Abstract:
In order to accurately illuminate a lighting target with planar light having few nonuniformities, a backlight ( 2 ) is provided with a light guide plate ( 5 ) having multiple leg members ( 531 ) protruding from an attachment part ( 53 ), which is the surface opposite of an emission part ( 52 ), and a chassis ( 4 ) having holding units ( 411 ) into which the leg members ( 531 ) are inserted.

Description:
TECHNICAL FIELD 
     The present invention relates to a backlight unit that supplies light to a non-light-emitting display panel such as a liquid crystal display panel and the like, and to a liquid crystal display device that includes the backlight unit. 
     BACKGROUND ART 
     In recent years, there are many cases where a liquid crystal display device is used as a display device of a notebook computer monitor, a television receiver and the like. The liquid crystal display device includes a liquid crystal display panel, and a backlight unit that is disposed to oppose a rear surface of the liquid crystal display panel, wherein the liquid crystal display panel adjusts penetration ratio (penetration amount) of light from the backlight unit and displays an image on a front surface of the liquid crystal display panel. 
     The backlight unit is roughly classified into two kinds. One is a light guide plate type (edge-light type) that uses a light guide plate and inputs light from a side of the light guide plate, and the other is a direct type in which a light source is disposed to oppose a rear surface of a liquid crystal module. Request for thickness reduction and size reduction of the liquid crystal display device is increasing, and more and more liquid crystal display devices are employing the edge-light type that is a backlight unit thinner than the direct type backlight unit. 
     A conventional backlight unit is described with reference to drawings.  FIG. 8  is an exploded perspective view of a conventional backlight unit that is used for a liquid crystal display device. A backlight unit  92  is a unit that outputs planar light to a rear surface of a liquid crystal display panel (not shown). As shown in  FIG. 8 , the backlight unit  92  includes: a backlight chassis  94 ; a plurality of LED lamps  96  that are light sources held by the backlight chassis  94 ; and a light guide plate  95  that is held by the backlight chassis  94  and outputs light from the LED lamp  96  as the planar light to the liquid crystal display panel. The light guide plate  95  is held by a side wall portion of the backlight chassis  94 . Here, as shown in  FIG. 8 , the backlight unit  92  has a rectangular shape when viewed from front. And, the LED lamp  96  is disposed to output the light to both end surfaces of the light guide plate  95  in a long-edge direction (see JP-A-2001-14923 and JP-A-2001-34185). 
     However, there are many cases where a gap is formed between the backlight chassis  94  and the light guide plate  95 . In these cases, it is impossible to position the light guide plate  95  with respect to the backlight chassis  94  with high accuracy, accordingly, the brightness of the light output from the backlight unit  92  declines and the light output position deviates. 
     There is a backlight unit that has a structure for positioning the backlight unit  94  and the light guide plate  95  with high accuracy.  FIG. 9  is a front view of an improved version of the backlight unit that is shown in  FIG. 8 . As shown in  FIG. 9 , there is the backlight unit  92  in which the light guide plate  95  is provided with a plurality of cutouts  950 ; and a stud pin  97  disposed in the backlight chassis  94  and the cutout  950  are applied to each other to be positioned. As described above, by using the plurality of cutouts  950  and the stud pins  97  disposed corresponding to the cutouts  950 , it is possible to correctly position the backlight chassis  94  and the light guide plate  95 , accordingly, it is possible to output correct planar light to the liquid crystal display panel (not shown). 
     CITATION LIST 
     Patent Literature 
     
         
         PLT1: JP-A-2001-14923 
         PLT1: JP-A-2004-34185 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     However, in the case where the positioning of the backlight chassis  94  and the light guide plate  95  is performed by using the cutout  950  and the stud pin  97 , the light traveling in the light guide plate  95  is blocked and irregularly reflected because of influence of the cutouts  950  and the stud pint  97 . As a result of this, brightness unevenness occurs in the planar light output from the light guide plate  95 . The brightness unevenness caused by these cutout  950  and the stud pin  97  is described with reference to drawings.  FIG. 10  is an enlarged view of a portion where the brightness unevenness is caused by the stud pin, and  FIG. 11  is an enlarged view of a portion where the brightness unevenness is caused by the cutout. 
