Abstract:
A backlight unit includes a plurality of lamps which generate a light; a case which houses the lamps; and a lamp holder that holds the lamps, the lamp holder being placed in the case, wherein a distance between first adjacent lamps is different from a distance between second adjacent lamps. Further preferred, a distance from a bottom surface of the case increases from an edge of the case to the center portion thereof.

Description:
This application claims the benefit of the Korean Patent Application No. 10-2004-49923 filed in Korea on Jun. 30, 2004, which is hereby incorporated by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a backlight unit, and more particularly to a backlight unit with a lamp holder. 
     2. Description of the Related Art 
     Generally, liquid crystal display devices (LCDs) are increasingly being used because they are light, thin and consume low power. For example, liquid crystal display devices are used in office automation equipment, audio/video equipment, and so on. In a liquid crystal display device, light transmission is controlled in accordance with video signals applied to a plurality of control switches arranged in a matrix, thereby displaying a desired picture on a screen. 
     The LCD is not a self-luminous display device. It requires a light source, such as a backlight. There are two types of backlight for the LCD device: a direct type backlight and an edge type backlight. The edge type backlight, which is used in medium to small size liquid crystal display panels, has a lamp installed at an outer area of a flat panel to irradiate light in such a way that the light generated from the lamp is incident on the entire surface of the liquid crystal display panel through a transparent light guide panel. The direct type backlight, which is used in medium to large size liquid crystal display panels, has a plurality of lamps arranged in a plane, and a diffusion plate installed between the lamp and a liquid crystal display panel in order to irradiate a light onto a large and middle sized liquid crystal display panel. 
       FIG. 1  is a perspective view of a liquid crystal display device with a direct type backlight according to the related art. Referring to  FIG. 1 , the direct type liquid crystal display device includes a liquid crystal display panel  2  and a backlight unit which irradiates light onto the liquid crystal display panel  2 . The liquid crystal display panel  2  has liquid crystal cells forming an active matrix between upper and lower glass substrates, and a thin film transistor installed for switching a video signal to each of the liquid crystal cells. A refractive index of each liquid crystal cell is changed in accordance with the video signal, thereby displaying a corresponding picture. A tape carrier package TCP (not shown) is stuck onto a lower substrate of the liquid crystal display panel  2 . A driver integrated circuit IC is mounted on the TCP to apply a drive signal to the thin film transistor. Further, polarizing sheets  8  and  18  are each installed in front and rear surfaces of the liquid crystal display panel  2 . The polarizing sheets  8  and  18  improve the viewing angle of a displayed picture. 
     The backlight unit includes a plurality of lamps  36 . The lamps  36  receive power from an external power source, irradiate light onto the liquid crystal display panel  2 , and are independently driven. A case  34  is provided to hold the lamps  36 . A reflection plate  14  is installed between the lamps  36  and the case  34 . The reflection plate  14  reflects light generated by the lamps  36 , thereby preventing light leakage. A diffusion panel  12  is provided to diffuse light generated from the lamp  36  or reflected by the reflection plate  14  toward the liquid crystal display panel  2 . A plurality of optical sheets  10  distributes the light diffused from the diffusion panel  12  onto the liquid crystal display panel  2 , thereby improving the light efficiency. 
     Each of the lamps  36  includes a glass tube, an inert gas inside the glass tube, a cathode and an anode which are installed at both ends of the glass tube. The inert gas is charged in the inside of the glass tube, and a phosphorus is spread over the inner wall of the glass tube. In each of the lamps  36 , an inverter applies an AC voltage to a high voltage electrode and a low voltage electrode. Electrons radiate from the low voltage electrode and collide with the inert gas inside the glass tube. The numbers of electrons increase according to a geometric progression. Then, the excess electrons cause an electric current to flow inside the glass tube, and excite the inert gas, which emits an ultraviolet ray. The emitted ultraviolet ray collides with luminous phosphorus spread over the inner wall of the glass tube, thereby radiating a visible ray. 
