Patent Publication Number: US-2017351142-A1

Title: Liquid crystal display device

Description:
TECHNICAL FIELD 
     The present invention relates to a liquid crystal display device. 
     BACKGROUND ART 
     Display devices including liquid crystal display panels as display components for displaying images (e.g., smartphones, tablet computers, television sets, digital cameras, and car navigation systems) have been known. Because the liquid crystal panels do not emit light, the liquid crystal panels are provides with lighting devices that apply light from the back of the liquid crystal panels (known as backlight devices). A lighting device including a light guide plate and light emitting diodes (LEDs) opposed to an end surface of the light guide plate has been known. Such a light device is referred to as an edge-light lighting device (or a side-light lighting device). This type of lighting device is suitable to reduce a thickness and power consumption. 
     In the edge-light lighting device, the end surface of the light guide plate is a light entering surface through which light from the light source enters and a plate surface of the light guide plate on the front side is a light exiting surface through which the light that has entered through the light entering surface exit toward the liquid crystal display panel. The light that has entered the light guide plate through the light entering surface travels through the light guide plate while repeating reflection and exit through the light exiting surface. 
     The lighting device includes an optical sheet that covers the light exiting surface. The optical sheet includes layers of a diffusing sheet, a prism sheet, and the like. The light exiting through the light exiting surface is transmitted through the optical sheet and converted into planar light. The planar light is supplied to the liquid crystal display panel. 
     As disclosed by Patent Document 1, a lighting device that includes a single prism sheet and a light guide plate that includes a light collecting portion has been known. The lighting device includes the single prism sheet that replaces a multilayered optical sheet to reduce the thickness. The light collecting portion includes prism sheets and cylindrical lenses on a light exiting surface or a surface opposite from the light exiting surface. The light collecting portion of the light guide plate includes multiple longitudinal prisms arranged in line on a front side or a rear side of the light guide plate with the longitudinal direction aligned with an optical axis of light from the light source. The prism sheet also includes multiple prisms arranged parallel to the light collecting portion of the light guide plate. 
     In such a lighting device, the light exiting from the light guide plate is collected due to optical effect of the light collecting portion and directed to the prism sheet. Furthermore, the light directed to the prism sheet is collected in the frontward direction according to the optical properties of the prisms. As a result, the light is converted into even planar light. The light collecting property of the light collecting portion of the light guide plate and the light collecting property of the prisms of the prism sheet are observed in arrangement directions of the light collecting portion and the prisms. 
     RELATED ART DOCUMENT 
     Patent Document 
     Patent Document 1: International Publication No. 2012/050121 
     Problem to be Solved by the Invention 
     In a liquid crystal display device including the lighting device having the light collecting property described above, light is emitted along a frontward direction relative to a display surface of a liquid crystal display panel (a direction normal to the display surface). The display surface may include areas through which the larger number of light rays exit and travel in direction angled to the frontward direction toward the display surface (i.e., in directions with small angles relative to the display surface), which form side lobe light, resulting in reduction in frontward brightness or uneven brightness. 
     DISCLOSURE OF THE PRESENT INVENTION 
     The present invention was made in view of the foregoing circumstances. An object is to provide a liquid crystal display device in which a reduction in frontward direction and uneven brightness are less likely to occur. 
     Means for Solving the Problem 
     A liquid crystal display device according to the present invention includes a light source, a light guide plate, a backlight unit, and a complex polarizing plate. The light guide plate is a plate shaped member that includes a light entering surface, a light exiting surface, and a light collecting portion. The light entering surface is an end surface of the plate shaped member and opposed to the light source. The light exiting surface is a front plate surface of the plate shaped member through which light entering through the light entering surface exits. The light collecting portion is formed in the light exiting surface and/or a rear plate surface of the plate shaped member. The light collecting portion is configured to collect light rays exiting from the light exiting surface with respect to a light collecting direction perpendicular to an optical axis of the light source to direct the light rays in a frontward direction. The backlight unit includes an optical sheet disposed to cover the light exiting surface and collecting the light rays exiting through the light exiting surface with respect to the light collecting direction to direct the light rays in the frontward direction while transmitting the light rays therethrough. The complex polarizing plate includes a selective reflection sheet and a polarizing plate. The selective reflection sheet includes a first transmission axis for passing linearly polarized light in a first condition along the first transmission axis and a reflection axis perpendicular to the first transmission axis for reflecting linearly polarized light in a second condition along the reflection axis. The polarizing plate includes a second transmission axis for passing the linearly polarized light in the first condition. The polarizing plate is laid on the selective reflection sheet with the second transmission axis parallel to the first transmission axis. The complex polarizing plate is laid on the backlight unit with the first transmission axis and the second transmission axis along a non-light collecting direction perpendicular to the light collecting direction. 
     Because the liquid crystal display device has the configuration described above, the selective reflection sheet of the complex polarizing plate actively reflects the light rays of the light exiting from the backlight unit which travel in directions angled to the frontward direction toward sides in the light collecting direction (side lobe light). The reflected light rays are multiply scattered and thus the polarization is canceled. The reflected light rays form a light flux that contributes to improvement of the brightness in the frontward direction. Therefore, the reduction in forward brightness or the uneven brightness is less likely to occur in the liquid crystal display device. 
     In the liquid crystal display device, the optical sheet may include a sheet base having a sheet shape and a prism sheet that includes a prism portion formed on a front surface of the sheet base opposed to the complex polarizing plate. The prism portion may include a plurality of unit prisms having elongated shapes that extend in the non-light collecting direction. The unit prisms may be arranged along the non-light collecting direction. 
     In the liquid crystal display device, each of the unit prisms may have a triangular cross section with a vertex having an angle of 90°. 
     In the liquid crystal display device, the sheet base may be made of material that does not have a birefringent property. 
     In the liquid crystal display device, the light collecting portion may include a plurality of unit light collecting portions having elongated shapes that extend along the non-light collecting direction and being arranged along the light collecting direction. 
     In the liquid crystal display device, each of the unit light collecting portions may have a triangular cross section with a vertex having an obtuse angle or a semicircular cross section. 
     In the liquid crystal display device, the light source may include a plurality of point light sources arranged in line along the light collecting direction. 
     In the liquid crystal display device, the backlight unit may include the light guide plate that is the plate shaped member disposed in a flipped position. 
     The liquid crystal display device may further include a light exiting-side polarizing plate opposed to the complex polarizing plate and a liquid crystal display panel disposed between the complex polarizing plate and the light exiting-side polarizing plate. 
     Advantageous Effect of the Invention 
     According to the present invention, a liquid crystal display device in which a decrease in frontward brightness and uneven brightness are less likely to occur is provided. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an exploded perspective view illustrating a schematic configuration of a liquid crystal display device according to a first embodiment of the present invention. 
         FIG. 2  is an exploded perspective view illustrating a schematic configuration of a backlight unit in the liquid crystal device. 
         FIG. 3  is a cross-sectional view of the liquid crystal display device along a longitudinal direction thereof (an X-axis direction) illustrating a cross-sectional configuration. 
         FIG. 4  is a cross-sectional view of the liquid crystal display device along a transverse direction thereof (a Y-axis direction) illustrating a cross-sectional configuration. 
         FIG. 5  is a plan view of a light guide plate. 
         FIG. 6  is a back view of the light guide plate. 
         FIG. 7  is a cross-sectional view of the backlight unit along a transverse direction thereof (the Y-axis direction) illustrating a cross-section configuration. 
         FIG. 8  is a cross-sectional view along line A-A in  FIG. 7 . 
         FIG. 9  is a perspective view schematically illustrating a complex polarizing plate. 
         FIG. 10  is an exploded perspective view schematically illustrating a positional relationship between a backlight unit and a complex polarizing plate in a testing device. 
         FIG. 11  is a graph illustrating a relationship between angle of a transmission axis of the complex polarizing plate and relative value of a frontward brightness in the testing device. 
         FIG. 12  is a perspective view schematically illustrating a relationship between the testing device and a coordinate system. 
         FIG. 13  is a diagram illustrating measured brightness distribution (light distribution characteristics) of light exiting from the testing device with a transmission axis of the complex polarizing plate at 90°. 
         FIG. 14  is a diagram illustrating measured brightness distribution (light distribution characteristics) of light exiting from the testing device with the transmission axis of the complex polarizing plate at 0°. 
         FIG. 15  is a perspective view schematically illustrating a relationship between the testing device and another coordinate system. 
         FIG. 16  is a diagram illustrating measured brightness distribution (light distribution characteristics) in a “12 o&#39;clock-to-6 o&#39;clock” direction in the testing device. 
         FIG. 17  is a diagram illustrating measured brightness distribution (light distribution characteristics) in a “3 o&#39;clock-to-9 o&#39;clock” direction in the testing device. 
         FIG. 18  is a graph illustrating a relationship between the brightness ratios of frontward light relative to side lobe light in the 3 o&#39;clock-to-9 o&#39;clock” direction in the testing device and angles of the transmission axis of the complex polarizing plate. 
         FIG. 19  is an exploded perspective view schematically illustrating a positional relationship between a backlight unit and a complex polarizing plate in a testing device in a liquid crystal display device according to a second embodiment of the present invention. 
         FIG. 20  is an exploded perspective view schematically illustrating a positional relationship between a backlight unit and a complex polarizing plate in a testing device in a liquid crystal display device according to a third embodiment of the present invention. 
