This application claims the priority benefit under 35 U.S.C. §119 to Japanese Patent Application No. JP2013-139561 filed on Jul. 3, 2013, which disclosure is hereby incorporated in its entirety by reference. 
     BACKGROUND 
     1. Field 
     The presently disclosed subject matter relates to a light guide plate and a side-edge type surface-emission optical apparatus including the light guide plate used in a liquid crystal display (LCD) unit. 
     2. Description of the Related Art 
     Side-edge type surface-emission optical apparatuses used for LCD units are advantageous in terms of their thin and light structures. 
       FIG. 15A  is a plan view illustrating a prior art side-edge type surface-emission optical apparatus, and  FIGS. 15B and 15C  are cross sectional views of the side-edge type surface-emission optical apparatus of  FIG. 15A  taken along the lines B-B and C-C, respectively, in  FIG. 15A  (see: FIGS. 1(a) and 9 of JP2009-81094A &amp; US2009/0086509A1). 
     In  FIGS. 15A, 15B and 15C , the side-edge type surface-emission optical apparatus is constructed by a light guide plate  1  made of transparent material such as acryl resin or polycarbonate resin with a light incident surface S in , a light distributing control surface S cont  perpendicular to the light incident surface S in  and a light emitting surface S out  opposing the light distributing control surface S cont . A light source  2  formed by a plurality of light emitting diodes (LEDs) is disposed on the light incident surface S in . Also, a prism sheet  3 , which is illustrated only in  FIG. 15C , is disposed on the light emitting surface S out  of the light guide plate  1 , and a Liquid crystal display (LCD) panel (not shown) is disposed outside of the prism sheet  3 . The prism sheet  3  is a single-face-deformed triangular prism sheet; however, the prism sheet  3  may have a cross section such as a curved cross section other than a triangular cross section. Further, a flexible reflective sheet  4 , which is illustrated only in  FIGS. 15B and 15C , is disposed on the light distributing control surface S cont . The flexible reflective sheet  4  is operated to return light leaked from the light distributing control surface S cont  thereto, thus enhancing the luminous intensity. In  FIGS. 15B and 15C , note that the flexible reflective sheet  4  is apart from the light guide plate  1 ; however, the flexible reflective sheet  4  is actually in close proximity to the light distributing control surface S cont  of the light guide plate  1 . 
     As illustrated in  FIG. 15A , a plurality of flat mirror-finish portions  11  are provided on the light distributing control surface S cont  of the light guide plate  1  and extend from the light incident surface S in . The flat mirror-finish portions  11  serve as means for spreading light to the inner part of the light guide plate  1 . In this case, the farther from the light incident surface S in  a location of the flat mirror-finish portions  11 , the smaller the width of the flat mirror-finish portions  11  at that location. A plurality of triangular prism sequences  12  are provided on areas of the light distributing control surface S cont  of the light guide plate  1  where the flat mirror-finish portions  11  are not provided. The triangular prism sequences  12  are protruded with respect to the flat mirror-finish portions  11 . Each of the triangular prism sequences  12  includes a plurality of equidistantly-arranged triangular prisms for bending the path of light. In this case, the farther from the light incident surface S in  a location of the triangular prism sequences  12 , the larger the width of the triangular prism sequences  12  at that location. Thus, much more light is totally reflected by the triangular prism sequences  12 , to realize a uniform surface emission. 
     In  FIG. 16 , which is a partly-enlarged cross sectional view of the light guide plate  1  of  FIGS. 15A, 15B and 15C , each prism  12 P of the triangular prism sequences  12  per pitch P 0  has an asymmetrical structure formed by a straight-type rising sloped surface  12   a  whose slope angle is defined by α0, and a straight-type falling sloped surface  12   b  whose slope angle is defined by α1 (≠α0). Particularly, the falling sloped surface  12   b  carries out a light distributing control to bend the path of light. In order to enhance the luminous intensity of a surface-emission along the normal direction to the prism sheet  3 , the slope angle α1 of the falling sloped surface  12   b  with respect to the flat mirror-finish portions  11  is preferably as small as possible, for example.
 
