Abstract:
There are provided a light source  16  that emits light, an optical sheet  14  that adjusts the optical path of the light emitted from the light source  16  in such a way that a liquid crystal panel  12  is irradiated with the light from behind, and a frame  13  that permits the optical sheet  14  to be kept in position behind the liquid crystal panel  12 . The optical sheet  14  is supported at one point on the frame  13 . When the optical sheet  14  is rectangular, advisably it is supported on the frame  13  substantially at the center of one side of the optical sheet  14 . For stable fitting, preferably the optical sheet  14  is supported on the frame  13  substantially at the center of that side of the optical sheet  14  which, in actual use, is located at the top with respect to the direction of gravity.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a backlight device, and to a liquid crystal display device provided therewith. More particularly, the present invention relates to a backlight device provided with an optical sheet having a predetermined optical function, and to a liquid crystal display device provided with such a backlight device.  
         [0003]     2. Description of Related Art  
         [0004]     Conventionally, display apparatuses such as television monitors have been typically using CRTs (cathode ray tubes). In recent years, flat panel displays have come to be used increasingly widely not only for their space-saving and power-saving advantages but also for their improved viewing angle, contrast, and color reproduction characteristics. Among such flat panel displays, liquid crystal display devices in particular are now so popular as if to supersede CRTs.  
         [0005]     In a liquid crystal display device, the liquid crystal itself does not emit light, and hence image display is achieved in the following manner. While a display area formed of liquid crystal is irradiated with backlight or external light, the alignment of the liquid crystal is controlled with a drive circuit, and thereby the amount of light transmitted through the liquid crystal is controlled in such a way as to produce an image. Here, for the purpose of irradiating the liquid crystal, which forms the display area, uniformly with the light from a light source such as a backlight, an optical sheet, such as a diffusive sheet or prism sheet, is laid between the backlight and the liquid crystal. Such optical sheets are mostly formed of resin, and hence they expand and contract as the ambient temperature varies. This requires one to take the expansion and contraction of an optical sheet into consideration when designing how to fix it inside a display device.  
         [0006]      FIG. 23  shows how an optical sheet is conventionally fitted. A frame  1  has a frame portion  1   a  and a stepped support portion  1   b . The stepped support portion  1   b  is thinner than the frame portion  1   a , and projects inward from the inner edge of the frame portion  1   a . When a rectangular optical sheet  2  is fitted to this frame  1 , first the optical sheet  2  is fitted inside the frame portion  1   a  of the frame  1 , and is then fixed in position with an outer edge portion thereof held between the stepped support portion  1   b  of the frame  1  and an unillustrated support member.  
         [0007]     When the optical sheet  2  is fitted to the frame  1  in this way, as the ambient temperature varies, the optical sheet  2  expands and contracts radially with respect to a central portion thereof, as indicated by arrows in  FIG. 23 . Consequently, stresses tend to concentrate in the central portion of the optical sheet  2 . This causes the optical sheet  2  to be deformed with wrinkles, badly affecting the optical characteristics of the liquid crystal display device.  
         [0008]     An attempt to avoid this is made in Japanese Patent Application Laid-Open No. H11-337942. According to this publication, as shown in  FIG. 24 , in an illumination device, for the purpose of preventing the wrinkling and warping of an optical sheet, projection-like engagement pins  5  are formed on a frame  6   b , and holes  4   a  are formed in a light-diffusing sheet  4 . The engagement pins  5  are put through the holes  4   a , and are then fitted into holes  7  in an upper frame  6 . In this way, the light-diffusing sheet  4  is fitted to the frame  6   b.    
         [0009]     In the embodiment specifically disclosed in the publication mentioned above, however, only two holes  4   a  are formed in the light-diffusing sheet  4 . Consequently, as the light-diffusing sheet  4  expands and contracts, stresses concentrate around the two holes  4   a , causing the light-diffusing sheet  4  to wrinkle or warp. Considering the current trend toward increasingly large liquid crystal display devices in the fields of television monitors and the like, the fitting method described above is insufficient to overcome the increasingly striking effects of the concentration of stresses resulting from the expansion and contraction of an optical sheet.  
       SUMMARY OF THE INVENTION  
       [0010]     In view of the conventionally encountered problems discussed above, it is an object of the present invention to provide a backlight device and a liquid crystal display device in which, even when an optical sheet expands or contracts as the ambient temperature varies, the optical sheet does not wrinkle or warp.  
         [0011]     According to one aspect of the present invention, a backlight device for use in a liquid crystal display device in which a liquid crystal panel is irradiated with light from behind is provided with: a light source that emits light; an optical sheet that adjusts the optical path of the light emitted from the light source in such a way that the liquid crystal panel is irradiated with the light from behind; and a frame that permits the optical sheet to be kept in position behind the liquid crystal panel. Here, the optical sheet is supported at one point on the frame. It should be understood that, in the present specification, an optical sheet denotes one that diffuses, condenses, or otherwise processes the light emitted from a light source by refracting, diffracting, reflecting, or otherwise acting upon it so that a liquid crystal panel is irradiated with the light from behind. Specifically, an optical sheet may be a diffusive sheet, prism sheet, polarization-selective reflective/transmissive sheet, or the like.  
