Abstract:
A light guide plate has a light introducing portion configured to confine light that is incident from an end surface, and a light guide-plate main body configured to emit incident light to an outside from a light emitting surface and provided continuously with the light introducing portion in a thickness smaller than a maximum thickness of the light introducing portion. The light introducing portion has an inclined surface inclined toward the light guide-plate main body from a surface comprising a larger thickness than a thickness of the light guide-plate main body, on at least one of a surface at a light emission side and a surface opposite to the surface at the light emission side. The inclined surface has a main-inclined surface configured to be positioned in front of a light source arranged opposite to the end surface.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to Japanese Patent Application No. 2012-283623 filed on Dec. 26, 2012, the entire contents of which are incorporated by reference herein. 
     BACKGROUND 
     1. Field 
     The present invention relates to a light guide plate and a surface light source device, and specifically relates to a surface light source device for efficiently making light incident to a light guide plate having a smaller thickness than a height of a light source, and to a light guide plate, and the like. 
     2. Related Art 
     In recent years, with a reduction in a thickness of a mobile device that has a surface light source device, a reduction in a thickness of the surface light source device is also being required. To reduce the thickness of the surface light source device, it becomes necessary to reduce a thickness of a light guide plate. However, even when the thickness of a planar light guide plate can be reduced, there is a limit to a reduction in a height of a light source composed of an LED. Therefore, when a thin planar light guide plate is used, the height of the light source becomes larger than a thickness of an end surface (a light-incident end surface) of the light guide plate, and the light source arranged opposite to the light-incident end surface of the light guide plate is protruded to above an upper surface of the light guide plate. When the light source is protruded to above the light guide plate, all of light emitted from the light source does not enter the light incident end face of the light guide plate but a part of the light leaks to the outside of the light guide plate, and light utilization efficiency becomes poor. 
       FIG. 1A  is a perspective view schematically illustrating a conventional surface light source device. A surface light source device  11  in  FIG. 1A  is configured by a light source  12  and a light guide plate  13 , and the light source  12  is arranged opposite to a light-incident end surface  19  of the light guide plate  13 . The light guide plate  13  is provided by integrally forming a light guide-plate main body  15  having substantially a uniform thickness and a light introducing portion  14  having a wedge shape. An inclined surface  16  is formed on an upper surface of the light introducing portion  14 , and a directional transform pattern  17  (light-leakage prevention patterns) formed of a plurality of V-grooves  18  is formed on the inclined surface  16 . A thickness of an end surface (the light-incident end surface  19 ) of the light introducing portion  14  is larger than a height of the light source  12 . As a surface light source device that uses a light guide plate having such a structure, there is a one disclosed in Japanese Unexamined Patent Publication No. 2006-171253, for example. 
     In the surface light source device  11 , light emitted from the light source  12  is incident from the light-incident end surface  19  into the light introducing portion  14 . Since the thickness of the end surface (the light-incident end surface  19 ) of the light introducing portion  14  is larger than the height of the light source  12 , the light emitted from the light source  12  is efficiently taken into the light introducing portion  14 . The light incident to the light introducing portion  14  is reflected by the upper surface (the inclined surface  16 ) or a lower surface of the light introducing portion  14 , is guided to the light guide-plate main body  15 , and is emitted to an outside from a light emitting surface  20  of the light guide-plate main body  15 . 
     In this case, when the inclined surface  16  is flat, there is a risk that a part of the light that is incident to the light introducing portion  14  leaks to an outside by being transmitted through the inclined surface  16 , without being reflected by the inclined surface  16 . Therefore, in the surface light source device  11 , light leakage from the inclined surface  16  is reduced by providing the directional transform pattern  17  on the inclined surface  16 . That is, in the surface light source device  11 , because the V-grooves  18  as shown in  FIG. 1B  are provided on the inclined surface  16 , an incidence angle of the light incident to the V-grooves  18  becomes large like light L indicated by solid line arrows, and the light L is reflected by the V-grooves  18  and does not easily leak from the inclined surface  16 . As a result, according to the surface light source device  11  in such a structure, it becomes possible to improve light utilization efficiency and reduce the thickness of the surface light source device. 
