Abstract:
Occurrence of an event can be prevented, whereby, when a backlight expands with heat, a light guide plate might move a group of optical sheets into contact with a middle frame and deform the sheets, thereby causing nonuniformity of luminance on a screen. In particular, a protrusion is formed at an edge of the plate, and the sheets are mounted on the plate to avoid the protrusion. The frame covers the protrusion and the edge of the sheets. Even if a spatial gap with respect to the frame is lost by expansion of the plate, the sheets suffer no deformation due to interference between an edge of the sheets and the frame. Therefore, the nonuniformity of luminance on the screen does not occur, either. Also, unusual sounds due to movement of the plate under vibration do not arise since a gap between the protrusion and the frame can be small.

Description:
CLAIM OF PRIORITY 
       [0001]    The present application claims priority from Japanese Patent Application JP 2012-278309 filed on Dec. 20, 2013, the content of which is hereby incorporated by reference into this application. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates generally to liquid crystal display devices, and more particularly to a vehicular, compact liquid crystal display device slimmed down in bezel region size for increased display region. 
         [0004]    2. Description of the Related Art 
         [0005]    A conventional liquid crystal display device include: a thin-film transistor (TFT) substrate formed with pixel electrodes, TFTs, and other elements in matrix form; a counter substrate disposed in facing relation with respect to the TFT substrate and formed with a color filter and other elements at locations corresponding to the pixel electrodes on the TFT substrate; and liquid crystals arranged between the TFT substrate and the counter substrate. The display device forms images by controlling, on a pixel-by-pixel basis, transmittance of light formed by the liquid-crystal molecules. 
         [0006]    Liquid crystal display devices can be made compact and lightweight, so that their applications are expanding in a wide variety of products, including TVs, cell phones, and many more. Recently, they also are commonly used as vehicle-mounted types of displays. For vehicular application, since vibration and the like are exerted, they require more effective anti-vibration preventive measures than those required in a normal type of liquid crystal display device. 
         [0007]    In addition to being exposed to a vibration-prone environment, vehicular types of liquid crystal display devices are used in a wide environmental temperature range from −30° C. to +85° C. Heat-related preventives are therefore necessary. Optical sheets, in particular, have a problem in that it swelling or waviness is caused by thermal expansion under a high-temperature atmosphere, the sheet will not return to a normal stare even after being cooled down. By way of example for comparison, displays for medical monitors range between 0° C. and +60° C. in operating environmental temperature. 
         [0008]    Vehicular liquid crystal display devices further need to be placed in a limited space. Since liquid crystals do not emit light themselves, a backlight is disposed in the back of a liquid crystal display panel. Disposing the backlight in a limited space requires downsizing a light source, so light-emitting diodes (LEDs) are used as the light source. These LEDs are arranged on a side of a light guide plate, then various optical sheets are arranged on the light guide plate, and these optical parts are accommodated in a mold, whereby the backlight is constructed. 
         [0009]    JP-A-2008-46430 describes a configuration that includes a light guide plate having a sloped surface to increase the amount of incidence of light from LEDs by arranging the LEDs on a side of the light guide plate that is thicker than any other section of the light guide plate. 
       SUMMARY OF THE INVENTION 
       [0010]    For vehicular liquid crystal display devices, there is also the strong demand to increase only a display region size while maintaining a constant entire outline. In normal liquid crystal display devices, the backlight is disposed in a resin mold, whereas in vehicular liquid crystal display devices, the backlight is rested in a metal-formed middle frame and lower frame to make the outline of the device smaller. 
         [0011]    Additionally, vehicular liquid crystal display devices need to provide high luminance compared with other liquid crystal display devices. Implementation of this, on the other hand, involves more consumption of electric power in the LEDs that form the light source, and hence a greater deal of heat emission from the LEDs. The backlight is preferably placed in a metal to release the LED-emitted heat more efficiently. 
