Abstract:
A liquid crystal display device includes a liquid crystal display panel having a liquid crystal layer sandwiched between a pair of upper and lower substrates, an illuminating light source disposed behind the liquid crystal display panel, an upper case made of metal having a sidewall bent back from a periphery of a front portion thereof and a window approximately corresponding to a display area of the liquid crystal display panel, a lower case made of resin for housing the illuminating light source in a recess thereof, and an inner frame made of metal having an opening for allowing light from the illuminating light source to be directed toward the liquid crystal display panel and interposed between the liquid crystal display panel and the lower case, the upper case and the lower case being clamped together after stacking the liquid crystal display panel, the inner frame and the illuminating light source in the order named between the upper case and the lower case.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a liquid crystal display device. A liquid crystal display panel employed in the liquid crystal display device comprises a pair of opposed substrates sealed along their peripheries to form an envelope for a layer of liquid crystal material and a plurality of electrodes for defining pixels on an inner surface of at least one of said pair of opposed substrates. Each pixel has a function of controlling the amount of light passing therethrough. 
     In general, the liquid crystal display devices are known which comprise a liquid crystal display panel using a pair of transparent substrates and a backlight disposed behind the liquid crystal display panel such that images are produced by controlling the amount of light passing through the liquid crystal display panel from the backlight. 
     The backlight is housed in a frame made of resin (hereinafter referred to as a lower case). The liquid crystal display panel is placed on the lower case so that the liquid crystal display panel is illuminated by the backlight. 
     The lower case serves mainly to house the backlight, supports the liquid crystal display panel and serves as a mask for projecting light from the backlight onto an area for displaying images of the liquid crystal display panel (hereinafter referred to as a display area). 
     Such a technique is disclosed in U.S. Pat. No. 5,432,626 (Application No. 29,622) issued on Jul. 11, 1995, for example, which employs a case for housing a backlight and an intermediate frame interposed between the case and a liquid crystal display panel. 
     But with a tendency toward large-sized display area, there arises a problem with the prior art liquid crystal display device that the mechanical strength of the lower case needs to be increased for housing a large-sized backlight and consequently the thickness of the lower case is required to be increased. This increases the width of the non-useful display area at the borders of the liquid crystal display panel (hereinafter referred to as a border area). 
     As explained above, the lower case supports the liquid crystal display panel and serves as a mask for projecting light from the backlight onto a display area of the liquid crystal display panel. With this structure, there is a possibility that shading occurs at the peripheries of the display area of the liquid crystal display panel viewed from the observer side. 
     Since the thickness of portions of the lower case serving as a mask is relatively greater, the portions block the light from the backlight and cause the shading at the peripheries at the display area of the liquid crystal display panel viewed from the observer side. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to solve the above problem with the prior art and to provide a liquid crystal display device capable of securing a sufficient mechanical strength of the lower case without increasing the border area of the large-sized liquid crystal display panel. 
     It is another object of the present invention to provide a liquid crystal display device free from occurrence of the shading at the peripheries of the display area of the liquid crystal display panel viewed from the observer side. 
     The above-mentioned objects and novel features of the present invention will become more apparent by reference to the following description taken in conjunction with the accompanying drawings. 
     For achieving the aforesaid object, a liquid crystal display device according to an embodiment of the present invention comprises: a liquid crystal display panel having a liquid crystal layer sandwiched between a pair of upper and lower substrates, an illuminating light source disposed behind the liquid crystal display panel, an upper case made of metal having a sidewall bent back from a periphery of a front portion thereof and a window approximately corresponding to a display area of the liquid crystal display panel, a lower case made of resin for housing the illuminating light source in a recess thereof, and an inner frame made of metal having an opening for allowing light from the illuminating light source to be directed toward the liquid crystal display panel and interposed between the liquid crystal display panel and the lower case, the upper case and the lower case being clamped together after stacking the liquid crystal display panel, the inner frame and the illuminating light source in the order named between said upper case and said lower case. 
     For achieving the aforesaid object, a liquid crystal display device according to another embodiment of the present invention comprises: a liquid crystal display panel having a liquid crystal layer sandwiched between a pair of upper and lower substrates, an illuminating light source disposed behind the liquid crystal display panel, an upper case made of metal having a sidewall bent back from a periphery of a front portion thereof and a window approximately corresponding to a display area of the liquid crystal display panel, a lower case made of resin for housing the illuminating light source in a recess thereof, an inner frame made of metal having an opening for allowing light from the illuminating light source to be directed toward the liquid crystal display panel and interposed between the liquid crystal display panel and the lower case, and a transparent plate fitted within the opening in the inner frame, the upper case and the lower case being clamped together after stacking the liquid crystal display panel, the inner frame and the illuminating light source in the order named between the upper case and the lower case. 