4°≦α1≦5°
 
As a result, spatial light distribution characteristics of the light guide plate  1  with a narrow full-width at half maximum as shown in  FIG. 17  can be realized. In  FIG. 17 , note that the full-width at half maximum is 25° with a range from 55° to 80°. Also, the slope angle α0 of the rising sloped surface  12   a  with respect to the flat mirror-finish portions  11  is
 
15°≦α0≦90°
 
     Thus, light distribution characteristics of the prism sheet  3  as shown in  FIG. 18  with a high emission along the normal direction to the prism sheet  3  can be realized. 
     In the side-edge type surface-emission optical apparatus of  FIGS. 15A, 15B and 15C , in order to further enhance the luminous intensity along the normal direction to the prism sheet  3 , the slope angle α1 of the falling sloped surface  12   b  with respect to the flat mirror-finish portions  11  is preferably smaller than 4″. As a result, the falling sloped surface  12   b  on the side of the light source  2  is approximately in parallel with the flat mirror-finish portions  11 . Therefore, as illustrated in  FIG. 19 , the flexible reflective sheet  4  is in broad contact with the falling sloped surface  12   b  so that a region R with no air gap may be generated between the falling sloped surface  12   b  and the flexible reflective sheet  4 . In the region R, the flexible reflective sheet  4  partly spreads light reflected at the falling sloped surface  12   b , so that a so-called wet-out phenomenon inviting irregular patterns as illustrated in  FIG. 20  may occur in the light emitting surface S out  of the light guide plate  1 , which would degrade the light distribution characteristics of the prism sheet  3  of  FIG. 18 . 
     SUMMARY 
     The presently disclosed subject matter seeks to solve one or more of the above-described problems. 
     According to the presently disclosed subject matter, in a light guide plate having a light incident surface, a light distributing control surface perpendicular to the light incident surface and a light emitting surface opposing the light distributing control surface, a flat mirror finishing portion is provided on a first area of the light distributing control surface, and a prism sequence is provided on a second area of the light distributing control surface where the flat mirror finishing portion is not provided. The prism sequence is protruded with respect to the flat mirror finishing portion. Each prism of the prism sequence has a rising sloped surface opposing the light incident surface, a first falling sloped surface connected to the rising sloped surface, and a second falling sloped surface connected to the first falling sloped surface. A slope of the first falling sloped surface is larger than a slope of the second falling sloped surface. 
     According to the presently disclosed subject matter, while the second falling sloped surface is not so sloped to improve the light distributing control efficiency, the first falling sloped surface is rapidly sloped, so that the angle between the first falling sloped surface and the rising sloped surface is reduced to form a sharper edge. As a result, when the light guide plate according to the presently disclosed subject matter is applied to a side-edge type surface-emission optical apparatus, the contact area between the light guide plate and the reflective sheet can be decreased to suppress the wet-out phenomenon. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other advantages and features of the presently disclosed subject matter will be more apparent from the following description of certain embodiments, taken in conjunction with the accompanying drawings, as compared with the prior art, wherein: 
         FIG. 1  is a cross-sectional view illustrating a side-edge type surface-emission optical apparatus including a first embodiment of the light guide plate according to the presently disclosed subject matter; 
         FIG. 2  is a partly enlarged cross-sectional view of the light guide plate of  FIG. 1 ; 
