Patent Publication Number: US-2012039078-A1

Title: Lighting device and displaying device

Description:
TECHNICAL FIELD  
     The present invention relates to a lighting device such as a backlight unit, and to a display device such as a liquid crystal display device. 
     BACKGROUND ART  
     When a display panel such as a liquid crystal display panel is a non light-emitting type, a backlight unit that supplies light (backlight light) to the liquid crystal display panel is mounted in a liquid crystal display device. And, the backlight unit is mounted with a light source in various ways. A direct lighting system is one example. In this direct lighting system, a plurality of LEDs (Light Emitting Diodes) are arranged in a matrix, and a group of the LEDs in a matrix is aligned in parallel with a panel surface of the liquid crystal display panel. 
     In such a backlight unit in which LEDs are mounted by a direct lighting system (a direct lighting backlight unit), a diffusion sheet is disposed over a surface where the LEDs are mounted. When a surface of the diffusion sheet is observed, an image such as the one shown in  FIG. 25  is viewed. 
     Here, as shown in this image, an unevenness in light amount in which an area directly above an LED is brighter than the other areas occurs in a direct lighting backlight unit. One measure to resolve such unevenness in light amount is to make the distance between each LED narrower. However, even though this measure suppresses unevenness in light amount, the number of LEDs is increased, and thereby increasing the cost of the backlight unit. 
     On the other hand, when attempting to suppress the unevenness in light amount while minimizing the number of LEDs in order to reduce the cost, it is necessary to increase the distance between the LED-mounted surface and the diffusion sheet (in other words, the thickness of the backlight unit needs to be increased). This is because if the distance between the LED-mounted surface and the diffusion sheet is too small, light is not likely to reach an upper side of an area between each LED, and the unevenness in light amount is not resolved. 
     Therefore, in order to resolve the unevenness in light amount (in other words, in order to secure luminance uniformity of light from a backlight unit), a backlight unit has to either increase in cost or increase in thickness. Here, a backlight unit of Patent Document 1 is one example of a measure to secure luminance uniformity while maintaining the cost relatively low and suppressing an increase in thickness, for example. In this backlight unit, as shown in  FIG. 26 , a prism sheet (transmission sheet) ps is disposed over a surface where LEDs  112  are mounted. 
     This way, as shown in  FIG. 27 , light (see arrow dashed lines) from the LED  112  travels in various directions from an outgoing face “is” of the prism sheet ps. Accordingly, an unevenness in light amount in which an area directly above the LED  112  is brighter than the other areas is not likely to occur. 
     RELATED ART DOCUMENTS 
     Patent Documents 
     Patent Document 1: Japanese Patent Application Laid-Open Publication 2002-049324 
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     However, as shown in  FIGS. 26 and 27 , a prism pr, which is formed in a light-reception face rs of the prism sheet ps, is a triangle prism pr that tapers off toward the LED  112 . Therefore, incident light to this triangle prism pr is only refracted by one of side surfaces (refractive surfaces) ss of the triangle prism pr. Accordingly, it cannot be said that incident light to the prism sheet pr travels sufficiently away from the LED  112  (that is to say, because the angle of refraction by only one refraction is limited, and light is not sufficiently separated from the LED  112 ). 
     The present invention was devised in order to resolve the above-mentioned problems. An object of the present invention is to provide a lighting device such as a backlight unit that emits backlight light with suppressed unevenness in light amount by increasing the amount of light that travels away from the light source, and also to provide a display device mounted with such a lighting device. 
     Means for Solving the Problems 
     The lighting device includes a light source, and a transmission sheet including a light-reception face for receiving light from the light source and an outgoing face for emitting light that has passed through the light-reception face. In this lighting device, the outgoing face includes a light refractive element at least having a first refractive face, which refracts light coming from the light-reception face, and a second refractive face, which refracts light coming from the first refractive face as side surfaces. 
     When light is transmitted between the first refractive face and the second refractive face this way, a light refractive element including these first refractive face and second refractive face as side surfaces has a shape that tapers off toward the side separating from the light-reception face. Accordingly, a large part of light coming from the light-reception face is refracted twice by the two refractive faces of the light refractive element formed in the outgoing face, and therefore, the amount of light traveling away from the light source is likely to increase. As a result, the lighting device no longer emits light with partial bright regions reflecting a shape of the light source, and the unevenness in light amount can be suppressed. 
     The lighting device that satisfies a formula below (F1) is especially preferable. 
     
       
         
           
             
               
                 
                   [ 
                   
                     Formula 
                      
                     
                         
                     
                      
                     F 
                      
                     
                         
                     
                      
                     1 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     
                       ( 
                       
                         
                           90 
                            
                           ° 
                         
                         - 
                         
                           θ 
                           2 
                         
                       
                       ) 
                     
                     + 
                     
                       
                         sin 
                         
                           - 
                           1 
                         
                       
                        
                       
                         [ 
                         
                           
                             n 
                             · 
                             sin 
                           
                            
                           
                             { 
                             
                               
                                 180 
                                  
                                 ° 
                               
                               - 
                               
                                 3 
                                 · 
                                 
                                   ( 
                                   
                                     
                                       90 
                                        
                                       ° 
                                     
                                     - 
                                     
                                       θ 
                                       2 
                                     
                                   
                                   ) 
                                 
                               
                             
                             } 
                           
                         
                         ] 
                       
                     
                   
                   &gt; 
                   
                     
                       sin 
                       
                         - 
                         1 
                       
                     
                      
                     
                       [ 
                       
                         
                           n 
                           · 
                           sin 
                         
                          
                         
                           { 
                           
                             
                               ( 
                               
                                 
                                   90 
                                    
                                   ° 
                                 
                                 - 
                                 
                                   θ 
                                   2 
                                 
                               
                               ) 
                             
                             - 
                             
                               
                                 sin 
                                 
                                   - 
                                   1 
                                 
                               
                                
                               
                                 ( 
                                 
                                   
                                     sin 
                                      
                                     
                                       ( 
                                       
                                         
                                           90 
                                            
                                           ° 
                                         
                                         - 
                                         
                                           θ 
                                           2 
                                         
                                       
                                       ) 
                                     
                                   
                                   n 
                                 
                                 ) 
                               
                             
                           
                           } 
                         
                       
                       ] 
                     
                   
                 
               
               
                 
                   ( 
                   F1 
                   ) 
                 
               
             
           
         
       
     
     Here, 
     
         
         θ: an angle formed by the first refractive face and the second refractive face 
         n: a refractive index of the transmission sheet. 
       
    
     Moreover, when the refractive index of the transmission sheet is 1.5, it is preferable that an angle θ formed by the first refractive face and the second refractive face satisfies a formula below (A1). 
       50°≦θ&lt;88°  formula (A1)
 
     The lighting device that satisfies a formula below (F2) is preferable. 
     
       
         
           
             
               
                 
                   [ 
                   
                     Formula 
                      
                     
                         
                     
                      
                     F 
                      
                     
                         
                     
                      
                     2 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     90 
                      
                     ° 
                   
                   &gt; 
                   
                     
                       ( 
                       
                         
                           90 
                            
                           ° 
                         
                         - 
                         
                           θ 
                           2 
                         
                       
                       ) 
                     
                     + 
                     
                       
                         sin 
                         
                           - 
                           1 
                         
                       
                        
                       
                         [ 
                         
                           
                             n 
                             · 
                             sin 
                           
                            
                           
                             { 
                             
                               
                                 180 
                                  
                                 ° 
                               
                               - 
                               
                                 3 
                                 · 
                                 
                                   ( 
                                   
                                     
                                       90 
                                        
                                       ° 
                                     
                                     - 
                                     
                                       θ 
                                       2 
                                     
                                   
                                   ) 
                                 
                               
                             
                             } 
                           
                         
                         ] 
                       
                     
                   
                   &gt; 
                   
                     
                       sin 
                       
                         - 
                         1 
                       
                     
                      
                     
                       [ 
                       
                         
                           n 
                           · 
                           sin 
                         
                          
                         
                           { 
                           
                             
                               ( 
                               
                                 
                                   90 
                                    
                                   ° 
                                 
                                 - 
                                 
                                   θ 
                                   2 
                                 
                               
                               ) 
                             
                             - 
                             
                               
                                 sin 
                                 
                                   - 
                                   1 
                                 
                               
                                
                               
                                 ( 
                                 
                                   
                                     sin 
                                      
                                     
                                       ( 
                                       
                                         
                                           90 
                                            
                                           ° 
                                         
                                         - 
                                         
                                           θ 
                                           2 
                                         
                                       
                                       ) 
                                     
                                   
                                   n 
                                 
                                 ) 
                               
                             
                           
                           } 
                         
                       
                       ] 
                     
                   
                 
               
               
                 
                   ( 
                   F2 
                   ) 
                 
               
             
           
         
       
     
     It is especially preferable that a formula below (A2) be satisfied. 
       50°≦θ&lt;76°  formula (A2)
 
     Further, it is preferable that a formula below (A3) be satisfied.) 
       65(°)≦θ76(°)   formula (A3)
 
     It is preferable that a formula below (B1) be satisfied when a first diffusion sheet for receiving light that has passed through the transmission sheet is included. 
       0.35&lt; D ( L−R )/ D ( L−DS )&lt;1   formula (B1)
 
     Here,
         D (L-R): the shortest distance from the light source to a connecting line of the first refractive face and the second refractive face of the light refractive element in the transmission sheet   D(L-DS): the shortest distance from the light source to the first diffusion sheet.       

     Moreover, it is especially preferable that a formula below (B2) be satisfied. 
       0.5 ≦D ( L−R )/ D ( L−DS )≦0.75   formula (B2)
 
     Further, it is preferable that a formula below (B3) be satisfied. 
       0.5 ≦D ( L−R )/ D ( L−DS )≦0.625   formula (B3)
 