     The construction of the present invention enables the metal inner frame to reinforce the lower case and consequently eliminates the need for increasing the thickness of the lower case, namely avoids the increase in the so-called border area of the liquid crystal display panel. 
     Employment of metal for the inner frame makes the thickness of the inner frame thin. In directing light from the backlight by the inner frame only onto the display area of the liquid crystal display panel, it is possible to prevent occurrence of the shading at the peripheries of the display area of the liquid crystal display panel when it is viewed from the observer&#39;s side by accurately positioning the edge of the opening of the inner frame. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings, in which like reference numerals designate similar components throughout the figures, and in which: 
     FIG. 1 is an exploded perspective view of an embodiment of the liquid crystal display device in accordance with the present invention; 
     FIG. 2 is a diagram showing a configuration of a liquid crystal display panel and its drive circuit used for an embodiment of a liquid crystal display device in accordance with the present invention; 
     FIG. 3 is a plan view of an example of a pixel in the liquid crystal display device of the present invention; 
     FIG. 4 is a cross-sectional view taken along line IV—IV of FIG. 3; 
     FIG. 5 is a cross-sectional view taken along line V—V of FIG. 3; 
     FIG. 6 is a cross-sectional view taken along line VI—VI of FIG. 3; 
     FIG. 7 is a perspective view of an example of a backlight unit in the liquid crystal display device of the present invention; 
     FIG. 8 is a perspective view of an example of an inner frame in the liquid crystal display device of the present invention; 
     FIG. 9 is a cross-sectional view taken along line IX—IX of FIG. 1 for explaining of the advantages of the present invention; 
     FIG. 10 is an illustration for explaining the disadvantages of the prior art; 
     FIG. 11 is an exploded perspective view of another embodiment of the liquid crystal display device in accordance with the present invention; 
     FIG. 12A is a cross-sectional view taken along line XIIA—XIIA of FIG. 11; 
     FIG. 12B is a cross-sectional view of a modification of FIG. 12A; 
     FIGS. 13A and 13B are cross-sectional views for explaining further modifications of the present invention, respectively; 
     FIG. 14 is a cross-sectional view for explaining another modification of the present invention; and 
     FIG. 15 is a cross-sectional view of another embodiment of a sag-proof plate in the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The embodiments of the liquid crystal display device in accordance with the present invention will now be described in detail with reference to the accompanying drawings. 
     Embodiment 1 
     Structure of the Liquid Crystal Display Device 
     The liquid crystal display device of this embodiment is an active matrix liquid crystal display device of a so-called horizontal electric field type (commonly called an in-plane switching (IPS) type, see Japanese Patent Publication No. Sho 63-21907 and U.S. Pat. No. 4,345,249) and its structure is shown in FIG.  2 . 
     A liquid crystal display panel  100  shown in FIG. 2 has a display section comprised of a plurality of pixels arranged in a matrix and each pixel is structured such that it can control or modulate the amount of light passing therethrough from a backlight unit  300  disposed behind the liquid crystal display panel  100  independently of other pixels. 
     Modulation of the amount of light at each pixel is performed by the in-plane switching (IPS) method which makes an electric field in a layer of liquid crystal material sandwiched between a pair of opposed transparent substrates  1 A, 1 B in parallel with the transparent substrates IA,LB. 
     This type of the liquid crystal display panel  100  provides a sharp image even when it is viewed at a large angle from the normal to its display surface and consequently it is known for its wide viewing angles. 
     Scanning signal lines  2  and reference signal lines  4  extend in an x direction (a row direction) and are arranged in parallel with each other in a y direction (a column direction) on an inner surface on the liquid crystal layer side of the one substrate  1 A of the opposing transparent substrates  1 A,  1 B with a layer of liquid crystal material therebetween. 
     In FIG. 2, scanning signal lines  2  and reference signal lines  4  are alternately arranged on the transparent substrate  1 A, such as, from the top of the transparent substrate  1 A, a scanning signal line  2 , a reference signal line  4  adjacent to the scanning signal line  2 , another scanning signal line  2  spaced a relatively large distance from the reference signal line  4 , another reference signal line  4  closely spaced from the another scanning signal line  2 , and so on. 
