Abstract:
A direct type backlight assembly includes at least one light lamp under a liquid crystal panel, having two substrates bonded with a liquid crystal layer therebetween. A light guide plate is positioned between adjacent light lamps. A reflective plate is under the light lamps and the light guide plate. At least one optical sheet covers an entire surface over the light guide plate. A case receives the above components. The backlight assembly prevents sagging of the optical sheets.

Description:
[0001]     This application claims the benefit of the Korean Application No. 10-2004-0021679 filed on Mar. 30, 2004, which is hereby incorporated by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to backlights of liquid crystal display devices and to liquid crystal display devices in general. More particularly, the present invention relates to a direct type backlight assembly and to a liquid crystal display device for preventing formation of an optical wrinkle caused by heat from a light lamp.  
         [0004]     2. Discussion of the Background Art  
         [0005]     Recently, research in the area of flat display devices has been focusing on Liquid Crystal Display Devices (LCDs), Field Emission Display Devices (FEDs), Electro-luminescence Display Devices (ELDs), and Plasma Display Panels (PDPs).  
         [0006]     In the field of flat display devices, applications pertaining to LCDs are expanding to notebook PC&#39;s, desktop monitors, and liquid crystal TVs, because of LCD features of great contrast ratio, suitability for gradation display and motion picture display, and low power consumption.  
         [0007]     Referring to  FIG. 1 , the LCD illustrated therein is provided with an upper substrate  101  having a color filter layer  112  formed thereon for displaying colors, a lower substrate  102  having switching devices for changing an orientation of liquid crystal molecules, and a liquid crystal layer  100  between the upper and lower substrates  101  and  102 .  
         [0008]     Besides the color filter layer  112 , the upper substrate  101  is also provided with a black matrix  111  for blocking a leakage of light, and a first common electrode  113  which is a first electrode for applying a voltage to the liquid crystal layer  100 , and besides the thin film transistor  121  (which is a switching device), the lower substrate  102  is also provided with pixel electrode  122  which is a second electrode for receiving a signal from the thin film transistor and applying a voltage to the liquid crystal layer  100 .  
         [0009]     There are spacers  115  between the upper and lower substrates  101  and  102  for maintaining a fixed gap between the two substrates, and a sealant  116  is provided at a periphery of the substrates for preventing leakage of the liquid crystal material outside of the substrates.  
         [0010]     However, since the LCD is a non-emissive type display device which emits no light from itself, a separate external light source is required for illumination. Particularly, in a case of a transmissive LCD, a separate illuminating device, i.e., a backlight assembly, is required in the rear of an LCD panel for emitting and guiding light without fail.  
         [0011]     There are a variety of backlight assemblies in use, and they range from a direct type to a light guide plate type. The light guide plate type is provided with a linear light source in tube form, such as a fluorescent lamp (e.g., a hot cathode, or a cold cathode type) at a side of the liquid crystal display panel, for projecting light from the light lamp to an entire surface of the liquid crystal display panel by using a light guide plate, and the direct type is provided with light lamps mounted under the liquid crystal display panel for diffusing light from the light lamps as the light is diffused by a diffuser sheet between the light lamps and the liquid crystal display panel.  
         [0012]     However, the light guide plate type is not competitive because of difficult sourcing of components, and the fabrication process has a problem of light leakage caused by close fitting of lamps and the lamp housing, and a drop in luminance caused by loss of light through the light guide plate.  
         [0013]     In order to solve these problems, a direct type of light assembly is suggested, in which the light lamps are mounted under the liquid crystal panel, for providing a higher luminance and a more uniform light distribution. A direct type light source is preferable for a display in which uniform luminance is required.  
         [0014]     In a case of the direct type, efforts for providing a high luminance backlight assembly has been an object of intense research, particularly directed to the development of a backlight assembly including a plurality of light lamps, or one curved lamp mounted under the display screen.  
         [0015]     A background art direct type backlight assembly will be described with reference to the attached drawings.  FIG. 2  illustrates a disassembled perspective view of a background art backlight assembly,  FIG. 3  illustrates a section across a line I-I′ in  FIG. 2 , and  FIG. 4  illustrates a section of a backlight assembly (for use in describing problems of the background art backlight assembly).  
         [0016]     Referring to  FIGS. 2 and 3 , the background art direct type backlight assembly is provided with a plurality of light lamps  51  for emitting light, a reflective plate  52  for converging the light from the light lamps  51  to a display part of the liquid crystal display panel, optical sheets having a diffuser sheet  55 , a prism sheet  56  and a protection sheet  57 , and a case  53  for fastening and supporting various components inclusive of the light lamps  51 .  
         [0017]     In addition to supporting the backlight assembly, the case  53  also supports the liquid crystal display panel above the backlight assembly. There is a bezel part comprised of stainless steel on an outside of the case  53  fitted to an edge of the liquid crystal panel to cover a periphery of the screen, excluding an effective area where a picture is displayed thereon.  
         [0018]     Optical sheets are provided for preventing the light lamps from showing on a display surface of the liquid crystal display panel, and providing a light source with a uniform luminance distribution, and includes the diffuser sheet  55 , the prism sheet  56 , and the protection sheet  57 .  