         [0012]     With the construction described above, where the optical sheet is supported at one point on the frame, even when the optical sheet expands and contracts as the ambient temperature varies, it does so with respect to the supporting point mentioned above. This prevents undue stresses from being applied to the optical sheet. Consequently, the optical sheet does not wrinkle or warp, and hence the light with which the liquid crystal panel is irradiated through the optical sheet from the backlight device remains uniform.  
         [0013]     Here, the frame may be so shaped as to surround the outer edge of the optical sheet. From the viewpoint of surely preventing the wrinkling or warping of the optical sheet resulting from the expansion and contraction thereof, it is preferable that, when the optical sheet is supported on the frame, a gap be left between the outer edge of the optical sheet and the inner edge of the frame.  
         [0014]     The optical sheet may be substantially rectangular, with the optical sheet supported on the frame substantially at the center of one side of the optical sheet. From the viewpoint of stable fitting, it is preferable that the optical sheet be supported on the frame substantially at the center of that side of the optical sheet which, when the backlight device is in actual use, is located at the top with respect to the direction of gravity.  
         [0015]     One way of supporting the optical sheet on the frame is as follows. In a portion of the frame adjoining the outer edge of the optical sheet, a recessed portion is formed that has an opening facing the outer edge of the optical sheet, and a portion of the outer edge of the optical sheet is extended outward to form a projecting portion. The projecting portion is placed in the recessed portion, and the optical sheet is supported on the frame as a result of at least one surface of the projecting portion placed in the recessed portion being pressed by a pressing member. With this construction, it is possible to reduce the distortion of the optical sheet in the direction of the thickness thereof, and to reduce the vibration of the optical sheet resulting from it being allowed to move freely. Moreover, pressing the projecting portion of the optical sheet, which projects therefrom, does not spoil the expansion and contraction of the optical sheet.  
         [0016]     From the viewpoint of easy positioning of the optical sheet, it is preferable that, as seen in a plan view, the projecting portion and the recessed portion be geometrically similar to each other. To restrict the horizontal movement of the optical sheet, it is advisable that the pointed end of the projecting portion and the portion of the recessed portion corresponding thereto make contact with each other. To allow ample margins in the dimensional and assembly accuracy of the optical sheet, it is advisable that the pointed end of the projecting portion and the portion of the recessed portion corresponding thereto be apart from each other. To permit the pressing member to surely support the optical sheet by pressing it, it is preferable that an elastic member be placed between the pressing member and the projecting portion.  
         [0017]     To reduce, without unduly restricting, the movement of the optical sheet in the direction of the thickness thereof, it is preferable to form, around the rim of the frame facing the optical sheet, a stepped portion deeper than the recessed portion so that, when the optical sheet is supported on the frame, a gap is left between the optical sheet and the stepped portion.  
         [0018]     Another way of supporting the optical sheet on the frame is as follows. A hole is formed in the optical sheet and a pin-like member is formed on the frame. The optical sheet is supported on the frame as a result of the pin-like member being put through the hole of the optical sheet. Here, it is preferable that a portion of the outer edge of the optical sheet be extended outward to form a projecting portion, with the hole formed in this projecting portion.  
         [0019]     Still another way of supporting the optical sheet on the frame is as follows. The frame has an upper frame and a lower frame that can be put together. The optical sheet is supported between the upper and lower frames as a result of the upper and lower frames being put together.  
         [0020]     Here, it is preferable that a portion of the outer edge of the optical sheet be extended outward to form a projecting portion so that, when the upper and lower frames are put together, the optical sheet is supported on the frame as a result of the projecting portion being held between the upper and lower frames.  
         [0021]     In this case, to permit the optical sheet to be surely supported, it is preferable to form, in a portion of at least one of the upper and lower frames adjoining the outer edge of the optical sheet, a recessed portion that has an opening facing the outer edge of the optical sheet, form a projection on at least one of the bottom surface of the recessed portion or the surface facing that surface, and place the projecting portion of the optical sheet in the recessed portion so that the optical sheet is supported on the frame as a result of, when the upper and lower frames are put together, the projection pressing the projecting portion.  
         [0022]     A hole may be formed in the optical sheet, with a pin-like member formed on at least one of the upper and lower frames, so that the optical sheet is supported on the frame as a result of the pin-like member being put through the hole of the optical sheet. Here, to allow ample margins in the dimensional and assembly accuracy of the upper and lower frames, it is preferable that a through hole through which to put the pin-like member or a groove in which to fit the pin-like member be formed in the upper or lower frame. It is preferable that a portion of the outer edge of the optical sheet be extended outward to form a projecting portion, with the hole formed in this projecting portion.  