         FIG. 3  is a cross-sectional view illustrating a contact state between the sloped surface of the light guide plate and the flexible reflective sheet of  FIG. 1 ; 
         FIG. 4  is a graph illustrating proper states of the spatial light distribution characteristics of the light guide plate of  FIG. 1 ; 
         FIG. 5  is a graph illustrating proper states of the light distribution characteristics of the prism sheet of  FIG. 1 ; 
         FIG. 6  is a graph illustrating improper states of the spatial light distribution characteristics of the light guide plate of  FIG. 1 ; 
         FIG. 7  is a graph illustrating improper states of the light distribution characteristics of the prism sheet of  FIG. 1 ; 
         FIG. 8  is a cross-sectional view illustrating a side-edge type surface-emission optical apparatus including a second embodiment of the light guide plate according to the presently disclosed subject matter; 
         FIG. 9  is a partly-enlarged cross-sectional view of the light guide plate of  FIG. 8 ; 
         FIG. 10  is a cross-sectional view illustrating a contact state between the sloped surfaces of the light guide plate and the flexible reflective sheet of  FIG. 8 ; 
         FIG. 11  is a cross-sectional view illustrating a side-edge type surface-emission optical apparatus including a third embodiment of the light guide plate according to the presently disclosed subject matter; 
         FIG. 12  is a partly-enlarged cross-sectional view of the light guide plate of  FIG. 11 ; 
         FIG. 13  is a cross-sectional view illustrating a contact state between the sloped surfaces of the light guide plate and the flexible reflective sheet of  FIG. 11 ; 
         FIG. 14  is a cross-sectional view illustrating a modification of the side-edge type surface-emission optical apparatus of  FIG. 12 ; 
         FIG. 15A  is a plan view illustrating a prior art side-edge type surface-emission optical apparatus; 
         FIGS. 15B and 15C  are cross-sectional views of the side-edge type surface-emission optical apparatus of  FIG. 15A  taken along the lines B-B and C-C, respectively, in  FIG. 15A ; 
         FIG. 16  is a partly-enlarged cross-sectional view of the light guide plate of  FIGS. 15A, 15B and 15C ; 
         FIG. 17  is a graph illustrating spatial light distribution characteristics of the light guide plate of  FIGS. 15A, 15B and 15C ; 
         FIG. 18  is a graph illustrating light distribution characteristics of the prism sheet of  FIGS. 15A, 15B and 15C ; 
         FIG. 19  is a cross-sectional view illustrating a contact state between the sloped surfaces of the light guide plate and the flexible reflective sheet of  FIGS. 15A, 15B and 15C ; and 
         FIG. 20  is a perspective view for explaining the wet-out phenomenon in  FIGS. 15A, 15B and 15C . 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     In  FIG. 1 , which is a cross-sectional view illustrating a side-edge type surface-emission optical apparatus including a first embodiment of the light guide plate according to the presently disclosed subject matter, the light guide plate  1  of  FIGS. 15A, 15B and 15C  is replaced by a light guide plate  1 A whose triangular prism sequences  12 A are different from the triangular prism sequences  12  of  FIGS. 15A, 15B and 15C . 
     In  FIG. 2 , which is a partly-enlarged cross-sectional view of the light guide plate  1 A of  FIG. 1 , each prism  12 PA of the triangular prism sequences  12 A per pitch P has an asymmetrical structure formed by a rising sloped surface  12   c  whose slope angle is defined by β0, and two straight-type falling sloped surfaces  12   d  and  12   e  whose slope angles are defined by β1 and β2, respectively. The falling sloped surfaces  12   d  and  12   e  are straightly-sloped. In this case, the rising sloped surface  12   c  can be either straightly-sloped or concavedly-sloped (curvedly-sloped). The slope angle β1 of the falling sloped surface  12   d  with respect to the flat mirror-finish portions  11  and the slope angle β2 of the falling sloped surface  12   e  with respect to the flat mirror-finish portions  11  satisfy the following:
 