     However, even when the directional transform pattern  17  formed of the plurality of V-grooves  18  is provided on the inclined surface  16 , light leakage from the inclined surface  16  occurs as shown in  FIG. 1B  (of the light L that is incident to the light introducing portion  14 , the light that leaks to an outside is indicated by broken-line arrows).  FIG. 1B  is a schematic view illustrating behaviors of the light L in high light intensity that is emitted from a light emission center  12   a . Of the light emitted from the light emission center  12   a , the light L emitted substantially forward from the light emission center  12   a  is reflected from both a groove slant surface  18   a  that faces a center surface C (a flat surface that passes through the light emission center C of the light source and is perpendicular to a light-incident end surface  19  and a light emitting surface  20  is referred to as the center surface C) and a groove slant surface  18   b  that faces opposite to the center surface C as indicated by solid-line arrows, and is not easy to leak from the inclined surface  16 . On the other hand, in the surface light source device  11 , the directional transform pattern  17  has V-grooves of the same shapes having bilaterally symmetrical cross-sectional shapes that are repeatedly arranged, as shown in  FIG. 1B . Therefore, the light L obliquely emitted from the light emission center  12   a  becomes easy to leak to an outside as indicated by the broken-line arrows. That is, when a position where the light is incident to the directional transform pattern  17  becomes far from the center surface C, the light L becomes incident to the groove slant surface  18   b , on which a normal line faces opposite to the center surface C, at an angle gradually nearer perpendicular to the groove slant surface  18   b . Accordingly, the light L easily leaks from the groove slant surface  18   b.    
     SUMMARY 
     One or more embodiments of the present invention makes light difficult to leak from directional transform patterns formed on inclined surfaces, in a surface light source device having the inclined surfaces at a light introducing portion of a light guide plate. 
     A light guide plate according to one or more embodiments of the present invention includes a light introducing portion configured to confine light that is incident from an end surface, and a light guide-plate main body configured to make incident light emit to an outside from a light emitting surface and provided continuously with the light introducing portion in a thickness smaller than a maximum thickness of the light introducing portion. The light introducing portion includes an inclined surface inclined toward the light guide-plate main body from a surface having a larger thickness than a thickness of the light guide-plate main body, on at least one of a surface at a light emission side and a surface opposite to the surface at the light emission side. The inclined surface includes a main-inclined surface positioned in front of a light source arranged opposite to the end surface, the main-inclined surface having an average direction of normal lines oriented to a direction perpendicular to the end surface when viewed from a thickness direction, and side-inclined surfaces positioned at both sides of the main-inclined surface, each side-inclined surface having an average direction of normal lines inclined to the main inclined surface when viewed from the thickness direction. Directional transform patterns are formed on the main-inclined surface and on the side-inclined surfaces at both sides of the main-inclined surface. According to one or more embodiments of the present invention, the side-inclined surfaces are formed continuously with the main-inclined surface. According to one or more embodiments of the present invention, the directional transform patterns are configured as a layout of pattern elements that linearly extend. 
     In a light guide plate according to one or more embodiments of the present invention, the side-inclined surfaces on which the average directions of normal lines are inclined to the main inclined surface when viewed from a thickness direction are provided at both sides of the main-inclined surface, and the directional transform patterns are formed on the main-inclined surface and on the side-inclined surfaces at both sides of the main-inclined surface. Therefore, light leakage from the inclined surface near a side surface of the light guide plate can be reduced. 
     In a light guide plate according to one or more embodiments of the present invention, the pattern elements that configure the directional transform patterns formed on the side-inclined surfaces have bilaterally asymmetrical cross-sectional shapes. According to one or more embodiments of the present invention, the directional transform patterns of the side-inclined surfaces are formed such that a ridge line and a valley line are alternately arranged and that a slant surface is formed between the ridge line and the valley line, and that, of adjacent ones of the slant surfaces, a slant surface on which an outward normal line is further oriented to a center of the light guide plate than a direction perpendicular to the pattern-element layout direction has a smaller width parallel to a pattern-element layout direction than that of a slant surface on which an outward normal line is oriented to an outer side of the light guide plate than a direction perpendicular to the pattern-element layout direction. According to the above, light leakage from the inclined surface near the side surface of the light guide plate can be further reduced. 