     As shown in  FIG. 10 , the stud pin  97  is disposed in the cutout  950 , accordingly, when viewing the light guide plate  95  from front, the stud pin  97  is disposed in the rectangular shape of the light guide plate  95 . In this state, when the light is output from the LED lamp, the stud pin  97  blocks the light, accordingly, as shown in  FIG. 10 , a V-shaped shadow Sd is formed. Because of this shadow, the brightness unevenness occurs in the planar light output from the backlight unit. 
     Besides, as shown in  FIG. 11 , because of the forming of the cutout  950 , the light in the light guide plate  95  is totally reflected by the cutout  950 , accordingly, the brightness at the cutout  950  increases and the brightness unevenness occurs. 
     As described above, according to the light guide plate  95  that has the structure which easily causes the brightness unevenness, it becomes hard to perform optimum designing for alleviating the brightness unevenness of the output light. 
     To avoid this, it is an object of the present invention to provide a backlight unit that is easy to produce and assemble and able to output planar light having alleviated brightness unevenness to an illumination target with high accuracy, and a liquid crystal display device that uses the backlight unit. 
     Solution to Problem 
     To achieve the above object, the present invention is a backlight unit that includes: a chassis; a light source that is mounted in the chassis; and a rectangular parallelepiped light guide plate that is mounted in the chassis, receives light from the light source via an end surface, and outputs the light as planar light from a front output portion; wherein the light guide plate includes a plurality of leg members that protrude from a mounted portion that is a surface opposite to the output portion; and the chassis includes holding portions into which the leg members are inserted. 
     According to this structure, the light guide plate is a rectangular parallelepiped and the leg member, which is an engagement structure that engages with the chassis of the light guide plate, has a shape that protrudes outside the light guide plate, accordingly, it is unnecessary to dispose the engagement structure in front of the light source. Besides, the cutout for the positioning is unnecessary. According to this, the light from the light source is not blocked by the engagement structure, and the light from the light source is not unevenly reflected by the cutout, accordingly, it is possible to alleviate brightness unevenness of the planar light output from the output portion. 
     Besides, the light guide plate has the structure which includes the leg member that protrudes from the rectangular parallelepiped, accordingly, the structure is simple and the production is easy. Besides, the output portion of the light guide plate which outputs the received light as the planar light is a rectangular parallelepiped, accordingly, optimum designing of the light guide plate for alleviating the brightness unevenness of the planar light is easy. 
     Besides, by inserting the leg member into the holding portion, the light guide plate is positioned in the chassis with high accuracy. According to this, the light guide plate is able to output the planar light form the output portion to an illumination target with high accuracy. 
     In the above structure, the light guide plate and the plurality of leg members may be formed integrally with each other, or the plurality of leg members may be attachable to and detachable from the light guide plate. If the light guide plate and the leg members are formed integrally with each other, the production of the light guide plate is easy. Besides, in a case where the light guide plate and the leg member are structured to be attachable and detachable, even in a case where it is necessary to change the position of the holding portion of the chassis, it is sufficient to change the position of the leg member of the light guide plate, accordingly, it is easy to deal with the modification of the chassis. 
     In the above structure, the plurality of leg members may be formed to be not-insertable into the holding portion when the light guide plate is rotated 180° about an axis that passes through a center of the mounted portion and meets the mounted portion at right angles. 
     According to this structure, only when the light guide plate is in a direction (direction that is not rotated 180°) that is correct with respect to the chassis, it is possible to mount the light guide plate in the chassis. According to this, even if the direction is not strictly confirmed, the light guide plate is mounted in the chassis in the correct direction. According to this, it is possible to mount the optimized light guide plate in the chassis in the suitable direction, and it is possible to alleviate the brightness decline and brightness unevenness of the planar light output from the light guide plate. 