     The video signal lines  3  extend in the y direction and are arranged in parallel with each other in the x direction, electrically insulated from the scanning signal lines  2  and the reference signal lines  4 . 
     A unit pixel is formed in a relatively large rectangular area enclosed by the scanning signal lines  2 , the reference signal lines  4  and the video signal lines  3 , and a plurality of unit pixels are arranged in a matrix to form a display area. The pixels will be explained in detail subsequently. 
     The liquid crystal display panel  100  is provided with external circuits such as a vertical scanning circuit  5  and a video signal drive circuit  6 . The vertical scanning circuit  5  supplies scanning signals (voltages) to each of the scanning signal lines  2  successively and the video signal circuit  6  supplies video signals (voltages) to the video signal lines  3  in synchronism with the scanning signals. 
     The vertical scanning circuit  5  and the video signal drive circuit  6  receive supply voltages from a liquid crystal drive power supply circuit  7  and receive display data and control data respectively separated by a controller  9  from video information from CPU  8 . Specially for the liquid crystal display panel  100 , the reference signal lines  4  are employed and are supplied with the reference voltage signal from the liquid crystal drive power supply circuit  7 . 
     Overall Structure of the Liquid Crystal Display Device 
     FIG. 1 is an exploded perspective view of an embodiment of the liquid crystal display device in accordance with the present invention. The liquid crystal display device of FIG. 1 is divided into five main sections, a liquid crystal display panel module  400 , a backlight unit  300 , a lower case  500 , an inner frame  700  and an upper frame  800 , which are assembled into a module. These sections will be explained below. 
     Liquid Crystal Display Panel Module 
     The liquid crystal display panel  400  corresponds to a section enclosed by a dot-and-dash line P in FIG.  2  and comprises the liquid crystal display panel  100 , the vertical scanning circuit  5  comprised of a plurality of semiconductor ICs mounted at the peripheries of the liquid crystal display panel  100 , the video signal drive circuit  6 , a flexible gate-circuit board  30  (see FIG. 1) connected to the input terminals for the vertical scanning circuit  5  and a flexible drain-circuit board  31  (see FIG. 1) connected to the input terminals for the video signal drive circuit  6 . 
     Outputs from a controller circuit board  33  which will be explained in detail subsequently are inputted to the vertical scanning circuit  5  and the video signal drive circuit  6  mounted on the liquid crystal display panel  100  via the gate-circuit board  30  and the drain-circuit board  31 , respectively, and the outputs of the two circuits are inputted to the scanning signal lines  2  and the video signal lines  3  of the liquid crystal display panel  100 , respectively. 
     The display area of the liquid crystal display panel  100  is formed of a large number of pixels arranged in a matrix as explained above, and FIG. 3 is an enlarged plan view of one of the pixels which corresponds to an area A enclosed by broken lines in FIG.  2 . FIG. 4 is a cross-sectional view of the pixel taken along line IV—IV in FIG. 3, FIG. 5 is a cross-sectional view of the pixel taken along line V—V in FIG. 3, and FIG. 6 is a cross-sectional view of the pixel taken along line VI—VI in FIG.  3 . 
     In FIG. 3, the reference signal line  4  extend in the x direction on the major surface of the transparent substrate  1 A and the scanning signal line  2  are spaced a relatively large distance from the reference signal line  4  in the negative y direction and extend in parallel with the reference signal line  4 . 
     Three reference electrodes  14  are formed integrally with the reference signal line  4 . Two of the three reference electrodes  14  extend close to and along a pair of adjacent video signal lines  3  defining one pixel area, respectively, in the negative y direction to the vicinity of the scanning signal lines  2 , and the other one of the three reference electrodes  14  extend in parallel with and between the two reference electrodes  14 . 
     An insulating film  15  made of silicon nitride, for example, is formed to cover the surface of the transparent substrate  1 A and the scanning signal lines  2 , the reference signal lines  4  and the reference electrodes  14  formed on the transparent substrate  1 A (see FIGS. 4,  5  and  6 ). The insulating film  15  serves as an interlayer insulating film between the video signal lines  3  and the scanning signal lines  2  or the reference signal lines  4  at their intersections, serves as a gate insulating film at areas forming a thin film transistor TFT and serves as a dielectric at areas forming a storage capacitor Cstg. 