         [0023]     To surely prevent the optical sheet from dropping off the upper and lower frames, and to permit the optical sheet to be fixed with the upper and lower frames put together, a fixing member may be provided that keeps the upper and lower frames fixed together, with an engagement portion that engages with the fixing member provided in a tip portion of the pin-like portion.  
         [0024]     To further prevent the optical sheet from wrinkling or warping, it is preferable that, when the upper and lower frames are put together, gaps be left between the optical sheet and the upper and lower frames in the direction along the axis of the pin-like member.  
         [0025]     From the viewpoint of facilitating the fitting of the optical sheet on the frame, on at least one of a side of the optical sheet and the side of the frame that faces that side of the optical sheet, a positioning projection may be formed to project toward the other. Here, it is preferable that the side on which the positioning projection is formed be at least one of the side of the optical sheet at which the optical sheet is supported and the side of the frame that faces that side of the optical sheet. It is preferable that, as the positioning projection, two positioning projections be formed on both sides of the supporting point.  
         [0026]     According to another aspect of the present invention, a liquid crystal display device is provided with the backlight device described above.  
         [0027]     According to still another aspect of the present invention, in an optical sheet that is designed for use in a liquid crystal display device and that is fitted behind the liquid crystal display device by being supported on a frame, a portion of the outer edge of the optical sheet is extended to form a projecting portion, and this projecting portion permits the optical sheet to be supported at one point on the frame. Here, a hole may formed in the projecting portion so that the optical sheet is supported at one point on the frame as a result of a pin-like member formed on the frame being put through the hole. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0028]      FIG. 1  is a sectional view schematically showing the liquid crystal display device of a first embodiment of the invention;  
         [0029]      FIG. 2  is a plan view, as seen from below, showing how the optical sheet is fitted on the frame;  
         [0030]      FIG. 3  is an enlarged detail view of  FIG. 2 ;  
         [0031]      FIG. 4  is an enlarged detail view of  FIG. 2 ;  
         [0032]      FIG. 5  is a sectional view of the recessed portion of the frame;  
         [0033]      FIG. 6  is a sectional view showing how the projecting portion of the optical sheet is fitted into the recessed portion of the frame;  
         [0034]      FIG. 7  is a sectional view showing how the projecting portion of the optical sheet is held;  
         [0035]      FIG. 8  is a plan view showing how the optical sheet is fitted on the frame in a second embodiment of the invention;  
         [0036]      FIG. 9  is an enlarged detail view of  FIG. 8 ;  
         [0037]      FIG. 10  is an enlarged detail view showing a third embodiment of the invention;  
         [0038]      FIG. 11 a  sectional view schematically showing the liquid crystal display device of a fourth embodiment of the invention;  
         [0039]      FIG. 12 a  sectional view schematically showing the liquid crystal display device of a fifth embodiment of the invention;  
         [0040]      FIG. 13  is a sectional view schematically showing a modified example of a liquid crystal display device;  
         [0041]      FIG. 14  is a perspective detail view showing another example of how the optical sheet is supported;  
         [0042]      FIG. 15  is a sectional view showing another embodiment of a liquid crystal display device according to the present invention;  
         [0043]      FIG. 16  is a perspective view showing how the optical sheet is supported;  
         [0044]      FIG. 17  is a vertical sectional view of  FIG. 16 ;  
         [0045]      FIG. 18  is a perspective view showing another example of how the optical sheet is supported;  
         [0046]      FIG. 19  is a perspective view showing another example of how the optical sheet is supported;  
         [0047]      FIG. 20  is a vertical sectional view showing another example of how the optical sheet is supported;  
         [0048]      FIG. 21  is a vertical sectional view showing another example of how the optical sheet is supported;  
         [0049]      FIG. 22  is a perspective view showing another example of the frame used in the present invention;  
         [0050]      FIG. 23  is a plan view showing a conventional example; and  
         [0051]      FIG. 24  is a diagram showing another conventional example. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0052]     Hereinafter, backlight devices and liquid crystal display devices according to the present invention will be described with reference to the drawings. It should be understood that these embodiments are not intended to limit the implementation of the present invention in any way.  
         [0053]      FIG. 1  is a sectional view schematically showing the backlight device and the liquid crystal display device of a first embodiment of the invention. In the liquid crystal display device  10  shown in this figure, a backlight device  11  having an optical sheet laid on the light-emitting side thereof and a liquid crystal panel  12  are held together by a metal bezel  30 .  