     It is preferable that a connecting line of the first refractive face and the second refractive face be perpendicular to a direction in which the light source is aligned. 
     This way, light coming from the first refractive face and light coming from the second refractive face is likely to reach an area between respective light sources. Therefore, a difference in luminance is not likely to occur between a light source and an area between light sources. 
     For example, when the light source is point light sources that are arranged two dimensionally, and if one direction and the other direction that are perpendicular to each other on a two dimensional surface are called a first direction and a second direction, it is preferable that the connecting line become perpendicular to one of the first direction and the second direction, which is a direction in which the point light sources are aligned. 
     This way, in a direction perpendicular to the connecting line, a difference in luminance is not likely to occur between the light sources and an area between the light sources. 
     Here, it is preferable that there be a plurality of the transmission sheets overlapping with each other, and the transmission sheet on a side close to the light source be called a first transmission sheet, and the transmission sheet on a side far from the light source be called a second transmission sheet, and that a connecting line of the first transmission sheet become perpendicular to one of the first direction and the second direction, and a connecting line of the second transmission sheet become perpendicular to the other one of the first direction and the second direction. 
     This way, a difference in luminance can be suppressed between the light sources and areas between the light sources in both of one direction and the other direction that are perpendicular to each other on the two dimensional surface. 
     Further, when there is a difference between a length of an arrangement interval of the point light sources along the first direction and a length of an arrangement interval of the point light sources along the second direction, it is preferable that the direction of a longer arrangement interval become perpendicular to the connecting line of the second transmission sheet. 
     When the distance of an arrangement interval of point light sources is long, light is usually not likely to reach an area between respective light sources. However, light refractive elements in the second transmission sheet, which is located on a side close to a viewer of light of the lighting device, can spread light to an area between respective light sources, and therefore, light emitted from the lighting device does not include unevenness in light amount. 
     The light source of the lighting device is not limited to point light sources, and it may be linear light sources that are aligned next to each other. 
     One example of the light refractive element is a prism. 
     It is preferable that the prism be a triangle prism having an isosceles triangle cross-section in which side surfaces are the first refractive face and the second refractive face having an equal length. 
     When the light refractive element is a prism, the prism may be a point-like prism including another refractive face as a side surface in addition to the first refractive face and the second refractive face. 
     Moreover, a surface of the prism may be a curved surface that is convex toward a light outgoing side of the transmission sheet. This is because light exiting from a curved surface is likely to travel in various directions, and unevenness in light amount can be further suppressed. 
     There are various ways to make light exiting from the transmission sheet travel in various directions. For example, recesses and projections for scattering light may be formed in at least a part of a surface of the transmission sheet. Or the transmission sheet may include a light diffusion material. 
     Further, if a second diffusion sheet for receiving light from the transmission sheet is included, light traveling in various directions from the transmission sheet is further diffused, and therefore, light from the lighting device does not include unevenness in light amount. 
     The light refractive element is not limited to a prism, and it may be a hologram, for example. 
     A display device including the lighting device described above and a display panel for receiving light from the lighting device is also the present invention. 
     Effects of the Invention 
     According to the present invention, incident light to the transmission sheet is released after being refracted multiple times by the light refractive element including two refractive faces formed in the outgoing face. Therefore, the amount of light that travels to be separated from the light source is likely to increase. As a result, a lighting device including such a transmission sheet no longer emits light with partial bright regions reflecting a shape of the light source, and unevenness in light amount can be suppressed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an exploded perspective view of a liquid crystal display device of Example 1. 
         FIG. 2  is a cross-sectional arrow view along the line A 1 -A 1 ′ of  FIG. 1 . 
         FIG. 3  is an exploded perspective view of a liquid crystal display device of a comparative example. 
         FIG. 4  is a cross-sectional arrow view along the line a 1 -a 1 ′ of  FIG. 3 . 
         FIG. 5  is a characteristics diagram showing a comparison between a vertex angle θ of a prism in a prism sheet and an outgoing angle  8  with respect to a prism sheet. 
         FIG. 6A  is a light path view showing a path of light that transmits through a prism sheet. 
         FIG. 6B  is a light path view showing a path of light that transmits through a prism sheet. 
         FIG. 6C  is a light path view showing a path of light that transmits through a prism sheet. 
         FIG. 6D  is a light path view showing a path of light that transmits through a prism sheet. 
         FIG. 7  is a light path view showing an example of light that transmits through a prism. 
         FIG. 8  is a light path view showing an example of light that transmits through a prism. 
         FIG. 9A  is a light path view showing light that transmits through a prism sheet. 
         FIG. 9B  is a light path view showing light that transmits through a prism sheet. 
         FIG. 9C  is a light path view showing light that transmits through a prism sheet. 
         FIG. 9D  is a light path view showing light that transmits through a prism sheet. 
         FIG. 10  is a light path view showing how light traveling in various directions from an LED transmits through the prism sheet. 
         FIG. 11  is a chart showing images of planar light that is viewed in Example 1 and the comparative example. 
         FIG. 12  is a light path view showing an example of light that transmits through a prism. 
         FIG. 13  is a light path view showing an example of light that transmits through a prism. 
         FIG. 14  is an exploded perspective view of a liquid crystal display device of Example 2. 
         FIG. 15  is a cross-sectional arrow view along the line A 2 -A 2 ′ of  FIG. 14 . 
         FIG. 16  is an exploded perspective view of a liquid crystal display device of Example 3. 
         FIG. 17  is a cross-sectional arrow view along the line A 3 -A 3 ′ of  FIG. 16 . 
         FIG. 18  is an explanatory view showing both an image of planar light viewed in Example 2 and a characteristics graph of positions and luminance in the planar light. 
         FIG. 19  is an explanatory view showing both an image of planar light viewed in Example 3 and a characteristics graph of positions and luminance in the planar light. 
         FIG. 20  is an exploded perspective view of a liquid crystal display device of Example 4. 
         FIG. 21  is a cross-sectional arrow view along the line A 4 -A 4 ′ of  FIG. 20 . 
         FIG. 22  is a cross-sectional view of a prism sheet in which prisms with curved side surfaces are arranged. 
         FIG. 23A  is an example of an image of planar light that has passed through a prism sheet in which prisms with curved side surfaces are arranged. 
         FIG. 23B  is an example of an image of planar light that has passed through a prism sheet in which prisms with curved side surfaces are arranged. 
         FIG. 23C  is an example of an image of planar light that has passed through a prism sheet in which prisms with curved side surfaces are arranged. 
         FIG. 23D  is an image of planar light that has passed through a prism sheet in which prisms with flat side surfaces are arranged. 
         FIG. 24  is an explanatory view showing the center of curvature of the curved surface, which is a side surface of a prism. 
         FIG. 25  is an image showing planar light that is emitted from a conventional backlight unit. 
         FIG. 26  is a cross-sectional view of a liquid crystal display device including a conventional backlight unit. 
         FIG. 27  is a light path view of light that transmits through a prism sheet of the backlight unit shown in  FIG. 26 . 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Embodiment 1 
     Embodiment 1 is described below based on the figures. Here, hatchings, member characters, and the like may be omitted for convenience, but in those cases, other figures should be referred to. A black dot along with arrows shown in the figures indicates directions perpendicular to the plane of paper. Further, numeric examples included here are only examples, and the present invention is not limited to those numbers. A unit used for angles included here is (°), and it may be omitted in some cases. 
     Further, a description is made below on a liquid crystal display device as an example of a display device (Example 1), but the present invention is not limited to this, and other display devices may also be used (of course, lighting devices other than a backlight unit, which will be described later, may be encompassed as well). 
       FIG. 1  is an exploded cross-sectional view of a liquid crystal display device  59 , and  FIG. 2  is a cross-sectional arrow view along the line A 1 -A 1 ′ of  FIG. 1 . As shown in  FIG. 1 , the liquid crystal display device  59  includes a liquid crystal display panel (display panel)  49 , a backlight unit (lighting device)  39 , and housings HG (front housing HG 1  and rear housing HG 2 ) that sandwiches the liquid crystal display panel and the backlight unit. 
     In the liquid crystal display panel  49 , an active matrix substrate  41  that includes switching elements such as TFTs (Thin Film Transistors) and an opposite substrate  42  facing this active matrix substrate  41  are attached together by a sealing material (not shown in the figure). Liquid crystal (not shown in the figure) is injected to a gap between these two substrates  41  and  42  (moreover, polarizing films  43  and  43  are attached so as to sandwich the active matrix substrate  41  and the opposite substrate  42 ). Such a liquid crystal display panel  49  displays an image by using a change in transmittance caused by inclination of liquid crystal molecules. 
     The backlight unit  39  overlaps with the liquid crystal display panel  49 , and emits light onto this non light-emitting liquid crystal display panel  49 . That is, the liquid crystal display panel  49  improves its display function by receiving light (backlight light) from the backlight unit  39 . Therefore, if the entire surface of the liquid crystal display panel  49  can be irradiated evenly with light from the backlight unit  39 , the display quality of the liquid crystal display panel  49  is improved. 
     This backlight unit  39  includes, as shown in  FIG. 1 , an LED module MJ, prism sheets PS (a first prism sheet PS 1  and a second prism sheet PS 2 ), a diffusion sheet  31 , and optical sheets  32  ( 32 A and  32 B). 
     The LED module MJ includes a mounting substrate  11 , LEDs (Light Emitting Diodes)  12 , and a reflective sheet  13 . Over the mounting substrate  11 , electrodes  11 E are arranged in a planar manner (in a matrix, for example), and the LEDs (light sources, light emitting elements)  12  are mounted on those electrodes  11 E. The mounting substrate  11  supplies current from a power source, which is not shown in the figure, to the LEDs  12  through the electrodes  11 E. 
     The LEDs  12  are point light sources that emit light by receiving current supplies, and are arranged corresponding to the electrodes  11 E over a mounting surface  11 U of the mounting substrate  11  (here, the direction of a light emitting surface  12 L of the LEDs  12  is same as the direction of the mounting surface  11 U on which the electrodes  11 E are arranged). As a result, the LEDs  12  are arranged over the mounting surface  11 U of the mounting substrate  11  in a planar manner (two dimensional arrangement), and generate planar light.  100481  One example of the arrangement of the LEDs  12  is a planar arrangement in a rectangular shape as well as in a matrix, as shown in  FIG. 1 . Here, for convenience, a long direction of the rectangle is called a X direction, a short direction is called a Y direction, and a direction crossing (a direction perpendicular to, for example) the X direction and the Y direction is called a Z direction. A surface formed by the LEDs  12  that are arranged in a matrix is called an LED-mounted surface XY (the LED-mounted surface XY, which is a two dimensional surface, is in the same direction as the direction of a surface formed by the X direction and the Y direction, and therefore, the character XY is used). An arrangement interval Px of the LEDs  12  in the X direction is same as an arrangement interval Py of the LEDs  12  in the Y direction. 
     Over the mounting surface  11 U, the reflective sheet (reflector)  13  is attached to a region other than the electrodes  11  E, and the reflective sheet  13  reflects a part of light from the LEDs  12 . In other words, the reflective sheet  13  makes light coming toward the mounting surface  11 U travel away from the mounting surface  11 U (one example of the reflective sheet  13  is the lumirror E6SV manufactured by TORAY INDUSTRIES, INC., for example). 
     There are two prism sheets PS, and they are laminated. A prism sheet PS located below is called a first prism sheet PS 1 , and a prism sheet PS located above is called a second prism sheet PS 2 . 
     The first prism sheet PS 1  includes a light-reception face RS (a first light-reception face RS 1 ), which receives light from the LEDs  12  and light from the reflective sheet  13 , and an outgoing face IS (a first outgoing face IS 1 ), which releases light that has passed through this first light-reception face RS 1 . Further, in the first prism sheet PS 1 , triangle prisms PR, which have a linear shape extending in the Y direction, are arranged in the first outgoing face IS 1  along the X direction (the triangle prism PR of the first prism sheet PS 1  is called a triangle prism PR 1 ). This first prism sheet PS 1  refracts light coming from the first light-reception face RS 1  by two side surfaces SS (a first refraction face and a second refraction face) of a surface of the triangle prism PR 1 , and releases the light to outside. 
     The second prism sheet PS 2  overlaps with the first outgoing face IS 1  of the first prism sheet PS 1 . Therefore, the second prism sheet PS 2  includes a light-reception face RS (a second light-reception face RS 2 ) that receives light coming from the first outgoing face IS 1  of the first prism sheet PS 1 , and an outgoing face IS (a second outgoing face IS 2 ) that releases light that has passed through this second light-reception face RS 2 . 
     Specifically, in the second prism sheet PS 2 , triangle prisms PR, which have a linear shape extending in the X direction, are arranged in the second outgoing face IS 2  along the Y direction (here, the triangle prism PR of the second prism sheet PS 2  is called a triangle prism PR 2 ). In a similar manner as the first prism sheet PS 1 , this second prism sheet PS 2  refracts light coming from the first light-reception face RS 1  by two side surfaces SS (a first refraction face and a second refraction face) of a surface of the triangle prism PR 2 , and releases the light to outside. 
     Furthermore, the two prism sheets PS 1  and PS 2  overlap with each other such that the first outgoing face IS 1  of the first prism sheet PS I and the second light-reception face RS 2  of the second prism sheet PS 2  face each other, and that ridge lines R of the triangle prisms PR 1  and PR 2  in the two prism sheets PS 1  and PS 2  are perpendicular to each other (here, the ridge lines R of the triangle prism PR 1  of the first prism sheet PS 1  are in a direction same as the Y direction, and the ridge lines R of the triangle prism PR 2  of the second prism sheet PS 2  are in a direction same as the X direction). Here, the ridge line R of the triangle prism PR is a connecting line that is formed by the connection between one side surface SS 1  and other side surface SS 2  of the side surfaces SS of the triangle prism PR. 
     The diffusion sheet  31  (a first diffusion sheet) overlaps with the second outgoing face IS 2  of the second prism sheet PS 2 . The diffusion sheet  31  receives and then diffuses light that has passed through the first prism sheet PS 1  and the second prism sheet PS 2 , and spread the light to the entire region of the liquid crystal display panel  49  (one example of the diffusion sheet  31  is PC-9391, which is polycarbonate manufactured by TEIJIN CHEMICALS LTD., for example). 
     The optical sheets  32  ( 32 A and  32 B) are luminance increasing sheets, for example. The optical sheets  32  for increasing the luminance focus light passing through the sheets by taking advantage of multiple reflection and refractive index of light to increase the luminance (an example of the optical sheets  32  for increasing luminance is BEF and DBEF manufactured by Sumitomo 3M Limited, for example). 
     Moreover, the optical sheets  32  are not limited to luminance increasing sheets, and they may be sheets for diffusing light such as the diffusion sheet  31  (an example of the optical sheets  32  for diffusing light is BS-912 manufactured by KEIWA INC., for example). 
     In the backlight unit  39  described above, light from the LEDs  12  travels through the first prism sheet PS 1 , the second prism sheet PS 2 , the diffusion sheet  31 , and the optical sheets  32  ( 32 A and  32 B), and then the light is emitted as backlight light with increased light-emitting luminance. This backlight light then reaches the liquid crystal display panel  49 , and the image quality of the liquid crystal display panel  49  is improved by the backlight light. 
     Here, in the backlight unit  39  described above, the LEDs  12  are arranged in a matrix (along the X direction as well as the Y direction) in the LED module MJ. Therefore, if light emitted from each LED  12  does not spread properly along the X direction and the Y direction, when the backlight light (planar light) is observed in a plan view, the area of the LEDs  12  becomes brighter than the other areas (the LEDs  12  are reflected on the planar light). That is to say, planar light having unevenness in light amount is generated. 
     As a measure for such unevenness in light amount, as shown in  FIG. 2 , it is preferable that light be emitted having a relatively large outgoing angle δ with respect to a normal direction N of a sheet surface direction of the first prism sheet PS 1  (a surface direction same as the direction of the first light-reception face RS 1 ), for example. And, the backlight unit  39  described above is designed such that light having a relatively large outgoing angle δ is emitted from the prism sheet PS. Therefore, unevenness in light amount can be suppressed in the backlight unit  39 . 
     Here, the backlight unit  39  in which such unevenness in light amount is suppressed (for convenience, it is referred to as the backlight unit  39  of Example 1; see  FIGS. 1 and 2 ) is described by using a backlight unit  39 ′ that emits planar light with unevenness in light amount as a comparative example (the comparative example is shown in the exploded perspective view of  FIG. 3 , and in  FIG. 4 , which is a cross-sectional arrow view along the line a 1 -a 1 ′ of  FIG. 3 ). 
     Further, the first prism sheet PS 1  is mainly described below, but it is needless to say that the second prism sheet PS 2  may be formed in a manner similar to the first prism sheet PS 1 , which will be described later. And, such a second prism sheet PS 2  has a similar functional effect as the first prism sheet PS 1 , which will be described later. 
     First, the comparative example is described (for convenience, in order to avoid confusion with Example 1, “′” may be attached to a component number and the like of the comparative example). In this backlight unit  39 ′ of the comparative example, over a first light-reception face RS 1 ′ of a first prism sheet PS 1 ′, triangle prisms PR 1 ′, which have a linear shape extending in the Y direction, are aligned along the X direction. In this first prism sheet PS 1 ′, light is refracted by one of two side surfaces SS 1   a ′ and SS 2   a ′ of a surface of the triangle prism PR 1 ′, and the light is then guided to a first outgoing face IS 1 ′ and is emitted to outside. 
     Further, in the backlight unit  39 ′ of the comparative example, the second prism sheet PS 2 ′ overlaps with the first outgoing face IS 1 ′ of the first prism sheet PS 1 ′. Accordingly, this second prism sheet PS 2 ′ includes a second light-reception face RS 2 ′, which receives light coming from the flat-surfaced first outgoing face IS 1 ′ of the first prism sheet PS 1 ′, and a second outgoing face IS 2 ′, which emits light that has passed through this second light-reception face RS 2 ′. 
     In the second prism sheet PS 2 ′, triangle prisms PR 2 ′, having a linear shape extending in the X direction, are aligned along the Y direction in the second light-reception face RS 2 ′. In this second prism sheet PS 2 ′, light is refracted by one of two side surfaces SS 1   b ′ and SS 2   b ′of a surface of the triangle prism PR 2 ′, and the light is then guided to the second outgoing face IS 2 ′ and is emitted to outside. 
     How the outgoing angle δ changes when the vertex angle θ of each triangle prism PR 1  (PR 1 ′) of the first prism sheet PS 1  (PS 1 ′) is changed will be compared between Example 1 and the comparative example (here, the vertex angle θ is an angle formed by one side surface and the other side surface in the prism). A graph showing the result of the comparison is  FIG. 5  (that is,  FIG. 5  is a graph showing outgoing angles δ corresponding to vertex angles θ of the triangle prism PR). Various numerical examples of Example 1 and the comparative example are as follows. 
     EXAMPLE 1  
     