         FIG. 21  is a graph illustrating a relationship between angle of a transmission axis of a complex polarizing plate and relative value of a frontward brightness in a testing device according to a comparative example. 
         FIG. 22  is a graph illustrating a relationship between angle of a transmission axis of a complex polarizing plate and relative value of a frontward brightness in a testing device according to another comparative example. 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     First Embodiment 
     A first embodiment will be described with reference to  FIGS. 1 to 18 . In this section, a liquid crystal display device  10  will be described. X-axes, Y-axes and Z-axes may be specified in the drawings. The axes in each drawing correspond to the respective axes in other drawings. The vertical direction is defined based on  FIGS. 3 to 5  and the upper side and the lower side in those drawings correspond to the front and the rear of the device, respectively. The front side of the liquid crystal display device  10  may be referred to as a front-face side of the liquid crystal display device  10 . In this specification, “a frontward direction” refers to a direction normal to (or perpendicular to) a display surface DS of the liquid crystal display device  10  to toward the front side. 
     The liquid crystal display device  10  may be used for an electronic device such as a tablet computer. As illustrated in  FIG. 1 , the liquid crystal display device  10  has a rectangular overall shape in a plan view. The liquid crystal display device  10  includes at least a liquid crystal display unit LDU, a touchscreen  14 , a cover panel  15  (a protection panel, a cover glass), and a case  16 . 
     The liquid crystal display unit LDU includes a liquid crystal display panel  11 , a backlight unit  12  (a lighting device), and a frame  13 . The backlight unit  12  is disposed behind the liquid crystal display panel  11  and configured to emit light toward the liquid crystal display panel  11 . The frame  13  presses down the liquid crystal display panel  11  from the front side. The touchscreen  14  and the cover panel  15  are held in the frame  13  of the liquid crystal display unit LDU from the front side. 
     The touchscreen  14  is disposed more to the front than the liquid crystal display panel  11  with a predefined distance apart from the display surface DS of the liquid crystal display panel  11  to cover the liquid crystal display panel  11 . The cover panel  15  is disposed to cover the touchscreen  14  from the front side. An anti-reflection film AR is disposed between the touchscreen  14  and the cover  15  (see  FIGS. 3 and 4 ). A case  16  is fixed to the frame to cover the liquid crystal display unit LDU from the rear side. 
     Next, the liquid crystal display panel  11  included in the liquid crystal display unit LDU will be described. The liquid crystal display panel  11  has a rectangular overall shape in the plan view. The liquid crystal display panel  11  includes a pair of substrates  11   a  and  11   b  and a liquid crystal layer (not illustrated). The liquid crystal layer is between the substrates  11   a  and  11   b . Each of the glass substrates  11   a  and  11   b  is a substantially transparent and has high light transmissivity. A sealant, which is not illustrated, is around the liquid crystal layer. The substrates  11   a  and  11   b  are bonded together with the sealant using a bonding force of the sealant. 
     The liquid crystal display panel  11  includes a display area AA in which images are displayed and a non-display area NAA in which no images are displayed. In  FIG. 1 , a longitudinal direction, a transverse direction, and a thickness direction of the liquid crystal display panel  11  correspond with the X-axis direction, the Y-axis direction, and the Z-axis direction, respectively. 
     One of the substrates  11   a  and  11   b  on the front side (on the front-face side) is a CF substrate  11   a  and one on the rear side (on the backside) is an array substrate  11   b . The CF substrate  11   a  is slightly smaller than the array substrate  11   b.    
     On the inner surface of the array substrate  11   b  (on the liquid crystal layer side), a number of thin film transistors (TFTs) that are switching components and a number of pixel electrodes are disposed in a matrix. Gate lines and source lines are routed in a grid to surround the TFTs and the pixel electrodes. Specific image signals are supplied from a control circuit, which is not illustrated, to the lines. Each pixel electrode surrounded by the gate lines and the source lines is a transparent electrode film of indium tin oxide (ITO) or zinc oxide (ZnO). 
     On an inner surface of the CF substrate  11   a  (on the liquid crystal layer side), a number of CFs are disposed to corresponding to pixels. The CFs are arranged such that three colors of R, G and B are repeatedly arranged. A black matrix (a light blocking layer) is formed in a grid pattern to surround the CFs for reducing color mixture. Common electrodes that are opposed to the pixel electrodes on the array substrate  11   b  are formed on surfaces of the CFs and the black matrix. The common electrodes are formed from the transparent metal film that forms the pixel electrodes. 
     On the inner surfaces of the substrates  11   a  and  11   b , alignment films for alignment of liquid crystal molecules in the liquid crystal layer are formed, respectively. 
     A polarizing plate  25  is bonded to the outer surface of the CF substrate  11   a . A complex polarizing plate  28  including layers of a polarizing plate  26  and a polarization selective reflection sheet  27  is bonded to the outer surface of the array substrate  11   b . The complex polarizing plate  28  will be described in detail later. 
     Next, the frame  13 , the touchscreen  14 , the cover panel  15 , and the case  16  included in the liquid crystal display unit LDU will be described. 
     The frame  13  is made of metal having high thermal conductivity such as aluminum. The frame  13  may be prepared by stamping. The frame  13  holds down the periphery of the liquid crystal display panel  11 . The frame  13  and a chassis of the backlight unit  12  (which will be described later) hold the liquid crystal display panel  11  and components of the backlight unit  12  (e.g., a light guide plate, which will be described later) therebetween. The frame  13  and the chassis of the backlight unit  12  are fixed together with screws SM. The frame  13  includes walls that extend from the front side to the rear side. The screws SM are screwed into the cantilever wall portions from the outer side to the inner side. 
     The frame  13  holds the touchscreen  14  and the cover panel  15  on the front side. The frame  13  receives the peripheries of the touchscreen  14  and the cover panel  15  from the rear. 
     A shock absorber  29  is disposed between a front surface of the periphery of the liquid crystal display panel  11  and a rear surface of the periphery of the frame  13 . A first fixing member  30  is fixed to a front surface of the inner periphery of the frame  13  and a rear surface of the outer periphery of the touchscreen  14  for fixing the peripheries of the frame  13  and the touchscreen  14  and for absorbing an impact. A second fixing member  31  is fixed to a front surface of the outer periphery of the frame  13  and a rear surface of the outer periphery of the cover panel  15  and for absorbing an impact. The shock absorber  29 , the first fixing member  30 , and the second fixing member  31  are double-side tapes and disposed to overlap the non-display area of the liquid crystal display panel  11 . 
     The touchscreen  14  is for inputting position information within the display surface DS of the liquid crystal display panel  11  using a fingertip of a user. The touchscreen  14  is driven using a projected capacitive touchscreen technology. The touchscreen  14  includes a predefined touchscreen pattern (an electrode pattern) formed on a glass substrate that is substantially transparent and has high light transmissivity. The touchscreen  14  has a rectangular shape in the plan view similar to the liquid crystal display panel  11 . 
     The cover panel  15  has a rectangular shape in the plan view similar to the touchscreen  14 . The cover panel  15  is a glass plate that is substantially transparent and has high light transmissivity. The cover panel  15  is formed over the touchscreen  14  via an antireflective film AR. The cover panel  15  has the rectangular shape slightly larger than the touchscreen  14 . The outer periphery of the cover panel  15  is located outer than the outer periphery of the touchscreen  14 . 
     A frame-shaped light blocking layer  32  is formed on the rear surface of the outer periphery of the cover panel  15 . The frame-shaped light blocking layer  32  is formed in a frame shape in the plan view along the outer periphery of the cover panel  15 . The frame-shaped light blocking layer  32  is a coating film made of black paint. The frame-shaped light blocking layer  32  is formed in a predefined area of the cover panel  15  using a printing technology such as screen printing and ink-jet printing. In the plan view, a portion of the liquid crystal display unit LDU inside the inner edge of the frame-shaped light blocking layer  32  corresponds to the display area AA of the display surface of the liquid crystal display panel  11 . A portion of the liquid crystal display unit LDU outside the inner edge of the frame-shaped light blocking layer  32  corresponds to the non-display area. 
     The case  16  is a component of the liquid crystal display device  10  to form the back of the liquid crystal display device  10 . The case  16  has a container-like shape with an opening on the front side. A bottom surface of the case  16  is curved to bulge from the front side to the rear side. The case  16  is made of resin or metal and formed into a predefined shape. The case  16  holding the liquid crystal display unit LDU therein is fixed to the frame  13  with an opening edge of the case  16  held to the frame  13  from the rear side. 
     Next, the backlight unit  12  will be described. As illustrated in  FIG. 1 , the backlight unit  12  has a rectangular thin block overall shape similar to the liquid crystal display panel  11 . As illustrated in  FIGS. 2 to 4 , the backlight unit  12  is a so-called edge light type (a side light type) lighting unit. The backlight unit  12  includes light emitting diodes (LEDs)  17  (an example of point light sources), an LED board  18  (a light source board), a light guide plate  19 , a reflection sheet  24  (a reflection member), an optical sheet  20 , a light blocking frame  21 , a chassis  22 , and a heat dissipation member  23 . The LEDs  17  are light sources. The LEDs  17  are mounted on the LED board  18 . Light from the LEDs  17  enters the light guide plate  19 . The reflection sheet  24  reflects light from the light guide plate  19 . The optical sheet  20  is layered on the light guide plate  19 . The light blocking frame  21  holds down the light guide plate  19  from the front side. The chassis  22  holds the LED board  18 , the light guide plate  19 , the optical sheet  20 , and the light blocking frame  21  therein. The heat dissipation member  23  is mounted to contact the outer surface of the chassis  22 . 