         [0012]      FIG. 8  shows an example of a plan view of a conventional vehicular liquid crystal display device. The liquid crystal display device in  FIG. 6  has an outline covered with a metal-formed upper frame  20 . A display region  300  of a liquid crystal display panel is forced internally to the upper frame  20 . A backlight not shown is formed in the back of the liquid crystal display panel. Referring to  FIG. 8 , a clearance from an outer edge or the display region to that of the upper frame, that is, a bezel is greater on short sides of the upper frame than on long sides thereof. In a configuration like this one, LEDs that form a light source are arranged near the short sides of the upper frame. Conversely in a case that a large bezel can be formed at the long sides, LEDs that form the light source are arranged near the long sides. 
         [0013]      FIG. 9  is a cross-sectional view of section A-A of the conventional vehicular liquid crystal display device, shown in  FIG. 8 . Referring to  FIG. 9 , a TFT substrate  100  and a counter substrate  200  are bonded together via a sealing material not shown, and liquid crystals not shown are interposed between the TFT substrate  100  and the counter substrate  200 . A lower polarizer  101  is attached to a lower surface of the TFT substrate  100 , and an upper polarizer  201  to an upper surface of the counter substrate  200 . A combination of the TFT substrate  100 , the counter substrate  200 , the lower polarizer  101 , and the upper polarizer  201 , is referred to as the liquid crystal display panel. 
         [0014]    The TFT substrate  100  is formed more largely than the counter substrate  200 , and a section of the TFT substrate  100  that lies as one sheet of material, serves as a terminal section. An IC driver  60  is mounted on the terminal section. In addition, a flexible wiring substrate  80  for supplying electric power and signals is connected to the liquid crystal display panel. A backlight is disposed in the back of the liquid crystal display panel. The backlight is interposed between a middle frame  30  and a lower frame  40 . Although a light source for the backlight is not shown, a plurality of LEDs are arranged on side faces of a light guide place  10 . 
         [0015]    Referring to  FIG. 9 , a reflecting sheet  12  is placed on the lower frame  40 , and the light guide plate  10  is placed on the reflecting sheet  12 . The reflecting sheet  12  and the light guide plate  10  work to direct LED-emitted incident light from the side faces of the light guide plate  10  so the liquid crystal display panel. Optical sheets, including a diffusion sheet and a prism sheet, are mounted on the light guide plate  10 . The optical sheets  18  function to raise utilization efficiency of the light, to reduce nonuniformity in luminance of the backlight, and to minimize moiré. The flexible wiring substrate  80 , connected to the terminal section of the liquid crystal display panel, extends to the rear of the lower frame  40  and is connected to a printed wiring substrate  90  via a connector  81 . The printed wiring substrate  90  is protected with a substrate cover  50  formed of a metal. A combination of such liquid crystal display panel and backlight as shown in  FIG. 9  is hereinafter called the liquid crystal display device. 
         [0016]      FIG. 10  is a cross-sectional view of the liquid crystal display device, the view obtained by magnifying the light guide plate  10 , optical sheets  18 , middle frame  30 , and lower frame  40  shown in  FIG. 9 . Referring to  FIG. 10 , a spatial gap g 2  is present between the middle frame  30  and the optical sheets  18  under which the light guide plate  10 , the reflecting sheet  12 , and the lower frame  40  are arranged in that order. 
         [0017]    When the liquid crystal display device is operated, however, various parts of the backlight expand with the LED-emitted heat or with heat from an engine side of a vehicle equipped with the display device. Thermal expansion coefficients are greater in resins than in metals. For example, the middle frame is formed from stainless steel, the lower frame from aluminum, and the light guide plate  10  and the optical sheets  18  from polycarbonate. The light guide plate  10 , in particular, expands and when the gap g 2  shown in  FIG. 10  becomes lost, the light guide plate  10  brings the optical sheets  18  into contact with the middle frame  30 . 