         [0054]     The backlight device  11  includes: a case  19  having a C-shaped cross-section; a reflective sheet  18  laid over the floor surface of the case  19  and formed of PET foam; a rectangular light guide plate  15  arranged on top of the reflective sheet  18  and formed by molding acrylic resin; a pair of linear light sources (cold cathode fluorescent lamps)  16  arranged along opposite ends of the light guide plate  15  so as to face each other; lamp holders  17  having a C-shaped cross-section and arranged to surround the linear light sources  16 ; an optical sheet  14  arranged on the light-exit side of the light guide plate  15 ; a rectangular frame  13  that holds the optical sheet  14  and other components inside it; a pressing member  20  that is placed between the top surface of the lamp holders  17  and the frame  13 ; and an elastic member  21  that is placed between the pressing member  20  and a projecting portion  14   a  (see  FIG. 2 ) of the optical sheet  14 . The light-exit-side surface  15   a  of the light guide plate  15  is made mat by sand blasting or the like so that the light exiting from the light guide plate  15  is diffused there.  
         [0055]     The liquid crystal panel  12  includes: transparent substrates  23  and  24  that are arranged so as to face each other; liquid crystal  22  that is sealed between the transparent substrates  23  and  24 ; and polarizer plates  25  and  26  that are laid over the outer surfaces of the transparent substrates  23  and  24  to make uniform the direction of the polarization plane of the incident and emergent light. Seen in a plan view, the liquid crystal panel  12  has a large number of pixels arrayed in a matrix.  
         [0056]     The liquid crystal panel  12  is driven by a driver  27 , to which is connected, through a flexible printed circuit board  28 , a circuit board  29  for controlling the alignment of the liquid crystal  22 .  
         [0057]     The liquid crystal display device  10  shown in  FIG. 1  adopts a backlight mechanism of the so-called edge light type, wherein the linear light sources  16  are arranged along opposite ends of the light guide plate  15 . It should be understood, however, that the present invention may be applied to any other type of backlight mechanism; for example it may be applied to a backlight mechanism of the direct-lit type, wherein a light source is arranged behind the display surface of the liquid crystal panel  12 . The light guide plate  15  may have any other shape than that of a flat plate; for example, it may have a shape with a wedge-like cross section.  
         [0058]     The optical sheet  14  serves to diffuse, condense, or otherwise process the light emerging from the light guide plate  15  by refracting, diffracting, reflecting, or otherwise acting upon it so that the liquid crystal panel  12  is irradiated with the light from behind. Examples of such optical sheets include: a prism sheet having a plurality of prisms, each having an isosceles-triangle-shaped cross section, arranged periodically on the light-exit-side surface thereof; a polarization-selective reflective/transmissive sheet (for example, the model “DBEF” manufactured by 3M) that selectively reflects light so that the light is not absorbed by the polarizer plate  26  of the liquid crystal panel  12 ; and a diffusive sheet (for example, the model “PC-03” manufactured by Keiwa Shoko) that diffuses light. Either a single sheet of one of these or a plurality of sheets of them laid over one another are used. Practical examples of how such optical sheets are combined together are, from the light guide plate to the liquid crystal panel: a diffusive sheet, then a prism sheet, and then a polarization-selective reflective/transmissive sheet; a diffusive sheet, then a prism sheet, and then a diffusive sheet; a diffusive sheet, then a diffusive sheet, and then a diffusive sheet; and a diffusive sheet, and then a diffusive sheet. These optical sheets are about 0.1 mm to 0.5 mm thick, and their size as seen in a plan view varies according to the size of the display screen. In  FIG. 1 , for the sake of convenience, only a single optical sheet  14  is used.  
         [0059]     As shown in  FIG. 2 , the optical sheet  14  has a projecting portion  14   a  that extends outward from the center of one side  14   c - 1  of the rectangular edge  14   c  thereof. As shown in  FIG. 3 , the tip end of the projecting portion  14   a  is triangular so as to have a pointed end  14   b.    
         [0060]     On the other hand, as shown in  FIG. 2 , the frame  13  has a rectangular frame portion  13   f  and a stepped portion  13   b . The stepped portion  13   b  is thinner than the frame portion  13   f , and projects inward from a lower part of the inner edge  13   f - 1  of the frame portion  13   f  Substantially at the center of one side of the frame portion  13   f , in the obverse surface  13   g  thereof, a recessed portion  13   a  is formed by being cut from the inner edge  13   f - 1  side. That is, the recessed portion  13   a  has an opening  13   j  on the inner edge  13   f - 1  side. As shown in  FIG. 3 , the floor surface  13   i  of the recessed portion  13   a  is shaped like a home base, with a pointed end  13   d  at the vertex. Moreover, as shown in  FIG. 5 , the depth d 1  of the recessed portion  13   a  is smaller than the depth of the stepped portion  13   b , but is greater than the thickness of the projecting portion  14   a  (shown in  FIG. 3 ) of the optical sheet  14 . Furthermore, as shown in  FIG. 2 , on the same side of the frame portion  13   f  in which the recessed portion  13   a  is formed, but in a different place therefrom, a positioning projection  13   c  having a arc-shaped tip end is formed so as to project inward from the inner edge  13   f - 1 . The recessed portion  13   a  is formed in a place that is located at the top when the liquid crystal display device  10  is actually used.  
