70° to 90°, the X axis direction component of the deflected light L 2  is deflected to the direction of substantially normal to the light emitting surface  13 . The transmitted light L 3  enters other prism  12 An located at the subsequent stage in the light guiding direction. Although the deflected light L 4  is emitted to the outside of the light guiding plate  10 , it will be returned into the light guiding plate  10  by a reflecting sheet, which is employed when the light guiding plate  10  is configured as a backlight unit.  
         [0058]     Thus, by the prism pattern formed on the light reflecting surface  12  of the light guiding plate  10 , the X axis direction component of the light that is incident from the light incident surface  11  and guided into the light guiding plate  10  is subjected to light distribution control, and then deflected and guided upward to the normal direction of the light emitting surface  13  of the light guiding plate  10 . Specifically, letting the normal of a light emitting surface  30  be 0°, if grooves are made so as to have for example an angle □=58° and an angle □=90°, the light guided into the light guiding plate  10  is guided upward by the prism  12 An formed on the light reflecting surface  12 , in the range of about 0° to about 25° in the X axis direction.  
         [0059]     Meanwhile, the respective light emitting diodes  21  constituting the light source section  20  of the light guiding plate  10  function respectively as a point light source. Therefore the light emitted from each light emitting diode  21  enters the light guiding plate  10 , while holding directionality of radially spreading in the Y axis direction, as shown in  FIG. 4 . The light so entered becomes a deflected light, whose X axis direction component is guided upward to the normal direction of the light emitting surface  13 , as described above.  
         [0060]     At this time, for example, of the deflected lights L 2   a  to L 2   g,  deflected lights L 2   a  and L 2   b  or deflected lights L 2   f  and L 2   g,  which are lights incident and deflected on the prisms  12 An at a wide angle from the light emitting diode  21 , have the component guided upward to the vicinity of the normal of the light emitting surface  13  in the X axis direction, whereas in the Y axis direction, they become deflected lights having a component greatly tilted away from the direction of normal.  
         [0061]     Accordingly, the front luminance of the light emitting surface  13  made by such a deflected light is brought into non-uniform one that is high in a line-like region K extending in the X axis direction. For example, as shown in  FIG. 5 , if the light emitting surface  13  is observed from the front by having light incident on the light incident surface  11  from the three light emitting diodes  21   a,    21   b  and  21   c  provided in the light source section  20 , line-like regions Ka, Kb and Kc would appear in form of stripe.  
         [0062]     Thus, the light guiding plate  10 , in which the front luminance of light outputted from the light emitting surface  13  would be increased in the line-like regions, fails to exhibit sufficient function as a light guiding plate of the backlight unit of a liquid crystal display.  
         [0063]     The reason why in the light guiding plate  10 , the front luminance of the light emitting surface  13  causes high regions in the form of line is that the light incident by the light emitting diode  21  enters the prism  12 An with directionality. This may produce some light that can be deflected to the direction of normal by the prism  12 An in the X axis direction of the light emitting surface  13 , but cannot be deflected to the direction of normal in the Y axis direction.  
         [0064]     Hence, as shown in  FIG. 1 , grooves  13 Am (m is a natural number), which have a curved surface of an arbitrary radius of curvature R and extend in the X axis direction, are formed at a predetermined pitch and in m rows in the Y axis direction on the light emitting surface  13 , so that the light incident and guided into the light guiding plate  10  is reflected by random internal diffusion in the Y axis direction.  
         [0065]     Thus, the grooves  13 Am formed on the light emitting surface  13  become a cylindrical shape. Although in  FIG. 1 , all of the radius of curvature R of the grooves  13 Am, the depth of the grooves, and the pitch P are fixed for illustration, it should be noted that the present invention allows to design arbitrarily the radius of curvature R of the grooves  13 Am, the depth of grooves, and the pitch P, in order to effectively perform the internal diffusion in the Y axis direction.  