         [0019]     The diffuser sheet  55  is formed of a PET (Polyethylene Terephthalate) sheet of approximately 130 μm thick having diffusing particles coated on both sides, and the prism sheet  56  is formed of lenses of approximately 170 μm thick with an apex angle of approximately 90°.  
         [0020]     The light lamp is a Cold Cathode Fluorescent Lamp (CCFL) provided with a glass tube having a fluorescent material coated on an inside circumferential surface, and a discharge gas of a mixed inert gas or mercury Hg injected therein, and electrodes at opposite ends connected to power lines  59  and  59   a  for applying power thereto for driving the lamp, thereby turning it on with an external power source.  
         [0021]     The backlight assembly is also provided with a lamp guide  60  fastened to an inside of the case  53  for holding a light lamp  51  with a gap from the reflective plate  52 , and support sides  58  fastened to opposite ends of the light lamp  51  for holding and supporting the light lamp  51 .  
         [0022]     The lamp guide  60  holds the light lamp, and prevents the optical sheets from sagging, and the support sides  58  fixes the light lamp and seats the diffuser sheet  55  thereon.  
         [0023]     The backlight assembly provides the light from the light lamp  51  to an information display surface through the diffuser sheet  55 , the prism sheet  56 , the protection sheet  57 , etc. in succession.  
         [0024]     However, the background art direct type backlight assembly has problems.  
         [0025]     That is, in a case of a monitor or a TV LCD that requires a high luminance, many light lamps are required, which generate heat. As shown in  FIG. 4 , the optical sheets (particularly the diffuser sheet  55 ) can expand and sag downward, or form a wrinkle due to heat from the light lamps  51 . The wrinkle refracts a path of the light, which results in a dark or bright part on the screen thereby causing a brightness of the screen to be non-uniform.  
       SUMMARY OF THE INVENTION  
       [0026]     It is an object of the invention to provide a direct type backlight assembly and a liquid crystal display device for preventing formation of an optical wrinkle caused by heat from a light lamp.  
         [0027]     Accordingly, the present invention is directed to a direct type backlight assembly that substantially obviates one or more of the problems due to limitations and disadvantages of the background art.  
         [0028]     The present invention provides a direct type backlight assembly in which heat from light lamps is minimized, to prevent optical sheets from wrinkling.  
         [0029]     Further, the present invention provides a direct type backlight assembly in which not only a number of light lamps, but also a number of lamp guides are reduced, for reducing fabrication costs significantly.  
         [0030]     Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent to those having ordinary skill in the art upon examination of the following, or may be learned from practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof, as well as the appended drawings.  
         [0031]     To achieve these objects and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described herein, the direct type backlight assembly comprises at least two lamps under a liquid crystal display panel and at least one light guide plate between adjacent two lamps of the at least two lamps. Further, a liquid crystal display device is provided comprising a direct type backlight assembly with the above-mentioned features.  
         [0032]     Thus, the number of light lamps needed in the device is reduced. This is achieved by providing one or more light guide plates arranged between respective two adjacent light lamps. Thus, the light emitted from the lamps is used in a more efficient manner, since the light is partially guided by the light guide plate. According to the invention, the lamps emit light at least partially directly onto the liquid crystal display panel. In other words, the lamps are provided facing the liquid crystal display panel, so that a kind of direct type backlight assembly is provided.  
         [0033]     Preferably, the direct type backlight assembly may further comprise a reflective plate under the at least two lamps and the at least one light guide plate. By taking this measure, light may reflected from the reflective plate to the liquid crystal display panel. Thus, the light emitted from the lamps may be used in a further improved and more efficient manner.  
         [0034]     The at least one light guide plate preferably has a thickness smaller than a distance between the reflective plate and the optical sheets. When optical sheets are sagging due to heat from the lamps, the light guide plate provided in a small distance below the optical sheets may be supported by the light guide plate, thus deformation of the optical sheets is prevented and the display quality of the LCD is improved.  
         [0035]     It is to be understood that both the foregoing description and the following detailed description of the present invention are exemplary and explanatory and are intended to provide further explanation of the invention claimed. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0036]     The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this application, illustrate embodiments of the invention and together with the description serve to explain the principle of the invention.  
         [0037]     In the drawings;  
         [0038]      FIG. 1  illustrates a section of a background art liquid crystal display panel.  
         [0039]      FIG. 2  illustrates a disassembled perspective view of a background art backlight assembly.  
         [0040]      FIG. 3  illustrates a section of the backlight assembly across a line I-I′ in  FIG. 2 .  
         [0041]      FIG. 4  illustrates a section of a backlight assembly for describing problems of the background art backlight assembly.  
         [0042]      FIG. 5  illustrates a section of a liquid crystal display panel in accordance with a preferred embodiment of the present invention.  
         [0043]      FIG. 6  illustrates a section of the backlight assembly across a line II-II′ in  FIG. 5 .  
         [0044]      FIG. 7  illustrates a configuration of light lamps and light guide plates in accordance with a preferred embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0045]     Reference will now be made in detail to preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings.  