         
         
           
             The refractive index of the first prism sheet PS 1  and the second prism sheet PS 2 =1.5 
             The critical angle CA at a boundary surface between the first outgoing face IS 1  and air≈42(°) 
             The critical angle CA at a boundary surface between the second outgoing face IS 2  and air) 
             The shortest distance D(L−R) from the LEDs  12  (light-emitting surface  12 L to be precise) to the ridge line R of the prism PR 1  of the first prism sheet PS 1 =10 (mm) 
             The shortest distance D(L−DS) from the LEDs  12  to a light-reception face  31 B of the diffusion sheet  31 =20 (mm) 
             The thickness of the first prism sheet PS 1 =2.0 (mm) 
             The arrangement interval of the triangle prisms PR 1  in the first prism sheet PS 1 =0.1 (mm) 
             The thickness of the second prism sheet PS 2 =0.2 (mm) 
             The arrangement interval of the triangle prisms PR 2  in the second prism sheet PS 2 =0.1 (mm) 
             The arrangement interval Px of the LEDs  12  in the X direction=55 (mm) 
             The arrangement interval Py of the LEDs  12  in the Y direction=55 (mm) 
           
         
       
    
     COMPARATIVE EXAMPLE  
     
         
         
           
             The refractive index of the first prism sheet PS 1 ’ and the second prism sheet PS 2 ′=1.5 
             The critical angle CA at a boundary surface between the first outgoing face IS 1 ′ and air≈42(°) 
             The critical angle CA at a boundary surface between the second outgoing face IS 2 ′ and air≈42(°) 
             The shortest distance D (L−R) from the LEDs  12 ′ to the ridge line R of the ridge line R′ of the prism PR 1 ′ in the first prism sheet PS 1 =10 (mm) 
             The shortest distance D (L-DS) from the LEDs  12 ′ to a light-reception face of the diffusion sheet  31 =20 (mm) 
             The thickness of the first prism sheet PS 1 ′=0.2 (mm) 
             The arrangement interval of the triangle prisms PR 1 ′ in the first prism sheet PS 1 ′=0.1 (mm) 
             The thickness of the second prism sheet PS 2 ′=0.2 (mm) 
             The arrangement interval of the triangle prisms PR 2 ′ in the second prism sheet PS 2 ′=0.1 (mm) 
             The arrangement interval Px′ of the LEDs  12 ′ in the X direction=55 (mm) 
             The arrangement interval Py′ of the LEDs  12 ′ in the Y direction=55 (mm) 
           
         
       
    
     &lt;Light Straightly Above LED&gt; 
     As shown in the result of the comparative example in  FIG. 5 , the larger the vertex angle θ of the triangle prism PR 1 ′ is, the smaller the outgoing angle δ becomes. This is because, as shown in  FIG. 4 , when light incoming nearly straight to the first prism sheet PS 1 ′ (the first light-reception face RS 1 ′ to be precise) from the LEDs  12 ′ transmits through without being totally reflected by one side surface SS 1   a ′ of the triangle prism PR 1 ′, the light reaches and transmits through the first outgoing face IS 1 ′ without reaching the other side surface SS 2   a ′ (see the dashed arrow line). 
     More specifically, in the comparative example, the tip of the triangle prism PR 1 ′ points toward the side of the LEDs  12 ′, and therefore, the triangle prism PR 1 ′ tapers off toward the LEDs  12 ′. Accordingly, when incident light to one side surface SS 1   a ′ of the triangle prism PR 1 ′ transmits through without being totally reflected, the transmitting light has a refractive angle smaller than the incident angle with respect to the one side surface SS 1   a ′. Therefore, the light that has passed through the one side surface SS 1   a ′ travels to the first outgoing face IS 1 ′without reaching the other side surface SS 2   a ′, which faces the one side surface SS 1   a ′, within the triangle prism PR 1 ′. 
     Further, a surface of the first outgoing face IS 1 ′ is facing a direction same as the direction of the LED-mounted surface XY. Therefore, light coming from the one side surface SS 1   a ′ enters the first outgoing face IS 1 ′ at an angle inversely proportional to the size of the refractive angle with respect to the one side surface SS 1   a ′. Then, light that transmits through the first outgoing face IS 1 ′ is also emitted having an outgoing angle δ that is proportional to an incident angle of light entering the first outgoing face IS 1 ′. 
     Accordingly, when a refractive angle with respect to one side surface SS 1   a ′ of the triangle prism PR 1 ′ is likely to become large, in other words, when a vertex angle θ of the triangle prism PR 1 ′ is relatively small, the outgoing angle δ is likely to become large. On the other hand, when the vertex angle θ of the triangle prism PR 1 ′ is relatively large, the outgoing angle δ becomes small. 
     Meanwhile, in Example  1 , the tip of the triangle prism (light refractive element) PR 1  points toward the side of the diffusion sheet  31 , and therefore, the triangle prism PR 1  tapers off as it becomes distant from the LEDs  12 . Therefore, as shown in  FIG. 2 , light that enters nearly straight to the first prism sheet PS 1  (the first light-reception face RS 1  to be precise) from the LEDs  12  reaches one side surface SS 1   a  of the triangle prism PR 1 , and the result at light has an outgoing angle δ in accordance with the vertex angle θ as shown in  FIG. 5 . From the results in  FIG. 5 , light paths shown in  FIGS. 6A to 6D  (see arrow dashed lines) are understood. 
       FIG. 6A  shows a light path in which light that has been refracted by the side surface SS 1   a  of the triangle prism PR 1  is refracted after reaching the other side surface SS 2   a , and travels so as to come back to the side surface SS 1  a, and then continues to transmit through the side surface SS 1   a.    
       FIG. 6B  shows a light path in which light that has been refracted by the side surface SS 1   a  of the triangle prism PR 1  reaches the other side surface SS 2   a , and continues to transmit through the other side surface SS 2   a.    
       FIG. 6C  shows a light path in which light that has been refracted by the side surface SS 1   a  of the triangle prism PR 1  is refracted after reaching the other side surface SS 2   a , and then travels toward the first light-reception face RS 1 . 
       FIG. 6D  shows a light path in which light that has been refracted by the side surface SS 1   a  of the triangle prism PR 1  continues to transmit through the side surface SS 1   a.    
     As for the triangle prism PR 1  that forms the light paths shown in  FIGS. 6A to 6D , the vertex angle θ becomes larger in the order of  FIGS. 6A to 6D . Then,  FIGS. 6A to 6D  correspond to ranges A to D of the vertex angle θ in  FIG. 5 . 
     These ranges A to D of the vertex angle θ are obtained by recognizing the cross-sectional shape of the triangle prism PR 1  (PR 1 ′) as a triangle shape (that is, an isosceles triangle cross-section) with isosceles side surfaces SS 1   a  and SS 2   a  (SS 1   a ′ and SS 2   a ′), as shown in  FIGS. 2 and 4 , for example. Specifically, the ranges A to D of the vertex angle θ are obtained by recognizing the triangle prism PR 1  (PR 1 ′) as an isosceles triangle at an XZ surface cross-section formed by the X direction and the Z direction (see  FIGS. 7 and 8 ). 
     First, a base angle of the triangle prism PR 1  is called “α(°)”, and it is assumed that light entering nearly straight to the first prism sheet PS 1  from the LEDs  12  is totally reflected by the side surface SS 1   a  and then transmits through the other side surface SS 2   a . The light exiting from the other side surface SS 2   a  may travel at an outgoing angle δ away from the tip of the triangle prism PR 1  (see  FIG. 7 ), or may travel at an outgoing angle δ so as to get closer to the tip of the triangle prism PR 1  (see  FIG. 8 ). 
     In the cases of  FIGS. 7 and 8 , an incident angle to the other side surface SS 1   a  becomes “α”, which is same as the base angles. Accordingly, in order for light to be totally reflected by the other side surface SS 2   a , a formula below (P 1 ) needs to be satisfied. 
       α≧CA   (P1)
 