     Each LED  17  includes an LED chip that is disposed on a board and sealed with a resin. The board is fixed to the LED board  18 . Each LED chip mounted on the board has a main wavelength of emitting light in a single color of blue. In the resin that seals the LED chip, phosphors that emit a certain color of light when excited by the blue light emitted by the LED chip are dispersed. An overall color of light emitted by the phosphors is substantially white. A surface of each LED  17  opposite from a mounting surface thereof that is mounted to the LED board  18  is a light emitting surface  17   a , that is, the LED  17  is a top surface light emitting type. 
     The LED board  18  has an elongated plate shape and is held inside the chassis  22  along the transverse direction of the light guide plate  19 . The LED board  18  is held in a position such that the longitudinal direction and the transverse direction correspond with the Y-axis direction and the Z-axis direction, respectively. The LEDs  17  are mounted on a plate surface  18   a  of the LED board  18  facing the light guide plate  18 . The LEDs  17  are arranged in line along the longitudinal direction of the LED board  18  (a transverse direction of the light guide plate  19 , the X-axis direction) at intervals. The LEDs  17  mounted on the LED board  18  are opposed to the end surface of the light guide plate  19  with respect to the transverse direction with a predefined gap. The LED board  18  is mounted to a portion of the short side of the chassis  22 , which will be described later. 
     A base of the LED board  18  is made of metal such as aluminum or ceramic. A trace for supplying driving power to the LEDs  17  is formed on a surface of the base via an insulating layer. The trace is made from a metal film such as a copper foil and connects the LEDs  17  in series. 
     The light guide plate  19  is made of substantially transparent synthetic resin having a refractive index sufficiently larger than that of the air and high light transmissivity (e.g., acrylic resin such as PMMA). The light guide plate  19  is made from a plate having a substantially rectangular thin block shape similar to the liquid crystal display panel  11 . The light guide plate  19  has a rectangular shape in the plan view similar to the liquid crystal display panel  11 . The light guide plate  19  is prepared by injection molding. In the drawings, the longitudinal direction, the transverse direction, and the thickness direction of the light guide plate  19  correspond with the X-axis direction, the Y-axis direction, and the Z-axis direction, respectively. 
     As illustrated in  FIGS. 3 and 4 , the light guide plate  19  is disposed directly below the liquid crystal display panel  11  and the optical sheet  20  such that the plate surfaces of the light guide plate  19  (the surfaces having larger areas) are located on the front side and the rear side in the chassis  22 . One of end surfaces on the short side of the light guide plate  19  (short end surfaces) is opposed to the LEDs  17 . An optical axis L of light emitted by each LED  17  (see  FIG. 10 ) extends perpendicular to the light emitting surface  17   a  and along the longitudinal direction of the light guide plate  19 . 
     The light guide plate  19  includes a front plate surface  19   a , a rear plate surface  19   b , a pair of short end surfaces  19   c  and  19   d , and a pair of long end surfaces  19   e  and  19   f . The short end surfaces  19   c  and  19   d  are parallel to each other along a transverse direction (the X-axis direction). The long end surfaces  19   e  and  19   f  are parallel to each other along a longitudinal direction (the Y-axis direction). The pair of the short end surfaces  19   c  and  19   d  and the pair of the long end surfaces  19   e  and  19   f  form a periphery of the light guide plate  19 . 
     The front plate surface  19   a  of the light guide plate  19  (on the light emitting side) is configured as a light exiting surface  19   a  through which light exits toward the optical sheet  20  and the liquid crystal display panel  11 . The rear plate surface  19   b  of the light guide plate  19  may be referred to as a rear surface  19   b  where appropriate. The rear surface  19   b  is opposed to the reflection sheet  24  within the chassis  22 . The rear surface  19   b  of the light guide plate  19  is formed from rear unit prisms  44   a  of a rear prism portion  44 , which will be described later. The rear unit prisms  44   a  include light reflecting portions  41 . 
     The short end surface  19   c  of the pair of the short end surfaces  19   c  and  19   d  opposed to the LEDs  17  is configured as a light entering surface  19   c  through which light emitted by the LEDs  17  enters. The other short end surface  19   d  is configured as an opposite end surface  19   d  that is arranged opposite from the light entering surface  19   c.    
     The long end surface  19   e  of the pair of the long end surfaces  19   e  and  19   f  on the right relative to the LEDs  17  may be referred to as a right end surface  19   e  where appropriate. The long end surface  19   f  on the left relative to the LEDs  17  may be referred to as a left end surface  19   f  where appropriate. 
     Rays of the light entering the light guide plate  19  through the light entering surface  19   c  may be totally reflected by the light exiting surface  19   a  or the rear surface  19   b  or repeatedly reflected by the reflection sheet  24  that covers the rear surface  19   b . With the reflection, the light transmits through the light guide plate  19 . 
     A material of the light guide plate  19  may be acrylic resin (e.g., PMMA). In this case, the light guide plate  19  may have a refractive index of about 1.49 and a critical angle of about 42°. 
     The reflection sheet  24  is disposed on the rear surface  19   b  of the light guide plate  19 . The reflection sheet  24  reflects the light rays that have entered the light guide plate  19  toward the front side (i.e., the light exiting surface  19   a  side). The reflection sheet  24  may be formed from a white thin foamed plastic sheet (e.g., a foamed polyethylene terephthalate sheet) and in a size to cover the entire rear surface  19   b . The reflection sheet  24  is sandwiched between a bottom plate  22   a  of the chassis  22  and the rear surface  19   b  of the light guide plate  19  and held in the chassis  22 . 
     An end portion of the reflection sheet  24  on the light entering surface  19   c  side is located outer than the light entering surface  19   c . The end portion outer than the light entering surface  19   c  reflects the light from the LEDs  71  to increase the light entering efficiency for the light entering surface  19   c.    
     As illustrated in  FIGS. 6 and 9 , the exiting light reflecting portion  41  is formed in the rear surface  19   b  of the light guide plate  19  for reflecting the light transmitting through the light guide plate  19  to increase light rays exiting through the light exiting surface  19   a . The exiting light reflecting portion  41  includes the unit reflecting portions  41   a  each having a groove shape (a triangular groove shape). Each unit reflecting portion  41   a  has a right-triangular notch shape in a cross section along the Y-axis direction. As illustrated in  FIGS. 5 and 6 , the unit reflecting portions  41   a  are arranged in lines at intervals along the X-axis direction (the transverse direction of the light guide plate  19 ). The lines of the unit reflecting portions  41   a  are arranged along the Y-axis direction (the longitudinal direction of the light guide plate  19 ). In this embodiment, the intervals of the unit reflecting portions  41   a  in the longitudinal direction of the light guide plate  19  (a direction parallel to the optical axis of the LED  17 ) are about constant. 
     The unit reflecting portions  41   a  include sloped surfaces  41   a    1  and vertical surfaces  41   a   2 . The sloped surfaces  41   a    1  decline from the rear surface  19   b  toward the light exiting surface  19   a  as approaching from the light entering surface  19   c  side of the light guide plate  19  toward the opposite end surface  19   d  side. The vertical surfaces  41   a   2  face the light entering surface  19   c  and extend from the rear surface  19   b  side toward the light exiting surface  19   a  side. 
     The sloped surfaces  41   a   1  of the unit reflecting portions  41   a  on the light entering surface  19   c  side reflect the light rays such that some of the rays have incidences that do not exceed the critical angle relative to the light exiting surface  19   a  to increase the rays that exit through the light exiting surface  19   a.    
     The unit reflecting portions  41  gradually increase in size as a distance from the light entering surface  19   c  (or the LEDs  17 ) increases in the longitudinal direction of the light guide plate  19  (the X-axis direction). Namely, the sloped surfaces  41   a    1  and the vertical surfaces  41   a   2  of the unit reflecting portions  41   a  gradually increase in area as the distance from the light entering surface  19   c  (or the LEDs  17 ) increases. 
     The light rays entering the light guide plate  19  through the light entering surface  19   c  and transmitting through the light guide plate  19  are reflected by the sloped surfaces  41   a    1  of the unit reflecting portions  41   a  of the exiting light reflecting portion  41  toward the front side. The light rays directed toward the front side have the incidences equal to or smaller than the critical angle relative to the light exiting surface  19   a  (or the front prism portion  43 ) and thus exit through the light exiting surface  19   a.    
     The exiting light reflecting portion  41  that includes the unit reflecting portions  41   a  is formed in the rear surface  19   b  of the light guide plate  19 . According to the configuration, the light rays entering through the light entering surface  19   c  are less likely to unevenly exit the light guide plate  19  in an area close to the light entering surface  19   c . Therefore, more light rays reach the opposite end surface  19   d  and spread in the longitudinal direction of the light guide plate  19 . The spreading light rays exit through the light exiting surface  19   a . Namely, the exiting light reflecting portions  41  have functions for collecting the light rays exiting from the light exiting surface  19   a  relative to the longitudinal direction of the light guide plate  19  (the X-axis direction) to direct the light rays to travel in directions closer to the frontward direction of the liquid crystal display device  10 . 