     In the above structure, a sectional shape, which is obtained by cutting, along a surface parallel to the mounted portion, at least one set of leg members disposed at symmetrical positions with respect to the center of the mounted portion, may be a shape that is not point-symmetrical. 
     According to this structure, only when the light guide plate is in the direction (direction that is not rotated 180°) that is correct with respect to the chassis, it is possible to mount the light guide plate in the chassis. According to this, even if the direction is not strictly confirmed, the light guide plate is mounted in the chassis in the correct direction. According to this, it is possible to mount the optimized light guide plate in the chassis in the suitable direction, and it is possible to alleviate the brightness decline and brightness unevenness of the planar light output from the light guide plate. 
     In the above structure, the backlight unit may include two or more kinds of leg members that have sectional shapes which are obtained by cutting along the surface parallel to the mounted portion and are different from each other. 
     According to this structure, only when the light guide plate is in the direction (direction that is not rotated 180°) that is correct with respect to the chassis, it is possible to mount the light guide plate in the chassis. According to this, even if the direction is not strictly confirmed, the light guide plate is mounted in the chassis in the correct direction. According to this, it is possible to mount the optimized light guide plate in the chassis in the suitable direction, and it is possible to alleviate the brightness decline and brightness unevenness of the planar light output from the light guide plate. 
     In the above structure, the light guide plate may have a structure that is dividable into a plurality of light guide members; and the light guide members each may have two or more leg members. According to this structure, it is possible to perform area control in which the light guide plate outputs light that has brightnesses different for the plurality of regions. Besides, it is possible to assemble the light guide plate by only inserting the leg member into the holding portion. According to this, the assembly of the light guide plate is easy. 
     In the above structure, the holding portion may be a through-hole, or a closed concave hole. 
     Besides, the backlight unit according to the present invention is usable as a planar illumination device of a liquid crystal display device. Besides, without being limited to the liquid crystal display device, the backlight unit according to the present invention is usable as a rear surface illumination device of a transmissive type image display device that does not emit light by itself. 
     Advantageous Effects of Invention 
     According to the present invention, it is possible to provide a backlight unit that is easy to produce and assemble and able to output planar light having less brightness unevenness to an illumination target with high accuracy, and a liquid crystal display device that uses the backlight unit. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       [ FIG. 1 ] is an exploded perspective view of an example of a liquid crystal display device that includes a backlight unit according to the present invention. 
       [ FIG. 2 ] is an exploded perspective view of a backlight unit used in the liquid crystal display device shown in  FIG. 1 . 
       [ FIG. 3 ] is a rear view of the backlight unit shown in  FIG. 2 . 
       [ FIG. 4 ] is a schematic view of another example of a light guide plate and a backlight chassis of a backlight unit according to the present invention. 
       [ FIG. 5 ] is a schematic view of another example of a light guide plate and a backlight chassis. 
       [ FIG. 6 ] is a schematic view of another example of a light guide plate and a backlight chassis. 
       [ FIG. 7 ] is a schematic view of another example of a light guide plate and a backlight chassis. 
       [ FIG. 8 ] is an exploded perspective view of a conventional backlight unit used in a liquid crystal display device. 
       [ FIG. 9 ] is a plan view of an improved version of the backlight unit shown in  FIG. 8 . 
       [ FIG. 10 ] is an enlarged view of a portion where brightness unevenness is caused by a stud pin. 
       [ FIG. 11 ] is an enlarged view of a portion where brightness unevenness is caused by a cutout. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments of the present invention are described with reference to the drawings.  FIG. 1  is an exploded perspective view of an example of a liquid crystal display device that includes a backlight unit according to the present invention. As shown in  FIG. 1 , a liquid crystal display device A includes: a liquid crystal display panel  1 ; and a backlight unit  2  that is disposed on a rear surface of the liquid crystal display panel  1 . 