     Here, CA is a critical angle (°) at a boundary surface between the side surface of the triangle prism PR 1  and air. 
     Further, in the case of  FIG. 7 , an incident angle that light coming from the side surface SS 1   a  has with respect to the other side surface SS 2   a  is “180−3α”. Therefore, in order for light coming from the side surface SS 1   a  to transmit through the other side surface SS 2   a  without being totally reflected by the other side surface SS 2   a , a formula below (P2) needs to be satisfied. 
       180−3α&lt;CA   (P2)
 
     Accordingly, the base angle α is expressed by a formula below (P3) using the critical angle CA. 
       α&gt;(180 −CA )/3   (P3)
 
     In the case of  FIG. 8 , an incident angle that light coming from the side surface SS 1   a  has with respect to the other side surface SS 2   a  is “3α−180”. In order for light coming from the side surface SS 1   a  to transmit through the other side surface SS 2   a  without being totally reflected by the other side surface SS 2   a , a formula below (P4) needs to be satisfied. 
       3α−180&lt; CA    (P4)
 
     Accordingly, the base angle α is expressed by a formula below (P5) using the critical angle CA. 
       α&lt;(180+ CA )/3   (P5)
 
     Further, it is also possible to have a formula (P6) from the formula (P1) and the formula (P5). 
         CA ≦α&lt;(180+ CA )/3   (P6)
 
     A formula (P7) is derived from the formula (P3) and the formula (P5). 
       (180 −CA )/3&lt;α&lt;(180+ CA )/3   (P7)
 
     When light is totally reflected by the side surface SS 1   a , and the totally reflected light transmits through the other side surface SS 2   a  by this formula (P7), if the critical angle) CA≈42(°), the base angle a stays within the range below. 
       46(°)&lt;α&lt;74(°)   (P8)
 
     According to the formula (P8), the vertex angle θ of the triangle prism PR 1  stays within the range of a formula below (P9). And, the range of the vertex angle θ shown in this formula (P9) corresponds to the range B in  FIG. 5 , and the light path corresponds to  FIG. 6B . 
       32(°)&lt;θ&lt;88(°)   (P9)
 
     Moreover, if the vertex angle θ (=180−2α) in  FIG. 8  becomes too small, light entering to the other side surface SS 2   a  is totally reflected, and travels so as to come back to the side surface SS 1   a , and transmits through the side surface SS 1   a . In this case, a formula below (P10) is derived, and a formula (P11) is further derived (here, the reason for using) 90(°) in the formula (P11) is that it is impossible for an isosceles triangle to have two base angles a that are equal to or larger than 90(°)). 
       3α−180 ≧CA    (P10)
 
       (180+ CA )/3≦α&lt;α&lt;90   (P11)
 
     Accordingly, in the case of the formula (P11), when light is totally reflected by the side surface SS 1   a , and when the totally reflected light is further totally reflected by the other side surface SS 2   a  and comes back to the side surface SS 1   a,  and transmits through the side surface SS 1   a,  if the critical angle CA≈42(°), the base angles a stays within the range below. 
       74(°)≦α&lt;90(°)   (P12)
 
     Accordingly, from the formula (P12), the vertex angle θ of the triangle prism PR 1  stays within the range of a formula below (P13). The range of the vertex angle θ shown in this formula (P13) corresponds to the range A in  FIG. 5 , and the light path corresponds to  FIG. 6A .) 
       θ≦32(°)   (P13)
 
     Moreover, if the vertex angle θ (=180−2α) of  FIG. 7  becomes too large, light entering to the other side surface SS 2   a  is totally reflected, and travels toward the first light-reception face RS 1 . In this case, a formula below (P14) is derived, and a formula (P15) is further derived (here, a is larger than CA in the formula (P15) because if this relationship is not established, the total reflection does not occur at the side surface SS 1   a ). 
       180−3α≧ CA    (P14)
 
         CA ≦α≦(180− CA )/3   (P15)
 
     In the case of the formula (P15), when light is totally reflected by the side surface SS 1   a , and when the totally reflected light is further totally reflected by the other side surface SS 2   a  and travels toward the first light-reception face RS 1 , if the critical angle CA≈42(°), the base angle α stays within the range below.) 
       42(°)≦α≦46(°)   (P16)
 
     Accordingly, from the formula (P16), the vertex angle θ of the triangle prism PR 1  stays within the range of a formula below (P17). And, the range of the vertex angle θ shown in this formula (P17) corresponds to the range C in  FIG. 5 , and the light path corresponds to  FIG. 6C .) 
       88(°)≦θ≦96(°)   (P17)
 
     Furthermore, light may transmit through the side surface SS 1   a  without being totally reflected. In this case, because an incident angle a to the side surface SS 1   a  is smaller than the critical angle CA (≈42°), the base angle a stays within the range of a formula below (P18). 
       0(°)&lt;α&lt;42(°)   (P18)
 
     Therefore, from the formula (P18), the vertex angle θ of the triangle prism PR 1  stays within the range of a formula below (P19). And, the range of the vertex angle θ shown in this formula (P19) corresponds to the range D in  FIG. 5 , and the light path corresponds to  FIG. 6D . 
       96(°)&lt;θ&lt;180(°)   (P19)
 
     The followings can be said from  FIG. 5  and  FIGS. 6A to 6D  described above. Comparing Example 1 and the comparative example, when the vertex angle θ of the triangle prism PR 1  is kept within the range A, the range B, and the range C, the comparative example often has a larger outgoing angle δ than Example 1. 
     However, in the case of the range B, if the vertex angle is equal to or larger than 50°, the outgoing angle δ of Example 1 becomes larger than the outgoing angle δ of the comparative example. In other words, when the vertex angle  0  is in the range of a formula below (A1), the outgoing angle δ of Example 1 becomes larger than the outgoing angle δ of the comparative example. 
       50(°)≦θ&lt;88(°)   (A1)
 
     When the vertex angle θ is within the range of this formula (A1), incident light nearly straight to the first prism sheet PS 1  travels at an angle largely inclined from the normal direction N, which overlaps the LEDs  12 . As a result, the LEDs  12  are no longer noticeable from outside, and backlight light with suppressed unevenness in light amount is generated. Therefore, when the vertex angle θ is within the range of the formula (A1), the first prism sheet PS 1  should have the prism surface IS 1  (light refractive element surface, the first outgoing face IS 1 ) in which the prisms PR 1 , which are light refractive elements, are arranged, facing not on the side of the LED-mounted surface XY, but on the opposite side. 
     Moreover, when the vertex angle θ is smaller than 50(°), incident light nearly straight to the first prism sheet PS 1  travels at an angle largely inclined from the normal direction N, which overlaps the LEDs  12 , more so in the comparative example than Example 1. However, the smaller the vertex angle θ of the triangle prism PR 1  is, the more difficult it becomes to manufacture (shape forming or the like) with high accuracy (moreover, the cost is also likely to increase due to the difficulty with manufacturing). Accordingly, the backlight unit  39  of Example 1 can use a prism sheet PS that is lower in cost and easier to manufacture than the backlight unit  39  of the comparative example. 
     Here, in Example 1, if the outgoing angle δ is equal to or larger than 90(°), light incoming from the surface SS 1   a  of the triangle prism PR 1  travels away from the second prism sheet PS 2  when it transmits through the other surface SS 2   a  (in other words, light exiting from the other surface SS 2   a  travels so as to get closer to the first light-reception face RS 1 ). 
     Therefore, light is not likely to reach the second prism sheet PS 2  directly from the other side surface SS 2   a . Accordingly, it is preferable to have an angle smaller than a vertex angle θ that makes 90(°) of the outgoing angle δ. Then, if a vertex angle θ corresponding to when the outgoing angle δ is 90(°) is 76(°), it is preferable that the vertex angle θ be smaller than the angle of 76(°), and therefore, a formula below (A2) is derived. 
       50(°)≦θ&lt;76(°)   (A2)
 
     Furthermore, in terms of manufacturing a prism sheet PS, it is easier to manufacture ones with a larger vertex angle θ. Accordingly, it is preferable that the prism sheet PS include a plurality of triangle prisms PR having a vertex angle θ within the range of a formula below (A3). This is because the backlight unit  39  including such a prism sheet PS can easily generate backlight light with suppressed unevenness in light amount at low cost.) 
       65(°)≦θ&lt;76(°)   (A3)
 