         [0018]    Meanwhile, the optical sheets  18  normally expand with heat in a planar direction as well. However, since the optical sheets  18  are held down at their edges between the middle frame  30  and the light guide plate  10 , the optical sheets  18  actually cannot expand in the planar direction and consequently as shown in  FIG. 11 , the optical sheets  18  suffer deformation such as corrugation. The deformation of the optical sheets  10  causes the nonuniformity of luminance on a display screen. 
         [0019]    Even in the conventional device structure of  FIG. 9 , the event that the light guide plate  10  thermally expands and brings the optical sheets  18  into contact with the middle frame  30  is avoidable if the gap g 2  between the optical sheets  18  and the middle frame  30 , shown in  FIG. 10 , is increased. If the gap g 2  in  FIG. 10  is increased, however, this makes the light guide plate  10  easily movable in its planar direction within the lower frame  40 , and if the liquid crystal display device vibrates during a travel of the vehicle, this vibration moves the light guide plate  10 , thus causing vibration sounds. 
         [0020]    An object of the present invention is to prevent the movement of the light guide plate  10  due to vibration and at the same time to avoid distortion of the optical sheets  16  by preventing the light guide light  10  from moving the optical sheets  18  into contact with the middle frame  30 , and thereby prevent the nonuniformity of luminance on the screen from occurring. 
         [0021]    The present invention addresses the above problems. The following outlines a more specific way to implement the conquest. 
         [0022]    A liquid crystal display device includes a liquid crystal display panel and a backlight. The backlight includes a light guide plate and a group of optical sheets which are disposed within a lower frame, the light guide plate being mounted on the light guide plate, the backlight including a middle frame covering an edge of the optical sheets and that of the light guide plate. The light guide plate is rectangular and formed with a protrusion at each of four corners thereof. A spatial gap g 1  between a front end of each protrusion of the light guide plate and a lower surface of the middle frame is smaller than a spatial gap g 2  present between an upper surface of the optical sheets and the lower surface of the middle frame. 
         [0023]    (2) The liquid crystal display device described in item (1) above, wherein the optical sheets are each notched at positions corresponding to the protrusions formed at the corners of the light guide plate. 
         [0024]    (3) The liquid crystal display device described in item (2) above, wherein the gap between the front ends of the protrusions of the light guide plate and the lower surface of the middle frame is 0.05 to 0.20 mm. 
         [0025]    (4) The liquid crystal display device described in item (1) above, wherein the liquid crystal display panel is mounted on the middle frame via a cushioning spacer. 
         [0026]    In accordance with the present invention, the protrusions at the four corners of the light guide plate prevent the light guide plate from moving the optical sheets into contact with the middle frame and distorting the optical sheets, and thereby prevent nonuniformity of luminance on a display screen from occurring. 
         [0027]    In addition, in accordance with the present invention, the gap between the middle frame and the light guide plate or the protrusions of the light guide plate does not need to be increased, which prevents the light guide plate iron; moving or generating vibration sounds, even in case of vibration. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]      FIG. 1  is a sectional view of a liquid crystal display device embodying the present invention; 
           [0029]      FIG. 2  is a sectional view showing an example of snap fitting; 
           [0030]      FIGS. 3P-3C  are detailed views of a light guide plate according to the invention; 
           [0031]      FIG. 4  shows an example of a group of optical sheets that becomes engaged with the light guide plate of  FIGS. 3A-3C ; 
           [0032]      FIG. 5  is a perspective view of the optical sheets; 
           [0033]      FIG. 6  is a sectional view that shows essential elements of the invention; 
           [0034]      FIG. 7  is a sectional view that shows the essential elements of the invention under a thermally expanded state; 
           [0035]      FIG. 8  is a plan view showing an example of a liquid crystal display device to which the invention is applied; 
           [0036]      FIG. 9  is a sectional view of a conventional liquid crystal display device; 
           [0037]      FIG. 10  is a sectional view showing a middle frame, light guide plate, and optical sheets as used in the conventional liquid crystal display device shown by way of example in  FIG. 9 ; and 
           [0038]      FIG. 11  is a sectional view that shows problems associated with the conventional liquid crystal display device shown by way of example in  FIG. 9 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0039]    Hereunder, details of the present invention will he described with reference to an embodiment. 