     As shown in FIG. 5, initially a semiconductor layer  16  is formed at an area for forming a thin film transistor TFT on the insulating film  15 . The semiconductor film  16  is made of amorphous silicon, for example, and is disposed on the scanning signal line  2  near the video signal line  3 , and consequently a portion of the scanning signal line  2  serves as a gate electrode of the thin film transistor TFT also. 
     As shown in FIG. 3, a plurality of video signal lines extending in the y direction are arranged in the x direction the insulating film  15 . A drain electrode  3 A is formed integrally with the video signal line  3  such that the drain electrode  3 A extends on a portion of the surface of the semiconductor layer  16  of the thin film transistor TFT. 
     A display electrode  18  is formed on the surface of the insulating film  15  in the pixel area. The display electrode  18  runs between the reference electrodes  14 . One end of the display electrode  18  serves as a source electrode  18 A of the thin film transistor TFT, from there the display electrode  18  extends in the positive y direction, then extends on the reference signal line  4  in the x direction and then extends in the negative y direction terminating in the other end of the display electrode  18 , to form a square letter U. 
     A portion of the display electrode  18  overlapping the reference signal lines  4  forms a storage capacitor Cstg with the reference signal lines  4  with the insulating film  15  therebetween serving as a dielectric. The storage capacitor provides the effect of storing image information in the display electrode  18  longer after switching off of the thin film transistor TFT, for example. 
     The surface of the semiconductor layer  16  forming an interface with the drain electrode  3 A and the source electrode  18 A of the thin film transistor TFT is doped with phosphorus to form heavily doped layers  16 A and to obtain ohmic contacts at the drain electrode  3 A and the source electrode  18 A. In this case, initially the heavily doped layer  16 A is formed over the entire surface area of the semiconductor layer  16 , and the drain electrode  3 A and the source electrode  18 A are formed on the heavily doped layer  16 A. The heavily doped layer  16 A is patterned by etching away the parts of the heavily doped layer  16 A not covered by the drain electrode  3 A and the source electrode  18 A serving as masks. 
     A protective film  19  made of silicon nitride, for example, is formed on the thin film transistor TFT, the video signal lines  3 , the display electrodes  18  and the insulating film  15  as shown in FIGS. 4,  5  and  6 , and then a liquid crystal molecule orientation film  20  is formed on the protective film  19  to complete the transparent substrate  1 A of the liquid crystal display panel  100 . 
     A polarizer  21  is attached to the surface of the transparent substrate  1 A on the side opposite from a layer of liquid crystal material LCL. 
     A light-blocking film  22  is formed on portions of the transparent substrate  1 B on the side thereof facing the liquid crystal layer LCL which correspond to borders around each pixel area as shown in FIG.  4 . The light-blocking film  22  has functions of preventing light from illuminating the thin film transistor TFT directly and improving the display contrast. The light-blocking film  22  is configured such that a window indicated by broken lines in FIG. 3 defines a substantial pixel area. 
     A color filter  23  is formed to cover the window in the light-blocking film  22  as shown in FIG.  4 . Any two color filters  23  in two pixel areas adjacent in the x direction in FIG. 3, respectively, have two colors different from each other, and have borders between the two color filters on the light-blocking film  22 . A planarizing film  24  made of resin, for example, is formed on the color filters  23  and a liquid crystal molecule orientation film  25  is formed on the planarizing film  24 . 
     A polarizer  26  is attached to the surface of the transparent substrate  1 B on the side thereof opposite from the liquid crystal layer LCL. 
     Backlight 
     Returning to FIG. 1, a backlight unit  300  is disposed behind the liquid crystal display panel module  400 . This backlight unit  300  is of the so-called head-on type, and comprises a plurality (eight in FIG. 1) of cold cathode lamps  35  each extending in the x direction and arranged in the y direction and a reflector  36  for directing light from the cold cathode lamps  35  toward the liquid crystal display panel module  400 . The surface of the reflector  36  is corrugated in the direction of the arrangement of the cold cathode lamps  35  (the y direction) as shown in detail in FIG. 7 such that its portions seating the respective cold cathode lamps  35  are recessed concavely and its portions between the respective cold cathode lamps  35  project to form sharp tips in cross-section such that all the light rays from the respective cold cathode lamps  35  are directed toward the liquid crystal display panel module  400  efficiently. 