     To summarize, when the first prism sheet PS 1  is included in the backlight unit  39 , the triangle prisms PR 1 , which refract light coming from the first light-reception face RS 1 , are formed in the first outgoing face IS 1  of the first prism sheet PS 1 . One surface (the side surface SS 1   a,  for example) out of the side surfaces SS 1   a  and SS 2   a  of the triangle prism PR 1  refracts light coming from the first light-reception face RS 1 , and the other side surface (the side surface SS 2   a , for example) refracts light coming from the one surface. 
     When light is transmitted between the side surfaces SS 1   a  and SS 2   a  this way, a prism including these side surfaces SS 1   a  and SS 2   a  has a shape that tapers off toward the side away from the first light-reception face RS 1 , such as the triangle prism PR 1 . Therefore, a large part of light coming from the first light-reception face RS 1  is refracted twice by the two side surfaces SS 1   a  and SS 2   a  of the triangle prism PR formed in the first outgoing face IS 1 , and therefore, the amount of light that travels at an angle largely inclined from the normal direction N, which overlaps the LEDs  12 , is relatively increased. 
     Particularly, because the ridge lines R of the triangle prisms PR 1  that are aligned in the first prism sheet PS 1  in the X direction extends in the Y direction, a large part of light along the X direction travels at relatively large inclination angles with respect to the normal direction N overlapping the LED  12 . 
     Further, the triangle prisms PR 2 , which refract light coming from the second light-reception face RS 2 , are formed in the second outgoing face IS 2  of the second prism sheet PS 2 . One surface (the side surface SS 1   b,  for example) out of the side surfaces SS 1   b  and SS 2   b  of the triangle prism PR 2  refracts light coming from the second light-reception face RS 2 , and the other side surface (the side surface SS 2   b , for example) refracts light coming from the one side surface. 
     This way, functional effects similar to those of the first prism sheet PS 1  are obtained. In other words, a large part of light traveling from the second light-reception face RS 2  is refracted twice by the two side surfaces SS 1   b  and SS 2   b  of the triangle prism PR 2  formed in the second outgoing face IS 2 , and therefore, the amount of light that travels at relatively large inclination angles with respect to the normal direction N, which overlaps the LED  12 , is relatively increased. 
     Particularly, because the ridge lines R of the triangle prisms PR 2  that are aligned in the second prism sheet PS 2  in the Y direction extend in the X direction, a large part of light along the Y direction travels at relatively large inclination angles with respect to the normal direction N, which overlaps the LED  12 . 
     Based on the description above, if at least one of the first prism sheet PS 1  and the second prism sheet PS 2  is mounted in the backlight unit  39 , light that travels at relatively large inclination angles with respect to the normal direction N, which overlaps the LED  12 , is increased. That is, light straightly above the LEDs  12  or the like is no longer noticeable from the outside, and backlight light with suppressed unevenness in light amount is generated (see  FIG. 10 , which will be described later). 
     &lt;Peripheral Light From LED&gt; 
     A description was made above on how incident light nearly straightly above the first prism sheet PS 1  (light directly above) is refracted by the triangle prisms PR 1 . This is because such light (light that travels nearly straight from the light emitting surface  12 L of the LEDs  12 ; light directly above) is likely to become a cause for unevenness in light amount because it has relatively high light intensity among light emitted from the LEDs  12 . 
     However, the LEDs  12  also emit light other than the straight light (peripheral light). Here, by using  FIGS. 9A to 9D , a description is made on when such peripheral light enters the first prism sheet PS 1 . Further, numerical values for the backlight unit  39  for these figures are similar to the examples of numerical values of the above-described Example 1. Here, the vertex angle θ of the triangle prism PR 1  is 70(°). For convenience, the vertex angle θ and the outgoing angle δ in these figures may be assigned with a character such that one side of the normal direction N (the right side on the paper) is “+”, and the other side (the left side on the paper) is “−”. 
     First, as shown in  FIG. 9A , light L 1  entering nearly straight to the first prism sheet PS 1  (the first light-reception face RS 1  to be precise) is totally reflected by one side surface SS of the triangle prism PR 1 , and then transmits through the other side surface SS. An outgoing angle (refractive angle) δ 1  of light when exiting from the other side surface SS is shown in a formula below (Q1). 
       δ1≈|78(°)|  formula (Q1)
 
     Here, it is preferable that an incident angle β 1  to the first light-reception face RS 1  be β 1 ≈±0(°). 
     Meanwhile, as shown in  FIG. 9B , light L 2 , which enters the first prism sheet PS 1  at an incident angle β 2  of approximately 20(°) in absolute value, is totally reflected by one side surface SS of the triangle prism PR 1 , and then transmits through the other side surface SS in a similar way as the light L 1 . However, the value of an outgoing angle δ 2  changes in accordance with the incident angle to the side surface SS. 
     To explain in more detail, the value of the outgoing angle δ 2  is different between when light enters a side surface SS that has a relatively small inclination with respect to the incident direction of light to the first light-reception face RS 1  (side surface that follows the incident direction), and when light enters to a side surface SS that has a relatively large inclination with respect to the incident direction of light to the first light-reception face RS 1 . 
     Specifically, when light enters the side surface SS that has a relatively small inclination with respect to the incident direction of light to the first light-reception face RS 1 , the outgoing angle δ 2  becomes approximately 58(°) in absolute value. On the other hand, when light enters the side surface SS that has a relatively large inclination with respect to the incident direction of light to the first light-reception face RS 1 , the outgoing angle δ 2  becomes approximately 100(°) in absolute value. 
     Therefore, a formula (Q2) and a formula (Q2′) below are derived. 
       δ2≈|58(°)|  formula (Q2)
 
       δ2≈|100(°)|  formula (Q2′)
 
     Further, when the formula (Q2) and the formula (Q2′) are established, it is preferable that the incident angle β 2  to the first light-reception face RS 1  be 0(°)&lt;β 2 ≦|20(°)|. 
     Next, as shown in  FIG. 9C , light L 3  that enters the first prism sheet PS 1  at an incident angle β 3  that is larger than 20(°) and is approximately 59(°) in absolute value is described as follows. 
     When the light L 3  enters one of the side surfaces SS of the triangle prism PR 1  at an incident angle β 3  of slightly larger than 20(°) in absolute value, the value of the outgoing angle δ 3  becomes different between when light enters the side surface SS that has a relatively small inclination with respect to the incident direction of light to the first light-reception face RS 1 , and when light enters the side surface SS that has a relatively large inclination with respect to the incident direction of light to the first light-reception face RS 1 . 
     Specifically, when light enters the side surface SS that has a relatively small inclination with respect to the incident direction of light to the first light-reception face RS 1 , the outgoing angle δ 3  becomes less than 58(°) in absolute value. On the other hand, when light enters the side surface SS that has a relatively large inclination with respect to the incident direction of light to the first light-reception face RS 1 , the outgoing angle δ 3  becomes approximately 35(°) in absolute value. 
     Further, when the light L 3  enters one of the side surfaces SS of the triangle prism PR 1  at the incident angle β 3  of approximately 59(°) in absolute value, regardless of which one of the side surfaces SS, the light L 3  transmits through the side surface SS at an outgoing angle of approximately 24(°) in absolute value. 
     As a result, a formula below (Q3) is derived. 
       |24(°)|≦δ3&lt;|58(°)|  formula (Q3)
 
     Further, when the formula (Q3) is established, it is preferable that the incident angle β 3  to the first light-reception face RS 1  satisfy |20(°)|&lt;β3≦|59(°)|. 
     Next, as shown in  FIG. 9D , light L 4  incident to the first prism sheet PS 1  at an incident angle β 4  of larger than 59(°) in absolute value enters one of the side surfaces SS of the triangle prism PR 1 , and then transmits through the side surface SS at an outgoing angle δ 4  of smaller than 35(°) in absolute value without being totally reflected. 
     As a result, a formula below (Q4) is derived. 
       |24(°)|&lt;δ4&lt;|35(°)|  formula (Q4)
 
     Further, when the formula (Q4) is established, it is preferable that the incident angle β 4  to the first light-reception face RS 1  satisfy |59(°)|&lt;β3&lt;|90(°)|. 
     To summarize, as shown in  FIGS. 9A and 9B , when the incident angle β is equal to or less than 20(°) in absolute value, for example, when light straightly above the LEDs  12  (light with relatively high light intensity) exits from the first prism sheet PS 1 , the light is directed away the LEDs  12  with certainty (light exits the first prism sheet PS 1  at the outgoing angle δ of equal to or larger than 58(°) in absolute value). Therefore, in the planar light, the luminance (light density) of a region overlapping with the LEDs  12  is suppressed while increasing the luminance of a region overlapping with a gap between LEDs  12 . 
     Meanwhile, as shown in  FIGS. 9C and 9D , in the case where the incident angle β is larger than 20(°) in absolute value, for example, when light (peripheral light; light with relatively low light intensity) other than light straightly above the LEDs  12  exits from the first prism sheet PS 1 , the light exits at an outgoing angle δ of smaller than 58(°) in absolute value (a large part of the light exits at an outgoing angle δ of smaller than 35(°) in absolute value, to be precise). 
     This outgoing angle δ is smaller than the outgoing angle δ when the incident angle β is smaller than 20(°) in absolute value. However, as shown in  FIG. 10  (a figure showing a part of the light paths shown in  FIGS. 9A to 9D  along with one LED  12 ), compared to the light directly above the LED  12 , peripheral light is directed away from the LED  12  before it reaches the first prism sheet PS 1 . 
     Therefore, even though the outgoing angle δ of the peripheral light is relatively small, the peripheral light reaches a region overlapping with the gaps between the LEDs  12  after it exits the first prism sheet PS 1 , and thereby increasing the luminance. Accordingly, the unevenness in light amount can be suppressed in the backlight unit  39  that includes the first prism sheet PS 1  in which the prism surface IS 1 (the first outgoing face IS 1 ) is not facing the LED-mounted surface XY, but facing the other side (the diffusion sheet  31 ). 
     &lt;Distance Between LED and Prism Sheet&gt; 
     Here, light from the LEDs  12  is usually diverged. Therefore, if the distance between the LEDs  12  and the prism sheet PS is too small, the shape of the LEDs  12  is reflected in the prism sheet PS, and thereby causes deterioration of the quality of the backlight light (that is, the luminance uniformity of planar light is deteriorated). 
     Here, Example 1 and the comparative example described above will be compared on the following conditions. To explain in more detail, images in which the luminance uniformity of planar light is recognizable are obtained and compared on the following conditions. And, the result is shown in  FIG. 11 . 
     EXAMPLE 1  
     
         
         
           
             The shortest distance D (L−R) from the LEDs  12  (the light emitting surface  12 L to be precise) to the ridge line R of the prisms PR 1  in the first prism sheet PS 1  is changed to 0, 5, 7, 10, 12.5, and 15 (mm). Here, 0 (mm) means that the LEDs  12  and the first prism sheet PS 1  are in close contact with each other. 
             The vertex angle θ of the prisms PR 2  in the first prism sheet PS 1  and the second prism sheet PS 2  is changed to 65(°), 70(°), and 90(°). 
           
         
       
    
     COMPARATIVE EXAMPLE  
     
         
         
           
             The shortest distance D(L−R) from the LEDs  12 ′ to the ridge line R′ of the prism PR 1 ′ in the first prism sheet PS 1 ′ is changed to 0, 5, 7, 10, 12.5, and 15 (mm). Here, 0 (mm) means that the LEDs  12 ′ and the first prism sheet PS 1 ′ are in close contact with each other. 
             The vertex angle θ of the triangle prism PR′ (PR 1 ′ and PR 2 ′) in the first prism sheet PS 1 ′ and the second prism sheet PS 2 ′ is fixed to 90(°). 
           
         
       
    
     As shown in  FIG. 11 , in both Example 1 and the comparative example, when the shortest distance D(L−R) between the LEDs  12  and the ridge line R of the prism PR 1  is 0 (mm), the LEDs  12  are reflected in the planar light, and therefore, the luminance uniformity is determined to be low. 
     Moreover, even when the shortest distance D(L−R) is 5 (mm), the LEDs  12  are reflected in the planar light in both Example 1 and the comparative example, and therefore, the luminance uniformity is determined to be low. 
     Also, when the shortest distance D(L−R) is 7 (mm), the LEDs  12  are reflected in planar light when the vertex angle θ is 65(°) or 90(°) in Example 1, and in the comparative example, and therefore, the luminance uniformity is determined to be low. However, when the vertex angle θ is 70(°) in Example 1, the LEDs  12  are not reflected as much as the other cases. 
     Further, in  FIG. 11 , in the area surrounded by a dashed line rectangular, that is, when the vertex angle θ is 70(°) and the shortest distance D (L−R) is 10 (mm), 12.5 (mm), or 15 (mm) in Example 1, the reflection of the LEDs  12  is further suppressed compared to when the vertex angle θ is 70(°) and the shortest distance D(L−R) is 7 (mm) in Example 1 (on the other hand, the LEDs  12  are reflected distinctively when the vertex angle θ is in other values in Example 1 and in the comparative example). Particularly, when the vertex angle θ is 70(°) and the shortest distance D(L−R) is 10 (mm) or 12.5 (mm) in Example 1, the LEDs  12  are barely reflected, and the luminance uniformity becomes very high (see the dashed line rectangular). 
     Furthermore, although not shown in  FIG. 11 , when the vertex angle θ is 70(°) and the shortest distance D(L−R) is 20 (mm) in Example 1, planar light that is approximately at the same level as when the vertex angle θ is 70(°) and the shortest distance D(L−R) is 7 (mm) in Example 1 is obtained. 
     From the result of Example 1 described above, a formula below (B1) is derived. That is, when a range satisfies the formula below (B1), the unevenness in luminance of planar light can be suppressed. 
       0.35 &lt;D ( L−R )/ D ( L−DS )&lt;1   formula (B1)
 
     Here,
         D(L−R) means the shortest distance from the LEDs  12  to the ridge line R of the prism PR 1  in the first prism sheet PS 1     D(L−DS) means the shortest distance from the LEDs  12  to the light-reception face  31 B of the diffusion sheet  31 .       