     Although the unit reflecting portions  41   a  have the functions to collect the light rays with respect to the longitudinal direction of the light guide plate  19  (the X-axis direction), the unit reflecting portions  41   a  rarely have functions to collect the light rays with respect to the transverse direction of the light guide plate  19  (the Y-axis direction). 
     Next, a light collecting function of the light guide plate  19  with respect to the transverse direction (the Y-axis direction) will be described. The light guide plate  19  includes the front prism portion  43  and the rear prism portion  44  as a light collecting portion for collecting the light rays with respect to the transverse direction (the Y-axis direction) to direct the light rays to travel in the frontward direction. 
     First, the front prism portion  43  (an example of a light collecting portion) which forms the light exiting surface  19   a  of the light guide plate  19  will be described. The front prism portion  43  is integrally formed with the light guide plate  19  as a portion of the light guide plate  19 . The front prism portion  43  has a function for collecting the light rays toward the frontward direction of the liquid crystal display device  10  in the transverse direction of the light guide plate  19  and directing the light rays from the light guide plate  19  to the optical sheet  20  on the front side. 
     The front prism portion  43  includes front unit prisms  43   a . Each front unit prism  43   a  has an elongated shape that extends along the longitudinal direction of the light guide plate  19  (the Y-axis direction). The front unit prisms  43   a  are adjacent to one another along the transverse direction of the light guide plate  19  (the X-axis direction). The front unit prisms  43   a  have the same shape and the same size. The front unit prisms  43   a  have dimensions in the transverse direction (widths) which are constant in the longitudinal direction. 
     The front unit prisms  43   a  protrude from the rear side toward the front side. Each front unit prism  43   a  has a right-triangular shape with a vertex on the front side when viewed in the X-axis direction. Each front unit prism  43   a  has a pair of sloped surfaces  43   a    1  and  43   a   2  that are adjacent to each other and form the vertex. The sloped surfaces  43   a    1  and  43   a   2  have elongated shapes. The sloped surfaces  43   a    1  and  43   a   2  have rectangular shapes that are elongated along the longitudinal direction of the light guide plate  19  (band shapes). The sloped surface  43   a    1  is arranged on the right end surface  19   e  side of the light guide plate  19  and the sloped surface  43   a   2  is arranged on the left end surface  19   f  side of the light guide plate  19 . 
     Angles θ 1  of the vertexes of the front unit prisms  43   a  are set to a predefined obtuse angle (i.e., larger than 90°). Specifically, the angles θ 1  are set in a range from 100° to 150°, more preferably, about 110°. The angles θ 1  of the vertexes of the front unit prisms  43   a  are set larger than angles θ 11  of vertexes of light exiting-side unit prisms  42   a  of the optical sheet  20 , which will be described later. 
     The front prism portion  43  having such a configuration adds anisotropic light collecting effects, which will be described below, to the light rays that have traveled through the light guide plate  19  and reached the light exiting surface  19   a.    
     If the light rays in the light guide plate  19  enter the sloped surfaces  43   a   1  and  43   a   2  of the front unit prisms  43   a  of the light exiting surface  19   a  with incidences equal to or smaller than the critical angle, the light rays are refracted at the sloped surfaces  43   a    1  and  43   a   2  and exit to the outside. The light rays are collected by the front unit prisms  43   a  of the front prism portion  43  with respect to the transverse direction of the light guide plate  19  to direct the light rays to travel in directions closer to the frontward direction. 
     If the light rays in the light guide plate  19  enter the sloped surfaces  43   a    1  and  43   a   2  of the front unit prisms  43   a  of the light exiting surface  19   a  with incidences larger than the critical angle, the light rays are totally reflected by the sloped surfaces  43   a   1  and  43   a   2  and returned toward the rear surface  19   b . The returned light rays may be reflected by the rear surface  19   b  of the light guide plate  19  or the reflection sheet  24  and directed toward the front side (the light exiting surface  19   a  side). 
     Because of effects of collecting light rays with respect to the transverse direction of the light guide plate  19  exerted by the front unit prisms  43   a  of the front prism portion  43 , the light rays exiting through the light exiting surface  19   a  are directed in the frontward direction of the liquid crystal display device  10  (a direction normal to the display surface DS). 
     Next, the rear prism portion  44  of the rear surface  19   b  of the light guide plate  19  (an example of a light collecting portion) will be described. The rear prism portion  44  is integrally formed with the light guide plate  19  as a portion of the light guide plate  19 . The rear prism portion  44  includes the rear unit prisms  44   a . Each rear unit prism  44   a  has an elongated shape that extends along the longitudinal direction of the light guide plate  19  (the Y-axis direction). The rear unit prisms  44   a  are adjacent to one another along the transverse direction of the light guide plate  19  (the X-axis direction). The rear unit prisms  44   a  have the same shape and the same size. The rear unit prisms  44   a  have dimensions in the transverse direction (widths) which are constant in the longitudinal direction. The dimensions of the rear unit prisms  44   a  in the transverse direction (the widths) are larger than those of the front unit prisms  43   a.    
     The rear unit prisms  44   a  protrude from the front side toward the rear side of the light guide plate  19 . Each rear unit prism  44   a  has a right-triangular shape with a vertex on the rear side when viewed in the X-axis direction. Each of the rear unit prisms  44   a  at ends of the lines of the rear unit prisms  44   a  in the transverse direction of the light guide plate  19  in this embodiment has a half shape of another rear unit prism  44   a  cut along a center line drawn from the vertex (i.e. a right triangle shape). Each of the rear unit prisms  44   a  includes either one of the sloped surfaces  44   a    1  and  44   a   2 . 
     Each rear unit prism  44   a  has a pair of sloped surfaces  44   a    1  and  44   a   2  that are adjacent to each other and form the vertex. The sloped surfaces  44   a    1  and  44   a   2  have elongated shapes. The sloped surfaces  44   a   1  and  44   a   2  have rectangular shapes that are elongated along the longitudinal direction of the light guide plate  19  (band shapes). The sloped surface  44   a   1  is arranged on the right end surface  19   e  side of the light guide plate  19  and the sloped surface  44   a   2  is arranged on the left end surface  19   f  side of the light guide plate  19 . 
     Angles θ 2  of the vertexes of the rear unit prisms  44   a  are set to a predefined obtuse angle (i.e., larger than 90°). Specifically, the angles θ 2  are set in a range from 100° to 150°, more preferably, about 140°. The angles θ 2  of the vertexes of the rear unit prisms  44   a  are set larger than the angles θ 1  of the front unit prisms  43   a  described earlier and angles θ 11  of vertexes of unit prisms  20   b   1  of the optical sheet  20 , which will be described later. 
     The rear prism portion  44  having such a configuration adds anisotropic light collecting effects, which will be described below, to the light rays that have traveled through the light guide plate  19  and reached the rear surface  19   b.    
     If the light rays in the light guide plate  19  enter the sloped surfaces  44   a   1  and  44   a   2  of the rear unit prisms  44   a  of the rear surface of the light guide plate  19  with incidences larger than the critical angle, the light rays are totally reflected by the sloped surfaces  44   a   1  and  44   a   2  and directed toward the front side of the light guide plate  19  on which the front prism portion  43  is formed. 
     If the light rays in the light guide plate  19  enter the sloped surfaces  44   a    1  and  44   a   2  of the rear unit prisms  44   a  of the rear surface of the light guide plate  19  with incidences equal to or smaller than the critical angle, the light rays are refracted at the sloped surfaces  44   a   1  and  44   a   2  and exit toward the reflection sheet  24 . The light rays exiting toward the reflection sheet  24  are reflected by the reflection sheet  24  and enter the light guide plate  19  through the sloped surfaces  44   a   1  and  44   a   2  of the rear unit prisms  44 . The light rays travel toward the front side of the light guide plate  19  on which the front prism portion  43  is formed. 
     The light rays traveling toward the front side of the light guide plate  19  as described above are repeatedly reflected inside the light guide plate  19  and finally reflected by the exiting light reflecting portion  41  formed in the rear surface  19   b  of the light guide plate  19 . Then, the light rays are refracted at the sloped surfaces  43   a    1  and  43   a   2  of the front unit prisms  43   a  and exit the light guide plate  19 . The light rays that have exited the light guide plate  19  are collected toward the frontward direction in the transverse direction of the light guide plate  19  because of the optical property of the rear prism portion  44 . 
     With the rear surface  19   b  of the light guide plate  19  including the rear prism portion  44 , the light rays exiting from the front prism portion  43  to the outside are collected toward the frontward direction in the transverse direction of the light guide plate  19 . 
     With the front prism portion  43  and the rear prism portion  44  included in the light guide plate  19 , the light rays transmitting through the light guide plate  19  are more likely to be repeatedly reflected. As a result, the light rays properly spread out inside the light guide plate  19 . 
     The unit reflecting portions  41   a  of the exiting light reflecting portion  41  are formed such that portions of the rear unit prisms  44   a  of the rear prism portion  44  are cut out. An amount of light reflected by the exiting light reflecting portion  41  (the unit reflecting portions  41   a ) tends to be proportional to a surface area thereof. Therefore, the size (the surface area) of the exiting light reflecting portion  41  (the unit reflecting portions  41   a ) is set to achieve a necessary amount of the reflected light. 