         [0061]     The pressing member  20  is formed of resin, and is shaped like a flat plate. The elastic member  21  is formed of rubber, resin, or the like, and is given a thickness equal to the thickness of the projecting portion  14   a  subtracted from the depth d 1  of the recessed portion  13   a.    
         [0062]     Next, how the optical sheet  14  is fitted to the frame  13  will be described. As shown in  FIG. 2 , the projecting portion  14   a  of the optical sheet  14  is placed in the recessed portion  13   a  of the frame  13 , and the optical sheet  14  is fitted inside the frame portion  13   f . Then the position of the optical sheet  14  is adjusted so that the edge  14   c  of the side  14   c - 1  of the optical sheet  14  where the projecting portion  14   a  is formed makes contact with the positioning projection  13   c.    
         [0063]     At this time, as shown in  FIG. 3 , the pointed end  14   b  of the projecting portion  14   a  makes point contact with the pointed end  13   d  of the recessed portion  13   a , and thereby restricts the movement of the optical sheet  14  in the X direction indicated in the figure. At the same time, as shown in  FIG. 4 , the edge  14   c  of the optical sheet  14  makes contact with the positioning projection  13   c , and thereby restricts the movement of the optical sheet  14  in the Y direction indicated in the figure. Here, since the positioning projection  13   c  is arc-shaped, it does not restrict the X-direction movement of the optical sheet  14 . Except at the pointed end  14   b , the outer edge of the projecting portion  14   a  makes no contact with the side wall of the recessed portion  13   a , and hence a gap S 4  is left between them. Needless to say, the projecting portion  14   a  and the recessed portion  13   a  may be closely fitted together.  
         [0064]     When the optical sheet  14  is fitted to the frame  13  in this way, as shown in  FIG. 2 , gaps S 1  and S 2  are left between the inner edge  13   f - 1  of the frame portion  13   f  and the bottom and both side edges of the optical sheet  14 .  
         [0065]     As shown in  FIG. 6 , the depth d 1  of the recessed portion  13   a  is greater than the thickness of the projecting portion  14   a  of the optical sheet  14 . Moreover, the depth of the stepped portion  13   b  is greater than the depth d 1  of the recessed portion  13   a . Hence, when the projecting portion  14   a  of the optical sheet  14  is kept in contact with the floor surface of the recessed portion  13   a , a gap d 2  is left between the optical sheet  14  and the stepped portion  13   b . This helps reduce, without unduly restricting, the movement of the optical sheet in the direction of the thickness thereof.  
         [0066]     As shown in  FIG. 7 , the projecting portion  14   a  of the optical sheet  14  is pressed against a thin portion  13   e  by the pressing member  20 , with the elastic member  21  placed between them. This permits the optical sheet  14  to be held by the frame  13 . Here, the total thickness of the projecting portion  14   a  and the elastic member  21  is equal to the depth d 1  of the recessed portion  13   a . Then, as shown in  FIG. 1 , the other components are fitted to make the liquid crystal display device  10 . It should be noted that the frame  13  is so arranged that the thin portion  13   e  of the recessed portion  13   a  is located on the liquid crystal panel  12  side of the optical sheet  14 .  
         [0067]     As described above, the optical sheet  14  is fitted to the frame  13  with the projecting portion  14   a  of the optical sheet  14  placed in the recessed portion  13   a  of the frame  13  and with only the projecting portion  14   a  pressed by the pressing member  20  and the elastic member  21 . Consequently, even when the optical sheet  14  expands and contracts as the ambient temperature varies, it does so freely with respect to the projecting portion  14   a . Hence, no undue stresses are applied to the optical sheet  14 , which thus does not wrinkle or warp. In this way, it is possible to make uniform the light with which the liquid crystal panel is irradiated from the backlight device.  
         [0068]      FIGS. 8 and 9  show the backlight device and the liquid crystal display device of a second embodiment of the invention. In the following description, no explanations will be repeated of such features as are found also in the previously described embodiment, and chiefly the differences therefrom will be discussed.  
         [0069]     The frame  33  has a rectangular frame portion  33   f  and a stepped portion  33   b . The stepped portion  33   b  is thinner than the frame portion  33   f , and projects inward from the inner edge  33   f - 1  of the frame portion  33   f  In the frame portion  33   f , a recessed portion  33   a  is formed that has the same shape as in the previously described embodiment. On the side where the recessed portion  33   a  is formed, on both sides thereof, positioning projections  33   c  are formed that each have the same shape as in the previously described embodiment.  