     In a light guide plate according to one or more embodiments of the present invention, the pattern elements that configure the directional transform pattern formed on the main-inclined surface have bilaterally asymmetrical cross-sectional shapes. According to one or more embodiments of the present invention, the directional transform pattern of the main-inclined surface is formed such that a ridge line and a valley line are alternately arranged and that a slant surface is formed between the ridge line and the valley line, and that, of adjacent ones of the slant surfaces, a slant surface on which an outward normal line is oriented to a center of the light guide plate has a larger width parallel to a pattern-element layout direction than that of a slant surface on which an outward normal line is oriented to an outside of the light guide plate. According to the above, light leakage from an inclined surface in a front region of the light source can be reduced. 
     For a structure of an inclined surface of which both sides are bent, upper end edges and lower end edges of the side-inclined surfaces may be inclined to the end surface, viewed from a thickness direction, for example. Further, upper end edges of the side-inclined surfaces may be inclined to the end surface, and lower end edges of the side-inclined surfaces may be parallel with the end surface, viewed from a thickness direction. Further, upper end edges of the side-inclined surfaces may be parallel with the end surface, and lower end edges of the side-inclined surfaces may be inclined to the end surface, viewed from a thickness direction. 
     A surface light source device according to one or more embodiments of the present invention includes the light guide plate according to one or more embodiments of the present invention, and a light source configured to transmit light to the end surface of the light introducing portion of the light guide plate. In the surface light source device according to one or more embodiments of the present invention, since the light guide plate according to one or more embodiments of the present invention is used, light leakage from the inclined surface can be reduced and brightness of the light emitting surface can be improved. 
     The light guide plate according to one or more embodiments of the present invention can be used for a liquid-crystal display device, and a screen of the liquid-crystal display device can be made bright by improving light utilization efficiency of the surface light source device. 
     The liquid-crystal display device according to one or more embodiments of the present invention can be also used in a mobile device such as a smartphone, a tablet computer, an electronic book reader, and an electronic dictionary. 
     Embodiments of the present invention are not limited to the above. Additionally, embodiments of the present invention may be formed by combining any of the above elements. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a perspective view of an example of a conventional surface light source device, and  FIG. 1B  is a schematic view for explaining behaviors of light incident to a directional transform pattern of the surface light source device in  FIG. 1A ; 
         FIG. 2A  is a perspective view of a surface light source device according to a first embodiment of the present invention, and  FIG. 2B  is a partially-broken-down plan view of the surface light source device in  FIG. 2A ; 
         FIG. 3A  is a perspective view of the surface light source device in  FIG. 2A  excluding a directional transform pattern of an inclined surface, and  FIG. 3B  shows a cross-sectional shape of each portion of the directional transform pattern in the surface light source device in  FIG. 2A ; 
         FIG. 4  is a schematic view for explaining behaviors of light incident to a directional transform pattern positioned at a front side viewed from a light source; 
         FIG. 5A  is a schematic view for explaining behaviors of light incident to a directional transform pattern positioned at the right side viewed from a light source, and  FIG. 5B  is a schematic view for explaining behaviors of light incident to a directional transform pattern in a comparative example; 
         FIGS. 6A ,  6 B, and  6 C are cross-sectional views illustrating different shape of directional transform patterns in the surface light source device in  FIG. 2A ; 
         FIG. 7A  is a perspective view of a surface light source device according to a second embodiment of the present invention, and  FIG. 7B  is a partially-broken-down plan view of the surface light source device in  FIG. 7A ; 
         FIG. 8  is a perspective view of the surface light source device in  FIG. 7A  excluding a directional transform pattern of an inclined surface; 
         FIG. 9A  is a perspective view of a surface light source device according to a third embodiment of the present invention, and  FIG. 9B  is a partially-broken-down plan view of the surface light source device in  FIG. 9A ; 
         FIG. 10  is a perspective view of the surface light source device in  FIG. 9A  excluding a directional transform pattern of an inclined surface; 
         FIG. 11  is a perspective view of a surface light source device according to a fourth embodiment of the present invention; 
         FIG. 12  is a schematic cross-sectional view of a liquid-crystal display device according to one or more embodiments of the present invention; and 
         FIG. 13  is a schematic front view of a mobile device according to one or more embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention are described below with reference to the attached drawings. In embodiments of the invention, numerous specific details are set forth in order to provide a more thorough understanding of the invention. However, it will be apparent to one of ordinary skill in the art that the invention may be practiced without these specific details. In other instances, well-known features have not been described in detail to avoid obscuring the invention. Additionally, the present invention is not limited to the following embodiments, and can be variously modified within a range without deviating from the scope of the present invention. 