     The reflector  36  is provided with sideboards  37  at its sides perpendicular to the longitudinal axes of the cold cathode lamps  35 , and both ends of the cold cathode lamps  35  are fitted into the slits  38  formed in the sideboards  37  to prevent the movement of the cold cathode lamps  35  in the direction of their arrangement. 
     Lower Case 
     A lower case  500  constitutes a part of an outer frame of the liquid crystal display device assembled as a module and houses the backlight unit  300 . The lower case  500  is in the form of a box having a bottom and sidewalls and the top ends of the sidewalls are configured such that they support a light diffusing plate  600  disposed to cover the backlight unit  300 . 
     The light diffusing plate  600  diffuses the light from the cold cathode lamps  35  of the backlight unit  300  to project light uniformly toward the liquid crystal display panel module  400 . The thickness of the lower case  500  is made relatively small because the resultant reduction of the mechanical strength of the lower case  500  is compensated for by an inner frame  700  to be described subsequently. 
     A DC/AC inverter circuit board  40  is attached to the rear surface of the lower case  500 , and is connected to the terminals of the cold cathode lamps  35  to provide a supply voltage to the cold cathode lamps  35  of the backlight unit  300 . 
     Inner Frame 
     An inner frame  700  is disposed between the liquid crystal display panel module  400  and the light diffusing plate  600 . The inner frame  700  is made of a metal plate of relatively small thickness, and has an opening  42  corresponding to a display area of the liquid crystal display panel module  400 . 
     The inner frame  700  has functions of pressing the light diffusing plate  600  against the lower case  500  and supporting the liquid crystal display panel module  400 . 
     The positioning spacers  44  made of resin and having an L-shaped cross-section, for example, are attached to positions on the inner frame  700  corresponding to the corners of the liquid crystal display panel  100  of the module  400  to be mounted on the inner frame  700  such that the liquid crystal display panel  100  is positioned accurately with respect to the inner frame  700 . 
     As shown in detail in FIG. 8, the inner frame  700  has sidewalls  46  integrally formed therewith, that is, is in the form of a general box made of metal and having the opening  42  in its bottom. 
     The inner frame  700  is structured such that it is fitted over the lower case  500  with the light diffusing plate  600  interposed therebetween, namely, the inner frame  700  is fitted over the lower case  500  such that the inner surface of the sidewalls  46  of the inner frame  700  face the outer surface of the sidewalls of the lower case  500 . 
     The inner metal frame  700  of this structure forms a frame structure in combination with the lower case  500  such that the mechanical strength of the frame structure is increased without increasing the thickness of the lower case  500 . Even if the mechanical strength of each of the inner frame  700  and the lower case  500  is not sufficient enough, when the inner frame  700  is fitted over the lower case  500  as described above, the mechanical strength of the combination of the two is improved, especially the strength against twisting around the diagonals of the box structure is increased. This provides an advantage of securing the sufficient mechanical strength of the liquid crystal display device without increasing the border areas of the liquid crystal display panel. 
     The mechanical strength of the inner frame  700  by itself is increased compared with a substantially flat inner frame without any sidewalls such that handling of the inner frame  700  is facilitated in early stages of the module assembly. 
     Incidentally, in this embodiment a control circuit board  33  and a DC/DC converter circuit board  47  are juxtaposed at portions of the sidewalls  46  of the inner frame  700 , namely, they are arranged in a plane perpendicular to the display surface of the liquid crystal display panel module  400  to reduce the border areas. 
     In this case, the control circuit board  33  is connected to the flexible gate-circuit board  30  and the flexible drain-circuit board  31  attached to the liquid crystal display panel module  400  and the drain-circuit board  31  is bent to provide the above arrangement. 
     The control circuit board  33  and the DC/DC converter circuit board  47  correspond to a section B enclosed by dot-and-dash lines in FIG.  2 . 
     Incidentally, with this structure, electromagnetic radiation from the control circuit board  33  is prevented by the sidewalls  46  of the inner frame  700  from being introduced into other electronic components. 
     In the above embodiment the inner frame  700  is in the form of a box, but the inner frame  700  need not be a perfect box, it may be a frame having a sidewall formed in at least one of its four sides. The inner frame  700  is provided with a bent portion integrally formed with it, which improves its mechanical strength. 