         [0070]     The optical sheet  34  is rectangular, and has a projecting portion  34   a  that projects outward from the center of one side  34   c  thereof. The tip portion of the projecting portion  34   a  is triangular so as to have a pointed end  34   b  (shown in  FIG. 9 ). As shown in  FIG. 9 , the length over which the projecting portion  34   a  projects is smaller than in the previously described embodiment, and the pointed end  34   b  thereof is kept apart from the pointed end  33   d  of the floor surface  33   i  of the recessed portion  33   a . Needless to say, the recessed portion  33   a  may be made deeper in order thereby to keep the pointed end  34   b  of the projecting portion  34   a  apart from the pointed end  33   d  of the recessed portion  33   a.    
         [0071]     When the optical sheet  34  constructed as described above is fitted to the frame  33 , the positioning projections  33   c  achieve the positioning of the optical sheet  34  in the Y direction indicated in the figure. On the other hand, with respect to the positioning in the X direction indicated in the figure, since the pointed end  34   b  of the projecting portion  34   a  makes no contact with the pointed end  33   d  of the recessed portion  33   a , the gap between them permits the optical sheet  34  to move over a distance d 3  in the X direction. This produces margins in the dimensional and assembly accuracy of the optical sheet  34  and the frame  33 .  
         [0072]      FIG. 10  shows the backlight device and the liquid crystal display device of a third embodiment of the invention. This embodiment differs from the first embodiment in that, instead of a positioning projection formed on the frame  43 , a positioning projection  44   d  is formed on the optical sheet  44 .  
         [0073]     Specifically, in the optical sheet  44  shown in  FIG. 10 , on the same side thereof where the projecting portion (not illustrated) is formed, but in a different place therefrom, an arc-shaped positioning projection  44   d  is formed, and this is kept in contact with the inner edge  43   f - 1  of the frame portion  43   f  of the frame  43  to achieve positioning.  
         [0074]      FIG. 11  shows the backlight device and the liquid crystal display device of a fourth embodiment of the invention. This embodiment differs from the first embodiment in that the depth of the recessed portion  53   a  in the frame  53  is made approximately equal to the thickness of the projecting portion  14   a  of the optical sheet  14 .  
         [0075]     With the construction described above, when the projecting portion  14   a  is placed in the recessed portion  53   a , the top surface of the frame  53  is level with the top surface of the projecting portion  14   a . This permits the projecting portion  14   a  of the optical sheet  14  to be held between the top surface of the lamp holders  17  and the recessed portion  53   a . Moreover, quite naturally, except in the projecting portion  14   a , the entire optical sheet  14  is left freely moving, preventing stresses from being applied to the optical sheet  14  even when it expands and contracts. The thickness of the projecting portion  14   a  of the optical sheet  14  may be made slightly greater than the depth of the recessed portion  53   a  of the frame  53 .  
         [0076]      FIG. 12  shows the backlight device and the liquid crystal display device of a fifth embodiment of the invention. This embodiment differs from the first embodiment in that a backlight  64  of the direct-lit type is used.  
         [0077]     Specifically, in the liquid crystal display device shown in  FIG. 12 , immediately behind the liquid crystal panel  12 , four linear light sources  60  are arranged at equal intervals, and the linear light sources  60  are covered from behind with a case  61  that has a reflective sheet  62  laid on the inner surface thereof and that has a substantially C-shaped cross section. Moreover, between the linear light sources  60  and the optical sheet  14 , a diffusive plate  63  is arranged to make uniform the light from the linear light sources  60 .  
         [0078]     Moreover, in this embodiment, as in the fourth embodiment described above, as shown in  FIG. 13 , the depth of the recessed portion  53   a  of the frame  53  may be made approximately equal to the thickness of the projecting portion  14   a  of the optical sheet  14 .  
         [0079]     In all the embodiments described thus far, the optical sheet is pressed by the pressing member so that the optical sheet is supported by being held between the frame and the pressing member. Alternatively, it is also possible to form a hole in the optical sheet and a pin-like member on the frame so that the optical sheet is supported on the frame as a result of the pin-like member being put through the hole.  FIG. 14  shows an example of this type of supporting structure.  
         [0080]     In  FIG. 14 , substantially at the center of the floor surface of the recessed portion  13   a  of the frame  13 , a cylindrical pin-like member  70  is formed to project upright therefrom. The height of the pin-like member  70  is equal to the depth of the recessed portion  13   a . On the other hand, in the projecting portion  14   a  formed on the optical sheet  14 , a through hole  141  is formed. The diameter of the through hole  141  is equal to or slightly greater than that of the pin-like member  70 .  
         [0081]     When the optical sheet  14  is supported on the frame  13 , the pin-like member  70  on the frame  13  is put through the through hole  141  in the optical sheet  14 . Then a predetermined component, for example the lamp holders  17  (shown in  FIG. 11 ), is fitted on top of the frame  13 . This permits the tip-end face of the pin-like member  70  to make contact with the surface of the predetermined component, and thereby permits the optical sheet  14  to be fitted on the frame  13  while preventing the optical sheet  14  from dropping off the pin-like member  70 .  