     First Embodiment 
     A structure of a surface light source device according to a first embodiment of the present invention is described below with reference to  FIGS. 2A and 2B , and  FIGS. 3A and 3B .  FIGS. 2A and 2B  are a perspective view and a partially-broken-down plan view of a surface light source device  31  according to the first embodiment of the present invention, respectively.  FIG. 3A  is a perspective view of the surface light source device  31  excluding a directional transform pattern  37  of an inclined surface  36  of the surface light source device  31 . That is,  FIG. 3A  is a perspective view of the surface light source device  31  in  FIG. 2A  in which the directional transform pattern  37  is replaced by a flat surface (an envelope surface) that is in contact with a ridge line of the directional transform pattern  37 .  FIG. 3B  is a schematic view of a cross-sectional shape of each portion (directional transform patterns  37   a ,  37   b , and  37   c ) of the directional transform pattern  37  formed in the surface light source device  31 . 
     The surface light source device  31  is formed of a light source  32  and a light guide plate  33 . The light source  32  incorporates one or a plurality of LEDs, and emits white light from a light emission window at a front side. This light source  32  is also referred to as a point light source. 
     The light guide plate  33  has a light introducing portion  34  provided at an end part of a light guide-plate main body  35 . The light guide plate  33  is formed of a transparent resin having a high refractive index such as an acrylic resin, a polycarbonate resin (PC), cycloolefin resin, and polymethylmethacrylate (PMMA). 
     The light introducing portion  34  is a portion having a large thickness in the light guide plate  33 , and the light source  32  is arranged opposite to a light-incident end surface  40  as an end surface of the light guide plate  33 . A thickness of the light-incident end surface  40  is equal to or larger than a height of the light emission window of the light source  32 . Therefore, light emitted from the light source  32  is efficiently incident from the light-incident end surface  40  into the light introducing portion  34 , and light utilization efficiency of the surface light source device  31  is improved. 
     As shown in  FIG. 3A , in front of a center portion of the light-incident end surface  40 , that is, in a front region of the light source  32 , a main-inclined surface  36   a  inclined from an upper surface of the light introducing portion  34  toward an upper surface of the light guide-plate main body  35  is provided. The main-inclined surface  36   a  is an inclined flat surface, and a direction of a normal line erected on the main-inclined surface  36   a  is oriented to a direction perpendicular to the light-incident end surface  40 , viewed from a thickness direction of the light guide plate  33  (a direction perpendicular to a light emitting surface  41 ). 
     At both sides of the main-inclined surface  36   a , there are provided side-inclined surfaces  36   b  and  36   c  that are inclined downward from the upper surface of the light introducing portion  34  toward the upper surface of the light guide-plate main body  35 . The side-inclined surfaces  36   b  and  36   c  are inclined flat surfaces, and directions of normal lines erected on the side-inclined surfaces  36   b  and  36   c  are inclined to a direction of the main-inclined surface  36   a  (or, the center surface C of the light guide plate  33 ), respectively, viewed from a thickness direction of the light guide plate  33 . 
     The side-inclined surfaces  36   b  and  36   c  are formed continuously with the main-inclined surface  36   a . Therefore, the inclined surface  36  formed of the main-inclined surface  36   a  and the side-inclined surfaces  36   b  and  36   c  at right and left sides has a shape that both sides are bent. Further, upper end edges of the inclined surface  36  are bent in a ship-bottom shape, and a horizontal surface  39  parallel with a lower surface of the light introducing portion  34  is formed at corner portions of the upper surface of the light introducing portion  34 . Lower end edges of the inclined surface  36 , that is, a boundary between the light introducing portion  34  and the light guide-plate main body  35 , are also bent in a ship-bottom shape. 