       First Embodiment 
       [0040]      FIG. 1  is a sectional view of a liquid crystal display device embodying the present invention.  FIG. 1  shows section A-A of the liquid crystal display device shown in  FIG. 8 . Description of  FIG. 8  is omitted here since the figure has already been described earlier in this specification. Referring to  FIG. 1 , a backlight, disposed in a middle frame  30  and a lower frame  40 , is present in the back of the liquid crystal display device, and the backlight is covered in its entirety with an upper frame  20 . In these respects, the liquid crystal display device is the same as that described in  FIG. 9 . An IC driver  60  is disposed at a terminal section of a TFT substrate  100 , a flexible wiring substrate  80  is connected to the terminal section, and the flexible wiring substrate  80  extends near the back of the lower frame  40  and is connected to a printed wiring substrate  90  in the back of the lower frame  40  via a connector  81 . In these respects, the liquid crystal display device is also the same as in  FIG. 9 . 
         [0041]    Referring to  FIG. 1 , the lower frame  40  accommodating the backlight is formed from aluminum to have a thickness of nearly 1 mm. This is because, in addition to accommodating the backlight, the lower frame  40  has a role of a heat release plate to release heat that originates from LEDs. The middle frame  30  and upper frame  10  in  FIG. 1  are both formed of nearly 0.3 mm thick stainless steel or iron, in order to keep the liquid crystal display device lightweight. 
         [0042]    Referring also to  FIG. 1 , the middle frame  30  and the lower frame  40  are assembled by snap-fitting not shown in  FIG. 1 .  FIG. 2  is a sectional view of the snap-fit section  25 . The lower frame  40  in  FIG. 2  is formed with a hole on one side face beforehand. When a cut is formed on one side of the middle frame  30  and this frame is bent inward at one section thereof, this bent section fits into the above hole to join the middle frame  30  and the lower frame  40  together. The middle frame  30  and the upper frame  20  are likewise assembled using substantially the same snap-fit section  25  as used in  FIG. 2 . 
         [0043]    One of the most significant differences between  FIG. 1  and  FIG. 8  which shows the conventional example exists in a shape of the light guide plate  10 . Referring to  FIG. 1 , a protrusion  11  is formed at an end portion of the light guide plate  10 . A clearance from a front end of the protrusion  11  of the light guide plane  10  to an inner surface of the middle frame  30  is smaller than an upper surface of the optical sheets  18  and an inner surface of the middle frame  30 . That is to say, even if the light guide plate  10  thermally expands and comes into contact with the middle frame  30 , one optical sheets  18  do not come into contact with the middle frame  30 . This prevents the middle frame  30  from holding down the edges of the optical sheets  18  and thus from deforming the optical sheets  18  into a corrugated form. In short, nonuniformity of luminance on a display screen due to such deformation of the optical sheets  18  is suppressed. 
         [0044]    Even if the thermal expansion of the light guide plate  10  pushes a flange section of the middle frame  30  upward, since a cushioning spacer  70  is disposed between the liquid crystal display panel and the middle frame  30 , the cushioning spacer  70  flexibly collapses and does not stress the liquid crystal display panel. 
         [0045]      FIGS. 3A-30  show an exemplary shape of the light guide plate  10  that is one feature of the present invention.  FIG. 3A  is a plan view of the light guide plate  10 ,  FIG. 3B  is a side elevation showing a long side of the light guide plate  10 , and  FIG. 3C  is a side elevation shoving a short side of the light guide plate  10 . The light guide plate  10  shown in  FIGS. 3A-3C  has a feature in that the protrusion  11  is formed at four corners. The optical sheets  18  to be mounted on the light guide plate  10  have a shape that avoids the protrusions  11  formed at the four corners of the light guide plate  10 .  FIG. 4  is a plan view of a lower diffusion sheet  13  that is a portion of the optical sheets  18 . As shown in  FIG. 4 , the lower diffusion sheet  13  is notched at four corners to mate with the protrusions  11  at the four corners of the light guide plane  10 . 