     A fixed gap is maintained between the lamps  36  in the holding case  34 . The reflection plate  14  is arranged between an upper surface of the case  34  and the lamps  36 , reflects the light generated from the lamps  36 , and irradiates the generated light toward the liquid crystal display panel  2 , thereby improving light efficiency. A diffusion pattern formed on the diffusion panel  12  diffuses the light generated from the lamps  36  or reflected by the reflection plate  14 . The diffusion pattern causes the incident light to propagate toward the liquid crystal display panel  2  with a wide angle. 
     The optical sheets  10  increase the brightness of the diffused light from the diffusion panel  12 , thereby improving the brightness of the liquid crystal display device. Further, light slantingly incident from the surface of the diffusion panel  12  and the reflection sheet  14  is vertically oriented by the optical sheets  10  toward the liquid crystal display panel  2 . Thus, the optical sheets  10  act set the propagation direction of the light exiting from the surface of the optical sheets  10 . 
     In the related art liquid crystal display device, a fixed distance is provided between the lamps  36 . Moreover, the lamps  36  are installed at a fixed distance from the surface of the case  34 . Accordingly, the related art liquid crystal device provides an uniform display brightness, thereby reducing visual fatigue as compared to a cathode ray tube CRT. On the other hand, experiments show that, for a user watching the screen, the focus of attention is the central portion of the screen. For example, with the amount of information transmitted to the user in mid to large size TV screen, the user focuses his attention on the central portion of the screen rather than on the edge. Thus, it is beneficial to emphasize brightness in the central portion of the screen. However, the arrangement of the lamps  36  in the related direct type backlight does not provide brightness enhancement in the central portion of the screen. The related art direct type backlight irradiates light with the same brightness toward the edges of the screen as in the center of the display. Thus, light from the related art direct type backlight is unnecessarily bright at the edges. Accordingly, power is wasted unnecessarily. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed to a backlight unit that substantially obviates one or more of the problems due to limitations and disadvantages of the related art. 
     An object of the present invention to provide a backlight unit that is brighter at a central portion of a liquid crystal display. 
     Additional features and advantages of the invention will be set forth in part in the description which follows, and in part will be apparent from the description, or may be learned by 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. 
     To achieve these objects and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described herein, a backlight unit includes a plurality of lamps which generate a light; a case that houses the lamps; and a lamp holder that holds the lamps, the lamp holder being placed in the case, wherein a distance between first adjacent lamps is different from a distance between second adjacent lamps. 
     In another aspect, a backlight unit includes a plurality of lamps which generate a light; a case that houses the lamps; a lamp holder that holds the lamps, the lamp holder being placed in the case, wherein a distance of each of the lamps from a bottom surface of the case increases from an edge of the case to a center portion thereof. 
     In another aspect, a liquid crystal display device includes a liquid crystal display panel; and a backlight unit that irradiates a light onto the liquid crystal display panel. The backlight unit includes a plurality of lamps that generate the irradiated light; a case that houses the lamps; a plurality of lamp holders at opposite ends of the case, the lamp holders holding the lamps, wherein a distance between first adjacent lamps is different from a distance between second adjacent lamps, and a distance of each of the lamps from a bottom surface of the case increases from an edge of the case to a center portion thereof. 
     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 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. 
         FIG. 1  is a perspective view representing a related art liquid crystal display device. 
         FIG. 2  is a perspective view of an exemplary liquid crystal display device according to an embodiment of the present invention. 
         FIG. 3  is a cross-sectional view of the exemplary liquid crystal display device along line III-III′ of  FIG. 2 . 
         FIG. 4  shows an exemplary lamp holder for the liquid crystal display device of  FIG. 2 . 
         FIGS. 5A to 5C  show exemplary shapes for the lamp holder according to embodiments of the present invention. 
         FIG. 6  is a diagram illustrating an exemplary arrangement of the lamps on the lamp holder according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. 
       FIG. 2  is a perspective view of an exemplary liquid crystal display device according to an embodiment of the present invention.  FIG. 3  is a cross-sectional view of the exemplary liquid crystal display device along line III-III′ of  FIG. 2 . Referring to  FIGS. 2 and 3 , a liquid crystal display device includes a liquid crystal display panel  102 , and a backlight unit that irradiates light onto the liquid crystal display panel  102 . 
