CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit under 35 U.S.C. §119(a) of Korean Patent Application No. 10-2005-0084074, filed in the Korean Intellectual Property Office on Sep. 9, 2005, the entire disclosure of which is hereby incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a backlight unit and a display device having the same. More particularly, the present invention relates to a backlight unit and a display device having the same which is capable of providing superior color reproducibility and low power consumption by using a point light source and a linear light source at substantially the same time. 
   2. Description of the Related Art 
   Recently, a flat panel display apparatus, such as a liquid crystal display (LCD), a plasma display panel (PDP), and an organic light emitting diode (OLED), has been developed to substitute for a conventional display such as a cathode ray tube (CRT). 
   An LCD comprises an LCD panel having a thin film transistor (TFT) substrate and a color filter substrate, and a liquid crystal disposed therebetween. Since the LCD panel does not emit light by itself, the LCD further comprises a backlight unit in back of the TFT substrate as a light source for providing light. The transmittance of the light emitted from the backlight unit is adjusted according to an arrangement of the liquid crystal. The LCD panel and the backlight unit are then typically accommodated in a chassis. 
   Depending on the location of the light source, the backlight unit may be classified as an edge type or a direct type backlight unit. The edge type backlight unit is provided with the light source at a lateral side of a light guiding plate and is typically used for relatively small sized LCDs, such as those used in laptops and desktop computers. The edge type backlight unit provides high light uniformity and good endurance, and is suitable for use in thin profile LCDs. 
   As the size of the LCD panel in the market has been increased, the development of the direct type backlight unit has been emphasized. The direct type backlight unit provides light on the entire surface of the LCD panel by disposing a plurality of light sources behind the LCD panel. The direct type backlight unit provides a high level of brightness by using a plurality of light sources, as compared with the edge type backlight unit, but the brightness is generally not sufficiently uniform. 
   A point light source such as a light emitting diode (LED) or a linear light source such as a lamp, has been recognized as a suitable light source for the edge type backlight unit. The lamp has low power consumption, but has poor color reproducibility. Alternatively, the LED has good color reproducibility, long life spans, and fast instant lighting, but has high power consumption and greater heat generation. 
   Accordingly, a need exists for a display device system and method that is capable of providing superior color reproducibility and having low power consumption. 
   SUMMARY OF THE INVENTION 
   Accordingly, it is an aspect of embodiments of the present invention to substantially solve the above and other problems, and to provide a backlight unit having superior color reproducibility and low power consumption. 
   Accordingly, it is another aspect of embodiments of the present invention to provide a display device comprising a backlight unit having superior color reproducibility and low power consumption. 
   Additional aspects and/or advantages of embodiments of the present invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by the practice of the present invention. 
   The foregoing and/or other aspects of embodiments of the present invention can be achieved by providing a backlight unit comprising a first light guide plate, a linear light source disposed along at least one side edge of the first light guide plate, a plurality of second light guide plates facing the first light guide plate, and a point light source disposed along at least one side edge of each second light guide plate. 
   According to an aspect of embodiments of the present invention, the first light guide plate comprises a light exiting surface, a first light incident surface provided on the opposite side of the light exiting surface and a second incident surface facing the linear light source, and a plurality of second light guide plates facing the first incident surface. 
   According to another aspect of embodiments of the present invention, the plurality of light exiting surfaces of the second light guide plates are parallel to the first incident surface of the first light guide plate. 
   According to another aspect of embodiments of the present invention, a distance between each light exiting surface of the second light guide plates and the first light incident surface of the first light guide plate is uniform. 
   According to another aspect of embodiments of the present invention, the backlight unit further comprises a reflecting plate disposed under the second light guide plate. 
   According to another aspect of embodiments of the present invention, the first light guide plate comprises a light exiting surface, a light reflecting surface provided on the opposite side of the light exiting surface and a light incident surface facing the linear light source, and a plurality of second light guide plates facing the light exiting surface of the first light guide plate. 
   According to another aspect of embodiments of the present invention, the plurality of light exiting surfaces of the second light guide plates are parallel to the light exiting surface of the first light guide plate. 
   According to another aspect of embodiments of the present invention, a distance between each light exiting surface of the second light guide plates and the light exiting surface of the first light guide plate is uniform. 
   According to another aspect of embodiments of the present invention, the backlight unit further comprises a reflecting plate disposed under the first light guide plate. 