         [0046]    Referring back to  FIGS. 3A-3C , when the light guide plate  10  has a diagonal dimension D of, for example, 4 inches wide across one corner, the light guide plate  10  is of the following dimensions, for example. The protrusions  11  at the four comers have a square plans that measures, as shown, in  FIG. 3A , between 0.5 mm and 1.0 mm in length “p” of one side, and as shown in  FIG. 3B , nearly 1.0 mm in height “t 2 ” of the protrusion  11 . The light guide plate  10  itself has a thickness “s” of nearly 3.0 mm. A display region  300  is formed internally to the protrusions  11 , and a spatial clearance “s” from an edge of the light guide plate  10  to that of the display region  300  is 2.0 mm, for example, 
         [0047]      FIG. 5  is a perspective view showing an example of the optical sheets  18 . The optical sheets  18  comprise a lower diffusion sheet  13 , a lower prism sheet  14 , an upper prism sheet  15 , and an upper diffusion sheet  16 , which ere mounted in this order on the light guide plate  10 . The lowest sheet in  FIG. 5  is the lower diffusion sheer  13 . Light that exits the light guide plate  10  and heads towards the liquid crystal display panel has slight nonuniformity of brightness in that the light is, for example, relatively bright near the LEDs and dark at sections distant from the LEDs or between one LED and another LED. The lower diffusion sheet  13  alleviates such nonuniformity of brightness to form the backlight of uniform luminance. 
         [0048]    The lower prism sheet  14  is overlaid upon the lower diffusion sheet  13 . As shown in  FIG. 5 , the lower prism sheet  14  includes prisms that are triangular in section, and these prisms extend in a longitudinal direction and are arrayed in a lateral direction. The prisms are pitched at nearly 50 μm. The fewer prism sheet  14  functions to raise utilization efficiency of the light by, as shown in  FIG. 5 , orienting it in a vertical direction of the lower prism sheet as the light attempts spreading in a direction of “a”. 
         [0049]    The upper prism sheet  15  is overlaid upon the lower prism sheet  14 . As shown in  FIG. 6 , the upper prism sheet  15  includes prisms that are triangular in section, and these prisms extend in a longitudinal direction, and are arrayed in a lateral direction. The prisms are pitched at nearly 50 cm. The upper prism sheet  15  functions to raise utilization efficiency of the light by, as shown in  FIG. 5 , orienting it in a vertical direction of the upper prism sheet as the light attempts spreading in a direction of “b”. 
         [0050]    Referring to  FIG. 5 , the upper diffusion sheet  16  is overlaid upon the upper prism sheet  15 . The upper diffusion sheet  16  works to suppress occurrence of moiré on the screen of the liquid crystal display device. That is to say, microscopically the brightness of the light which has exited the lower prism sheet  14  or the upper prism sheet  15  is periodically changing according to the particular arrayal pitch of the prisms. 
         [0051]    Meanwhile, scan lines extend in a lateral direction, for example, and are arrayed in, for example, a longitudinal direction on the TFT substrate  100  of the liquid crystal display panel. Therefore, a section that periodically transmits the light in the longitudinal direction, and a section that acts as a shield against the light, occur according to the particular scan line. Image signal lines also extend in the longitudinal direction and are arrayed in the lateral direction on the TFT substrate  100  of the liquid crystal display panel. Therefore, a section that periodically transmits the light in the lateral direction, and a section that acts as a shield against the light, occur according to the particular image signal line. 
         [0052]    Interference of light then occurs between the light that has passed through the lower prism sheet  14  and the upper prism sheet  15 , and the scan lines or image signal lines on the TFT substrate  100  of the liquid crystal display panel, and moiré, occurs as a result. The upper diffusion sheet  16  has a role to alleviate the interference with the scan lines or image signal lines on the TFT substrate  100 , by alleviating an intensity level of the light which has passed through the prism sheets, and thus suppress the occurrence of moiré. The upper diffusion sheet  16  is omitted if the moiré does hot become a problem. The lower diffusion sheet  13  and the upper diffusion sheet  16  are, for example, 140 μm thick, and the lower prism sheet  14  and the upper prism sheet  15  are, for example, 155 μm thick. 