     Next, the optical sheet  20  will be described in detail. The optical sheet  20  has a light collecting function for collecting the light rays exiting from the light guide plate  19  in the transverse direction of the light guide plate  19  to adjust directions of the light rays closer to the frontward direction. 
     The optical sheet  20  has a rectangular shape in a plan view similar to the liquid crystal display panel  11 . The optical sheet  20  is laid on the light guide plate  19  to cover the light exiting surface  19   a . The optical sheet  20  is between the liquid crystal display panel  11  and the light guide plate  19 . The optical sheet  20  passes the light rays exiting from the light guide plate  19  therethrough. The optical sheet  20  adds the specific optical effects to the light rays that are passed through the optical sheet  20  and directs the light rays toward the liquid crystal display panel  11 . 
     The optical sheet  20  is a prism sheet including a sheet base and prisms that are formed on the front side of the sheet base. The optical sheet  20  includes a sheet base  20   a , a light entering surface  20   a   1 , and a prism portion  20   b . The sheet base  20   a  has a rectangular sheet shape in a plan view. The light entering surface  20   a    1  forms the rear surface of the sheet base  20   a . The light rays exiting from the light guide plate  19  enters through the light entering surface  20   a   1 . The prism portion  20   b  is formed on the front surface of the sheet base  20   a . The prism portion  20   b  has light-collecting anisotropy (light collecting property in the transverse direction). 
     The sheet base  20   a  is made of substantially transparent synthetic resin such as polyethylene terephthalate (PET). The sheet base  20   a  has a refractive index of about 1.67. The sheet base  20   a  in this embodiment is made of PET. 
     The prism portion  20   b  includes unit prisms  20   b   1 . The unit prism  20   b   1  is integrally formed with a front surface of the sheet base  20   a . The unit prisms  20   b   1  are made of material including photo-curable resin such as ultraviolet curable resin. The resin used for the unit prisms  20   b   1  may include acrylic resin such as PMMA. The refractive index of each unit prism  20   b   1  may be about 1.59. 
     The unit prisms  20   b   1  protrude from the rear side toward the front side of the sheet base  20   a . Each unit prism  20   b   1  has a right-triangular shape with a vertex on the front side when viewed in the X-axis direction. Each unit prism  20   b   1  has a pair of sloped surfaces  20   b   2  and  20   b   3  that are adjacent to each other and form the vertex. The sloped surfaces  20   b   2  and  20   b   3  have elongated shapes. The sloped surfaces  20   b   2  and  20   b   3  have rectangular shapes that are elongated along the longitudinal direction of the sheet base  20   a  (band shapes). The sloped surface  20   b   2  is arranged on the right relative to the LEDs  17  and the sloped surface  20   b   3  is arranged on the left relative to the LEDs  17 . 
     Widths (dimensions in the transverse direction) of the unit prisms  20   b   1  are constant in the longitudinal direction. The widths of the unit prisms  20   b   1  are smaller than the widths of the front unit prisms  43   a  of the light guide plate  19 . The unit prisms  20   b   1  are arranged without gaps in the transverse direction of the sheet base  20   a.    
     The angles θ 11  of the vertexes of the unit prisms  20   b   1  are set to about an right angle (about 90°). The angles θ 11  of the vertexes of the unit prisms  20   b   1  of the optical sheet  20  are smaller than the angles θ 1  of the vertexes of the front unit prisms  43   a  of the light guide plate  19 . 
     The light rays directed by the light guide plate  19  toward the optical sheet  20  having such a configuration transmit through an air layer between the light guide plate  19  and the optical sheet  20  and enter the sheet base  20   a  of the optical sheet  20  with refraction at the light entering surface  20   a   1 . The light rays that have entered the sheet base  20   a  and transmitted through the sheet base  20   a  are refracted at an interface between the sheet base  20   a  and the prism portion  20   b  according to incidences. The refracted light rays enter the unit prisms  20   b   1  and reach the sloped surfaces  20   b   2  and  20   b   3 . If incidences of the light rays that have reached the sloped surfaces  20   b   2  and  20   b   3  are equal to or larger than the critical angle, the light rays are totally reflected to the sloped surfaces  20   b   2  and  20   b   3  and returned to the sheet base  20   a . If the incidences are smaller than the critical angle, the light rays are refracted at the sloped surfaces  20   b   2  and  20   b   3  and exit to the outside. 
     The light rays exiting from the sloped surfaces  20   b   2  and  20   b   3  to the outside and traveling to the adjacent unit prisms  20   b   1  enter the adjacent unit prisms  20   b   1  to which the light rays travel, that is, the light rays are returned to the sheet base  20   a.    
     The light rays that have transmitted through the prism portion  20   b  of the optical sheet  20  and exited to the front side are collected with respect to the transverse direction (the Y-axis direction) to direct the light rays to travel in directions closer to the frontward direction. 
     The angles θ 11  of the vertexes of the unit prisms  20   b   1  of the optical sheet  20  are smaller than the angles θ 1  of the vertexes of the front unit prisms  43   a  of the light guide plate  19  and the angles θ 2  of the vertexes of the rear unit prisms  44   a . Therefore, the prism portion  20   b  retroreflects the larger number of the light rays and limits angles of the exiting light rays to a narrower range in comparison to the front prism portion  43  and the rear prism portion  44 . Namely, the prism portion  20   b  has the strongest light collecting property. 
     Next, the light blocking frame  21 , the chassis  22 , and the heat dissipating member  23  will be described. 
     The light blocking frame  21  has a frame shape that covers a periphery of the light guide plate  19 . The light blocking frame  21  has a function for holding the periphery of the light guide plate  19  from the front side. The light blocking frame  21  is in black and has light blocking property. The light blocking frame  21  is a processed piece made of synthetic resin. The light blocking frame  21  is fixed to the chassis  22  using members that are not illustrated. 
     The light blocking frame  21  includes a covering portion  21   a  that is disposed between the LEDs  17  on the LED board  18  and the end portions of the liquid crystal display panel  11  and the optical sheet  20 . The covering portion  21   a  has a function of a visor that covers the light entering surface  19   c  of the light guide plate  19 . Some of the light rays emitted by the LEDs  17  do not enter the light guide plate  19  through the light entering surface  19   b  or leak to the outside through the rear surface  19   b , the right end surface  19   e , or the left end surface  19   f . The covering portion  21   a  has a function for restricting those light rays from directly enter the end portions of the liquid crystal display panel  11  and the end portions of the optical sheet  20 . 
     The chassis  22  has a shallow container overall shape with an opening on the front side. The chassis  22  are formed from a sheet metal product having high thermal conductivity such as an aluminum sheet or an electrogalvanized steel sheet (SECC). The bottom plate  22   a  of the chassis  22  has a rectangular shape in a plan view similar to the liquid crystal display panel  11 . Side plates  22   b  project upright from the edges of the bottom plate  22   a.    
     The chassis  22  holds the reflection sheet  24 , the light guide plate  19 , the optical sheet  20 , and the liquid crystal display panel  11  that are laid in this sequence on the bottom plate  22   a . The side plates  22   b  are disposed to surround those components that are laid on one another. 
     The LED board  18  is fixed to an inner surface of the side plate  22   b  with a double-sided adhesive tape attached to a plate surface of the LED board  18  opposite from the mounting surface  18   a  on which the LEDs  17  are mounted. A driver circuit board for controlling driving of the liquid crystal display panel  11 , an LED driver circuit board for supplying driving power to the LEDs  17  (not illustrated), and a touchscreen driver circuit board for controlling driving of the touchscreen  14  (not illustrated) are attached to a rear plate surface of the bottom plate  22   a  of the chassis  22 . 
     The heat dissipating member  23  is formed from a sheet metal having high thermal conductivity such as an aluminum sheet. The heat dissipating member  23  has an elongated shape that extends along one of short edges of the chassis  22 . As illustrated in  FIG. 3 , the heat dissipating member  23  has an L-shaped cross section when viewed in the Y-axis direction. The heat dissipating member  23  is fixed to the frame  13  and the bottom plate  22   a  to connect the frame  13  to the bottom plate  22   a  of the chassis  22  with screws SW. The heat dissipating member  23  is configured to release heat generated by the LEDs  17  to the bottom plate  22   a  of the chassis  22 . 
     As described above, the liquid crystal display device  10  includes the backlight unit  12  having the light collecting functions for collecting the light rays with respect the X-axis direction and the Y-axis direction to direct the light rays to travel in directions closer to the frontward direction. The light collecting function with respect to the X-axis direction is performed by the exiting light reflecting portion  41  formed in the rear surface  19   b  of the light guide plate  19 . 
     The light collecting functions with respect to the Y-axis direction is performed by the front prism portion  43  and the rear prism portion  44  of the light guide plate  19  and the prism portion  20   b  of the optical sheet  20 . In this embodiment, if the light rays from the light guide plate  19  enter the light entering surface  20   a    1  formed from the rear surface of the optical sheet  20  (the rear surface of the sheet base) with incidences in a range from 23° to 40° with respect to the Y-axis direction (the transverse direction of the light guide plate  19  and the optical sheet  20 ), the angles of the light rays exiting from the sloped surfaces  20   b   2  and the  20   b   3  of the unit prisms  20   b   1  on the front side of the optical sheet  20  are in a range ±10° of the frontward direction, where the angle of the exiting light ray parallel to the frontward direction is defined 0°. 