   According to another aspect of embodiments of the present invention, the first light guide plate is a flat plate shape. 
   According to another aspect of embodiments of the present invention, the first light guide plate is a wedge shape. 
   According to another aspect of embodiments of the present invention, the linear light source is a cold cathode fluorescent lamp (CCFL) or an external electrode fluorescent lamp (EEFL). 
   According to another aspect of embodiments of the present invention, the point light source is a light emitting diode. 
   The foregoing and/or another aspects of embodiments of the present invention can be achieved by providing a display device comprising a display panel, a first light guide plate disposed in back of the display panel, a linear light source disposed along at least one side edge of the first light guide plate, a plurality of second light guide plates disposed in back of the first light guide plate, and a point light source disposed along at least one side edge of each second light guide plate. 
   According to an aspect of embodiments of the present invention, a light exiting surface of the second light guide plate is parallel to a light exiting surface of the first light guide plate. 
   According to another aspect of embodiments of the present invention, a distance between each light exiting surface of the second light guide plates and the light exiting surface of the first light guide plate is uniform. 
   According to another aspect of embodiments of the present invention, the first light guide plate is a flat plate shape and the second light guide plate comprises a wedge shape. 
   According to another aspect of embodiments of the present invention, the point light source is a light emitting diode. 
   According to another aspect of embodiments of the present invention, the display panel is a liquid crystal display panel. 
   It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory, and are intended to provide further explanation of the invention as claimed. 


   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and/or other aspects and advantages of embodiments of the present invention will become apparent and more readily appreciated from the following description of the exemplary embodiments, taken in conjunction with the accompany drawings, in which: 
       FIG. 1  is a perspective view of an exemplary LCD according to a first embodiment of the present invention; 
       FIG. 2  is a sectional view of an exemplary LCD according to the first embodiment of the present invention; 
       FIG. 3  is a sectional view of an exemplary LCD according to a second embodiment of the present invention; 
       FIG. 4  is a sectional view of an exemplary LCD according to a third embodiment of the present invention; 
       FIG. 5  is a sectional view of an exemplary LCD according to a fourth embodiment of the present invention; and 
       FIG. 6  is a sectional view of an exemplary LCD according to a fifth embodiment of the present invention. 
   


   Throughout the drawings, like reference numerals will be understood to refer to like parts, components and structures. 
   DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
   Reference will now be made in detail to embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. The embodiments are described below so as to explain the present invention by referring to the figures. 
   An exemplary liquid crystal display according to a first embodiment of the present invention will now be described with reference to the  FIGS. 1 and 2 . 
     FIG. 1  is an exploded perspective view of a liquid crystal display according to a first embodiment of the present invention, and  FIG. 2  is a sectional view of the liquid crystal display according to the first embodiment of the present invention. 
   A liquid crystal display (LCD)  1  comprises an LCD panel  20  and a backlight unit  100  providing light for the LCD panel  20 . The backlight unit  100  comprises a light regulating part  30 , a first light guide plate  41 , a second light guide plate  45 , a lamp  51 , a light emitting diode (LED)  55 , and a reflecting plate  65 . 
   The LCD panel  20 , the light regulating part  30 , the first light guide plate  41 , the second light guide plate  45 , the lamp  51 , the LED  55 , and the reflecting plate  65  are accommodated between an upper chassis  10  and a lower chassis  70 . 
   The LCD panel  20  comprises a TFT substrate  21  on which TFTs are formed, a color filter substrate  22  facing the TFT substrate  21 , a sealant  23  adhering the two substrates  21  and  22  and forming a cell gap, and a liquid crystal layer  24  surrounded by the two substrates  21  and  22 , and the sealant  23 . The LCD panel  20  according to the first embodiment is provided as a rectangular shape having a long side and a short side, but is not limited thereto. The LCD panel  20  controls the arrangement of the liquid crystal layer  24 , thereby forming an image thereon. However, the LCD panel  20  is supplied with light from the backlight unit  100  disposed at its rear, as the LCD panel  20  does not emit light by itself. 