     The liquid crystal display panel  1  has: a liquid crystal panel  11  in which liquid crystal is injected; a light polarization plate  12  that is attached to a front surface (viewer side) of the liquid crystal panel  11 ; and a light polarization plate  13  that is attached to a rear surface (backlight unit  2  side) of the liquid crystal panel  11 . The liquid crystal panel  11  includes: an array substrate that includes switching devices such as a TFT (thin film transistor) and the like; an opposite substrate that is disposed to oppose the array substrate and is provided with a color filter; and liquid crystal that is injected between the array substrate and the opposite substrate (none of them are shown). Here, the liquid crystal panel  1  may have a structure in which the color filter is formed on the array substrate and a transparent electrode is formed on the opposite substrate. 
     As shown in  FIG. 1 , in the liquid crystal display device A, an optical sheet unit  3  is disposed between the liquid crystal display panel  1  and the backlight unit  2 . In the liquid crystal display device A shown in  FIG. 1 , the optical sheet unit  3  includes three optical sheets, which however is not limiting, and may include more sheets, or may be composed of a smaller number of sheets. The optical sheet unit  3  includes, for example, a diffusion sheet, a lens sheet and the like. The diffusion sheet diffuses input light inside. In other words, the light entering the diffusion sheet is diffused in the diffusion sheet and is output as planar light that has less brightness unevenness. The lens sheet is a sheet that deflects (collects light) a light radiation characteristic; entering light is collected, whereby light emission brightness per unit area improves. Here, there is a case where an optical sheet other than these is used. 
     The light output from the backlight unit  2  enters the optical sheet unit  3 . The light entering the optical sheet unit  3  is diffused and collected, whereby the brightness unevenness is reduced, the light emission brightness is raised, and the light enters the liquid crystal display panel  1 . 
     The backlight unit  2  is described in detail with reference to drawings.  FIG. 2  is an exploded perspective view of a backlight unit used in the liquid crystal display device shown in  FIG. 1 , and  FIG. 3  is a rear view of a light guide plate that is used in the backlight unit shown in  FIG. 2 . The backlight unit  2  is a backlight unit of edge-light type. As shown in  FIG. 2 , the backlight unit  2  includes: a backlight chassis  4  that is a chassis; a light guide plate  5  that is held in the backlight chassis  4 ; and a plurality of LED lamps  6  (here, five each on the left and right, that is, ten in total) that are light sources held in the backlight chassis  4  and output light to the light guide plate  5 . 
     As shown in  FIG. 2 , the backlight chassis  4  includes a rectangular rear surface portion  41  and a side wall portions  42  that are formed to rise from four edges of the rear surface portion  41 . A reflection sheet (not shown) is mounted to cover the rear surface portion  41  and side wall portions  42  of the backlight chassis  4 . The reflection sheet is intended to reflect and return light leaking from the light guide plate  5  back to the light guide plate  5 , and usually, a white sheet is used. It is preferable that the reflection sheet is fixed to the backlight chassis  4  not to deviate upward. Because of this, as the fixing method for the reflection sheet, for example, there is a method for bonding a rear surface of the reflection sheet to the rear surface portion  41  and the side wall portions  42  by means of a double-adhesion type adhesive sheet. 
     The light guide plate  5  is a transparent rectangular parallelepiped plate-shaped member. The light guide plate  5  is provided, on side surfaces, with light reception portions  51  that receive the light from the LED lamp  6 . Here, in the light guide plate  5 , both end surfaces in a long-edge direction are the light reception portions  51 . Besides, one of main surfaces of the light guide plate  5  defines an output portion  52  from which the light input from the light reception portion  51  exits. Besides, a surface opposite to the output portion  52  defines a mounted portion  53  that opposes the rear surface portion  41  of the backlight chassis  4 . The light guide plate  5  and the LED lamp  6  are mounted in the backlight chassis  4  where the reflection sheet is disposed. 