         [0066]     Alternatively, it may also be possible to configure the light guiding plate of the present embodiment at irregular intervals or regularly changing intervals, in the following manner that the radius of curvature R and the depth of grooves are changed per groove  13 Am, while fixing the pitch P, or that the pitch P is changed per adjacent groove  13 Am while fixing the radius of curvature R and the depth of grooves.  
         [0067]     When the light guiding plate  10  is cut by the line B-B shown in  FIG. 1 , the cross section of the grooves  13 Am is as shown in  FIG. 6 . The radius of curvature R of the grooves  13 Am and the pitch P can be designed as follows: the radius of curvature R=0.1 mm, and the pitch P=0.15 mm, for example.  
         [0068]     The cylindrical shape pattern of the grooves  13 Am formed on the light emitting surface  13  is formed in an injection molding die and formed at the time of injection molding of the light guiding plate  10 . Alternatively, the grooves  13 Am may be formed by so-called direct cutting, by which the light guiding plate  10  is directly machined.  
         [0069]     In the case of performing injection molding of the light guiding plate  10 , the prisms  12 An formed on the aforesaid light reflecting surface  12 , and the cylindrical shape grooves  13 Am formed on the light emitting surface  13  are integrally molded on the light guiding plate  10  at the same time.  
         [0070]     Thus, by forming the grooves  13 Am in m rows on the light emitting surface  13  of the light guiding plate  10 , the light incident on the internal surface of the light emitting surface  13  at an angle not less than the critical angle at which the light is guided within the light guiding plate  10 , hits any location on the curved surfaces of the grooves  13 Am, as shown in  FIG. 7 , and becomes random because a plurality of the grooves  13 Am are formed at regular intervals or irregular intervals. As a result, the reflected light is spread and reflected in the Y axis direction of the light reflecting surface  12 . The light reflected by the curved surfaces of the grooves  13 Am is incident on the prisms  12 An formed on the light reflecting surface  12 , and then guided upward such that the component in the X axis direction becomes the normal direction of the light emitting surface  13 , as described above.  
         [0071]     The light reflected by the curved surfaces of the grooves  13 Am and guided upward by the prisms  12 An is also spread in the Y axis direction, as shown in  FIG. 8 , and the front luminance of the light emitted from the light emitting surface  13  can be homogenized over the entire surface of the light emitting surface  13 . Therefore, there is the effect of diffusing emission lines that appear in the form of stripe, as described above by using  FIG. 4  and  FIG. 5 .  
         [0072]     The light incident on the curved surface portions of the grooves  13 Am within the light guiding plate  10  is one that the light emitted from the light emitting diodes  21  is directly incident thereon, and light whose X axis direction component is guided upward in the normal direction of the light emitting surface  13  by the prisms  12 An. In either light, the grooves  13 Am have the effect of diffusing line-like emission lines that occur in the form of stripe.  
         [0073]     Thus, by forming the grooves  13 Am on the light emitting surface  13  of the light guiding plate  10 , it is able to suppress non-uniformity of luminance in which the light guided upward by the prisms  12 An formed on the light reflecting surface  12  of the light guiding plate  10  appears as emission lines in the form of stripe, if a plurality of the light emitting diodes  21  that are point light sources are employed as the light source section  20 .  
         [0074]     For example, as described above, when a plurality of grooves, whose cross-sectional shape is a triangle with the light reflecting surface  12  side as a base, wherein one of the base angles on the light incident surface  11  side of the aforesaid triangle is an angle □=50° to 60°, and the other base angle is an angle □=70° to 90°, are made on the light reflecting surface  12  so as to form the prisms  12 An on the light reflecting surface  12 , the upward-reflection of the light in the X axis direction is performed effectively, so that emission lines in the form of stripe notably appear, which is especially effective.  
         [0075]     Even when forming a plurality of grooves whose cross-sectional shape is a triangle having an angle ε=45° and an angle □=45°, which is generally employed as a conventional technique, forming the grooves  13 Am on the light emitting surface  13  of the light guiding plate  10  enables to suppress non-uniformity of luminance that appears as emission lines in the form of stripe when the light emitting diodes  21  are used as a point light source.  