β1&gt;β2
 
     That is, the falling sloped surface  12   d  is more sloped than the falling sloped surface  12   e . Thus, a sharper edge is formed by the rising sloped surface  12   c  and the falling sloped surface  12   d . On the other hand, the slope angle β0 of the rising sloped surface  12   c  with respect to the flat mirror finish portions  11  is the same as the slope angle α0 of  FIG. 16 , i.e.,
 
15°≦β0≦90°
 
     In  FIG. 2 , in order to suppress the wet-out phenomenon, the slope angle β1 of the falling sloped surface  12   d  satisfies the following formula (1):
 
3°&lt;β1&lt;8°  (1)
 
     On the other hand, in order to enhance the light distributing control efficiency, the slope angle β2 of the falling sloped surface  12   e  satisfies the following formula (2):
 
0.5°&lt;β2&lt;3°  (2)
 
     Further, the following formula (3) is satisfied:
 
2 ·S   12d   ≦S   12e ≦6 ·S   12d   (3)
         where S 124  is the area of the falling sloped surface  12   d ; and   S 12e  is the area of the falling sloped surface  12   e.          

     If S 12e /2·S 12d , the falling sloped surface  12   d  would adversely affect the light distributing control efficiency, to increase the full-width at half maximum of the spatial light distributing characteristics. On the other hand, if S 12e &gt;6·S 12d , the falling sloped surface  12   d  is increased to reduce the suppressing effect of the wet-out phenomenon. 
     Also, the pitch P of the prisms  12 PA is
 
15 μm≦P≦250 μm
 
     The minimum height of the falling sloped surface  12   d  is about 1 μm in order to sufficiently exhibit the suppressing effect of the wet-out phenomenon. In this case, the minimum value of the pitch P is about 15 μm. On the other hand, if the pitch P is larger than the maximum value 250 μm, each prism  12 PA can be recognized as dots in the LCD unit. 
     According to the first embodiment, as illustrated in  FIG. 3 , due to a sharper edge formed by the rising sloped surface  12   c  and the falling sloped surface  12   d , the contact area between the flexible reflective sheet  4  and the rising sloped surface  12   c  as well as the contact area between the flexible reflective sheet  4  and the falling sloped surface  12   d  is reduced to suppress the wet-out phenomenon. 
     In  FIG. 4 , which is a graph illustrating proper states of the spatial light distribution characteristics of the light guide plate  1 A, a state I to satisfy the formulae (1), (2) and (3) is defined by
 
β1=7°
 
β2=1°
 
 S   12e =5 ·S   12d  
 
     Also, a state II to satisfy the formulae (1), (2) and (3) is defined by
 
β1=4°
 
β2=2°
 
 S   12e =2 ·S   12d  
 
     In the state I, the full-width at half maximum of the spatial light distribution characteristics of the light guide plate  1 A as illustrated in  FIG. 4  is reduced to 14° with a range from 68° to 82° as compared with the prior art where the full-width at half maximum is 25° with a range from 55° to 80°. As a result, as illustrated in  FIG. 5 , the luminous intensity of a surface-emission along the normal direction to the prism sheet  3  can be increased as compared with the prior art. 
     Also, in the state II, the full-width at half maximum of the spatial light distribution characteristics of the light guide plate  1 A as illustrated in  FIG. 4  is reduced to 18° with a range from 63° to 81° as compared with the prior art where the full-width at half maximum is 25° with a range from 55° to 80°. As a result, as illustrated in  FIG. 5 , the luminous intensity of a surface-emission along the normal direction to the prism sheet  3  can be increased as compared with the prior art. 
     In  FIG. 6 , which is a graph illustrating improper states of the spatial light distribution characteristics of the light guide plate  1 A, a state III to satisfy the formulae (1) and (2) and not satisfy the formula (3) is defined by
 
β1=6°
 
β2=2°
 
 S   12e =3 ·S   12d  
 
     Also, a state IV to satisfy the formulae (2) and (3) and not satisfy the formula (1) is defined by
 