     As shown in  FIG. 3 , the mounted portion  53  of the light guide plate  5  is provided with leg members  531 . The leg member  531  is a cylindrical member that meets the mounted portion  53  having a rectangular flat surface at right angles. Besides, the leg members  531  are disposed at four corners and at intermediate portions of respective edges of the rectangular mounted portion  53 . On the other hand, the rear surface portion  41  of the backlight chassis  4  is provided with hold members  411 , that is, through-holes, at positions which correspond to the leg members  531  when the light guide plate  5  is disposed in the backlight chassis  4 . Besides, the reflection sheet also is provided with a through-hole which the leg member  531  penetrates. 
     The leg member  531  penetrates the through-hole of the reflection sheet and the holding portion  411  of the rear surface portion  41 , whereby the light guide plate  5  is positioned at a proper position of the backlight chassis  4 . Besides, the leg member  531  is inserted into the hold member  411 , accordingly, the light guide plate  5  is unlikely to deviate. According to this, it is possible to reduce fixing members such as an adhesive, an adhesive sheet and the like that are intended to fix the light guide plate  5  to the backlight chassis  4 . Here, it is preferable that the leg member  531  has enough strength such that the leg member  531  is not deformed nor broken even if force acts on the light guide plate  5 . 
     The leg member  531  may be formed integrally with the light guide plate  5  or may be detachably mounted on the light guide plate  5 . Besides, the leg member  531  is a cylindrical member, but may be a pillar member that is tapered to become slender toward its tip end. By tapering the leg member  531 , it is possible to insert the leg member  531  into the holding portion  411  without performing fine adjustment. According to this, it is possible to easily mount the light guide plate  5  in the backlight chassis  4 . In the above backlight unit  2 , the light guide plate  5  includes the eight leg members  531 ; however, this is not limiting, and may include more or less than eight in accordance with the size of the light guide plate  5  if it is possible to stably dispose the light guide plate  5  in the backlight chassis  4 . 
     Another example of the backlight unit according to the present invention is described with reference to a drawing.  FIG. 4  is a schematic view of another example of a light guide plate and a backlight chassis of the backlight unit according to the present invention. A light guide plate  5 A shown in  FIG. 4  has the same structure as the light guide plate  5  shown in  FIG. 3  except that the position of a leg member  531   a  is different. Besides, a backlight chassis  4 A has the same structure as the backlight chassis  4  shown in  FIG. 2  except that the position of a holding portion  411   a  is different. Here, in  FIG. 4 , the substantially same portions are indicated by the same reference numbers, and detailed description of the same portions is skipped. Besides, for comparison with the light guide plate  5  shown in  FIG. 3 , the leg member  531  is indicated by means of a two-dot-one-bar line. Besides, a corresponding relationship between the leg member  531   a  and the holding portion  411   a  is indicated by means of a one-dot-one-bar line. 
     As shown in  FIG. 4 , the leg members  531   a  are disposed near the four corners and near the central areas of the respective edges of the rectangular mounted portion  53 . However, unlike the light guide plate  5 , the eight leg members  531   a  are disposed such that the positions of the respective leg members  531   a  do not become point-symmetrical with respect to the center of the mounted portion  53 . And, the rear surface portion  41  of the backlight chassis  4  is provided with the holding portion  411   a  at the position that corresponds to the leg member  531   a  when the direction of the light guide plate  5 A is a correct mount direction. When the direction of the light guide plate  5 A is correct with respect to the backlight chassis  4 A, the leg member  531   a  is inserted into the holding portion  411   a  , and the light guide plate  5 A is positioned and mounted in the backlight chassis  4 A. 
     In a case where the light guide plate  5 A rotates 180° about an axis that passes through the center of the mounted portion  53  and meets the mounted portion  53  at right angles, the leg member  531   a  and the holding portion  411   a  do not overlie each other when viewed from front, accordingly, the leg member  531   a  is not insertable into the holding portion  411   a  . In other words, the light guide plate  5 A is mountable in the backlight chassis  4 A in the correct direction only. According to this, without strictly checking the direction of the light guide plate  5 A, the light guide plate  5 A is mountable in the backlight chassis  4 A in the correct direction. 