     The Y-axis direction corresponds with an arrangement direction of the front prism portion  43  (the front unit prisms  43   a ) and the rear prism portion  44  (the rear unit prisms  44   a ) of the light guide plate  19  and an arrangement direction of the prism portion  20   b  (the unit prisms  20   b   1 ) of the optical sheet  20 . 
     When the light rays are collected with respect to the Y-axis direction (the transverse direction of the light guide plate  19 ) to direct the light rays in the frontward direction using a combination of the optical sheet  20  and the light guide plate  19 , some of the light rays emitted by the backlight unit  12  and traveling in directions largely angled to the frontward direction in the Y-axis direction may concentrate in an area. Therefore, in the liquid crystal display device  10  including the backlight unit  12  having the function for collecting the light rays with respect to the Y-axis direction, a transmission axis of the complex polarizing plate  28  disposed on the rear side of the liquid crystal display panel  11  is aligned with the X-axis direction so that the concentration of the light rays and a decrease in frontward brightness are less likely to occur. 
     The complex polarizing plate  28  will be described.  FIG. 9  is a perspective view schematically illustrating the complex polarizing plate  28 . The complex polarizing plate  28  mainly includes a light entering-side polarizing plate  26  and a polarization selective reflection sheet  27  that are laid on each other. The polarizing plate  26  and the polarization selective reflection sheet  27  are bonded together with an adhesive layer. The polarization selective reflection sheet  27  is disposed on the rear side of the polarizing plate  26 . Namely, the polarization selective reflection sheet  27  is disposed on the light guide plate  19  (or the optical sheet  20 ) side and the polarizing plate  26  is disposed on the array board  11   b  side of the liquid crystal display panel  11 . The complex polarizing plate  28  is attached to the array board  11   b  of the liquid crystal display panel  11  with an adhesive layer. 
     In the complex polarizing plate  28 , the polarizing plate  26  and the polarization selective reflection sheet  27  are laid on each other with the transmission axis of the polarizing plate  26  (a second transmission axis) and the transmission axis of the polarization selective reflection sheet  27  (a first transmission axis) corresponding with each other. In the complex polarizing plate  28 , the transmission axis of the polarizing plate  26  (the second transmission axis) is parallel to the transmission axis of the polarization selective reflection sheet  27  (the first transmission axis). Namely, the complex polarizing plate  28  has the transmission axis  28 A that corresponds with (or parallel to) the transmission axis of the polarization selective reflection sheet  27  (the first transmission axis) and the transmission axis of the polarizing plate  26  (the second transmission axis). 
     The polarizing plate  26  is formed by mixing absorbers such as iodine and dichromatic dye into polymer resin and orientating the absorbers by stretching. The polarizing plate  26  is not limited to the above as long as the polarizing plate  26  is capable of converting non-polarization to linear polarization. The polarizing plate  26  has a function for passing the linearly polarized light rays (linearly polarized light rays in a first condition) entering the polarizing plate  26  with a direction of polarization (a vibration plane) parallel to the transmission axis (the second transmission axis). 
     The polarization selective reflection sheet  27  has a function for reflecting the linearly polarized light rays (linearly polarized light rays in a second condition) entering the polarization selective reflection sheet  27  with a direction of polarization (a vibration plane) parallel to a reflection axis. The polarization selective reflection sheet  27  has a function for passing the linearly polarized light rays (the linearly polarized light rays in the first condition) entering the polarization selective reflection sheet  27  with the direction of polarization parallel to the transmission axis (the first transmission axis). The transmission axes of the polarization selective reflection sheet  27  are perpendicular to the reflection axis. 
     A brightness enhancement film such as a DBEF (by 3M Company) and a Nippokusu APCF (by Nitto Denko Corporation) may be used for the polarization selective reflection sheet  27 . 
     It may be described that the complex polarizing plate  28  has a reflection axis  28 B perpendicular to the transmission axis  28 A. The orientation of the reflection axis  28 B corresponds with the orientation of the reflection axis of the polarization selective reflection sheet  27 . 
     The light exiting-side polarizing plate  25  is disposed on the front side of the liquid crystal panel  11 . The polarizing plate  25  is attached to the CF substrate  11   a  with an adhesive with the transmission axis perpendicular to the transmission axis  28 A of the complex polarizing plate  28 . 
       FIG. 10  is an exploded perspective view schematically illustrating a positional relationship between the backlight unit  12  and the complex polarizing plate  28  in a testing device T. As illustrated in  FIG. 10 , the complex polarizing plate  28  is placed over the optical sheet  20  with the transmission axis  28 A corresponding with the X-axis direction. The testing device T includes the complex polarizing plate  28  placed over the optical sheet  20  of the backlight unit  12  in the liquid crystal display device  10  according to the first embodiment of the present invention. 
     The optical axis L of the light from the LEDs  17  is along the X-axis direction. In this specification, a direction perpendicular to the optical axis L (the Y-axis direction) is defined as a “light collecting direction”) of the light collecting portion (the front prism portion  43 , the rear prism portion  44 ) in the backlight unit  12  and a direction perpendicular to the light collecting direction (i.e., a direction along the optical axis L) is defined as a (non-light collecting direction). 
     With the complex polarizing plate  28 , the frontward brightness of the light exiting from the light guide plate  19  improves. 
     The orientation of the transmission axis  28 A of the complex polarizing plate  28  may be described with reference to a dial plate of an imaginary clock as in  FIG. 10 . Specifically, “6 o&#39;clock” of the dial plate of the imaginary clock is on the LED  17  side of the light guide plate  19  and “12 o&#39;clock” is on the opposite end surface  19   d  side of the light guide plate  19  (i.e., a direction along the optical axis of the LED  17 ). A direction along the X-axis direction corresponds with a “12 o&#39;clock-to-6 o&#39;clock direction” and a direction along the Y-axis direction corresponds with a “3 o&#39;clock-to-9 o&#39;clock direction.” 
     Furthermore, the orientation of the transmission axis  28 A of the complex polarizing plate  28  along the “3 o&#39;clock to 9 o&#39;clock” on the dial plate of the imaginary clock may be defined as “at an angle of 0°.” The orientation of the transmission axis  28 A along the “12 o&#39;clock to 6 o&#39;clock,” that is, rotated in clockwise from the above position on the X-Y plane (i.e., along the light exiting surface  19   a  of the light guide plate  19 ) may be defined as “at an angle of 90°.” 
     The angle of the transmission axis  28 A of the complex polarizing plate  28  and the frontward brightness will be described with reference to  FIG. 11 .  FIG. 11  is a graph illustrating a relationship between the angle of the transmission axis  28 A of the complex polarizing plate  28  and relative value of the frontward brightness in the testing device. The horizontal axis of the graph in  FIG. 11  represents the angle of the transmission axis  28 A of the complex polarizing plate  28  and the vertical axis represents the brightness (relative brightness in %) of the light from the light guide plate  19  and transmitted through the complex polarizing plate  28  in the frontward direction (a direction normal to the complex polarizing plate  28 ). 
     In the testing device including the complex polarizing plate  28  placed over the optical sheet  20  in the backlight unit  12 , light is supplied from the backlight unit  12  to the complex polarizing plate  28  and the frontward brightness of the light transmitted through the complex polarizing plate  28  was measured while the angle of the transmission axis  28 A was altered. The transmission axis  28 A along “3 o&#39;clock to 9 o&#39;clock” was defined as “at an angle of 0°” and the transmission axis  28 A rotated clockwise 180° from the above position was defined as “at an angle of 180°.” 
     As illustrated in  FIG. 11 , when the transmission axis  28 A was at an angle of 90°, the frontward brightness of the light exiting from the complex polarizing plate  28  was the highest. It was observed that the frontward brightness of the exiting light varied substantially symmetrically about the transmission axis  28 A at 90°. When the transmission axis  28 A was at 90°, the reflection axis  28 B of the complex polarizing plate  28  (i.e., the reflection axis of the polarization selective reflection sheet  27 ) was along the “3 o&#39;clock-to-9 o&#39;clock” direction (the Y-axis direction). The complex polarizing plate  28  actively reflects the light rays emitted by the backlight unit  12  in directions largely angled to the frontward direction toward the plate surface of the complex polarizing plate  28  (i.e., directions at small angles relative to the plate surface of the complex polarizing plate  28 ). It is assumed that the reflected light rays contribute to improvement of the brightness in the frontward direction. 
     Next, a brightness distribution (light distribution characteristics) when the transmission axis  28 A of the complex polarizing plate  28  is at 90° (i.e., the transmission axis  28 A is along the “12 o&#39;clock-to-6 o&#39;clock” direction, the X-axis direction) will be described. 
     A relationship between a coordinate system representing the brightness distribution and the testing device T including the backlight unit  12  and the complex polarizing plate  28  will be described with reference to  FIG. 12 .  FIG. 12  is a perspective view schematically illustrating the relationship between the testing device T and the coordinate system. As illustrated in  FIG. 12 , a hemispherical grid was set over a light exiting surface  28   a  in the testing device T (a front surface of the complex polarizing plate  28 ). A center of the hemispherical grid was at the center of the light exiting surface  28   a . The angle of 180° was set on the left of the testing device T that was viewed from the LED  17  side and the angle of 0° was set on the opposite side. The angle of 270° was set on the LED  17  side and the angle of 90° was set on the opposite side. 