     Further, when the formula below (B2) is satisfied, the unevenness in luminance of planar light is even more suppressed, and when the formula below (B3) is further satisfied, the unevenness in luminance of planar light is suppressed with certainty. 
       0.5≦ D ( L−R )/ D ( L−DS )≦0.75   formula (B2)
 
       0.5 ≦D ( L−R )/ D ( L−DS )≦0.625   formula (B3)
 
     &lt;Refractive Index&gt; 
     The first prism sheet PS 1  and the second prism sheet PS 2  with a refractive index of 1.5 were described above as an example. However, the refractive index (n) is not limited to this value. Here, light straightly above the LEDs  12  is described by using new figures  FIGS. 12 and 13 , which are based on  FIG. 2  (Example 1) and  FIG. 4  (comparative example).  FIG. 12  is based on  FIG. 2 , and  FIG. 13  is based on  FIG. 4  (however, the refractive index of a prism sheet shown in  FIGS. 12 and 13  is not particularly limited). 
     The vertex angle θ and the base angles a of the triangle prism PR 1  in the first prism sheet PS 1  in  FIG. 12  have the same values as the vertex angle θ and the base angles α of the triangle prism PR 1 ′ in the first prism sheet PS 1 ′ in  FIG. 13 . 
     As shown in  FIG. 12 , an angle that light existing from the first prism sheet PS 1  has with respect to the normal direction NL 2  of the side surface SS 2   a  of the triangle prism PR is called a refractive angle P, and an outgoing angle δ (an angle that light has with respect to the normal direction N to a sheet surface direction of the first prism sheet PS 1 ) of the exiting light is called an outgoing angle δp. An angle formed by the normal direction NL 2  with respect to the side surface SS 2   a  of the triangle prism PR and by the normal direction N with respect to the sheet surface direction of the first prism sheet PS 1  is the same angle as the base angle α of the triangle prism PR. 
     Usually, at a boundary surface of different mediums, Snell&#39;s law is established. Accordingly, in the case of the first prism sheet PS 1  shown in  FIG. 12 , a formula below (E1) is established. 
       [Formula E1] 
       1·sin  P=n ·sin ( 180°−3α)    (E1)
 
     Thus, a formula below (E2) can be obtained from this formula (E1). 
       [Formula E2] 
         P =sin −1   {n ·sin (180°−3α)}  (E2)
 
     Moreover, the outgoing angle δp is a total of the base angle α and the refractive angle P as shown in  FIG. 12 , and therefore, a formula below (E3) is obtained. Here, the formula (E3) is expressed using the vertex angle θ (=180−2α). 
     
       
         
           
             
               
                 
                   [ 
                   
                     Formula 
                      
                     
                         
                     
                      
                     E 
                      
                     
                         
                     
                      
                     3 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     
                       
                         
                           δ 
                            
                           
                               
                           
                            
                           p 
                         
                         = 
                           
                          
                         
                           α 
                           + 
                           P 
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           
                             ( 
                             
                               
                                 90 
                                  
                                 ° 
                               
                               - 
                               
                                 θ 
                                 2 
                               
                             
                             ) 
                           
                           + 
                           
                             
                               sin 
                               
                                 - 
                                 1 
                               
                             
                              
                             
                               { 
                               
                                 n 
                                 · 
                                 
                                   sin 
                                    
                                   
                                     ( 
                                     
                                       
                                         180 
                                          
                                         ° 
                                       
                                       - 
                                       
                                         3 
                                          
                                         α 
                                       
                                     
                                     ) 
                                   
                                 
                               
                               } 
                             
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           
                             ( 
                             
                               
                                 90 
                                  
                                 ° 
                               
                               - 
                               
                                 θ 
                                 2 
                               
                             
                             ) 
                           
                           + 
                           
                             
                               sin 
                               
                                 - 
                                 1 
                               
                             
                              
                             
                               [ 
                               
                                 
                                   n 
                                   · 
                                   sin 
                                 
                                  
                                 
                                   { 
                                   
                                     
                                       180 
                                        
                                       ° 
                                     
                                     - 
                                     
                                       3 
                                       · 
                                       
                                         ( 
                                         
                                           
                                             90 
                                              
                                             ° 
                                           
                                           - 
                                           
                                             θ 
                                             2 
                                           
                                         
                                         ) 
                                       
                                     
                                   
                                   } 
                                 
                               
                               ] 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   E3 
                   ) 
                 
               
             
           
         
       
     
     Meanwhile, in the first prism sheet PS 1 ′ shown in  FIG. 13 , angles are defined as follows. That is, in the first prism sheet PS 1 ′, an angle with respect to the normal direction NL 1  of the side surface SS 1   a ′ of the triangle prism PR 1 ′ included in the first light-reception face RS 1 ′ is called an incident angle X, a refractive angle at the side surface SS 1   a ′ of incident light at the incident angle X is called a refractive angle Y, an angle that light traveling at the refractive angle Y has when entering to the light outgoing face IS 1 ′ is called an incident angle Z, and an outgoing angle of light when existing the first outgoing face IS 1 ′ of the first prism sheet PS 1 ′ is called an outgoing angle δc.  101481  Accordingly, as shown in  FIG. 13 , an external surface of the side surface SS 1   a ′ with respect to a horizontal surface is the same angle as the base angle α of the triangle prism PR 1 ′, and therefore, the incident angle X with respect to the normal direction NL 1  of the side surface SS 1   a ′ is the same angle as the base angle α. 
     Furthermore, according to Snell&#39;s law, the formula below (E4) is established, and the refractive angle Y satisfies the formula below (E5). 
     
       
         
           
             
               
                 
                   [ 
                   
                     Formula 
                      
                     
                         
                     
                      
                     E 
                      
                     
                         
                     
                      
                     4 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     
                       1 
                       · 
                       sin 
                     
                      
                     
                         
                     
                      
                     α 
                   
                   = 
                   
                     
                       n 
                       · 
                       sin 
                     
                      
                     
                         
                     
                      
                     Y 
                   
                 
               
               
                 
                   ( 
                   E4 
                   ) 
                 
               
             
             
               
                 
                   [ 
                   
                     Formula 
                      
                     
                         
                     
                      
                     E 
                      
                     
                         
                     
                      
                     5 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   Y 
                   = 
                   
                     
                       sin 
                       
                         - 
                         1 
                       
                     
                      
                     
                       ( 
                       
                         
                           sin 
                            
                           
                               
                           
                            
                           α 
                         
                         n 
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   E5 
                   ) 
                 
               
             
           
         
       
     
     If an auxiliary surface that reaches the first outgoing face IS 1 ′ of the first prism sheet PS 1 ′ and that extends from the side surface SS 1   a ′ is formed, an angle formed by the auxiliary surface and the first outgoing face IS 1 ′ within the first prism sheet PS 1 ′ becomes the same angle as the base angle α. Accordingly, from this angle α, and an angle “90°-Y” that is an angle formed by light traveling toward the light outgoing face IS 1 ′ at the refractive angle Y and by the side surface SS 1   a ′, the incident angle Z satisfies a formula below (E6). 
     
       
         
           
             
               
                 
                   [ 
                   
                     Formula 
                      
                     
                         
                     
                      
                     E 
                      
                     
                         
                     
                      
                     6 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   Z 
                   = 
                   
                     α 
                     - 
                     
                       
                         sin 
                         
                           - 
                           1 
                         
                       
                        
                       
                         ( 
                         
                           
                             sin 
                              
                             
                                 
                             
                              
                             α 
                           
                           n 
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   E6 
                   ) 
                 
               
             
           
         
       
     
     Accordingly, between the incident light of the incident angle Z and the outgoing light of the outgoing angle δc, a formula (E7) according to Snell&#39;s law is established, and the outgoing angle δc can be obtained by a formula below (E8). 
     
       
         
           
             
               
                 
                   [ 
                   
                     Formula 
                      
                     
                         
                     
                      
                     E 
                      
                     
                         
                     
                      
                     7 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     
                       n 
                       · 
                       sin 
                     
                      
                     
                       { 
                       
                         α 
                         - 
                         
                           
                             sin 
                             
                               - 
                               1 
                             
                           
                            
                           
                             ( 
                             
                               
                                 sin 
                                  
                                 
                                     
                                 
                                  
                                 α 
                               
                               n 
                             
                             ) 
                           
                         
                       
                       } 
                     
                   
                   - 
                   
                     
                       1 
                       · 
                       sin 
                     
                      
                     
                         
                     
                      
                     δ 
                   
                 
               
               
                 
                   ( 
                   E7 
                   ) 
                 
               
             
             
               
                 
                   [ 
                   
                     Formula 
                      
                     
                         
                     
                      
                     
                       E 
                        
                       8 
                     
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     
                       
                         
                           δ 
                            
                           
                               
                           
                            
                           c 
                         
                         = 
                           
                          
                         
                           
                             sin 
                             
                               - 
                               1 
                             
                           
                            
                           
                             [ 
                             
                               
                                 n 
                                 · 
                                 sin 
                               
                                
                               
                                 { 
                                 
                                   α 
                                   - 
                                   
                                     
                                       sin 
                                       
                                         - 
                                         1 
                                       
                                     
                                      
                                     
                                       ( 
                                       
                                         
                                           sin 
                                            
                                           
                                               
                                           
                                            
                                           α 
                                         
                                         n 
                                       
                                       ) 
                                     
                                   
                                 
                                 } 
                               
                             
                             ] 
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           
                             sin 
                             
                               - 
                               1 
                             
                           
                            
                           
                             [ 
                             
                               
                                 n 
                                 · 
                                 sin 
                               
                                
                               
                                 { 
                                 
                                   
                                     ( 
                                     
                                       
                                         90 
                                          
                                         ° 
                                       
                                       - 
                                       
                                         θ 
                                         2 
                                       
                                     
                                     ) 
                                   
                                   - 
                                   
                                     
                                       sin 
                                       
                                         - 
                                         1 
                                       
                                     
                                      
                                     
                                       ( 
                                       
                                         
                                           sin 
                                            
                                           
                                             ( 
                                             
                                               
                                                 90 
                                                  
                                                 ° 
                                               
                                               - 
                                               
                                                 θ 
                                                 2 
                                               
                                             
                                             ) 
                                           
                                         
                                         n 
                                       
                                       ) 
                                     
                                   
                                 
                                 } 
                               
                             
                             ] 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   E8 
                   ) 
                 
               
             
           
         
       
     
     When comparing the outgoing angle δp and the outgoing angle δc described above, the followings can be said. That is, as described above (see  FIG. 5 ), it is preferable that the outgoing angle δp of light from the first prism sheet PS 1  shown in  FIG. 12  be larger than the first prism sheet PS 1 ′ shown in  FIG. 13  (δp&gt;δc). Accordingly, a formula below (F1) is derived. 
     