     Still another example of the backlight unit according to the present invention is described with reference to a drawing.  FIG. 5  is a schematic view of another example of a light guide plate and a backlight chassis of the backlight unit. A light guide plate  5 B shown in  FIG. 5  has the same structure as the light guide plate  5  shown in  FIG. 3  except that the light guide plate  5 B includes a leg member  531   b . Besides, a backlight chassis  4 B has the same structure as the backlight chassis  4  shown in  FIG. 2  except that the shape of a holding portion  411   b  of the backlight chassis  4   b  is different. The substantially same portions are indicated by the same reference numbers, and detailed description of the same portions is skipped. 
     As shown in  FIG. 5 , the leg members  531   b  are disposed at the four corners and in the central areas of the respective edges of the mounted portion  53 . Here, as described above, the position of the leg member  531   b  is the same as the leg member  531  of the light guide plate  5 . And, a section shape cut along the plane parallel to the mounted portion  53  of the leg member  531   b  is a regular pentagon. Besides, the rear surface portion  41  of the backlight chassis  4 B is provided, at the position corresponding to the leg member  531   b  of the light guide plate  5 B, with the holding portion  411   b  that has the same regular pentagon as the sectional shape of the leg member  531   b . Only when the light guide plate  5 B is in the correct direction with respect to the backlight chassis  4 B, the leg member  531   b  is inserted into the holding portion  411   b  , and the light guide plate  5 B is positioned and held in the backlight chassis  4 B with high accuracy. 
     In a case where the light guide plate  5 B rotates 180° about the axis that passes through the center of the mounted portion  53  and meets the mounted portion  53  at right angles, the leg members  531   b  each move to positions that are symmetrical with the center of the mounted portion  53 . At this time, the sectional shape of the leg member  531   b  is the regular pentagon and is not point-symmetrical. And, the shapes of the leg member  531   b  and the holding portion  411   b  are rotated 180° from each other to be in a reverse relationship with each other. Accordingly, the leg member  531   b  is not insertable into the holding portion  411   b  . In other words, the light guide plate  5 B is mountable in the backlight chassis  4 B in the correct direction only. According to this, without strictly checking the direction of the light guide plate  5 B, the light guide plate  5 B is mountable in the backlight chassis  4 B in the correct direction. 
     Here, the example is described, in which in the above backlight unit, the sectional shape of the leg member  531   b  and the shape of the holding portion  411   b  are the regular pentagons; however, this is not limiting, and a shape, which is a triangle, a fan shape and the like that are not point-symmetrical (do not match each other when the light guide plate is rotated 180° as described above), may be widely usable. Besides, in the above backlight unit, the sectional shapes of all the leg members  531   b  and the shape of the holding portion  411   b  are the regular pentagons; however, this is not limiting, and even if the shape of at least one set of the leg member positions that are symmetrical with respect to the center of mounted portion  53  and the shape of the holding portion  411   b  corresponding to this leg member are shapes that are not point-symmetrical, the same effects are obtainable. In other words, without strictly checking the light guide plate, the light guide plate is mountable in the backlight chassis in the correct direction. 
     Still another example of the backlight unit according to the present invention is described with reference to a drawing.  FIG. 6  is a schematic view of another example of a light guide plate and a backlight chassis of the backlight unit. A light guide plate  5 C shown in  FIG. 6  has the same structure as the light guide plate  5  shown in  FIG. 3  except that one of the eight leg members  531  is a leg member  531   c  . Besides, a backlight chassis  4 C has the same structure as the backlight chassis  4  shown in  FIG. 2  except that the shape of a holding portion  411   c  of the backlight chassis  4 C is different. The substantially same portions are indicated by the same reference numbers, and detailed description of the same portions is skipped. 