     A directional transform pattern  37   a  is formed on the main-inclined surface  36   a . The directional transform pattern  37   a  is configured by a layout of pattern elements  38  having V-groove shapes, and slant surfaces (groove slant surfaces  38   a  and  38   b ) are formed between ridge lines and valley lines. Each pattern element  38  extends to a direction perpendicular to the light-incident end surface  40 , viewed from a thickness direction of the light guide plate  33 , and is arranged in a width direction of the main-inclined surface  36   a  so as to become parallel to each other. As shown in  FIG. 3B , the pattern elements  38  of the directional transform pattern  37   a  have bilaterally asymmetrical cross-sectional shapes. 
     Directional transform patterns  37   b  and  37   c  are formed on the side-inclined surfaces  36   b  and  36   c , respectively. Each of the directional transform patterns  37   b  and  37   c  is configured by a layout of the pattern elements  38  having V-groove shapes, and the slant surfaces (groove slant surfaces  38   a  and  38   b ) are formed between ridge lines and valley lines. Each pattern element  38  extends to a direction perpendicular to the light-incident end surface  40 , viewed from a thickness direction of the light guide plate  33 , and is arranged in a width direction of the directional transform patterns  37   b  and  37   c  so as to become parallel to each other. As shown in  FIG. 3B , the pattern elements  38  of the directional transform patterns  37   b  and  37   c  have bilaterally asymmetrical cross-sectional shapes. 
     In this way, the directional transform pattern  37  formed of the directional transform patterns  37   a ,  37   b , and  37   c  is formed on a whole of the inclined surface  36 . 
     Although not shown in the drawings, a light emitting unit for emitting light that guides in the light guide-plate main body  35  from the light emitting surface  41  is provided on a surface opposite to the light emitting surface  41  of the light guide-plate main body  35 . The light emitting unit is formed in a prism-shaped pattern, by sandblasting, spreading ink, and in a diffraction grating pattern, and the like (see  FIG. 12 ). At a lower surface side of the light guide plate  33 , there is provided a reflection plate that makes light, which leaked from the lower surface of the light guide plate  33 , incident again to the light guide plate  33  (see  FIG. 12 ). 
     Work of the directional transform pattern  37  in the inclined surface  36  will be described next. In the directional transform pattern  37   a , each pattern element  38  has a bilaterally asymmetrical cross-sectional shape in a cross section perpendicular to the ridge line of the pattern element  38  (hereinafter, simply referred to as a cross section). That is, as shown in  FIG. 4 , in two groove slant surface  38   a  and  38   b  that constitute the pattern element  38  in a V-groove shape, a width Ba of the groove slant surface  38   a  on which a normal line N erected is oriented to the center surface C is larger than a width Sa of the groove slant surface  38   b  on which a normal line N erected is oriented to opposite to the center surface C. 
     Therefore, in the directional transform pattern  37   a , a proportion of an area (a total value) of each groove slant surface  38   a  becomes large, and a proportion of an area (a total value) of each groove slant surface  38   b  becomes small. Therefore, a large portion of light emitted from a light emission center  32   a  is totally reflected by the groove slant surface  38   a , and even when the light emitted from the light emission center  32   a  leaks from the groove slant surface  38   b , a leakage amount of the light becomes small. 
     In the directional transform patterns  37   b  and  37   c , a cross-sectional shape of each pattern element  38  is also bilaterally asymmetrical. While  FIG. 5A  illustrates the directional transform pattern  37   c , the directional transform pattern  37   b  has a shape symmetrical with the directional transform pattern  37   c  with respect to the center surface C. In the directional transform patterns  37   b  and  37   c , the width Bc of the groove slant surface  38   b  on the surface of which the normal line N erected is further oriented to opposite to the center surface C than a normal line M with respect to the normal line M erected on the side-inclined surface  36   b  is larger than the width Sc of the groove slant surface  38   a  on which the normal line erected is further oriented to the center surface C than the normal line M with respect to the normal line M erected on the side-inclined surface  36   b.    