       
         
           
             
               
                 
                   [ 
                   
                     Formula 
                      
                     
                         
                     
                      
                     F 
                      
                     
                         
                     
                      
                     1 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     
                       ( 
                       
                         
                           90 
                            
                           ° 
                         
                         - 
                         
                           θ 
                           2 
                         
                       
                       ) 
                     
                     + 
                     
                       
                         sin 
                         
                           - 
                           1 
                         
                       
                        
                       
                         [ 
                         
                           
                             n 
                             · 
                             sin 
                           
                            
                           
                             { 
                             
                               
                                 180 
                                  
                                 ° 
                               
                               - 
                               
                                 3 
                                 · 
                                 
                                   ( 
                                   
                                     
                                       90 
                                        
                                       ° 
                                     
                                     - 
                                     
                                       θ 
                                       2 
                                     
                                   
                                   ) 
                                 
                               
                             
                             } 
                           
                         
                         ] 
                       
                     
                   
                   &gt; 
                   
                     
                       sin 
                       
                         - 
                         1 
                       
                     
                      
                     
                       [ 
                       
                         
                           n 
                           · 
                           sin 
                         
                          
                         
                           { 
                           
                             
                               ( 
                               
                                 
                                   90 
                                    
                                   ° 
                                 
                                 - 
                                 
                                   θ 
                                   2 
                                 
                               
                               ) 
                             
                             - 
                             
                               
                                 sin 
                                 
                                   - 
                                   1 
                                 
                               
                                
                               
                                 ( 
                                 
                                   
                                     sin 
                                      
                                     
                                       ( 
                                       
                                         
                                           90 
                                            
                                           ° 
                                         
                                         - 
                                         
                                           θ 
                                           2 
                                         
                                       
                                       ) 
                                     
                                   
                                   n 
                                 
                                 ) 
                               
                             
                           
                           } 
                         
                       
                       ] 
                     
                   
                 
               
               
                 
                   ( 
                   F1 
                   ) 
                 
               
             
           
         
       
     
     Further, as described above (see  FIG. 5 ), it is preferable that the outgoing angle δp be smaller than 90°. This way, a formula below (F2) is also led. 
     
       
         
           
             
               
                 
                   [ 
                   
                     Formula 
                      
                     
                         
                     
                      
                     F 
                      
                     
                         
                     
                      
                     2 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     90 
                      
                     ° 
                   
                   &gt; 
                   
                     
                       ( 
                       
                         
                           90 
                            
                           ° 
                         
                         - 
                         
                           θ 
                           2 
                         
                       
                       ) 
                     
                     + 
                     
                       
                         sin 
                         
                           - 
                           1 
                         
                       
                        
                       
                         [ 
                         
                           
                             n 
                             · 
                             sin 
                           
                            
                           
                             { 
                             
                               
                                 180 
                                  
                                 ° 
                               
                               - 
                               
                                 3 
                                 · 
                                 
                                   ( 
                                   
                                     
                                       90 
                                        
                                       ° 
                                     
                                     - 
                                     
                                       θ 
                                       2 
                                     
                                   
                                   ) 
                                 
                               
                             
                             } 
                           
                         
                         ] 
                       
                     
                   
                   &gt; 
                   
                     
                       sin 
                       
                         - 
                         1 
                       
                     
                      
                     
                       [ 
                       
                         
                           n 
                           · 
                           sin 
                         
                          
                         
                           { 
                           
                             
                               ( 
                               
                                 
                                   90 
                                    
                                   ° 
                                 
                                 - 
                                 
                                   θ 
                                   2 
                                 
                               
                               ) 
                             
                             - 
                             
                               
                                 sin 
                                 
                                   - 
                                   1 
                                 
                               
                                
                               
                                 ( 
                                 
                                   
                                     sin 
                                      
                                     
                                       ( 
                                       
                                         
                                           90 
                                            
                                           ° 
                                         
                                         - 
                                         
                                           θ 
                                           2 
                                         
                                       
                                       ) 
                                     
                                   
                                   n 
                                 
                                 ) 
                               
                             
                           
                           } 
                         
                       
                       ] 
                     
                   
                 
               
               
                 
                   ( 
                   F2 
                   ) 
                 
               
             
           
         
       
     
     And, when the backlight unit  39  is mounted with at least one prism sheet that satisfies the formula (F1) or the formula (F2) described above (that is, at least either the first prism sheet PS 1  that satisfies the formula (F1) or the formula (F2), or the second prism sheet PS 2  that satisfies the formula (F1) or the formula (F2) is mounted) light that travels at relatively large inclination angles with respect to the normal direction N, which overlaps the LED  12 , is increased as described above. As a result, light straightly above the LEDs  12  and the like becomes no longer noticeable from outside, and backlight light with suppressed unevenness in light amount is generated. 
     Embodiment 2 
     Embodiment 2 will be described. Here, members having the similar functions as the members used in Embodiment 1 are assigned with the same reference characters, and the description of them is omitted. 
     In Example 1 of Embodiment 1, the arrangement interval (arrangement pitch) of the LEDs  12  was same in the X direction and the Y direction (Px=Py=55 (mm)). Here, in Embodiment 2, two examples in which the arrangement interval of the LEDs  12  is different in the X direction and in the Y direction (Example 2 and Example 3) are described. A difference between Example 2 and Example 3 is that the alignment direction of the triangle prisms PR 1  in the first prism sheet PS 1  is different, and the alignment direction of the triangle prisms PR 2  in the second prism sheet PS 2  is also different. 
     To explain in more detail, in Example 2, as shown in  FIGS. 14 and 15  (the cross-sectional arrow view along the line A 2 -A 2 ′ of  FIG. 14 ), the first prism sheet PS 1  has the triangle prisms PR 1 , which have a linear shape extending in the Y direction, aligned in the first outgoing face IS 1  along the X direction. Moreover, the second prism sheet PS 2  has the triangle prisms PR 2 , which have a linear shape extending in the X direction, aligned in the second outgoing face IS 2  along the Y direction. 
     That is to say, in Example 2 and Example 1, the extending directions (direction of the ridge line R) of the triangle prisms PR 1  and PR 2  in the first prism sheet PS 1  and the second prism sheet PS 2  are the same, and the alignment directions of the triangle prisms PR 1  and PR 2  are also the same. 
     However, the ridge lines R of the triangle prisms PR 1  in the first prism sheet PS 1  become perpendicular to the X direction, which is the direction of the shorter arrangement interval of the LEDs  12 , and the ridge lines R of the triangle prisms PR 2  in the second prism sheet PS 2  become perpendicular to the Y direction, which is the direction of the longer arrangement interval of the LEDs  12  (in other words, the alignment direction of the triangle prisms PR 1  in the first prism sheet PS 1  is the same direction as the&#39;X direction, which is the direction of a short arrangement interval, and the alignment direction of the triangle prisms PR 2  in the second prism sheet PS 2  is the same direction as the Y direction, which is the direction of a long arrangement interval). 
     Meanwhile, in Example 3, as shown in  FIGS. 16 and 17  (the cross-sectional arrow view along the line A 3 -A 3 ′ in  FIG. 16 ), the first prism sheet PS 1  has the triangle prism PR 1 , which have a linear shape extending in the X direction, aligned in the first outgoing face IS 1  along the Y direction. The second prism sheet PS 2  has the triangle prisms PR 2 , which have a linear shape extending in the Y direction, aligned in the second outgoing face IS 2  along the X direction. 
     That is, the extending direction of the triangle prisms PR 1  as well as the alignment direction of the triangle prisms PR 1  in the first prism sheet PS 1  of Example 3 become perpendicular to (cross) the extending direction of the triangle prisms PR 1  as well as the alignment direction of the triangle prisms PR 1  in the first prism sheet PS 1  of Example 2. 
     Similarly, the extending direction of the triangle prisms PR 2  as well as the alignment direction of the triangle prisms PR 2  in the second prism sheet PS 2  of Example 3 become perpendicular to (cross) the extending direction of the triangle prisms PR 2  as well as the alignment direction of the triangle prisms PR 2  in the second prism sheet PS 2  of Example 2. 
     Further, the ridge lines R of the triangle prisms PR 1  in the first prism sheet PS 1  become perpendicular to the Y direction, which is the direction of a long arrangement interval of the LEDs  12 , and the ridge lines R of the triangle prisms PR 2  in the second prism sheet PS 2  become perpendicular to the X direction, which is the direction of a short arrangement interval of the LEDs  12  (in other words, the alignment direction of the triangle prisms PR 1  in the first prism sheet PS 1  is in the same direction as the Y direction, which is the direction of a long arrangement interval of the LEDs  12 , and the alignment direction of the triangle prisms PR 2  in the second prism sheet PS 2  is in the same direction as the X direction, which is the direction of a short arrangement interval of the LEDs  12 ). 
     Numeral examples that are common in Example 2 and Example 3 are as follows. 
     EXAMPLE 2, EXAMPLE 3  
     
         
         
           
             The refractive index of the first prism sheet PS 1  and the second prism sheet PS 2 =1.5 
             The critical angle CA at a boundary surface between the first outgoing face IS 1  and air=≈42(°) 
             The critical angle CA at a boundary surface between the second outgoing face IS 2  and air≈42(°) 
             The shortest distance D(L−R) from the LEDs  12  (the light emitting surface  12 L to be precise) to the ridge lines R of the prisms PR 1  in the first prism sheet PS 1 =10 (mm) 
             The shortest distance D(L−DS) from the LEDs  12  to the light reception face  31 B of the diffusion sheet  31 =20 (mm) 
             The thickness of the first prism sheet PS 1 =2.0 (mm) 
             The vertex angle θ of the triangle prism PR 1  in the first prism sheet PS 1 =70(°) 
             The arrangement interval of the triangle prisms PR 1  in the first prism sheet PS 1 =0.1 (mm) 
             The thickness of the second prism sheet PS 2 =0.2 (mm) 
             The vertex angle θ of the triangle prism PR 2  in the second prism sheet PS 2 =70(°) 
             The arrangement interval of the triangle prisms PR 2  in the second prism sheet PS 2 =0.1 (mm) 
             The arrangement interval Px of the LEDs  12  in the X direction=48 (mm) 
             The arrangement interval Py of the LEDs  12  in the Y direction=55 (mm) 
           
         
       