     As shown in  FIG. 6 , the light guide plate  5 C includes the leg member  531   c  in an upper right area of the mounted portion  53  when viewed from rear. And, the leg members  531  are disposed at the remaining corners and in the central areas of the respective edges of the mounted portion  53 . Here, as described above, the positions of the leg member  531   c  and the leg member  531  are the same as the leg member  531  of the light guide plate  5 . And, the rear surface portion  41  of the backlight chassis  4 C is provided, in an upper left area when viewed from front, with the holding portion  411   c . Besides, at the positions corresponding to the remaining leg members  531 , the same holding portions  411  as in the backlight chassis  4  are formed. The sectional shape of the leg member  531   c  cut along the plane parallel to the mounted portion  53  is a square. Likewise, the shape of the hold member  411   c  also is a square. As shown in  FIG. 6 , when the direction of the light guide plate  5 C is correct with respect to the backlight chassis  4 C, the leg member  531   c  corresponds to the holding portion  411   c  , and the light guide plate  5 C is positioned and held in the backlight chassis  4 C with high accuracy. 
     In a case where the light guide plate  5 C rotates 180° about the axis that passes through the center of the mounted portion  53  and meets the mounted portion  53  at right angles, the leg member  531   c  opposes the holding portion  411  at a lower right portion when viewed from front. At this time, the sectional shape of the leg member  531   c  is the square and the shape of the holding portion  411  at the lower right portion is the circle, accordingly, the leg member  531   c  is not insertable into the holding portion  411 . Likewise, the leg member  531 , which opposes the holding portion  411   c  because of the rotation, has the circular sectional shape that is a shape different from the square holding portion  411   c  , accordingly, is not insertable. In other words, the light guide plate  5 C is positioned and held in the backlight chassis  4 C in the correct direction only with high accuracy. According to this, without strictly checking the direction of the light guide plate  5 C, the light guide plate  5 C is mountable in the backlight chassis  4 C in the correct direction. 
     Here, the sectional shape of the leg member  531  is the circle and the sectional shape of the leg member  531   c  is the square, accordingly, when the sectional shape of the leg member  531  is larger than the sectional shape of the leg member  531   c  , the leg member  531   c  is insertable into the holding portion  411 . Even in this case, the leg member  531  is not insertable into the holding portion  411   c  . In a case where the section of the leg member  531  is smaller than the section of the leg member  531   c  , reversely, even if the leg member  531  is insertable into the holding portion  411   c  , the leg member  531   c  is not insertable into the holding portion  411 . In other words, irrespective of the size relationship between the leg member  531   c  and the leg member  531 , the light guide plate  5 C is mountable in the backlight chassis  4 C in the correct direction. 
     Here, in the above description, the example is described, which includes one leg member that has the different sectional shape; however, this is not limiting, and the light guide plate may have a structure that includes two or more kinds of leg members whose sectional shapes are different from one another. At this time, of the sets of the leg members that are disposed symmetrically with respect to the center of the mounted portion, at least one set is a set of leg members that has sectional shapes different from each other. 
     Further, in the above example, to prevent the leg member from being inserted into the holding portion when the light guide plate rotates, the sectional shape of the leg member is modified; however, the sectional shape may be the same, while the sectional size may be modified. In this case as well, the leg member having a large sectional shape is not insertable into the holding portion into which the leg member having a small sectional shape is inserted. According to this, only when the light guide plate is in the correct direction, the light guide plate is positioned and held in the backlight chassis with high accuracy. 
     Still another example of the backlight unit according to the present invention is described with reference to a drawing.  FIG. 7  is a schematic view of another example of a light guide plate and a backlight chassis of the backlight unit. A light guide plate  7  shown in  FIG. 7  includes a plurality of light guide members  70  (here, five; however, this is not limiting), and the light guide members  70  are ranged and disposed in the backlight chassis  4 . 
     The light guide members  70  are a transparent rectangular parallelepiped plate-shaped member and ranged in a short-edge direction to compose the light guide plate  7 . In the light guide member  70 , a side surface defines a light reception portion  701  that receives the light from the LED lamp  6 . Here, in the light guide member  70 , both end surfaces in a long-edge direction are the light reception portions  701 . Besides, one of main surfaces of the light guide member  70  defines an output portion from which the light received by light reception member  701  exits. Besides, a surface opposite to the output portion defines a mounted portion  703  that opposes the rear surface portion  41  of the backlight chassis  4 . 