         [0082]     Needless to say, it is also possible to form the through hole  141  at the edge of the optical sheet  14  without forming the projecting portion  14   a  thereon and then fit the optical sheet  14  on the frame  13 . Alternatively, as will be described later, it is also possible to give the pin-like member  70  a height greater than the depth of the recessed portion  13   a  and form, in a predetermined member (for example, the lamp holders  17  (shown in  FIG. 11 )) that is fitted on top of the frame  13 , a through hole through which to put the pin-like member  70  or a groove in which to fit the pin-like member  70  (see  FIG. 20 ).  
         [0083]     Next, another embodiment of a backlight device and a liquid crystal display device according to the invention will be described. In the devices of the embodiments described thus far, the optical sheet is supported by being held between the frame and the pressing portion. By contrast, in the embodiments described below, a frame is used that is composed of separate pieces, namely an upper frame and a lower frame, that can be put together, and an optical sheet is supported by being held between those upper and lower frames.  
         [0084]      FIG. 15  is a sectional view schematically showing the backlight device and the liquid crystal display device of another embodiment of the invention. Here, such members and portions as are found also in the device shown in  FIG. 1  are identified with common reference numerals. In the following description, explanations of such structures and functions as are found also in the device shown in  FIG. 1  will not be repeated.  
         [0085]     In the device shown in  FIG. 15 , a frame  7  is used that includes an upper frame  71  and a lower frame  72 . On the upper frame  71 , an overhanging portion  71   a  is formed that projects inward from the inner edge thereof. A liquid crystal panel  12  is fitted on this overhanging portion  71   a . On the other hand, the lower frame  72  is rectangular, and is open at the top and bottom faces thereof. Inside the lower frame  72  are arranged the following components: a light guide plate  15 ; linear light sources  16  that are arranged along opposite ends of the light guide plate  15 ; lamp holders  17  that are arranged so as to envelop the linear light sources  16 ; and a reflective sheet  18  that is laid on the bottom surface of the light guide plate  15 . With the upper and lower frames  71  and  72  put together, the frame  7  is surrounded by a metal bezel  30 . The optical sheet  14  is held between the recessed portion  13   a  of the lower frame  72  and the bottom surface of the upper frame  71 . How the optical sheet  14  is held between the upper and lower frames  71  and  72  is shown in  FIG. 16 .  
         [0086]      FIG. 16  is an enlarged detail view showing how the optical sheet  14  is fitted. In the lower frame  72 , a recessed portion  13   a  is formed so as to be open at the inner edge of the frame, and, substantially at the center of the floor surface of the recessed portion  13   a , a hemispherical projection  721  is formed. Moreover, on the upper frame  71 , in a position corresponding to the projection  721  on the lower frame  72 , a similar projection  711  (shown in  FIG. 17 ) is formed.  
         [0087]     As shown  FIG. 17 , to support the optical sheet  14  with the upper and lower frames  71  and  72 , first, the projecting portion  14   a  formed on the optical sheet  14  is placed in the recessed portion  13   a  in the lower frame  72  ((a) in the figure). Then, the upper and lower frames  71  and  72  are put together ((b) in the figure). This permits the projecting portion  14   a  of the optical sheet  14  to be held between the projection  711  on the upper frame  71  and the projection  721  on the lower frame  72 .  
         [0088]     There is no particular restriction on the shape and number of projections  711  and  721  formed. In this embodiment, a projection is formed on each of the upper and lower frames. It should be understood, however, that the present invention does not absolutely require a projection; so long as the optical sheet can be held between the upper and lower frames, a projection may be formed only on one of the upper and lower frames, or no projection at all may be formed. In a case where the optical sheet is held with the upper and lower frames with no projection formed, the recessed portion formed in the upper and lower frames needs to be given a depth smaller than the thickness of the optical sheet.  
         [0089]     In this embodiment, the optical sheet  14  is held as a result of the projecting portion  14   a  formed on the optical sheet  14  being held between the upper and lower frames  71  and  72 . Instead of the projecting portion formed on the optical sheet  14 , as shown in  FIG. 18 , projecting portions  715  and  722 , each having a projection  721  formed thereon, may be formed on the upper and lower frames  71  and  72  so as to face each other so that the optical sheet  14  is held between those projecting portions  715  and  722 .  
         [0090]      FIG. 19  shows another example of how the optical sheet is supported between the upper and lower frames  71  and  72 . In the embodiment shown in this figure, substantially at the center of the floor surface of the recessed portion  13   a  of the lower frame  72 , a cylindrical pin-like member  70  is formed to project upright therefrom, and, in the projecting portion  14   a  formed on the optical sheet  14 , a through hole  141  is formed. The diameter of the through hole  141  is equal to or slightly greater than that of the pin-like member  70 . The pin-like member  70  on the lower frame  72  is put through the through hole  141  in the optical sheet  14 , and the lower and upper frames  72  and  71  are put together. This permits the optical sheet  14  to be fitted to the frame  7 .  