     Upper Frame 
     An upper frame  800  presses the liquid crystal display panel module  400 , the inner frame  700  and the light diffusing plate  600  against the lower case  500 , and forms an outer frame of the module of the liquid crystal display device with the lower case  500 . 
     The upper frame  800  made of metal is of the shape of a general box having an opening (a display window)  48  corresponding to the display area of the liquid crystal display panel module  400  and is clamped to the lower case  500  a by engagement with it, for example. The upper frame  800  also has a function of electrostatic shielding. 
     Cross-sectional Configuration of the Module 
     FIG. 9 is a cross-sectional view of the liquid crystal display device taken along line IX—IX in FIG.  1 . It is apparent from FIG. 9 that the inner frame  700  also has a function of a mask for projecting the light from the backlight unit  300  onto the display area of the liquid crystal display panel module  400  and reducing the amount of light leaking into the areas around the display area, in addition to supporting the liquid crystal display panel module  400 . For this purpose, the contour of the opening  42  in the inner frame  700  needs to be positioned accurately with respect to the liquid crystal display panel module  400 . 
     In this embodiment, as explained above, the thickness of the inner frame  700  is relatively small, and consequently this embodiment provides the advantages that setting of the inner frame  700  is accurately performed and consideration of influences by the thickness of the inner frame  700  is not required in setting of the inner frame  700 . 
     FIG. 10 is a cross-sectional view of a liquid crystal display panel module in which the thickness of the inner frame  700  is made greater than that of the inner frame  700  shown in FIG.  9 . It is apparent from FIG. 10 that the masking function of the lower case  500  is greatly influenced by its thickness and consequently the masking function is not sometimes performed sufficiently due to an inappropriate position of the contour of the opening  52  in the lower case  500  and the shading occurs around the display area of the liquid crystal display panel  100  when viewed from the observer side. 
     In FIGS. 9 and 10, D denotes the thickness of the inner frame, δ is a spacing between the inner frame and the transparent substrate  1 A, which is determined by a cushion layer, for example, d is the thickness of the transparent substrate  1 A, and L is a distance from the edge of the opening of the inner frame to the edge of the display area. 
     Prior art liquid crystal display panels have small viewing angles, and consequently there is a problem that gray scale images viewed at off-normal angles differ from those by normal viewing. 
     To solve this problem, the present inventors have adopted the above-explained in-plane switching for the liquid crystal display panel, but found that, since the liquid crystal display panel of the in-plane switching type has wide viewing angles, the shading around the display area caused by the inner frame becomes pronounced when viewed at angles near θ indicated in FIGS. 9 and 10. 
     In FIGS. 9 and 10, consider a ray which leaves the display surface of the liquid crystal display panel  100  at an angle θ with respect to the normal to the display surface and which also passes the edge of the display area of the panel  100 , if the length L is chosen such that a ray path formed by tracing the ray backward is not intercepted by the inner frame, the shading is not observed when the liquid crystal display panel is viewed at angles within the angle θ with respect to the panel normal. 
     It is desirable that the angle θ is as large as possible and the distance L is as short as possible, but the distance L increases with increase in the angle θ. 
     By tracing backward a ray which leaves the display surface of the panel  100  at an angle θ with respect to the display surface normal and also passes the edge of the its display area, it is found that adjustment of the total distance (D+σ+d) of the thickness D of the inner frame, the spacing σ and the thickness d of the transparent substrate  1 A can reduce the distance L with the angle θ being fixed. 
     In this embodiment, the employment of the inner metal. frame  700  enables reduction of the thickness D of the inner frame  700  and consequently reduction of the distance L without degrading the mechanical strength of the module. 
     In the liquid crystal display device of this embodiment, reduction of the thickness of the inner frame  700  enables the liquid crystal display panel module  400  to be disposed closer to the backlight unit  300  as shown in FIG.  9  and also provides an advantage that brightness of the display is increased. 
     Embodiment 2 
     FIG. 11 is an exploded perspective view of another embodiment of the liquid crystal display device in accordance with the present invention, and corresponds to FIG.  1 . The same reference numerals as utilized in FIG. 1 designate corresponding portions in FIG.  11 . The structure of the liquid crystal display device in this embodiment may be substantially the same as that of FIG. 1, except that there is a plate  50  (hereinafter referred to as a sag-proof plate) for prevention of sagging of the liquid crystal display panel module  400  disposed behind it. The sag-proof plate  50  is a transparent plate made of acrylic resin, for example. 