β1=8°
 
β2=1°
 
 S   12e =3 ·S   12d  
 
     In the state III, the full-width at half maximum of the spatial light distribution characteristics of the light guide plate  1 A as illustrated in  FIG. 6  is 24° with a range from 56° to 80° which is almost the same as that of the prior art where the full-width at half maximum is 25° with a range from 55° to 80°. As a result, as illustrated in  FIG. 7 , the luminous intensity of a surface-emission along the normal direction to the prism sheet  3  is almost the same as that of the prior art. 
     Also, in the state IV, the full-width at half maximum of the spatial light distribution characteristics of the light guide plate  1 A as illustrated in  FIG. 6  is 23° with a range from 57° to 80° which is almost the same as that of the prior art where the full-width at half maximum is 25° with a range from 55° to 80°. As a result, as illustrated in  FIG. 7 , the luminous intensity of a surface-emission along the normal direction to the prism sheet  3  is almost the same as that of the prior art. 
     In  FIG. 8 , which is a cross-sectional view illustrating a side-edge type surface-emission optical apparatus including a second embodiment of the light guide plate according to the presently disclosed subject matter, the light guide plate  1 A of  FIG. 1  is replaced by a light guide plate  1 B whose triangular prism sequences  12 B are different from the triangular prism sequences  12 A of  FIG. 1 . 
     In  FIG. 9 , which is a partly-enlarged cross-sectional view of the light guide plate  1 B of  FIG. 8 , the straight-type falling sloped surface  12   d  of  FIG. 2  is replaced by a concave-type falling sloped surface  12   d ′. The concave-type falling sloped surface  12   d ′ is tangentially-curved and extended from the straight-type falling sloped surface  12   e ; however, the concave-type falling sloped surface  12   d ′ is not always tangentially-curved. Also, the concave-type falling sloped surface  12   d ′ is curvedly-sloped with one radius of curvature. In any case, the concave-type falling sloped surface  12   d ′ is more sloped than the straight-type falling sloped surface  12   e . Thus, a sharper edge is formed by the rising sloped surface  12   c  and the concaved-type falling sloped surface  12   d′.    
     In  FIG. 9 , in order to suppress the wet-out phenomenon, the slope angle β1′ of a line segment between the ends of the concave-type falling sloped surface  12   d ′ satisfies the following formula (1′) in the same way as the formula (1):
 
3°&lt;β1′&lt;8°  (1′)
 
     On the other hand, in order to enhance the light distributing control efficiency, the slope angle β2 of the straight-type falling sloped surface  12   e  satisfies the following formula (2′) in the same way as the formula (2):
 
0.5°&lt;β2&lt;3°  (2′)
 
     Further, the following formula (3′) in the same way as the formula (3) is satisfied:
 
2 ·S   12d   ′≦S   12e ≦6· S   12d ′  (3′)
         where S 12d ′ is the area of the concave-type falling sloped surface  12   d ′; and   S 12e  is the area of the straight-type falling sloped surface  12   e.          

     The formula (3′) can be modified to the following formula:
 
2· L   12d   ≦L   12e ≦6 ·L   12d  
         where L 12d  is the length of the circular segment of a cross section of the concave-type falling sloped surface  12   d ′; and   L 12e  is the length of the line segment of a cross section of the straight-type falling sloped surface  12   e.          

     Also, since the minimum height of the concave-type falling sloped surface  12   d ′ is about 1 μm to exhibit the suppressing effect of the wet-out phenomenon, the pitch P of the prisms  12 PB is
 
15 μm≦P≦250 μm
 
     In the above-described second embodiment, since the straight-type falling sloped surface  12   e  is the same as that of the first embodiment, the light distributing control effect is the same as that of the first embodiment. However, since the slope angle β1′ of the concave-type falling sloped surface  12   d ′ is smaller than the slope angle β1 of the straight-type falling sloped surface  12   d  of the first embodiment to form a sharper edge by the rising sloped surface  12   c  and the falling sloped surface  12   d ′, the contact area between the concave-type falling sloped surface  12   d ′ and the flexible reflective sheet  4  is further decreased as illustrated in  FIG. 10  to further improve the suppressing effect of the wet-out phenomenon. 
     In  FIG. 11 , which is a cross-sectional view illustrating a side-edge type surface-emission optical apparatus including a third embodiment of the light guide plate according to the presently disclosed subject matter, the light guide plate  1 A of  FIG. 1  is replaced by a light guide plate  1 C whose triangular prism sequences  12 C are different from the triangular prism sequences  12 A of  FIG. 1 . 
     In  FIG. 12 , which is a partly-enlarged cross-sectional view of the light guide plate  1 C of  FIG. 11 , the straight-type falling sloped surface  12   d  of  FIG. 2  is replaced by a concave-type falling sloped surface  12   d ″. Also, the straight-type falling sloped surface  12   d  of  FIG. 2  is replaced by a concave-type falling sloped surface  12   e ″. Cross sections of the concave-type falling sloped surfaces  12   d ″ and  12   e ″ are curved. In this case, the radius of curvature of the concave-type falling sloped surface  12   d ″ is smaller than that of the concave-type falling sloped surface  12   e ″, so that the concave-type falling sloped surface  12   d ″ is more sloped than the concave-type falling sloped surface  12   e ″. Thus, a sharper edge is formed by the rising sloped surface  12   c  and the concave-type falling sloped surface  12   d″.    
     In  FIG. 12 , in order to suppress the wet-out phenomenon, the slope angle β1″ of a line segment between the ends of the concave-type falling sloped surface  12   d ″ satisfies the following formula (1″) in the same way as the formula (1):
 