    
     In Example 2 and Example 3 described above, similarly to Example 1, the first prism sheet PS 1  has the triangle prisms PR 1 , which refract light coming from the first light-reception face RS 1 , formed in the first outgoing face IS 1 , and one surface (SS 1   a,  for example) of the side surfaces SS 1   a  and SS 2   a  of the triangle prism PR 1  refracts light coming from the first light-reception face RS 1 , and the other side surface (SS 2   a , for example) refracts light coming from the one side surface. 
     Moreover, in Example 2 and Example 3, similarly to Example 1, the second prism sheet PS 2  has the triangle prisms PR 2 , which refract light coming from the second light-reception face RS 2 , formed in the second outgoing face  152 , and one surface (SS 1   b,  for example) of the side surfaces SS 1   b  and SS 2   b  of the triangle prism PR 2  refracts light coming from the second light-reception face RS 2 , and the other side surface (SS 2   b , for example) refracts light coming from the one side surface. 
     Therefore, Example 2 and Example 3 have functional effects similar to those of Example 1. However, when Example 2 and Example 3 are compared, there is a difference between them.  FIGS. 18 and 19  show the difference. There figures show an image in which the luminance uniformity of planar light is recognizable, and a graph showing positions and luminance (nt) per direction of two directions perpendicular to each other in the image ( FIG. 18  corresponds to Example 2, and  FIG. 19  corresponds to Example 3). 
     When  FIG. 18 , which is Example 2, and  FIG. 19 , which is Example 3, are compared, the followings become clear. To explain details, when comparing a graph showing the luminance along the Y direction in Example 2 and a graph showing the luminance along the Y direction of Example 3, in areas shown by arrows in the graphs, an area with a lowered luminance was formed in Example 3, and such an area with a lowered luminance is not formed in Example 2 (in other words, recessed areas are not formed in the curved graph line of Example 2, but the recessed areas are formed in a part of the curved graph line of Example 3). 
     Such an area with a lowered luminance in Example 3 is called a dark line, and it is one example of unevenness in light amount of planar light. In the LEDs  12  arranged in a matrix, when the arrangement interval of the LEDs  12  along one of the two directions (X direction and Y direction), which are perpendicular to each other, is different from the arrangement interval of the LEDs  12  along the other direction, such a dark line is likely to occur along the direction of the shorter arrangement interval. 
     The reason is that, when the arrangement interval is long, light of the LEDs  12  is not likely to reach near the center of the arrangement interval, and an area darker than the surrounding area is created, and that area is further aligned in the direction along the shorter arrangement interval. 
     In order to resolve such a dark line, light should be spread all the way in the direction along the longer arrangement interval on a side that is as close as possible to a viewer of planar light. And, an example in which such a measure was implemented is Example 2. 
     That is, in Example 2, the triangle prisms PR 2  of the second prism sheet PS 2 , which is located on a side as close as possible to a viewer of planar light, become perpendicular to the Y direction, which is a direction of the long arrangement interval, and light is spread along the Y direction (the point is that, by making the ridge lines R of the triangle prisms PR 2  in the second prism sheet PS 2  become perpendicular to the Y direction, light is spread along the Y direction). This way, an area darker than the surrounding area is eliminated in the arrangement interval of the LEDs along the Y direction, and furthermore, a dark line is not formed in the X direction, which is the direction along the short arrangement interval. 
     On the other hand, in Example 3, the triangle prisms PR 2  of the second prism sheet PS 2  is perpendicular to the X direction, which is the direction of the short arrangement interval so that light is spread along the X direction, and light is not fully spread in the Y direction. Therefore, an area darker than the surrounding area is formed in the arrangement interval of the LEDs  12  along the Y direction, and further, a dark line is formed in the X direction, which is the direction along the short arrangement interval. However, even though some dark lines are formed in Example 3, unevenness in light amount is more suppressed than the comparative example. 
     Embodiment 3 
     Embodiment 3 is described. Here, members having a similar function as the members used in Embodiments 1 and 2 are attached with the same reference characters, and the description of them are omitted. 
     In Embodiments 1 and 2, the LEDs  12  were used as a light source in the backlight unit  39 . However, a light source is not limited to the LED  12 . For example, as shown in  FIGS. 20 and 21  (the cross-sectional arrow view along the line A 4 -A 4 ′ in  FIG. 20 ), the light source may be fluorescent tubes  17 . 
     In the backlight unit  39  (Example 4) shown in these figures, the fluorescent tubes  17  extend along the Y direction, and are aligned along the X direction. Moreover, unlike Examples 1 to 3, in Example 4, only the first prism sheet PS 1  in which the triangle prisms PR 1  are aligned in the same direction as the alignment direction of the fluorescent tubes  17  is mounted, and other prism sheets such as the second prism sheet PS 2  are not mounted. 
     This is because the fluorescent tube  17  is a linear light source, and therefore, it is relatively easy light to spread along the linear direction (that is, the Y direction), but it is difficult for light to spread in a direction in which the fluorescent tubes  17  are aligned (that is, the X direction) (particularly, the larger the distance between the fluorescent tubes  17  becomes, the more difficult it becomes for light to spread). Accordingly, in the case of Example 4, as long as a prism sheet PR that is capable of spreading light along the alignment direction of the fluorescent tubes  17  is included, it is unnecessary to include a prism sheet in which the triangle prisms PR extending in the X direction are aligned in the Y direction. Various numeral examples relating to this Example 4 are as follows. 
     EXAMPLE 4  
     
         
         
           
             The refractive index of the first prism sheet PS 1 =1.5 
             The critical angle CA at a boundary surface between the first outgoing face IS 1  and air≈42(°) 
             The shortest distance D(L−R) from the LEDs  12  (the light emitting surface  12 L to be precise) to the ridge lines R of the prisms PR 1  in the first prism sheet PS 1 =10 (mm) 
             The shortest distance D(L−DS) from the radical center of the fluorescent tube  17  to the light-reception face  31 B of the diffusion sheet  31 =20 (mm) 
             The thickness of the first prism sheet PS 1 =2.0 (mm) 
             The arrangement interval of the triangle prisms PR 1  in the first prism sheet PS 1 =0.1 (mm) 
             The vertex angle θ of the triangle prism PR 1  in the first prism sheet PS 1 =70(°) 
             The arrangement interval Px of the fluorescent tubes  17 =55 (mm) 
           
         
       
    
     In Example 4 described above, similarly to Example 1, the triangle prisms PR 1 , which refract light coming from the first light-reception face RS 1 , are formed in the first outgoing face IS 1  of the first prism sheet PS 1 , and one surface of the side surfaces SS 1   a  and SS 2   a  of the triangle prism PR 1  refracts light coming from the first light-reception face RS 1 , and the other side surface SS 2   a  refracts light coming from the one surface. Therefore, Example 4 also has functional effects similar to those of Example 1. 
     Other Embodiments 
     Furthermore, the present invention is not limited to the above-mentioned embodiments, and various modifications are possible without departing from the scope of the present invention. 
     For example, the prism is not limited to the triangle prism PR having a triangle cross-section. As one example, a prism having other polygonal cross-section (such as a quadrangular or pentagonal cross-section) may be used as well. This is because such a polygonal prism PR includes at least two surfaces, which are a side surface (first refractive face) that refracts light coming from the light-reception face RS, and another side surface (second refractive face) that refracts light coming from the side surface. 
     Moreover, point-like prisms PR may also be arranged in a matrix instead of aligning the linear-shaped triangle prisms PR. For example, it is also acceptable to use a square pyramidal prism PR, which has a total of four side surfaces including two more side surfaces SS in addition to the two side surfaces SS of the triangle prism PR. 
     When using a prism sheet PS in which such square pyramidal prisms PR are arranged on the side of the diffusion sheet  31 , the two prism sheets PS 1  and PS 2  are unnecessary unlike Examples 1 to 3. This is because such a prism sheet PS in which the square pyramidal prisms PR are arranged in a matrix can spread light in two directions (the X and Y directions, for example). 
     Further, the shape of the prism PR is not limited to a square pyramidal shape, and other polygonal pyramidal shapes (or conical shape) may also be used. A trapezoid pyramidal or conical shape instead of a pyramidal or conical shape may also be used for the prism PR. The point is that any prism PR that is capable of spreading light along a direction causing unevenness in light amount such as a dark line may be used. 
     Further, a surface of the prism PR does not have to be flat. For example, as shown in  FIG. 22 , the surface (side surfaces SS) of the prism PR may be a curved surface that is convex toward the light outgoing side of the prism sheet PS. 
     When the side surfaces SS are flat such as the triangle prism PR, an outgoing direction from the side surfaces SS is usually almost fixed in accordance with an incident angle to the prism sheet PS. However, when the side surfaces SS are curved surfaces, light transmitting through the curved surfaces travels in many directions. Therefore, when the side surfaces SS of the prism PR are curved surfaces, the occurrence of unevenness in light amount is even more suppressed. 
       FIGS. 23A to 23D  show a difference between the unevenness in light amount caused when the side surfaces of the prism PR are flat, and the unevenness in light amount caused when the side surfaces are curved. These figures are simulation images showing the luminance uniformity on the diffusion sheet  31 , and  FIGS. 23A to 23C  are examples of when the side surfaces SS of the prism PR are curved, and  FIG. 23D  is an example when the side surfaces SS of the prism PR are flat. From these figures, it is clear that the occurrence of unevenness in light amount is even more suppressed when the side surfaces SS of the prism PR are curved. 
     Here, as shown in the cross-sectional view in FIG,  24 , “φ” in the figures means a central angle φ, which is derived from a circular arc that is a curved line connecting one end A and another end B of curved side surfaces SS (or a circular arc that is a curved line connecting one end A and another end C), and a center of curvature CP located on the side of the light-reception face RS (light-reception side that is opposite to the light outgoing side). 
     Further, minute recesses and projections for scattering light may also be formed in a part (the side surfaces SS of the prism PR, for example) of the surface of the prism sheet PS. When the surface is in this way, outgoing light from the prism sheet PS is easily spread to various directions. Further, not only on the side surfaces SS of the prism PR, but the minute recesses and projections may also be formed in other areas. The point is that it is acceptable as long as the recesses and projections are formed at least in a part of the surface of the prism sheet PS. 
     Moreover, if the prism sheet PS includes a light diffusion material, light exiting from the prism sheet PS is likely to spread in various directions. Acrylic resin is one example of a material for the prism sheet PS and the prism, but it is not limited to this. 
     Moreover, in order to further suppress unevenness in light amount, another diffusion sheet (second diffusion sheet) other than the main diffusion sheet  31  may also be laminated over the prism sheet PS. 
     The prism sheet PS has been used as an example of a sheet that transmits light from the LEDs  12  or the fluorescent tubes  17  in the description above. However, the present invention is not limited to the prism sheet PS, and the transmission sheet may also be a sheet including hologram (light refractive element) that refracts light, for example. 
     DESCRIPTION OF REFERENCE CHARACTERS 
     
         
         
           
             PS Prism sheet (transmission sheet) 
             PS 1  First prism sheet (first transmission sheet) 
             PS 2  Second prism sheet (second transmission sheet) 
             PR Triangle prism (light refractive element, prism) 
             PR 1  Triangle prism formed in the first prism sheet 
             PR 2  Triangle prism formed in the second prism sheet 
             SS side surface of triangle prism 
             SS 1   a  One of side surfaces of a triangle prism formed in the first prism sheet (first refractive face/second refractive face) 
             SS 2   a  The other side surface of the side surfaces of a triangle prism formed in the first prism sheet (second refractive face/first refractive face) 
             SS 1   b  One of side surfaces of a triangle prism formed in the second prism sheet (first refractive face/second refractive face) 
             SS 2   b  The other side surface of the side surfaces of a triangle prism formed in the second prism sheet (second refractive face/first refractive face) 
             R Ridge line of the prism (connecting line) 
             RS Light-reception face of the prism sheet 
             RS 1  First light-reception face of the first prism sheet 
             RS 2  Second light-reception face of the second prism sheet 
             IS Outgoing face of the prism sheet 
             IS 1  First outgoing face of the first prism sheet 
             IS 2  Second outgoing face of the second prism sheet 
             MJ LED module 
               11  Mounting substrate 
               11 U Mounting surface 
               12  LED (light source, point light source) 
               12 L Light emitting surface of LED 
               13  Reflective sheet 
               31  Diffusion sheet (first diffusion sheet) 
               32  Optical sheet 
             D(L−R) Shortest distance from LED to the ridge line of prism in prism sheet (shortest distance from light source to a connecting line between a first refractive face and a second refractive face of light refractive element in transmission sheet) 
             D(L−DS) Shortest distance from LED to diffusion sheet (shortest distance from light source to the first diffusion sheet) 
             θ Vertex angle of triangle prism 
             δ Outgoing angle with respect to prism sheet 
             CA Critical angle 
             X Alignment direction (first direction/second direction) of one direction of LEDs that are arranged in a matrix (two dimensional arrangement) 
             Y Alignment direction (second direction/first direction) of the other direction of LEDs that are arranged in a matrix (two dimensional arrangement) 
             Z A direction crossing X direction and Y direction 
             XY LED-mounted surface (two dimensional surface) 
             P LED arrangement interval 
             Px Arrangement interval of LEDs along X direction 
             Py Arrangement interval of LEDs along Y direction 
               39  Backlight unit (lighting device) 
               49  Liquid crystal display panel (display panel) 
               59  Liquid crystal display device (display device)