         [0076]     A backlight unit  40  provided with the above-mentioned light guiding plate  10  and the light source section  20  is shown in  FIG. 9 . In the backlight unit  40 , the light source section  20  is disposed on a light incident surface  11  side of the light guiding plate  10 , and a reflecting sheet  31  is disposed on a light reflecting surface  12  side of the light guiding plate  10 , and a diffusing sheet  32  and a lens sheet  33  are disposed by overlying them in order of mention. The light guiding plate  10 , the light source section  20 , the reflecting sheet  31 , the diffusing sheet  32 , the lens sheet  33 , which are so disposed, are assembled securely in frames  41  and  42 , each serving as a casing of the backlight unit  40 .  
         [0077]     In this backlight unit  40 , the light emitted from the respective light emitting diodes  21  possessed by the light source section  20  is incident and guided from the light incident surface  11  of the light guiding plate  10 , and reflected by the light reflecting surface  12  and the reflecting sheet  31 , and then emitted from the light emitting surface  13 .  
         [0078]     At this time, in the light reflecting surface  12  within the light guiding plate  10 , the X axis direction component of the light incident on the prisms  12 An is subjected to light distribution control, and guided upward and condensed in the normal direction of the light emitting surface  13 . The light guided within the light guiding plate  10  is reflected due to internal diffusion in the Y axis direction at the curved surface portions of the grooves  13 Am formed on the light emitting surface  13 , and then emitted to the prisms  12 An. In the light emitted by the prisms  12 An, the X axis direction component is guided upward and condensed in the normal direction of the light emitting surface  13  as described above.  
         [0079]     Thus, within the light guiding plate  10 , the upward reflection in the X axis direction by the prisms  12 An, and the internal diffusion by the grooves  13 Am are repeated to guide light, and the light is emitted from the light emitting surface  13 . The light emitted from the light emitting surface  13  is diffused into homogeneous light by the diffusing sheet  32 , and then subjected to light distribution control by the lens sheet  33 , so that the Y axis direction component of the light is condensed in the normal direction of the light emitting surface  13 . The light emitted from the lens sheet  33  is irradiated through an opening portion  42 A opened in the frame  42  to a liquid crystal display, for example.  
         [0080]     Thus, the backlight unit  40  does not require the second lens sheet  104  for condensing the component of the X axis direction light in the normal direction of the light emitting surface  83 , which is possessed by the backlight unit  100  illustrated for example in the conventional technique. It is therefore able to considerably reduce the cost and also thin the aforesaid backlight unit  40 .  
         [0081]     The reason for this is as follows. The upward reflection in the X axis direction of guided light becomes sufficient to obtain substantially the same front luminance as in the case of using a lens sheet for condensing in the X axis direction, by making light distribution control under such an appropriate design that a cross-sectional shape of grooves made for forming the prisms  12 An on the light reflecting surface  12  of the light guiding plate  10  becomes a triangle in which, letting the light reflecting surface  12  side be a base, the base angle on the light incident surface  11  side is an angle □=50° to 60°, and the other base angle is an angle □=70° to 90°. Thereby, the backlight unit  40  is able to obtain sufficient front luminance over the entire surface of the light emitting surface  13 . Thus, when the light guiding plate  10  is configured as the back light unit  40 , the light emitted from the light reflecting surface  12  to the outside of the light guiding plate  10  is reflected by the reflecting sheet  31  and returned to the inside of the light guiding plate  10 . The directivity of the light emitting diodes  21  is reflected so that the light reflected by the reflecting sheet  31  has directivity.  
         [0082]     However, as described above, the light guided within the light guiding plate  10  is subjected to interval diffusion at the curved surface portions of the grooves  13 Am formed on the light emitting surface  13  within the light guiding plate  10 , so that the directivity thereof is suppressed and becomes light random in the Y axis direction and then enters the prisms  12 An. Therefore, the light, which is emitted to the outside of the light guiding plate  10  and reflected by the reflecting sheet  31  and then again incident on the light guiding plate  10 , is similarly subjected to internal diffusion and used effectively.  