     The brightness distribution (the light distribution characteristics) of the light exiting from the testing device T was measured with an optical goniometer (EZContrast by ELDIM). The results are presented in  FIG. 13 .  FIG. 13  illustrates the brightness distribution (the light distribution characteristics) of the light exiting from the testing device T with the transmission axis  28 A of the complex polarizing plate  28  at 90°. 
     A comparative experiment was performed. The results of the comparative experiment illustrating a brightness distribution of light exiting from the testing device T with the transmission axis  28 A of the complex polarizing plate  28  at 0° (i.e., the transmission axis  28 A is along the “3 o&#39;clock-to-9 o&#39;clock” direction, the Y-axis direction) are presented in  FIG. 14 .  FIG. 14  illustrates the brightness distribution (the light distribution characteristics) of the light exiting from the testing device T with the transmission axis  28 A of the complex polarizing plate  28  at 0°. 
     In  FIGS. 13 and 14 , R 1  indicates a region having the highest brightness level. R 2 , R 3 , R 4 , R 5 , and R 6  indicate regions having brightness levels that become smaller in this sequence. 
     As illustrated in  FIG. 13 , when the transmission axis  28 A of the complex polarizing plate  28  is at 90°, the brightness level in the frontward direction is the highest. Regions (regions R 5 ) on the right and the left relative to the frontward direction have brightness levels slightly higher than brightness levels of surrounding regions. Such differences in brightness level are too small to be recognized by human eyes. Namely, when the transmission axis  28 A of the complex polarizing plate  28  is at 90°, the light exiting from the testing device T is less likely include light rays that travel in directions angled to the frontward direction toward the light exiting surface (side lobe light) in the “3 o&#39;clock-to-9 o&#39;clock” direction. 
     As illustrated in  FIG. 13 , when the transmission axis  28 A of the complex polarizing plate  28  is at 90°, the brightness level in the frontward direction is the highest and brightness levels do not become unnecessary high in regions on the right and the left relative to the frontward direction. The side lobe light is actively reflected by the light elective reflection sheet  27  of the complex polarizing plate  28  and the polarization is canceled when reflected light rays are multiply scattered. The reflected light rays form a light flux that contributes to improvement of the brightness in the frontward direction. Therefore, the brightness levels do not become unnecessary high. 
     When the transmission axis  28 A of the complex polarizing plate  28  is at 0°, the regions (the regions R 4  and R 5 ) on the right and the left relative to the frontward direction have the brightness levels higher than the brightness levels of the surrounding regions as illustrated in  FIG. 14 . The regions look brighter than the surrounding regions, that is, the differences in brightness level can be recognized by human eyes. When the transmission axis  28 A of the complex polarizing plate  28  is at 0°, the light exiting from the testing device T includes light rays travel in directions angled to the frontward direction toward the light exiting surface (side lobe light) in the “3 o&#39;clock-to-9 o&#39;clock” direction. In a condition illustrated in  FIG. 14 , the brightness level in the forward direction is the highest. 
     Next, the brightness distribution (the light distribution characteristics) of the light exiting from the testing device T was measured with the optical goniometer (EZContrast by ELDIM) while the angle of the transmission axis  28 A of the complex polarizing plate  28  was altered. Specifically, the brightness distribution (the light distribution characteristics) of the light exiting surface in the testing device T in the “12 o&#39;clock-to-6 o&#39;clock” direction and the brightness distribution (the light distribution characteristics) of the light exiting surface in the testing device T in the “3 o&#39;clock-to-9 o&#39;clock” direction were measured while the angle of the transmission axis  28 A of the complex polarizing plate  28  was altered from 0° to 30°, 60°, 90°, 120°, and 150° in this sequence. 
       FIG. 15  is a perspective view schematically illustrating a relationship between the testing device T and another coordinate system. In  FIG. 15 , an observation angle (a polar angle) d is an angle relative to an imaginary line that passes the center of the light exiting surface  28   a  (the front surface of the complex polarizing plate  28 ) in the testing device T and perpendicular to the light exiting surface  28   a . In the “12 o&#39;clock-to-6 o&#39;clock” direction, the LED  17  side is defined as −90° and the opposite side is defined as +90°. In the “3 o&#39;clock-to-9 o&#39;clock” direction, the right side viewed from the LED  17  side is defined as +90° and the opposite side is defined as −90°. 
       FIG. 16  illustrates the measured brightness distribution (the light distribution characteristics) in the “12 o&#39;clock-to-6 o&#39;clock” direction in the testing device T. In  FIG. 16 , the horizontal axis represents the observation angle d (deg.) in the 12 o&#39;clock-to-6 o&#39;clock” direction and the vertical axis represents the brightness (relative brightness) of the light exiting from the light exiting surface  28   a  in the 12 o&#39;clock-to-6 o&#39;clock” direction. 
     As illustrated in  FIG. 16 , the brightness (the relative brightness) in the 12 o&#39;clock-to-6 o&#39;clock” direction barely changed even though the angle of the transmission axis  28 A of the complex polarizing plate  28  was altered. 
       FIG. 17  illustrates the measured brightness distribution (the light distribution characteristics) in the 3 o&#39;clock-to-9 o&#39;clock” direction. In  FIG. 17 , the horizontal axis represents the observation angle d (deg.) in the 3 o&#39;clock-to-9 o&#39;clock” direction and the vertical axis represents the brightness (relative brightness) of the light exiting from the light exiting surface  28   a  in the 3 o&#39;clock-to-9 o&#39;clock” direction. 
     As illustrated in  FIG. 17 , in the brightness distribution (the light distribution characteristics) in the 3 o&#39;clock-to-9 o&#39;clock” direction, the brightness was locally increased in regions around d=about 60° to 70° and d=about −70° to −60° away from the frontward direction. The light was directed to such regions. As illustrated in  FIG. 17 , when the transmission axis  28 A of the complex polarizing plate  28  was at 90°, the brightness (the relative brightness) in the regions around d=about 60° to 70° and d=about −70° to −60° was significantly reduced in comparison to other regions. 
     Next, relationships between brightness ratios (side lobe light/frontward light) and the angles of the transmission axis  28 A of the complex polarizing plate  28  will be described with reference to  FIG. 18  based on the frontward light (d=−45° to +450) and the side lobe light (other than the frontward light) extracted from the brightness distribution (the light distribution characteristics) in the 3 o&#39;clock-to-9 o&#39;clock” direction illustrated in  FIG. 17 .  FIG. 18  is a graph illustrating a relationship between brightness ratio of the frontward light relative to the side lobe light in the 3 o&#39;clock-to-9 o&#39;clock” direction in the testing device and angle of the transmission axis  28 A of the complex polarizing plate  28 . 
     In  FIG. 18 , the horizontal axis represents the angle (deg.) of the transmission axis  28 A of the complex polarizing plate  28  and the vertical axis represents the brightness ratio between the frontward light and the side lobe light (=side lobe light/frontward light). The brightness ratio is a relative ratio. As illustrated in  FIG. 18 , in the brightness distribution (the light distribution characteristics) in the 3 o&#39;clock-to-9 o&#39;clock” direction, when the transmission axis  28 A of the complex polarizing plate  28  was at 90° (deg.), the brightness ratio was the smallest (i.e., the ratio of the side lobe light relative to the frontward light was the smallest). 
     As described above, in the liquid crystal display device  10  according to this embodiment, the transmission axis  28 A of the complex polarizing plate  28  disposed on the rear side of the liquid crystal display panel  11  corresponds with the X-axis direction. Namely, the linearly polarized light having the vibration plane (the polarization direction) parallel to the X-axis direction among the light exiting from the backlight unit  12  transmits through the complex polarizing plate  28  and the linearly polarized light having the vibration plane (the polarization direction) parallel to the reflection axis  28 B of the complex polarizing plate  28  is reflected by the polarization selective reflection sheet  27  of the complex polarizing plate  28 . 
     By setting the transmission axis  28 A of the complex polarizing plate  28  as above, the light rays exiting from the display surface DS of the liquid crystal display panel  11  and travel in the directions angled to the frontward direction toward the sides (the side lobe light) are reduced. Therefore, the uneven brightness in the light exiting from the display surface DS of the liquid crystal display panel  11  is less likely to occur. 
     By setting the transmission axis  28 A of the complex polarizing plate  28  disposed on the rear side of the liquid crystal display panel  11  corresponding with the X-axis direction, the reduction in frontward brightness is less likely to occur. 
     Even through the touchscreen  14  and the cover panel  15  are disposed to cover the display surface DS of the liquid crystal display panel  11 , the uneven brightness is less likely to occur in the light exiting from the display surface DS as described above. Therefore, the reduction in frontward brightness is less likely to occur. 
     Second Embodiment 
     A second embodiment will be described with reference to  FIG. 19 . Structures similar to those of the first embodiment will be indicated by the same symbols and will not be described in detail hereinafter. 
       FIG. 19  is an exploded perspective view schematically illustrating a positional relationship between a backlight unit  120  and the complex polarizing plate  28  in a testing device T 1  corresponding to a liquid crystal display device according to the second embodiment of the present invention. The testing device T 1  in this embodiment includes a light guide plate  190 , which is a difference from the testing device T in the first embodiment. Specifically, the light guide plate  190  in the testing device T 1  in this embodiment includes a front surface and a rear surface that correspond to the rear surface and the front surface of the light guide plate  19  in the first embodiment, respectively. Other configurations are basically similar to those of the first embodiment. 