     The liquid crystal display panel  102  has liquid crystal cells forming an active matrix between an upper substrate and a lower glass substrate, and a thin film transistor that switches a video signal applied to each of the liquid crystal cells. A refractive index of each liquid crystal cell is changed in accordance with the applied video signal to display a picture corresponding to the video signal. A tape carrier package TCP (not shown), is stuck onto a lower substrate of the liquid crystal display panel  102 . A driver integrated circuit IC (not shown) is mounted on the TCP. The IC applies a drive signal to the thin film transistor. Polarizing sheets  108  and  118  are installed at the front and rear surfaces of the liquid crystal display panel  102 , respectively. Herein, the polarizing sheets  108  and  118  improve the viewing angle of the liquid crystal display. 
     The backlight unit includes a plurality of lamps  136  powered by an external power source. The lamps  136  irradiate light onto the liquid crystal display panel  102 , and are independently driven. A lamp holder  150  is provided to hold the lamps  136 . The lamps  136  and the lamp holder  150  are placed in a case  134 . A reflection plate  114  is installed between the lamp  136  and the case  134  to prevent light generated by the lamps  136  from leaking from under the case  134 . A diffusion panel  112  diffuses light generated from the lamp  136 , or reflected by the reflection plate  114 , toward the liquid crystal display panel  102 . A plurality of optical sheets  110  irradiates the diffused light from the diffusion panel to the liquid crystal display panel  102  and improves an output light efficiency of the diffusion panel  112 . 
     Each of the lamps  136  includes a glass tube, a cathode and an anode. The cathode and the anode are provided at both ends of the glass tube, respectively. Phosphorus is spread over the inner wall of the glass tube. The glass tube is filled with an inert gas. An inverter is provided to apply an AC voltage to each of the lamps. Electrons radiated from the low voltage electrode collide with the inert gas inside the glass tube. The collisions caused the number of electrons to increase according to a geometric progression. Then, an electric current flows inside the glass tube due to the increase in the number of electrons. The electrons excite the inert gas. The excited inert gas emits an ultraviolet ray. The emitted ultraviolet ray collides with luminous phosphorus spread over the inner wall of the glass tube, thereby radiating visible rays. In embodiments of the present invention, the lamps  136  can have various shapes. For example, the lamps  136  can have a “U” shape or other shapes. 
     The lamp holder  150  supports and holds the lamps  136 . The lamp holder  150  is placed in and fixed to the case  134 . The case  134  houses the lamps  136  and the lamp holder  150 . The reflection plate  114  is arranged between a bottom surface of the case  134  and the lamps  136 . The reflection plate  114  reflects light generated from the lamps  136 , and irradiates the reflected light toward the liquid crystal display panel  102 , thereby improving light efficiency. The diffusion panel  112  is provided with a light pattern to disperse light generated from the lamps  136  or reflected by the reflection plate  114 . The diffused light from the diffusion panel  112  progresses toward the liquid crystal display panel  102  with a wide angle. 
     The optical sheets  110  increase the brightness of the diffused light from the diffusion panel  112 , thereby improving the brightness of the liquid crystal display device. Further, the optical sheets  110  transmits slantingly incident light, which is diffused from the diffusion panel  112  or reflected from the reflection sheet  114 , vertically toward the liquid crystal display panel  102 . In other words, the optical sheets  110  form a propagation direction for light exiting from the surface of the optical sheets  110 . 
       FIG. 4  shows an exemplary lamp holder for the liquid crystal display device of  FIG. 2 . Referring to  FIG. 4 , the lamp holder  150  includes a plurality of clipping parts  152  and supporting parts  154 , and a base  156 . The clipping parts  152  hold the ends of the lamps  136 . Each of the supporting parts  154  is attached at one side of a corresponding one of the clipping part  152  and supports the corresponding one of the clipping parts  152 . The base  156  fixes the plurality of the supporting parts  154 . 