     The sag-proof plate  50  is positioned within the opening  42  in the inner frame  700 , and is supported on the lower case  500  via light-collecting sheets  60 A,  60 B and the light diffusing plate  600 . 
     As shown in FIG. 12A, which is a cross-sectional view the liquid crystal display device taken along line XII A —XII A  in FIG. 11, the light diffusing plate  600  is disposed to bridges the opening in the lower case  500 , and the liquid crystal display panel module  400  is placed in contact with the light collecting sheet  60 A positioned on the light diffusing plate  600 . In this case, the sag-proof plate  50  is fitted within the opening  42  in the inner frame  700 , and consequently is prevented from moving horizontally within the opening by the inner frame  700  and from moving vertically by the light diffusing plate  600 , the light collecting sheets  60 A,  60 B and the liquid crystal display panel module  400 , resulting in elimination of the need of a particular device for positioning the sag-proof plate  50 . 
     With this structure of the liquid crystal display device, the rear surface of the liquid crystal display panel module  400  is pressed against the sag-proof plate  50  directly or with other optical components interposed therebetween such that the liquid crystal display panel exerts a force over at least the entire display area of the liquid crystal display panel toward the backlight unit  300 . The sag-proof plate  50  solves a problem that the liquid crystal display panel easily sags due to its own weight, for example, since the area of the liquid crystal display panel module  400  is increased drastically, although its thickness is not increased proportionately, as explained in the above embodiment. The sag-proof plate  50  is supported on the lower case  500  serving as part of an outer frame of the liquid crystal display device (in the embodiment, via the light collecting sheets  60 A,  60 B and the light diffusing plate  600 ), and it prevents sagging in the liquid crystal display panel module  400  sufficiently if it has some rigidity. 
     Incidentally, in the above embodiment, the sag-proof plate  50  contact the entire display area of the liquid crystal display panel module  400  directly or indirectly, but it is not necessary that the sag-proof plate  500  is in area contact with the liquid crystal display panel module. For example, the sag-proof plate  500  may contact the display area of the liquid crystal display panel module  400  at several points for prevention of sagging, because it is sufficient that the liquid crystal display panel exerts a force over the display area of the liquid crystal display panel module  400  toward the backlight unit  300 . 
     FIG. 12B is a cross-sectional view of a liquid crystal display device employing the sag-proof plate  50  of FIG. 12A with its ends modified. The sag-proof plate  50  is provided with steps at its ends such that its ends extend under the top portions of the inner frame  700  and onto the support portions of the lower case  500 . 
     With this structure, forces exerted on the ends of the sag-proof plate  50  are opposed by the support portions of the lower case such that deformation of the light diffusing plate  600  is prevented. 
     Embodiment 3 
     FIGS. 13A and 13B are schematic cross-sectional views for explaining modifications improved over the liquid crystal display device of Embodiment 2. 
     FIG. 13A illustrates a relationship between the sag-proof plate  50  and the transparent substrates  1 A,  1 B of the liquid crystal display panel module  400  in cross-section, and an electrically conductive transparent film  70  made of ITO (indium-tin-oxide), for example, is formed on at least an area of the sag-proof plate  50  facing the display area of the liquid crystal display panel module  400 . 
     As described above, the liquid crystal display panel module  400  in the above embodiments is of the so-called in-plane switching type in which each pixel controls the light transmission through the liquid crystal layer by controlling electric fields approximately parallel with the major surface of the transparent substrates. 
     In this case, the electric fields are very weak, and are easily influenced by external fields, and therefore the transparent conductive film  70  is intended to serve as antistatic means. Especially, the liquid crystal display panel module  400  is disposed to oppose the sag-proof plate  50  directly or indirectly as described above, electrification by friction easily influences the electric fields for driving the liquid crystal display panel, and therefore it is very effective to provide the above antistatic means on the surface of the sag-proof plate  50 . 
     For the same purpose, it is also effective to provide antistatic means such as this on the surface of the liquid crystal display panel module  400  on the side of the sag-proof plate  50 . 
     Incidentally, external static electricity to enter easily the surface of the liquid crystal display panel module  400  on the observer&#39;s side, that is, the surface of the liquid crystal display panel module  400  on the side opposite from the backlight unit  300 , and if antistatic means such as application of transparent conductive films is employed taken on this side, image display is not produced , and therefore antistatic means has been employed on this side. 