     When the directional transform pattern  37   a  extends to an end of the light guide plate  33  in a width direction, as shown in  FIG. 5B , the light L easily leaks from the groove slant surface  38   b . However, when the side-inclined surfaces  36   b  and  36   c  are bent in advance as shown in  FIG. 5A , the light does not easily leak. That is, the groove slant surface  38   a  from which the light L easily leaks is shielded by the groove slant surface  38   b , and light becomes not easily incident to the groove slant surface  38   a . Therefore, light leakage is reduced. 
     In a surface light source device  31  according to one or more embodiments of the present invention, since light leakage is reduced as described above, loss of light reduces, and light utilization efficiency improves. 
     (Modification) 
       FIGS. 6A ,  6 B, and  6 C are cross-sectional views illustrating different shapes of the directional transform patterns  37 . In  FIG. 6A , the directional transform pattern  37   a  formed on the main-inclined surface  36   a  is configured by the pattern elements  38  having bilaterally symmetrical cross-sectional shapes. The directional transform patterns  37   b  and  37   c  formed on the side-inclined surfaces  36   b  and  36   c  are also configured by the pattern elements  38  having bilaterally symmetrical cross-sectional shapes. In the directional transform pattern  37  in such a structure, an incidence angle of light incident to one groove slant surface  38   b  can be also made small by providing the directional transform patterns  37   b  and  37   c  at both the right and left side portions, and light becomes not easily incident to the other groove slant surface  38   a . Therefore, light leakage from the directional transform patterns  37   b  and  37   c  can be made small. 
     In  FIG. 6B , the directional transform pattern  37   a  formed on the main-inclined surface  36   a  is configured by the pattern elements  38  having bilaterally symmetrical cross-sectional shapes. The directional transform patterns  37   b  and  37   c  formed on the side-inclined surfaces  36   b  and  36   c  are configured by the pattern elements  38  having bilaterally asymmetrical cross-sectional shapes in a similar manner to that in the first embodiment. In the directional transform pattern  37  in such a structure, light leakage from the directional transform patterns  37   b  and  37   c  at both the right and left side portions can be made small in a similar manner to that in the first embodiment. 
     In  FIG. 6C , the directional transform pattern  37   a  formed on the main-inclined surface  36   a  is configured by the pattern elements  38  having bilaterally asymmetrical cross-sectional shapes in a similar manner to that in the first embodiment, and the directional transform patterns  37   b  and  37   c  formed on the side-inclined surfaces  36   b  and  36   c  are configured by the pattern elements  38  having bilaterally symmetrical cross-sectional shapes. In the directional transform pattern  37  in such a structure, light leakage from the directional transform pattern  37   a  at a center can be made small, and light leakage from the directional transform patterns  37   b  and  37   c  at both the right and left side portions can be made small. 
     Second Embodiment 
       FIGS. 7A and 7B  are a perspective view and a partially-broken-down plan view of a surface light source device  51  according to the second embodiment of the present invention, respectively.  FIG. 8  is a perspective view of the surface light source device  51 , showing only the inclined surface  36  by excluding the directional transform pattern  37 . That is,  FIG. 8  is a perspective view of the surface light source device  31  in  FIG. 7A  in which the directional transform pattern  37  is replaced by a flat surface (an envelope surface) that is in contact with a ridge line of the directional transform pattern  37 . 
     In the surface light source device  51 , the side-inclined surfaces  36   b  and  36   c  are also provided at both sides of the main-inclined surface  36   a , as shown in  FIG. 8 . Average directions of normal lines erected on the side-inclined surfaces  36   b  and  36   c  are inclined to a direction of the main-inclined surface  36   a  (or the center surface C of the light guide plate  33 ), viewed from a thickness direction of the light guide plate  33 . Therefore, the inclined surface  36  formed of the main-inclined surface  36   a  and the side-inclined surfaces  36   b  and  36   c  at right and left sides has a shape that both sides are bent. 
     Viewed from a thickness direction of the light guide plate  33 , upper end edges of the inclined surface  36  are a linear, and lower end edges of the inclined surface  36 , that is, a boundary between the light introducing portion  34  and the light guide-plate main body  35 , are bent in a ship-bottom shape. In this case, the side-inclined surfaces  36   b  and  36   c  may be flat surfaces, or twisted curved surfaces. 
     In the surface light source device  51 , light leakage from both side portions of the inclined surface  36  can be also reduced. 