         [0046]      FIG. 5  illustrates a section of a liquid crystal display panel in accordance with a preferred embodiment of the present invention,  FIG. 6  illustrates a section of the backlight assembly across a line II-II′ in  FIG. 5 , and  FIG. 7  illustrates a configuration of light lamps and light guide plates in accordance with a preferred embodiment of the present invention.  
         [0047]     Referring to  FIGS. 5 and 6 , the direct type backlight assembly includes tubular light lamps  11 , such as fluorescent lamps, arranged at regular intervals for emitting light, a light guide plate  20  between adjacent light lamps  11  for directing light from the light lamp toward the liquid crystal panel and adjusting the quantity and direction of the light, a reflective plate  12  under the light guide plate  20  for reflecting light leaked from a bottom of the light guide plate toward the liquid crystal display panel uniformly, optical sheets of a diffuser sheet  15 , a prism sheet  16 , a protection sheet  17  on an entire surface over the light guide plate  20  (in succession), a case  13  for receiving and holding the above components, and support sides  18  fastened to the case  13  for fixing a plurality of the light lamps  11  and the light guide plate  20 .  
         [0048]     The light lamp  11  has a discharge gas, such as neon Ne, argon Ar, mercury Hg, or the like injected therein, a fluorescent material coated on an inside circumferential surface thereof, and power lines  19  and  19   a  at both ends for supplying power to turn on the light lamp. The power lines  19  and  19   a  pass through the support sides  19 , and are connected to an external driving circuit.  
         [0049]     The light lamp  11  is mounted at a height approximately 4˜6 mm from the reflective plate  12  without being in contact therewith for enabling light emission from an entire surface. The light lamp  11  is inserted in and held at holes of the support sides  18 .  
         [0050]     At least one light guide plate  13  is arranged along a length direction of the light lamps  11 , wherein at least one light guide plate  20  is arranged between adjacent light lamps  11 . The light lamps may be arranged alternately with one of the light lamps  11  or two of the light lamps  11 , depending on a size of a liquid crystal model or liquid crystal display device. For example, ends of the light lamps  11  may be partially overlapped to form staggered rows of light lamps, such as two or three rows, depending on the size of the LCD.  
         [0051]     For reference, the light guide plate  20  is formed of an acryl resin and the like, in a form of a thin film, and may have a milky white scattering pattern (not shown) printed on an underside surface thereof for light scattering.  
         [0052]     If the light guide plates  20  are arranged on a part where the light lamps  11  are normally arranged, a number of light lamps  11  can be reduced. Therefore, there is a reduction of heat from light lamps, thereby preventing sagging of the optical sheets caused by heat.  
         [0053]     The use of the light guide plate which (adjusts a light quantity) compensates for the drop in luminance caused by the reduced number of light lamps.  
         [0054]     Referring to  FIG. 7 , it is preferred that a width ‘B’ of the light guide plate  20  is smaller than a distance ‘A’ between adjacent light lamps  11 , and a thickness t of the light guide plate  20  is smaller than a distance ‘C’ between the reflective plate  12  and the optical sheets.  
         [0055]     The optical sheets include the diffuser sheet  15 , the prism sheet  16 , and the protection sheet  17 , wherein the diffuser sheet  15  diffuses the light from the light lamp  11  uniformly, the prism sheet  16  on the diffuser sheet  15  converges the light diffused at the diffuser sheet and directs it to the liquid crystal panel, and the protection sheet  17  is on the prism sheet  16  for protecting the optical sheets.  
         [0056]     Since the diffuser sheet  15  is seated and held under the guide of the support sides  18 , and is supported on the light guide plates  20 , sagging of the diffuser sheet  15  caused by heat is prevented. Also, the light guide plates  20  and the diffuser sheet  15  are spaced a distance apart for prevention of a Moiré phenomenon.  
         [0057]     The necessity of the lamp guide in the background art provided for supporting the light lamps  11  and the diffuser sheet is reduced because of the reduced number of light lamps and the arrangement of the light guides. Therefore, the lamp guides may be either reduced in number or totally dispensed with.  
         [0058]     The direct type backlight assembly of the present invention has the following advantages. First, the arrangement of the light guide plate or plates between the light lamps permits a reduction in the number of light lamps which reduces total heat generation by the light lamps. The heat reduction reduces the risk of forming wrinkles or a sagging of the optical sheets, which cause poor outer appearance and projection of the form of the light lamp on a screen of a display. Moreover, refraction of a light path caused by a wrinkle in the optical sheets is prevented. Therefore, screen luminance is uniform, and picture quality is good.  
         [0059]     Second, the arrangement of the light guide plate or plates in place of the light lamps permits a reduction in a number of light lamps along with the number of lamp guides for supporting the light lamps. The reduced number of parts leads to a reduction in process costs. Third, the supporting of the optical sheets with the light guide plate or plates permits dispensing with the lamp guide required for supporting the optical sheets in the background art. Thus, a direct type backlight assembly of high efficiency can be provided.  
         [0060]     It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention. Thus, it is intended that the present invention covers the modifications and variations of this invention which fall within the scope of the appended claims and their equivalents.