   A driving part  25  is disposed on a side of the TFT substrate  21  for applying driving signals to the LCD panel  20 . The driving part  25  comprises a flexible printed circuit (FPC)  26 , a driving chip  27  seated on the flexible printed circuit  26 , and a printed circuit board (PCB)  28  connected on a side of the FPC  26 . Here, the driving part  25  shown in  FIG. 1  is a chip on film (COF) type, but is not limited thereto. Other types of driving parts may be used, such as, tape carrier package (TCP) or chip on glass (COG) type. Alternatively, the driving part  25  may be formed on the TFT substrate  21 . 
   The light regulating part  30  disposed in back of the LCD panel  20  can comprise a diffusion sheet  31 , a prism film  32 , and a protection film  33 . 
   The diffusion sheet  31  comprises a base plate and a coating layer having beads formed on the base plate, but is not limited thereto. The diffusion sheet  31  diffuses light through the first light guide plate  41 , thereby improving the uniformity of brightness. 
   Triangular prisms are placed on the prism film  32  in a predetermined arrangement. The prism film  32  concentrates the light diffused from the diffusion sheet  31  in a direction perpendicular to a surface of the LCD panel  20 . Typically, two prism films  32  are used, and the micro prisms formed on the prism film  32  form a predetermined angle with each other. The light passing through the prism film  32  progresses vertically, thereby forming a uniform brightness distribution. If necessary, a reflective polarizing film (not shown) can also be used along with the prism film  32 , or only the reflective polarizing film can be used without the prism film  32 . 
   The protection film  33 , positioned at the top of the light regulating part  30 , protects the prism film  32 , which can be vulnerable to scratching. 
   The first light guide plate  41  is disposed beneath the diffusion sheet  31 . The first light guide plate  41  has a flat plate shape and comprises a light exiting surface  41   a  opposite to the diffusion sheet  31 , a first light incident surface  41   b  facing the light exiting surface  41   a , and a pair of second light incident surfaces  41   c  facing the lamps  51 . The incident light received through the first light incident surface  41   b  and the second light incident surfaces  41   c , exits at the light exiting surface  41   a  toward the diffusion sheet  31 . The light from a second light guide plate  45  disposed beneath the first light guide plate  41  enters the first light incident surface  41   b , and the light from the lamps  51  enter the second light incident surfaces  41   c . The first light guide plate  41  can be comprised of any number of suitable materials, such as polymethylmetacrylate (PMMA) of an acrylic resin type. 
   The lamps  51  are disposed along side edges of the first light guide plate  41 . Each lamp  51  can be comprised of a cold cathode fluorescent lamp (CCFL), an external electrode fluorescent lamp (EEFL), or the like. A lamp cover  61  covering each lamp  51  reflects the light from the lamp  51  toward the second light incident surfaces  41   c  of the first light guide plate  41 . The lamp cover  61  can be comprised of an aluminum plate or the like, and a material such as silver (Ag) having good reflectivity can be coated on a side thereof facing the lamp  51 . 
   The plurality of second light guide plates  45  are disposed beneath the first light guide plate  41 . The second light guide plates  45  having a wedge shape each comprises a light exiting surface  45   a  opposite to the first light incident surface  41   b , a light incident surface  45   b  facing the LED  55 , and a reflecting surface  45   c  facing the light exiting surface  45   a . The light exiting through the light exiting surface  45   a  enters the first light incident surface  41   b  of the first light guide plate  41 , and is then mixed with the light from the lamp  51  of the first light guide plate  41 . The mixed light is then transferred to the diffusion sheet  31  through the light exiting surface  41   a  of the first light guide plate  41 . 
   The second light guide plates  45  have a regular size and the plurality of light exiting surfaces  45   a  are disposed in parallel with each other. Each light exiting surface  45   a  is disposed at a uniform distance with respect to the first light guide plate  41 . The second light guide plate  45  can also be comprised of polymethylmetacrylate (PMMA) of an acrylic resin type. 
   Each LED  55  facing the light incident surface  45   b  of the second light guide plate  45  emits red, green and blue light and provides white light through color mixing. The LED  55  and the light incident surface  45   b  are spaced apart at a predetermined distance so as to perform the color mixing. The LED  55  is seated on an LED circuit board  56 . The core of the LED circuit board  56  can be comprised of aluminum (Al) or any suitable material having good heat transfer rates. 
   The reflecting plates  65  are provided beneath the reflecting surface  45   c  of the second light guide plates  45 . The reflecting plate  65  reflects the incident light entering the reflecting surface  45   c  of the light from the LED  55  to be directed toward the light exiting surface  45   a . The reflecting plates  65  can be made of any suitable materials such as polyethylene terephthalate (PET) or polycarbonate (PC), and/or can be coated with silver (Ag) or aluminum (Al). 