     Although the backlight unit  120  includes the light guide plate  190  that includes the front surface and the rear surface that correspond to the rear surface and the front surface of the light guide plate  19 , respectively, the backlight unit  120  has a light collecting function similar to the first embodiment with respect to the Y-axis direction. Light rays that have entered the light guide plate  190  (the light guide plate  19 ) through the light entering surface  19   c  exit the light guide plate  190  through the rear surface (corresponding to the light exiting surface  19   a  of the light guide plate  19 ) toward the reflection sheet  24 . The exiting light rays are most likely to be totally reflected by the reflection sheet  24  without cancellation of the polarization. The reflected light rays are repeatedly reflected inside the light guide plate  190  and exit the light guide plate  190  through the front surface (corresponding to the rear surface  19   b  of the light guide plate  19 ) toward the optical sheet  20 . 
     In the backlight unit  120  having such a configuration, the number of reflection inside the light guide plate  190  is larger than that of the first embodiment. Therefore, further evenly spreading planar exiting light is achieved. Such a backlight unit  120  has a characteristic that fine foreign substances or forming irregularity is less recognizable even if the fine foreign substances enter the backlight unit  120  or the forming irregularity (e.g., burrs) occurs in production of the backlight unit  120 . In such a backlight unit  120 , the exiting light rays are collected with respect to the Y-axis direction with optical effects of the optical sheet  20  and the light guide plate  190  to direct the light rays in the frontward direction. 
     In such a backlight unit  120 , when the transmission axis  28 A of the complex polarizing plate  28  is orientated to correspond with the 12 o&#39;clock-to-6 o&#39;clock” direction (the X-axis direction, the optical axis direction of the LED  17 ), the light rays exiting from the light exiting surface  28   a  in the testing device T 1  traveling in directions angled to the frontward direction toward the sides (side lobe light) are reduced. Therefore, the uneven brightness in light exiting from the light exiting surface  28   a  is less likely to occur. 
     Third Embodiment 
     A third embodiment according to the present invention will be described with reference to  FIG. 20 .  FIG. 20  is an exploded perspective view schematically illustrating a positional relationship between a backlight unit  121  and the complex polarizing plate  28  in a testing device T 2  corresponding to a liquid crystal display device according to the third embodiment of the present invention. 
     The testing device T 2  according to this embodiment includes an optical sheet  200 , which is only difference between the testing device T 2  and the testing device T according to the first embodiment. Specifically, the optical sheet  200  in the testing device T 2  includes a sheet base  200   b  replaced with the sheet base  20   a  of the optical sheet  12  made of PET in the first embodiment. The sheet base  200   b  is made of material that does not have a birefringent property (e.g., polycarbonate, acrylic resin). Other configurations are basically similar to those of the first embodiment. 
     With such an optical sheet  200 , when the transmission axis  28 A of the complex polarizing plate  28  is at 90°, the frontward brightness level of light exiting from the testing device T 2  is the highest. Furthermore, the frontward brightness levels change symmetrically about the transmission axis  28 A at 90° more precisely than the first embodiment (see  FIG. 11 ). 
     A relationship between an angle of a transmission axis of a complex polarizing plate in a testing device according to a comparative example and a brightness level in the frontward direction (a frontward brightness level) will be described with reference  FIG. 21 .  FIG. 21  is a graph illustrating the relationship between angle of the transmission axis of the complex polarizing plate and relative value of the frontward brightness in the testing device according to the comparative example. The testing device according to the comparative example includes an optical sheet that includes a sheet base made of PET having a birefringent property. Configurations of the optical sheet other than the sheet base are similar to the first embodiment. The frontward brightness level of light exiting from the testing device according to the comparative example was the highest when the transmission axis of the complex polarizing plate was at 90°. The frontward brightness levels of the exiting light change asymmetrically about the transmission axis  28 A at 90°. 
     A relationship between angle of a transmission axis of a complex polarizing plate in a testing device according to another comparative example and brightness level in the frontward direction (frontward brightness level) will be described with reference to  FIG. 22 .  FIG. 22  is a graph illustrating the relationship between angle of the transmission axis of the complex polarizing plate in the testing device according to the comparative example and relative value of the frontward brightness. The testing device according to the other comparative example includes a sheet base made of PET having a birefringent property. Configurations other than the sheet base are similar to the first embodiment. The frontward brightness level of light exiting from the testing device according to the other comparative example was not the highest when the transmission axis of the complex polarizing plate was at 90° as illustrated in  FIG. 22 . The brightness level was the maximum when the transmission axis was at about 70°. The frontward brightness levels of the exiting light changed asymmetrically about the transmission axis at 90°. 
     As illustrated in  FIGS. 21 and 22 , with the sheet base of the optical sheet having the birefringent property, the brightness level of the exiting light is not the maximum when the transmission axis of the complex polarizing plate is at 90°. Furthermore, the frontward brightness levels of the exiting light do not change symmetrically about the transmission axis at 90°. In general, the sheet base made of PET is less likely to have the birefringent property as in the first embodiment. However, the sheet base made of PET may have the birefringent property due to a different producing method or a portion of an original sheet of taken for the sheet base. Therefore, it is preferable to use a material that is less likely to have the birefringent property for the material for the sheet base of the optical sheet. 
     Other Embodiment 
     The present invention is not limited to the above embodiments described with reference to the drawings. The following embodiments may be included in the technical scope of the present invention. 
     (1) In each of the above embodiments, the light guide plate  19  includes the front surface and the rear surface that include the light collecting portions (the front prism portion  43 , the rear prism portion  44 ), respectively. However, the present invention is not limited to such a configuration. For example, only the front surface of the light guide plate  19  may include a light collecting portion such as the front prism portion  43  or only the rear surface of the light guide plate  19  may include a light collecting portion such as the rear prism portion  44 . 
     (2) In each of the above embodiments, each of the unit light collecting portions (the front unit prisms  43   a , the rear unit prisms  44   a ) of the light collecting portions of the front surface and the rear surface of the light guide plate  19  has the triangular cross section when viewed along the optical axis L. However, the present invention is not limited to such a configuration. The cross-sectional shape is not limited to a particular shape as long as the unit light collecting portions have the light collecting properties to collect the light rays with respect to the direction perpendicular to the optical axis L (the light collecting direction) to direct the light rays to travel in directions closer to the frontward direction and supply light to the optical sheet  20  so that the optical sheet  20  can exert the designed light collecting effects. For example, the cross-sectional shape may be a substantially semi-circular shape including a semicircular shape and a semi-elliptical shape. 
     (3) In each of the above embodiments, the optical sheet  20  includes the prism portion  20   b  that includes the unit prisms  20   b   1  each having the triangular cross section when viewed along the optical axis L. However, the optical sheet  20  may include cylindrical lenses that extend along the optical axis L and have substantially semicircular cross sections instead of the unit prisms  20   b   1 . The shapes and the sizes of the prism portion  20   b  and the light collecting portions of the optical sheet such as the cylindrical lenses are not limited to any shapes and sizes as long as the light rays exiting from the optical sheet gather with respect to the direction perpendicular to the optical axis L (the light collecting direction) to direct the light rays in the frontward direction. 
     (4) In each of the above embodiments, the light from the light source enters the light guide plate  19  through one end surface (the light entering surface  19   c ). In each of the other embodiments, the opposite end surface  19   d  opposite from the light entering surface  19   c  may be configured as another light entering surface. 
     (5) In each of the above embodiments, the LEDs are used as the light source. However, other types of light sources such as organic ELs may be used in the other embodiments. 
     (6) In each of the above embodiments, the transmission axis of the polarizing plate  26  on the light entering side and the transmission axis of the polarizing plate  25  on the light exiting side are perpendicular to each other (a crossed Nichol configuration). However, the present invention is not limited to such a configuration. The transmission axis of the polarizing plate  25  on the light exiting side may be orientated as appropriate (e.g., parallel Nicole) according a liquid crystal mode. 
     (7) In each of the above embodiments, the optical sheet  20  includes a single prism sheet. However, another type of optical sheets (e.g., a diffuser sheet, a prism sheet) may be includes in the other embodiments as long as the effects of the present invention can be achieved. 
     EXPLANATION OF SYMBOLS 
     
         
         
           
               10 : Liquid crystal display device 
               11 : Liquid crystal display panel 
               12 : Backlight unit 
               13 : Frame 
               14 : Touchscreen 
               15 : Cover panel 
               17 : LED (light source, point light source) 
               19 : Light guide plate 
               19   a : Light exiting surface 
               19   b : Rear surface 
               19   c : Light entering surface 
               19   d : Opposite end surface 
               20 : Optical sheet (prism sheet) 
               20   a : Sheet base 
               20   b : Prism portion 
               20   b   1 : Unit prism 
               24 : Reflecting sheet 
               28 : Complex polarizing plate 
               28 A: Transmission axis 
               28 B: Reflection axis 
               43 : Front prism portion (light collecting portion) 
               43   a : Front unit prism 
               44 : Rear prism portion (light collecting portion) 
               44   a : Rear unit prism 
             L: Optical axis