         [0083]     The prism pattern formed on the light reflecting surface  12  may be any shape other than that of the prisms  12 An, and the present invention is not limited to this prism pattern.  
         [0084]     For example, when the prisms  12 An are formed on the light reflecting surface  12  by making grooves whose cross-sectional shape is a triangle, a prism pattern may be formed by changing the values of the angle □ and the angle □ to a value other than the angle □=50° to 60° and the angle □=70° to 90°. Alternatively, there may be used a prism pattern formed by making grooves having such a cross-sectional shape as described below.  
         [0085]     For example, a prism pattern may be used which is formed by employing, as the cross-sectional shape of grooves made on the light reflecting surface  12 , a rounded trapezoid-like cross-sectional shape  45  as shown in  FIG. 10 , or a cross-sectional shape  46  composed of two triangles as shown in  FIG. 11 , or a trapezoid-like cross-sectional shape  47 , one side of which has arbitrary irregularities as shown in  FIG. 12 .  
         [0086]     Alternatively, a prism may be formed at the portions of the light incident surface  11  of the light guiding plate  10  at which light emitting surfaces  22  of the respective light emitting diodes  21  make contact. For example, a prism pattern may be formed by making a plurality of grooves, whose cross-sectional shape is a triangle with having its lengthwise direction in the Z axis direction of the light guiding plate  10 , at the above-mentioned portions of the light incident surface  11 , at the time of injection molding of the light guiding plate  10 , or by direct cutting. Accordingly, the intensity distribution of the light incident from the respective light emitting diodes  21  can be controlled, and the surface emission, which is homogeneous free from non-uniformity of luminance, can intentionally be changed to make correction.  
       EXAMPLES  
       [0087]     Next, in order to verify effects of the internal diffusion in the Y axis direction by virtue of the grooves  13 Am formed on the light emitting surface  13  of the above-mentioned light guiding plate  10 , there were prepared, as examples of the present invention, a light guiding plate in which prisms are formed by making, on a light reflecting surface, grooves with having a lengthwise direction in the Y axis direction, the cross-sectional shape of the grooves being a triangle with a base in the light reflecting surface side, wherein one of the base angles on a light incident surface side is an angle □=58°, and the other base angle is an angle □=90°; and a light guiding plate in which prisms are formed by making grooves having the same cross-sectional shape, and a plurality of grooves having a radius of curvature R are formed at a predetermined pitch P on a light emitting surface. The X axis direction luminance and the Y axis direction luminance of the respective light guiding plates were measured to determine an emission angle distribution.  
         [0000]     EXPERIMENT 1  
         [0088]     First, as EXPERIMENT 1, there was prepared a light guiding plate in which prisms were formed by making, on a light reflecting surface, grooves whose cross-sectional shape was a triangle with a base in a light reflecting surface side, wherein one of the base angles on a light incident surface side was an angle □=58°, and the other base angle was an angle □=90°. Then, an emission angle distribution was measured from a luminance measurement.  
         [0089]     As shown in  FIG. 13 , in a light guiding plate  50  prepared by injection molding with an injection molding die, prisms  52 An were formed by making n (n is a natural number) grooves, whose cross-sectional shape was a triangle, on the light reflecting surface  52 . The cross-sectional shape of the grooves made for forming the prisms  52 An was a triangle with a base in a light reflecting surface side  52  side, wherein one of the base angles on a light incident surface  51  side was an angle □=58°, and the other base angle was an angle □=90°.  
         [0090]     The light from a light source (not shown) composed of three light emitting diodes disposed at a predetermined pitch was guided through the light incident surface  51  of the light guiding plate  50 , and the X axis direction luminance of the light emitted from the light emitting surface  53 , and the Y axis direction luminance were measured to find the emission angle distribution. Letting the measuring point be a central position of the light emitting surface  53 , on the X axis, the light incident surface  51  side is a negative direction, and the light guide direction is a positive direction, whereas on the Y axis, the right side with respect to the light guide direction is a positive direction, and the left side is a negative direction.  