   The reflecting plate  65  is elongated along the back of the LED circuit board  56  and can further cover a portion of the light exiting surface  45   a  in yet other exemplary embodiments of the present invention. In addition to the reflecting plate  65 , an LED cover enclosing the LED circuit board  56  can be provided. 
   According to the above described first embodiment, the backlight unit  100  uses the lamp  51  having superior power consumption, and the LED  55  having superior color reproducibility, substantially at the same time. Accordingly, the power consumption can be decreased compared with a case wherein only the LED  55  is used in the backlight unit  100 , and the color reproducibility can be increased compared with a case wherein only the lamp  51  is used in the backlight unit  100 . 
   To acquire the equivalent brightness, the LED  55  has a power consumption of 150% as compared with the lamp  51 . In the first embodiment, the power consumption to acquire the equivalent brightness can decrease to a level between that of the lamp  51  and that of the LED  55 . If the lamp  51  is used, the color reproducibility is about 72% of the national television system committee (NTSC) standard, but the performance of the first embodiment of the present invention can be increased up to 10% compared with the NTSC standard. If the LED  55  is used, there can be a matter of heat generation. However, the matter of heat generation is decreased since the LEDs  55  are separated according to the first embodiment. 
   Alternatively, since the second light guide plate  45  in which the LED  55  provides the light is provided as a plurality, driving of the LED  55  can be modified and varied. For example, the brightness of the LED  55  can be adjusted according to a contrast of each portion of a screen. In addition, a color of the LED  55  can be adjusted according to a color of each portion of the screen. In another example, the LED  55  can be driven sequentially and repeatedly according to a scan of the screen. In such a driving method, the LED  55  is effectively driven by portions, since interference between adjacent second light guide plates  45  is minimal. 
   The backlight unit  100  according to embodiments of the present invention can have various modifications and variations as shown in  FIGS. 3 through 5 . 
   According to a second embodiment shown in  FIG. 3 , the second light guide plate  45  is disposed above the first light guide plate  41 . 
   The first light guide plate  41  having a flat plate shape, comprises the light exiting surface  41   a  facing the second light guide plate  45 , a reflecting surface  41   d  provided on the opposite side of the light exiting surface  41   a , and a pair of light incident surfaces  41   e  facing the lamps  51 . 
   The second light guide plate  45  having a wedge shape comprises the light exiting surface  45   a  exiting light therefrom, a first incident surface  45   d  facing the LED  55 , and a second incident surface  45   e  facing the first light guide plate  41 . 
   The light generated from the lamp  51  exits the light exiting surface  41   a  of the first light guide plate  41  and enters the second incident surface  45   e  of the second light guide plate  45  and is then mixed with light generated from the LED  55 . The mixed light exits the light exiting surface  45   e  of the second light guide plate  45 . 
   A reflecting plate  62  is provided beneath the light reflecting surface  41   d  of the first light guide plate  41 . 
   According to a third embodiment of the present invention as shown in  FIG. 4 , a first light guide plate  42  has a wedge shape and the lamp  51  is disposed on one side of the first light guide plate  42 . The second light guide plate  45  is disposed above the first light guide plate  42 . 
   According to a fourth embodiment of the present invention as shown in  FIG. 5 , the second light guide plate  45  is disposed on both sides of the first light guide plate  41 . The reflecting plate  65  is disposed beneath the lower second light guide plate  45  which is disposed beneath the first light guide plate  41 . 
   According to a fifth embodiment of the present invention as shown in  FIG. 6 , the second light guide plate  45  is flat plate shape and disposed below the first light guide plate  41 . The light emitting diode  55  is disposed below the second light guide plate  45 . 
   It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the invention. Thus it is intended that the present invention cover the modifications and variations of this invention within the scope of the appended claims and their equivalents.
A backlight unit and method of use are provided having a first light guide plate, a linear light source disposed along at least one side edge of the first light guide plate, a plurality of second light guide plates facing the first light guide plate, and a point light source disposed along at least one side edge of each second light guide plate. Accordingly, the present invention can provide a backlight unit using both a point light source and a linear light source at substantially the same time, and having superior color reproducibility and low power consumption.