     Structures of the directional transform patterns  37   a ,  37   b , and  37   c  provided on the main-inclined surface  36   a  and the side-inclined surfaces  36   b  and  36   c  are similar to those in the first embodiment and the modification of the first embodiment (see  FIGS. 4 to 6 ). 
     Third Embodiment 
       FIGS. 9A and 9B  are a perspective view and a partially-broken-down plan view of a surface light source device  61  according to a third embodiment of the present invention, respectively.  FIG. 10  is a perspective view of the surface light source device  61 , showing only the inclined surface  36  excluding the directional transform pattern  37 . 
     In the surface light source device  61 , the side-inclined surfaces  36   b  and  36   c  are also provided at both sides of the main-inclined surface  36   a , as shown in  FIG. 10 . Average directions of normal lines erected on the side-inclined surfaces  36   b  and  36   c  are inclined to a direction of the main-inclined surface  36   a  (or the center surface C of the light guide plate  33 ), viewed from a thickness direction of the light guide plate  33 . Therefore, the inclined surface  36  formed of the main-inclined surface  36   a  and the side-inclined surfaces  36   b  and  36   c  at right and left sides has a shape that both sides are bent. 
     Further, viewed from a thickness direction of the light guide plate  33 , upper end edges of the inclined surface  36  are bent in a ship-bottom shape, and lower end edges of the inclined surface  36 , that is, boundaries between the light introducing portion  34  and the light guide-plate main body  35 , are in a liner shape. In this case, the side-inclined surfaces  36   b  and  36   c  may be flat surfaces, or twisted curved surfaces. 
     In the surface light source device  61 , light leakage from both side portions of the inclined surface  36  can be also reduced. 
     In the third embodiment, structures of the directional transform patterns  37   a ,  37   b , and  37   c  provided on the main-inclined surface  36   a  and the side-inclined surfaces  36   b  and  36   c  are also similar to those in the first embodiment and the modification of the first embodiment (see  FIGS. 4 to 6 ). 
     Fourth Embodiment 
       FIG. 11  is a perspective view of a surface light source device  71  according to a fourth embodiment of the present invention. In the surface light source device  71 , a plurality of light sources  32  are used for one light guide plate  33 . That is, the plurality of light sources  32  are arranged at a constant interval opposite to the light-incident end surface  40  of the light guide plate  33 . On the light guide plate  33 , inclined surfaces  36  and directional transform patterns  37  are formed opposite to respective light sources  32 . According to the fourth embodiment, it is possible to manufacture a surface light source device having a large illumination region. A light guide plate structure that becomes a basis of the surface light source device  71  may be a light guide plate according to one or more embodiments of the present invention. 
     Fifth Embodiment 
       FIG. 12  is a schematic cross-sectional view of a liquid-crystal display device  81  using the surface light source device according to one or more embodiments of the present invention (the surface light source device  31  in the first embodiment, for example). The liquid-crystal display device  81  has a diffusion plate  82 , a prism sheet  83 , and a liquid crystal panel  84  that are superposed opposite to a light-emitting surface side of the light guide plate  33 , and has a reflection sheet  85  arranged at a rear-surface side of the light guide plate  33 . According to the liquid-crystal display device  81 , it is possible to take advantage of a characteristic of the surface light source device according to one or more embodiments of the present invention, and it becomes possible to facilitate watching the screen by improving light utilization efficiency of the liquid-crystal display device  81  and also becomes possible to reduce the thickness of the liquid-crystal display device  81 . 
     Sixth Embodiment 
       FIG. 13  is a plan view of a mobile device, that is, a smartphone  91  that uses the surface light source device or the liquid-crystal display device according to one or more embodiments of the present invention. The smartphone  91  includes a liquid-crystal display device  92  having a touch panel in the front. When the surface light source device according to one or more embodiments of the present invention is used for the smartphone  91 , brightness of the liquid-crystal display device  92  can be increased by improving light utilization efficiency of the surface light source device. Further, the surface light source device according to one or more embodiments of the present invention can be also applied to a mobile device such as a tablet computer, an electronic dictionary, and an electronic book reader, in addition to a portable telephone such as a smartphone. 
     While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.