3°&lt;β1″&lt;8°  (1″)
 
     On the other hand, in order to enhance the light distributing control efficiency, the slope angle β2 of a line segment between the ends of the concave-type falling sloped surface  12   e ″ satisfies the following formula (2″) in the same way as the formula (2):
 
0.5°&lt;β2&lt;3°  (2″)
 
     Further, the following formula (3″) in the same way as the formula (3) is satisfied:
 
2· S   12d   ″≦S   12e ″≦6· S   12d ″  (3″)
 
where S 12d ″ is the area of the concave-type falling sloped surface  12   d ″; and
 
S 12e ″ is the area of the concave-type falling sloped surface  12   e″.  
 
     The formula (3″) can be modified to the following formula:
 
2 ·L   12d   ″≦L   12e ″≦6 ·L   12 ″
         where L 12d ″ is the length of the circular segment between the ends of the concave-type falling sloped surface  12   d ″; and   L 12e ″ is the length of the circular segment between the ends of the concave-type falling sloped surface  12   e″.          

     Further, a distance D between a center of the concave-type falling sloped surface  12   d ″ and a center of the line segment of the concave-type falling sloped surface  12   d ″ satisfies the following:
 
3/1000· L   12d   ″≦D≦ 2/100 ·L   12d ″
 
     Still further, since the minimum height of the concave-type falling sloped surface  12   d ″ is about 1 μm to exhibit the suppressing effect of the wet-out phenomenon, the pitch P of the prisms PC is
 
15 μm≦ P≦ 250 μm
 
     In the above-described third embodiment, since the concave-type falling sloped surface  12   e  is the same as that of the first embodiment, the light distributing control effect is the same as that of the first embodiment. However, since the angle between the rising sloped surface  12   c  and the concave-type falling sloped surface  12   d ″ becomes smaller, the contact area between the concave-type falling sloped surface  12   d ″ and the flexible reflective sheet  4  is further decreased as illustrated in  FIG. 13  to further improve the suppressing effect of the wet-out phenomenon. 
     In  FIG. 14 , which illustrates a modification of the light guide plate  1 C of  FIG. 12 , the concave-type falling sloped surfaces  12   d ″ and  12   e ″ are formed by a single concave-type falling sloped surface  12   de  whose concaved cross section has one radius of curvature. In this case, the farther from the light source  2  a location of the concave-type falling sloped surface  12   de , the larger the slope of the concave-type falling sloped surface  12   de . As a result, the contact area between the concave-type falling sloped surface  12   de  and the flexible reflective sheet  4  is decreased to further improve the suppressing effect of the wet-out phenomenon while maintaining the light distributing control effect due to the small slope of a part of the concave-type falling sloped surface  12   de.    
     It will be apparent to those skilled in the art that various modifications and variations can be made in the presently disclosed subject matter without departing from the spirit or scope of the presently disclosed subject matter. Thus, it is intended that the presently disclosed subject matter covers the modifications and variations of the presently disclosed subject matter provided they come within the scope of the appended claims and their equivalents. All related or prior art references described above and in the Background portion of the present specification are hereby incorporated in their entirety by reference.
In a light guide plate having a light incident surface, a light distributing control surface perpendicular to the light incident surface and a light emitting surface opposing the light distributing control surface, a flat mirror-finish portion is provided on a first area of the light distributing control surface, and a prism sequence is provided on a second area of the light distributing control surface where the flat mirror-finish portion is not provided. The prism sequence is protruded with respect to the flat mirror-finish portion. Each prism of the prism sequence has a rising sloped surface opposing the light incident surface, a first falling sloped surface connected to the rising sloped surface, and a second falling sloped surface connected to the first falling sloped surface, A slope of the first falling sloped surface is larger than a slope of the second falling sloped surface.