         [0091]     Here, the length of the pin-like member  70  is made equal to the depth of the recessed portion  13   a  so that, when the upper and lower frames  71  and  72  are put together, the top face of the pin-like member  70  just makes contact with the bottom surface of the upper frame  71 . This prevents the optical sheet  14  from dropping off the pin-like member  70 . The problem here is that, since each optical sheet is as thin as 0.1 mm to 0.5 mm, it is in practical terms often difficult to assemble the top face of the pin-like member  70  and the upper frame  71  with accuracy finer than that thickness. For this reason, it is recommended, as shown in  FIG. 20 , to make the pin-like member  70  longer and form, in the upper frame  71 , a through hole  712  ((a) in the figure) through which to put the pin-like member  70  or a groove  713  ((b) in the figure) in which to fit the pin-like member  70 . With this structure, it is possible to surely prevent the optical sheet  14  from dropping off the pin-like member  70  without the need to precisely adjust the length of the pin-like member  70 .  
         [0092]     Moreover, as shown in  FIG. 20 , it is preferable that, when the upper and lower frames  71  and  72  are put together, a gap d 4  be left between the optical sheet  14  and the upper and lower frames  71  and  72 . That is, by permitting the optical sheet  14  to move along the axis of the pin-like member  70 , even when the optical sheet  14  expands and contracts as the ambient temperature varies, it is possible to reduce the stresses produced and thereby to prevent the optical sheet from wrinkling or warping.  
         [0093]     Needless to say, as in the example shown in  FIG. 18 , it is also possible, instead of forming the projecting portion  14   a  on the optical sheet  14 , to fit the optical sheet  14  to the frame  7  by the use of a through hole  141  formed at the edge of the optical sheet  14 .  
         [0094]     After the upper and lower frames  71  and  72  are put together, they are kept in that state by being fixed together with an unillustrated fixing member. On the other hand, since the upper and lower frames  71  and  72  are typically formed by molding resin, if they receive an external impact, they may be so deformed as to cause the optical sheet  14  to drop off the pin-like member  70 . To prevent this, it is recommended to provide, in a tip portion of the pin-like member  70 , an engagement portion that engages with the fixing member so that the upper and lower frames  71  and  72  are put together and fixed together by the use of the pin-like member  70 . This simultaneously helps prevent the optical sheet  14  from dropping off the pin-like member  70 . A practical example of such a structure is shown in  FIG. 21 .  
         [0095]      FIG. 21  is a vertical sectional view around the pin-like member  70 . At (a) in the figure is shown the structure corresponding to the one shown at (a) in  FIG. 20 . When the upper and lower frames  71  and  72  are put together, the pin-like member  70  is put through the through hole  712 , and the tip portion of the pin-like member  70  is exposed at the top surface of the upper frame. In the tip-end face of the pin-like member  70 , a hole (engagement portion)  701  is formed that has an unillustrated internal thread formed on the interior surface thereof. Into this hole  701 , a screw (fixing member)  81  is screwed with a washer  82  placed in between. This permits the upper and lower frames  71  and  72  that are put together to be fixed together, and simultaneously surely prevents the optical sheet  14  from dropping off the pin-like member  70 .  
         [0096]     At (b) in the figure is shown the structure corresponding to the one shown at (b) in  FIG. 20 . When the upper and lower frames  71  and  72  are put together, the pin-like member  70  fits into the groove  713  formed in the upper frame  71 . In the tip-end face of the pin-like member  70 , a hole (engagement portion)  701  is formed that has an unillustrated internal thread formed on the interior surface thereof. Moreover, in the upper frame  71 , a through hole  714  is formed coaxially with the hole  701 . Through this hole  714 , a screw  81  is screwed into the hole  701  with a washer  82  placed in between. As in the example described just above, this permits the upper and lower frames  71  and  72  that are put together to be fixed together, and simultaneously surely prevents the optical sheet  14  from dropping off the pin-like member  70 .  
         [0097]     The engagement between the fixing member and the pin-like member may be achieved by any conventionally known engagement method other than that specifically described above. For example, it is possible to form an external thread around the exterior surface of the tip portion of the pin-like member, form the tip portion of the pin-like member in such a way that it protrudes from the upper frame when the upper and lower frames are put together, and put a nut around the protruding tip portion of the pin-like member.  
         [0098]     In the embodiments described above, the frame  7  is frame-shaped. It should be understood, however, that there is no particular restriction on the shape of the frame used in the present invention so long as it permits the optical sheet  14  to be fitted behind the liquid crystal panel  12 . For example, as shown in  FIG. 22 , the frame may be one that is composed of an upper frame  71  and a lower frame  72  both in the shape of an elongate plate so that the projecting portion  14   a  on the optical sheet  14  is held between the recessed portion  13   a  in the lower frame  72  and the upper frame  71 .