     As shown in  FIG. 7 , the light guide member  701  is provided, near both end portions in a long-edge direction of the mounted portion  703 , with leg members  704  that protrude in a direction that meets the mounted portion  703  at right angles. The rear surface portion  41  of the backlight chassis  4  is provided, at positions that correspond to the respective leg members  704 , with holding portions  412 , that is, through-holes into which the leg members  704  are inserted when the light guide members  70  are ranged and disposed in the backlight chassis  4 . By inserting the leg members  701  into the holding portions  412  to range and dispose the light guide members  70 , it is possible to easily form the light guide plate  7  in the backlight chassis  4 . 
     Besides, by adjusting brightness of the light from the LED lamp  6  for every light guide member  70 , it is possible to perform area control in which the light having the brightness different for every region is output. Here, by adding the features of the above shape, disposition, size and the like to the leg member  704  and the holding portion  412 , it is possible to dispose the light guide member  70  in the backlight chassis  4  in the correct direction. Here, illustration and description are skipped; however, a reflection sheet is disposed between the light guide member  70  and the backlight chassis  4 . And, the leg member  704  penetrates a through-hole that is formed through the reflection sheet. 
     Here, the holding portion of the backlight chassis in each of the above embodiments is the through-hole formed through the rear surface portion  41 ; however, may be a closed hole other than the through-hole. For example, if the backlight chassis is formed of a metal plate, it is possible to produce by means of extrusion molding of the metal plate. In addition, besides this, a method for forming a hole into which the leg member is smoothly insertable may be used. 
     In the backlight unit described in each of the above embodiments, the light guide plate (or the light guide member) has the rectangular parallelepiped shape, and the leg member, which is the engagement structure engaging with the backlight chassis of the light guide plate, has the shape that protrudes outside the light guide plate, and the light from the light source is not blocked by the engagement structure and is not unevenly reflected (irregular reflection) at the portion where the cutout and the like are formed, accordingly, it is possible to alleviate the brightness unevenness of the planar light output from the output portion. 
     Besides, by inserting the leg member into the holding portion, it is possible to position the light guide plate in the backlight chassis with high accuracy, accordingly, it is possible to output the planar light from the output portion to the region of the liquid crystal display device where the image display is performed. 
     Besides, the light guide plate is mountable in the backlight chassis in the correct direction, accordingly, the light reception portion is disposed correctly to oppose the light source, and it is possible to receive the light from the light source by means of the light reception portion with no loss. According to this, it is possible to increase the brightness of the planar light output from the backlight unit. 
     Further, in the above embodiments, the equal number of the LED lamps, that is, the light sources, are disposed at the left and right of the light guide plate; however, because of restriction on the space and a device mounted on the backlight unit, there is a case where it is impossible to perform the disposition with good balance. In this case as well, also, the light guide plate has the structure which includes the leg member that protrudes from the rectangular parallelepiped, accordingly, the structure is simple and the production is easy. Besides, the light guide plate has the simple structure, accordingly, the optimum designing for efficiently transmitting the light in the light guide plate is easy. Besides, it is possible to alleviate the light guide plate rotating and the light guide plate being mounted in the backlight chassis in a different direction. According to this, it is possible to easily demonstrate the performance of the optimized light guide plate. 
     Industrial Applicability 
     The present invention is usable as a display device of apparatuses such as a flat-screen television device, a flat-screen display device, a mobile phone and the like. 
     Reference Signs List 
       1  liquid crystal display panel 
       2  backlight unit 
       3  optical sheet unit 
       4  backlight chassis (chassis) 
       41  rear surface portion 
       411  holding portion 
       42  side wall portion 
       43  reflection sheet 
       5  light guide plate 
       51  light reception portion 
       52  output portion 
       53  mounted portion 
       531  leg member 
       6  LED lamp 
       7  light guide plate 
       70  light guide member 
       701  light reception portion 
       703  mounted portion 
       704  leg member