     Each of the clipping part  152  is shaped as a ring. Moreover, each of the clipping part  152  has an opening at one side. The clipping part  152  is made of an insulating material, for example, when the lamp  136  is a cold cathode fluorescent lamp CCFL. The clipping part  152  can be made of a conductive material to supply power to the lamp  136 , for example, when the lamp  136  is of an external electrode type. 
     The base  156  is bent upward. The lamps  136  are concentrated at a central portion of the base  156 . Each of the clipping part  152  and the corresponding supporting part  154  are disposed so that the distance between lamps  136  positioned at an edge is different from the distance between centrally positioned lamps. 
       FIGS. 5A to 5C  show exemplary shapes for the lamp holder according to embodiments of the present invention. As shown in  FIGS. 5A to 5C , the lamp holder can have various shapes. For example, as shown in  FIG. 5A , the lamp holder  250  may have H-shape. That is, the base  256  comprises of a lower base  256   a  and an upper base  256   b . A pair of supporting parts  252  is linked between the lower base  256   a  and the upper base  256   b . Further, each of the pair of supporting parts  252  has a half circled part at the middle portion so that a clipping part is formed by combining the faced two half circled middle parts of the pair of supporting parts  252 . In another example, as shown in  FIG. 5B , the lamp holder  350  may include a supporting part  352  in a plate shape extended from a base  356 . A clipping hole  354  is formed at the center of the supporting part  352  for holding the lamp. Each of the lamps  136  can be inserted into a clipping hole  354  of the supporting part  352 . In a further example, as shown in  FIG. 5C , the lamp holder  450  may include a base  456  which has a rectangular block shape and is provided with a plurality of clipping holes  454 . In this example, as the base and the supporting part are integrated in one body, the base  456  also plays a role of supporting part. The clipping holes  454  are provided at a front surface of the base  456 . The two bases are faced each other with the front surface having the clipping holes. Each of the lamps  136  can be inserted into one of the clipping holes  454  provided on the base  456 . 
       FIG. 6  is a diagram illustrating an exemplary arrangement of the lamps on the lamp holder according to an embodiment of the present invention. Referring to  FIGS. 3 and 6 , lamp holders  150  are fixed at both edges of the case  134  (see  FIG. 3 ). The lamps  136  are forcefully inserted into the clipping part  152  of the lamp holder  150 . In this embodiment of the present invention, the clipping part  152  are similar in size to or slightly bigger than the diameter of the lamps  136 , to prevent the lamps  136  from moving freely. The distance between adjacent clipping parts  152  is shorter at the central portion of the base  156  than at the edge of the base  156 . Accordingly, the distance between the lamps  136  inserted into the clipping part  152  increases toward the edges of the base  156  and is shorter toward the central portion of the base  156 . Thus, the lamps  136  have a higher concentration in the central portion of the lamp holder. More lamps  136  are concentrated on the central portion of the case  134 . Thus, the central portion of the display device has a higher brightness. In contrast, fewer lamps  136  are arranged at the edges of the case  134 . Thus, the edges of the display device are dimmer than in the central portion of the display device. 
     The lamps  136  are installed at a specific height from the case  134 . In this embodiment of the present invention, the heights of the supporting part  154  are different from each other. Thus, the lamps  136  are positioned at different distances from the case  134 . Specifically, the supporting part  154  located at the edge are shorter than the supporting part  154  positioned in the central portion of the display device. Accordingly, the light irradiated from the lamps  136  in the central portion is brighter than the light irradiated from the lamp  136  located at the edges, with respect to the rear surface of the diffusion panel  112 . 
     In embodiments of the present invention, as described above, the distance between adjacent lamps is shorter at a central portion of the display than at edges of the display, thereby providing higher brightness in the central portion. Accordingly, the perceived brightness is higher in the central portion, thereby providing an improved picture quality for the same power. Further, the light amount concentrated on the central portion is controlled and the power consumption at the edge part is reduced, thereby decreasing the overall power consumption. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the liquid crystal display panel having built-in driving circuit of 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 provided they come within the scope of the appended claims and their equivalents.