     FIG. 13B is a cross-sectional view of a structure in which the light collecting sheets  60 A,  60 B are disposed between the sag-proof plate  50  and the liquid crystal display panel module  400 . At least a portion of the light collecting sheets  60 A,  60 B facing the display area is provided with a transparent conductive film  70  made of ITO, for example, as antistatic processing. The surfaces of the light collecting sheets  60 A,  60 B are roughened. Interposition of the light collecting sheets  60 A,  60 B between the sag-proof plate  50  and the liquid crystal display panel module  400  prevents problems such as Newton&#39;s rings caused by contact of the sag-proof plate  50  and the liquid crystal display panel module  400 . 
     To prevent occurrence of optical non-uniformity due to a small separation between the sag-proof plate  50  and the liquid crystal display panel module  400  caused by the contact of the two opposing optically flat surfaces of the sag-proof plate  50  and the liquid crystal display panel module  400 , an antiglare treatment may be applied to roughen and make lusterless the surface of the polarizer attached to the liquid crystal display panel module  400  and in contact with the sag-proof plate  50 , or the surface of the sag-proof plate  50  may be roughened. 
     Embodiment 4 
     FIG. 14 is a schematic cross-sectional view for explaining a modification improved over the liquid crystal display device of Embodiment  2 . FIG. 14 illustrates the liquid crystal display panel module  400  and a sag-proof plate  50  made integral with the liquid crystal display panel module  400 . The sag-proof plate  50  is fixed on the display area of the liquid crystal display panel module  400 . 
     This structure reduces the number of components for the module of the liquid crystal display device and care need not be taken to position the sag-proof plate  50  with respect to the liquid crystal display panel module  400  in assembling of the module. For the similar purpose, the sag-proof plate  50  may be formed integral with the light diffusing plate  600 . For example, the surface of the sag-proof plate  50  on the side of the backlight unit  300  may be treated such that the surface has a function similar to the light diffusing plate  600 . 
     Embodiment 5 
     FIG. 15 is a cross-sectional view of another embodiment of the sag-proof plate  50  in accordance with present invention. a The sag-proof plate  50  is provided with metal films  80 , for example, on its side faces for the purpose of reflecting light. 
     As an alternative, white tapes or flexible spacers may be attached to the side faces of the sag-proof plate  50 . The flexible spacers may be fixed to the side faces of the sag-proof plate  50  with adhesive, or may be disposed on the light-collecting sheets  60 A,  60 B to be in contact with the side faces of the sag-proof plate  50 . 
     Light from the backlight unit  300  is projected toward the liquid crystal display panel module  400  via the relatively thick sag-proof plate  50 . If the above-described light-reflecting means is not disposed at the side faces, light directed toward the side faces of the sag-proof plate  50  does not reach the eyes of the observer and is wasted. In this embodiment, the above-mentioned disadvantage is eliminated and consequently the display area may be increased, although to some degree. 
     Incidentally, white tapes or spacers provided at the side faces absorb stresses between the sag-proof plate  50  and the inner frame  700  caused by thermal expansion of the sag-proof plate  50  or vibration of the liquid crystal display device during its transportation and consequently prevent deformation of the sag-proof plate  50 , and further, they prevent the sag-proof plate  50  being broken, and prevent occurrence of dust from the broken sag-proof plate  50 . The white tapes or spacers provided at the side faces prevent foreign particles from going into the gap between the sag-proof plate  50  and the inner frame  700  by filling the gap. 
     It is preferable that the flexible spacers are white for the purpose of reflecting light, but when the flexible spacers are used in combination with the metal film  80 , they need not be white. 
     Although the above-explained embodiments use the so-called in-plane switching, the present invention is not limited to this, but the present invention is also applicable to the liquid crystal display device of the so-called vertical field type, the twisted nematic type, for instance, because the main difference between the two types is in the structure of pixels, and the other structures are substantially similar. 
     Incidentally, in the liquid crystal display device of the vertical field type, the transmission of light at each pixel is controlled by a vertical electric field applied across a layer of liquid crystal material sandwiched between opposed transparent electrodes on the inner surfaces of the opposed transparent substrates. 
     It is apparent from the above explanation that the liquid crystal display device in accordance with present invention is capable of securing the mechanical strength of the lower case without increasing the so-called border areas irrespective of the increased size of the liquid crystal display device, and eliminating occurrence of shading at the peripheries of the display area of the liquid crystal display panel when it is viewed from the observer&#39;s side.