         [0053]    The optical sheets in  FIG. 5  are only shown as an example, and in another example, the upper diffusion sheet may be replaced by a DBEF (Dual Brightness Enhancement Film). The DBEF first reflects light, whose phase differs from that of a direction in which a lower polarizer transmits the light, and then the DBEF once again reflects the light via optical parts provided below, and causes the light to pass through for enhanced light utilization efficiency of the backlight. The DBEF has a thickness off for example, between 280 μm and 400 μm. 
         [0054]    In yet another example of optical sheets used in a vehicular liquid crystal display device, a so-called louver film may be used that imparts directivity to the light emitted from the display. 
         [0055]      FIG. 6  is an enlarged sectional view that shows essential elements of the present invention, this sectional view representing a relationship between the light guide plate  10 , the optical sheets  18 , and the middle frame  20 . Referring to  FIG. 6 , the optical sheets  18  are mounted on the light guide plate  10 . The optical sheets  10  comprise the lower diffusion sheet  13 , the lower prism sheet  14 , the upper prism sheet  15 , and the upper diffusion sheet  16 , The protrusion  11  is formed at an edge of the light guide plate  10 , and height of the protrusion  11  is greater than total thickness of the optical sheets  18 . A horizontal clearance “d” between the optical sheets  18  and the protrusion  11  of the light guide plate  10 , in  FIG. 6 , is the clearance between the notches  17  of the optical sheets  18  and the protrusion  11  of the light guide plate  10 , and “d” is a dimensional margin provided, so that the optical sheets  18  are easily mounted on the light guide plate  10 . 
         [0056]    A spatial gap g 1  between the protrusion  11  of the light guide plate  10  and a lower surface or the middle frame  30  is nearly between 0.05 mm end 0.20 mm. This clearance is of such a level that the light guide plate  10  does not shift in position within the lower frame  40 , even under a normal state. A spatial gap g 2  between the lower surface of the middle frame  30  and the upper surface of the optical sheets  18  is nearly 0.2 mm. However, g 2 &gt;g 1  always holds. In other words, the upper surface of the optical sheets  18  are always lower than the front end of the protrusion  11  of the light guide plate  10 . 
         [0057]      FIG. 7  shows a case in which the LEDs raise a temperature of the parts of the backlights and bring the protrusion  11  of the light guide plate  10  and the middle frame  30  into contact. In this case, the upper surface of the optical sheets  13  is also lower than the front end of the protrusion  11  of the light guide plate  10 , so the problem can be avoided that as in the conventional device, the fight guide plate  10  might, press the optical sheets  18  against the middle frame  30 , deform the optical sheets  18  into a corrugated form, and cause the nonuniformity of luminance on the screen. 
         [0058]    If the light guide plate  10  thermally expands too much, this pushes the middle frame  30  upward as denoted by a white arrow, but since the cushioning spacer  70  present on one middle frame  30  is formed from a spongy material to readily become collapsed by compressive force, no stress is applied to the liquid crystal display panel mounted on the cushioning spacer  70 . 
         [0059]    In this way, in accordance with the present invention, since the protrusion  11  is formed at the four corners of the light guide plate  10 , even when a temperature of the backlight rises for reasons such as heat from the LEDs, the nonuniformity of luminance due to the deformation of the optical sheets  18  as a result of their edges becoming sandwiched between the middle frame  30  and the light guide plate  10  can be prevented from occurring. In addition, since the gap between the middle frame  30  and the protrusion  11  of the light guide plate  10  can be reduced to a small level, the movement of the light guide plate  10  due to vibration can be prevented and unusual sounds due to the movement of the light guide plate  10  under vibration of the liquid crystal display device can be prevented from occurring.