         [0091]     The emission angle distribution in the X axis direction of the light guiding plate  50 , and the emission angle distribution in the Y axis direction are shown in FIGS.  14 ( a ) and  14 ( b ), respectively. The symbol 0° shown in FIGS.  14 ( a ) and  14 ( b ) indicates the normal direction of the light emitting surface  53 . For example, if the luminance distribution is concentrated in the vicinity of 0° in each of FIGS.  14 ( a ) and  14 ( b ), it can be said that both of the X axis direction component and the Y axis direction component in the light emitted from the light emitting surface  53  are condensed to the normal of the light emitting surface  53 .  
         [0092]     As shown in  FIG. 14 ( a ), the distribution in the vicinity of −20° to 0° is high in the X axis direction. This shows that the light was condensed in the normal direction of the light emitting surface  53  in the X axis direction.  
         [0093]     As shown in  FIG. 14 ( b ), the distribution is increased in the form of peak, around −40°, around 0°, and around 33° in the Y axis direction. This shows that in the Y axis direction, the directivity of the three light emitting diodes (not shown) was directly reflected to cause non-uniformity of luminance in the light emitted from the light emitting surface  53 .  
         [0094]     Next, a description will be given of a light guiding plate in which prisms are formed by making, on a light reflecting surface, grooves whose cross-sectional shape was a triangle with a base in the light reflecting surface side, wherein one of the base angles on a light incident surface side is an angle □=58°, and the other base angle is an angle □=90°, and a plurality of grooves having a radius of curvature R were formed at a predetermined pitch P on the light emitting surface. As a light guiding plate used in the experiments, two types were prepared by fixing the radius of curvature R of a plurality of grooves formed on a light emitting surface and changing the pitch P. The experiments for finding the respective emission angle distributions of two light guiding plates obtained by changing the pitch P of the plurality of grooves formed on a light emitting surface are referred to as “EXPERIMENT 2” and “EXPERIMENT 3,” respectively.  
         [0000]     EXPERIMENT 2  
         [0095]     As shown in  FIG. 15 , a light guiding plate  60  prepared by injection molding with an injection molding die has the same size as the light guiding plate  50 , and prisms  62 An are formed by making the same grooves as in the light guiding plate  50 , on a light reflecting surface  62 . Further, grooves  63 Am having a radius of curvature R=0.1 mm, in m (m is a natural number) rows were formed at intervals of pitch P=0.1 mm on a light emitting surface  63  of the light guiding plate  60 .  
         [0096]     The light from a light source (not shown) composed of three light emitting diodes disposed at a predetermined pitch was guided through a light incident surface  61  of the light guiding plate  60 , and the X axis direction luminance of the light emitted from the light emitting surface  63 , and the Y axis direction luminance were measured to find an emission angle distribution. Like the case of the light guiding plate  50 , letting the measuring point be a central position of the light emitting surface  63 , on the X axis, the light incident surface  61  side is a negative direction, and the light guide direction is a positive direction, whereas on the Y axis, the right side with respect to the light guide direction is a positive direction, and the left side is a negative direction.  
         [0097]     The emission angle distribution in the X axis direction of the light guiding plate  60 , and the emission angle distribution in the Y axis direction are shown in FIGS.  16 ( a ) and  16 ( b ), respectively. The symbol 0° shown in FIGS.  16 ( a ) and  16 ( b ) indicates the normal of the light emitting surface  63 . For example, if the luminance distribution is concentrated in the vicinity of 0° in each of FIGS.  16 ( a ) and  16 ( b ), it can be said that both of the X axis direction component and the Y axis direction component in the light emitted from the light emitting surface  63  are condensed to the normal of the light emitting surface  63 .  
         [0098]     As shown in  FIG. 16 ( a ), the distribution in the vicinity of −20° to 0° is high in the X axis direction, as in the case of the light guiding plate  50 . This shows that the light is condensed in the normal direction of the light emitting surface  63  in the X axis direction.  
         [0099]     As shown in  FIG. 16 ( b ), the distribution in the vicinity of −60° to 50° is high in the Y axis direction. Especially, the luminance level is stable in the vicinity of −30° to 30°. This clearly shows the effect of the formation of the plurality of the grooves  63 Am, and shows that at the curved surface portions of the grooves  63 Am within the light guiding plate  60 , the light guided within the light guiding plate  60  is reflected so as to cause internal diffusion in the Y axis direction, and incident on the prisms  62 An, and then emitted from the light emitting surface  63 .  
         [0100]     Therefore, this indicates that in the light guiding plate  60 , the non-uniformity of luminance in the Y axis direction, which was observed in the light guiding plate  50 , is completely eliminated, and the light luminance in the Y axis direction is homogenized and emitted from the light emitting surface  63 .  
         [0000]     EXPERIMENT 3  
         [0101]     As shown in  FIG. 17 , a light guiding plate  60  prepared by injection molding with an injection molding die has the same size as the light guiding plate  70 , and prisms  72 An are formed by making the same groove as in the light guiding plate  50 , on a light reflecting surface  72 . Further, grooves  73 Am having a radius of curvature R=0.1 mm, in m (m is a natural number) rows are formed at intervals of pitch P=0.15 mm on a light emitting surface  73  of the light guiding plate  70 .  
         [0102]     The light from a light source (not shown) composed of three light emitting diodes disposed at a predetermined pitch was guided through a light incident surface  71  of the light guiding plate  70 , and the X axis direction luminance of the light emitted from the light emitting surface  73 , and the Y axis direction luminance were measured to find an emission angle distribution. Like the case of the light guiding plate  50 , letting the measuring point be a central position of the light emitting surface  73 , on the X axis, the light incident surface  71  side is a negative direction, and the light guide direction is a positive direction, whereas on the Y axis, the right side with respect to the light guide direction is a positive direction, and the left side is a negative direction.  
         [0103]     The emission angle distribution in the X axis direction of the light guiding plate  70 , and the emission angle distribution in the Y axis direction are shown in FIGS.  18 ( a ) and  18 ( b ), respectively. The symbol 0° shown in FIGS.  18 ( a ) and  18 ( b ) indicates the normal of the light emitting surface  73 . For example, if the luminance distribution is concentrated in the vicinity of 0° in each of FIGS.  18 ( a ) and  18 ( b ), it can be said that both of the X axis direction component and the Y axis direction component in the light emitted from the light emitting surface  73  are condensed to the normal of the light emitting surface  73 .  
         [0104]     As shown in  FIG. 18 ( a ), the distribution in the vicinity of −20° to 0° is high in the X axis direction, as in the case of the light guiding plate  50 . This shows that the light is condensed in the normal direction of the light emitting surface  73  in the X axis direction.  
         [0105]     As shown in  FIG. 18 ( b ), the distribution in the vicinity of −60° to 60° is high in the Y axis direction. Especially, the luminance level is: stable in the vicinity of −20° to 10°, and the luminance is far higher than that in the light guiding plate  60 .  
         [0106]     This clearly shows the effect of the formation of the plurality of the grooves  73 Am, and shows that at the curved surface portions of the grooves  73 Am within the light guiding plate  70 , the light guided within the light guiding plate  70  is reflected so as to cause internal diffusion in the Y axis direction, and incident on the prisms  72 An, and then emitted from the light emitting surface  73 .  
         [0107]     Therefore, this indicates that in the light guiding plate  70 , the non-uniformity of luminance in the Y axis direction, which was observed in the light guiding plate  50 , is completely eliminated, and the light luminance in the Y axis direction is homogenized and emitted from the light emitting surface  63 . Additionally, in the light guiding plate  70 , the luminance in the Y axis direction is extremely higher than the light guiding plate  60  having improvements in the non-uniformity of luminance of the light guiding plate  50 . This indicates that the Y axis direction component of the light emitted from the light emitting surface  73  has a high luminance.  
         [0108]     It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and alterations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof.