Abstract:
A backlight unit includes a lamp housing, a plurality of elongate lamps received therein, and a cooling member having a heat-absorbing part and a heat-radiating part. The heat-absorbing part includes a plurality elongate heat-absorbing portions having a light reflecting function and arranged alternately with the elongate lamps. The heat-radiating part extends from the elongate heat-absorbing portions of the heat-absorbing part and is disposed outside the lamp housing. The cooling member has a heat-radiation function as well as a luminescence assistance function.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a backlight unit of direct emission type in which light sources are arranged at the rear side of the light emitting surface of the backlight unit. The present invention also relates to a LCD device including such a backlight unit. 
     2. Description of the Related Art 
     Backlight units are known in the art as planar light-emitting devices. The backlight units are generally classified into two types. The first type is a direct emission unit, in which a plurality of light sources (i.e., lamps) are arranged at the rear side of the light-emitting surface of the backlight unit. The second type is an edge-light emission unit, in which the light emitted by the light sources is guided toward the light-emitting surface by an optical guide plate. The direct emission unit has a larger light-emitting surface and thus can attain a higher luminance, compared to the backlight unit of the edge-light emission type. Having these advantages, the direct emission unit is suitable for use in, particularly, a large-screen LCD device. 
       FIG. 15  is an exploded perspective view illustrating the structure of a conventional backlight unit of the direct emission type. As shown in the drawing, the backlight unit  201  includes therein a reflecting plate  211 , a plurality of lamps  212 , a lamp-supporting base  215 , a diffusion plate  216 , an optical sheet  217 , and a backlight chassis  218 . The lamps  212  are connected at one end to an inverter  213 , and at the other end to a return substrate  214   a . A ground potential is applied to the return substrate  214   a  through a return cable  214   b . Part of the light emitted from the lamps  212  is directly irradiated onto the diffusion plate  216 , and other part of the light is reflected by the reflecting plate  211  and then irradiated onto the diffusion plate  216 . The outer surface of the diffusion plate  216  generally defines the light-emitting surface of the backlight unit  201 . 
       FIG. 16  is an exploded perspective view depicting a LCD device  200  that includes the backlight unit  201 . In the LCD device, a liquid crystal panel  202  is connected to an X-direction drive circuit  204  and a Y-direction drive circuit  205  by respective TCPs (Tape Carrier Packages)  206 . The panel  202  is clamped and held between a shield front frame  203  and the backlight unit  201 . The liquid crystal panel  202  is mounted on the light-emitting surface of the backlight unit  201 , or more precisely, on the optical sheet  217  thereof. The panel  202  controls the transmission of the backlight emitted by the backlight unit  210 , to thereby display desired images. 
     In the backlight unit  201 , the lamps  212  are interposed between the reflecting plate  211  and the diffusion plate  216 . Therefore, the heat generated by the lamps  212  is hardly radiated outside. The heat generated by the lamps  212  heats the diffusion plate  216 . Mounted on the diffusion plate  216 , the liquid crystal panel  202  is heated by the heat it receives from outside, and also by the heat radiated from the diffusion plate  216 . Consequently, in the LCD device  200 , the liquid crystal panel  202  may be heated up to an undesired temperature, particularly in a higher-temperature ambient, and may fail to display images of desired quality. In other words, the LCD device  200  may have its rated operating temperature lowered by the backlight unit. 
     As in most cases, the lamps  212  exhibit a temperature characteristic. That is, the luminescence efficiency of the lamps  212  increases as the temperature rises, reaches a peak efficiency at a specific temperature, and decreases as the temperature falls from the specific temperature. Thus, in the backlight unit  201 , there is a problem in that the lamps  212  have a reduced luminescence efficiency when their operating temperature rises above the specific temperature, due to the heat they generate. The backlight unit  201  has another problem in that when the lamps  212  are used at high temperatures, the lamps are degraded due to heat and thus the lifetime of the lamps  212  is reduced. As described heretofore, the backlight unit  201  has problems due to the heat that the lamps  212  itself radiate. 
     Various techniques of radiating the heat generated by the lamps  212  of the backlight unit are described in, for example, Jpn. Pat. Appln. Publication Nos. 2002-196326, 2003-84280, and 8 (1996)-29785. The technique described in Pat. Publication No. 2002-196326 is to radiate the heat of the lamps  212  through the ventilation holes that are formed in the reflecting plate  211  disposed at the rear side of the lamps  212 . The technique described in Publication No. 2003-84280 is to radiate the heat of the lamps  212  from the reflecting plate  211 , via a heat-radiating body that contacts the lamps  212  with the reflecting plate  211 . The technique described in Publication No. 8-29785 is to provide a transparent substrate  220  with a stripe member  219  having a comparatively higher thermal conductivity, between the diffusion plate  216  and the lamps  212  as shown in  FIG. 17 , and to radiate the heat absorbed in the transparent substrate  220  through a heat-radiating member  221 . 
       FIG. 18  is a sectional view taken parallel to the Y-Z plane in  FIG. 16 , and illustrates the LCD device  200  being assembled. It is desired in the LCD device  200  that the width (width D, in  FIG. 18 ) of frame of the LCD device be reduced so that the LCD device  200  may become smaller without reducing the effective display area. To this end, the TCPs  206  are bent in the LCD device  200  to dispose the X-direction drive circuit  204  and the Y-direction drive circuit  205  on the rear side of the backlight unit  201  in the vicinity of the reflecting plate  211  of the backlight unit  201 . In this configuration, however, if the heat is radiated from the reflecting plate  211  as is the case described in Publications Nos. 2002-196326 and 2003-84280, the X-direction drive circuit  204  and the Y-direction drive circuit  205  will be heated, thereby reducing the reliability of the drive circuits  204  and  205 . If a transparent substrate having a stripe member is provided between the diffusion plate  216  and the lamps  212 , as is the case described in Publication No. 8-29785, the luminance at the light-emitting surface will be reduced. 
     Another type of the backlight unit is also known. This type is called double-surface backlight unit, which has another light-emitting surface at the rear side thereof, in addition to the light-emitting surface on the front side. A double-surface backlight unit is described in, for example, Jpn. Pat. Appln. Publication No. 2000-338483.  FIG. 19  is an exploded perspective view of the double-surface backlight unit  201   a  described therein. The double-surface backlight unit  201   a  includes a front part and a rear part, which are symmetrical with respect to the lamps  212 . Each of the front and rear parts has a lamp-supporting base  215 , a diffusion plate  216 , an optical sheet  217 , and a backlight chassis  218 . The double-surface backlight unit  201   a  has no component that corresponds to the reflecting plate  211  ( FIG. 15 ) that is opposed to the diffusion plate  216 . The lamps  212 , which are arranged in a row, emit light through both the light-emitting surfaces. 
     In the double-surface backlight unit  201   a , the heat generated from the lamps  212  involves a problem as in the case of the single-surface backlight unit  201 . Having no reflecting plate  211  at the rear side of the lamps  212 , the double-surface backlight unit  201   a  cannot adopt a structure that radiates heat from the rear side, differently from the backlight units described in Publications Nos. 2002-196326 and 2003-84280. If the double-surface backlight unit  201   a  has a transparent substrate  220  provided between the diffusion plate  216  and the lamps  212  as is the case described in Publication No. 8-29785, the luminescence efficiency at the light-emitting surfaces will decrease similarly to the single-side backlight unit  210  described above. 
     SUMMARY OF THE INVENTION 
     In view of the problems in the conventional backlight units as described above, it is an object of the present invention to provide a backlight unit that solves the problems in the conventional backlight units. More specifically, it is an object of the present invention to provide a backlight unit that can radiate the heat generated from the light source while preventing the luminescence efficiency at the light-emitting surface from being degraded, even though the heat is not radiated from the rear side of the backlight unit. 
     The present invention provides a backlight unit including: a lamp housing having a light transmittance plate at a front surface of the lamp housing: at least one lamp received in the lamp housing for emitting light through the light transmittance plate toward outside the lamp housing: a cooling member having a heat-absorbing part received in the lamp housing and a heat-radiating part extending from the heat-absorbing part to outside the lamp housing for radiating heat absorbed by the heat-absorbing part to outside the lamp housing. 
     In accordance with the backlight unit of the present invention, the cooling member radiates the heat generated by the lamp to improve the luminescence efficiency of the backlight unit without using the rear side of the backlight unit. 
     The above and other objects, features and advantages of the present invention will be more apparent from the following description, referring to the accompanying drawings 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an exploded perspective view showing a backlight unit according to a first embodiment of the present invention; 
         FIG. 2  is an exploded perspective view illustrating the structure of a LCD device that includes the backlight unit shown in  FIG. 1 ; 
         FIG. 3  is a top plan view of the backlight unit shown in  FIG. 1 , as observed in the Z-axis direction; 
         FIG. 4  is a top plan view depicting the structure of a backlight unit according to a second embodiment of the present invention; 
         FIG. 5  is a top plan view showing the structure of a backlight unit according to a third embodiment of the present invention; 
         FIG. 6  is a top plan view showing the structure of a backlight unit according to a fourth embodiment of the present invention; 
         FIG. 7  is a top plan view illustrating the structure of a backlight unit according to a fifth embodiment of the present invention; 
         FIG. 8  is a top plan view depicting the structure of a backlight unit according to a sixth embodiment of the present invention; 
         FIG. 9  is an exploded perspective view showing the structure of a backlight unit according to a seventh embodiment of the present invention; 
         FIG. 10  is a top plan view of a backlight unit having a heat sink that is exposed outside the housing; 
         FIG. 11  is a top plan view of a backlight unit whose housing has ventilation holes; 
         FIG. 12  is a top plan view of a backlight unit that radiates heat through the housing; 
         FIG. 13  is a top plan view of a backlight unit that has a heat-absorbing member filled with liquid; 
         FIG. 14  is a top plan view of a backlight unit the housing of which has ventilation holes and which has no return substrate; 
         FIG. 15  is an exploded perspective view illustrating the structure of a conventional backlight unit; 
         FIG. 16  is an exploded perspective view depicting a LCD device of ordinary type; 
         FIG. 17  is a sectional view showing the conventional backlight unit described in Jpn. Pat Appln. Laid-Open Publication No. 8-29785; 
         FIG. 18  is a sectional view showing the structure of the LCD device shown in  FIG. 16 ; and 
         FIG. 19  is an exploded perspective view of a conventional double-surface backlight unit. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention will be now described in detail, with reference to the accompanying drawing. 
       FIG. 1  is an exploded perspective view showing a double-surface backlight unit  101  according to a first embodiment of the present invention. The double-surface backlight unit  101  includes a front part and a rear part, which are symmetrical with respect to lamps  112 . Each of the front and rear parts has a lamp-supporting base  115 , a diffusion plate  116 , an optical sheet  117 , and a backlight chassis  118 . The double-surface backlight unit  101  differs from the conventional double-surface backlight unit  201   a  ( FIG. 19 ) in that it has a heat-radiation member  119 . The heat-radiation member  119  is used as a cooling device.  FIG. 2  is an exploded perspective view of a LCD device  100  that includes the double-surface backlight unit  101 . The LCD device  100  has a pair of liquid crystal panels  102  and a pair of front shields  103 , in addition to the double-surface backlight unit  101 . In the front and rear parts of the backlight unit  101 , the front shield  103  holds the liquid crystal panel  102 . 
     As shown  FIG. 1 , the lamps  112  are used as a light source of the backlight unit  101 . The lamps  112  are connected at one of the terminals to an inverter  113 , which applies an AC voltage of about 1000 V to 1600 V, generally known as lighting initiation voltage, to the lamps  112 . The lamps  112  are connected at the other terminal to the ground by a return substrate  114   a  and a return cable  114   b . The diffusion plates  116  define the light-emitting surfaces of the backlight unit  101 . The diffusion plates  116  allow the light emitted from the lamps  112  somewhat uniform in intensity and then output the light. The optical sheets  117  are configured by a diffusion sheet, a lens sheet, a reflecting polarized film, or the like. These members also allow the light provided from the light-emitting surfaces (i.e., diffusion plates  116 ) more uniform and output the light. 
       FIG. 3  is a top plan view of the backlight unit  101  shown if  FIG. 1 , as observed in the Z-axis direction. In  FIG. 3 , the diffusion plates  116 , optical sheets  117  and backlight chassises  118  are not illustrated. The heat-radiation member  119  is made of a material that has a superior thermal conductivity. The heat-radiation member  119  may be made of a metal such as copper. It may be a pipe (tubular member) that is filled with liquid for conducting heat. Alternatively, it may be a heat pipe that is a sealed vessel vacuum-filled with a small amount of liquid and having a capillary structure on the inner wall. The heat-radiation member  119  is supported by the lamp-supporting bases  115 , together with the lamps  112 . 
     The heat-radiation member  119  has two rod-shaped parts  119   a . The rod-shaped parts  119   a  extend substantially parallel to the lamps  112 . Each rod-shaped part  119   a  extends between two adjacent lamps  112 . An optical film is wrapped around each rod-shaped part  119   a . The surface of each rod-shaped part  119   a  can therefore scatter or reflect light. Instead, optical coating may be applied to each rod-shaped part  119   a . In either case, the shadow of the heat-radiation member  119  does not appear on either diffusion plate  116 . Each rod-shaped part  119   a  extends in the X-axis direction, with one end thereof protruding outside the lamp-supporting base  115 . The rod-shaped parts  119   a  are connected to each other by a coupling member  119   b , outside the lamp-supporting base  115 . 
     In the first embodiment, the rod-shaped parts  119   a  of the heat-radiation member  119  absorbs the heat generated from the lamps  112 . The heat is therefore conducted to the coupling member  119   b  provided outside the lamp housing that is defined by the lamp-supporting bases  115 , reflecting plates  111 , and the front and rear diffusion plates  116 , and radiated outside the lamp housing through the coupling member  119   b  which functions as a heat-radiating part. Accordingly, in the double-surface backlight unit  100 , of which the heat tends to be stuffed in the lamp housing, the temperature in the lamp housing can be prevented from rising. Thus, the temperature in the lamp housing can remain within a range over which the lamps  112  have a high luminescence efficiency. This helps to enhance the luminance at the light-emitting surfaces of the backlight unit  101 . This can also reduce the heat conducted to the liquid crystal panels  102  that are mounted on the diffusion plates  116 . The LCD device  100  that has the backlight unit  101  can therefore display high-quality images. 
     If the reflecting plate has ventilation holes for radiating heat as is described in Publication No. 2002-196326, dust or foreign matter may enter the lamp housing through the holes. The first embodiment can radiate the heat from the lamp housing, without the necessity of forming ventilation holes in the reflecting plate. Thus, neither the dust nor foreign matter enters the lamp housing. Hence, the backlight unit  101  does not degrade the images that the LCD device  100  displays. If a heat-radiating body that contacts the reflecting plate and lamps is provided as described in Publication No. 2003-84280, the temperature of the lamps may be excessively lower, if the ambient temperature is relatively low. In the first embodiment, the lamps  112  will not be excessively cooled even if the ambient temperature is low. This is because the heat-radiation member  119  is not disposed in contact with the lamps  112 . 
     As described before, a stripe member having a relatively higher thermal conductivity is provided between the diffusion plate and the lamps in the technique described in Publication No. 8-29785. The stripe member inevitably lowers the luminance at the light-emitting surface of the backlight unit. In the first embodiment, the lamp-supporting bases  115  support the lamps  112 , as well as the heat-radiation member  119  that extends parallel to the lamps  112  and alternately therewith. In addition, the rod-shaped parts  119   a  of the heat-radiation member  119  are surface-treated to reflect or scatter the light from the lamps  112 . Therefore, the luminance at the light-emitting surfaces of the backlight unit  101  does not decrease. Rather, rod-shaped parts  119   a  increase the light irradiated onto the diffusion plates  116 . This increases the luminance at both the light-emitting surfaces. Thus, the adverse effect that the heat-radiation member  119  imposes on the luminance uniformity at the light-emitting surfaces is small. 
       FIG. 4  shows a backlight unit  101   a  according to a second embodiment of the present invention, in a top plan view thereof as viewed in the Z-axis direction similarly to  FIG. 3  that illustrates the first embodiment. In the second embodiment, a heat sink  120  is secured to the coupling member  119   b  of the heat-radiation member  119  and located outside the lamp housing. The heat sink  120  is a heat-radiating member and made of a metal such as aluminum or copper. In the backlight unit  101   a , heat is radiated from the lamp housing, mainly through the heat sink  120 . Attached to the heat-radiation member  119 , the heat sink  120  can enhance the heat-radiating efficiency of the heat-radiation member  119 . 
       FIG. 5  is a top plan view showing a backlight unit  101   b  according to a third embodiment of the present invention, as viewed in the Z-axis direction. The third embodiment is similar to the second embodiment except that the heat-radiation member  119  is a ring shape or an endless member. The rod-shaped parts  119   a  of the heat-radiation member  119  are connected to one another at both ends thereof, by coupling members  119   b  that lie outside the lamp housing. The coupling members  119   b  are attached to heat sinks  120  at two ends of each rod-shaped part  119   a  in the X-axis direction. In the third embodiment, heat is radiated from the lamp housing through both ends of the heat-radiation member  119  as viewed in the X-axis direction. Hence, the heat-radiating efficiency is higher than in the second embodiment. 
       FIG. 6  is a top plan view showing a backlight unit  101   c  according to a fourth embodiment of the present invention, in a top plan view thereof as viewed in the Z-axis direction. The fourth embodiment is similar to the second embodiment except that the heat sink  120  is arranged at one end in the Y-axis direction. The heat-radiation member  119  has a bent part  119   c , in addition to rod-shaped parts  119   a  and a coupling member  119   b . The free end of the bent part  119   c  is coupled to a heat sink  120 . Thus, the heat-radiation member  119  is meandering in a plane, as viewed in the Z-axis direction. In the fourth embodiment thus configured, heat can be radiated from the lamp housing in the Y-axis direction. Since the bent part  119   c  of the heat-radiation member  119  extends in the Y-axis direction and is spaced apart from the coupling member  119   b  in the X-axis direction, the heat is dissipated more efficiently than in the case where the bent part  119   c  extends in the Y-axis direction on the same side as the coupling member  119   b.    
       FIG. 7  is a top plan view showing a backlight unit  101   d  according to a fifth embodiment of the present invention, in a top plan view thereof as viewed in the Z-axis direction. The fifth embodiment is similar to the third embodiment except for two points. First, the ring-shaped heat-radiation member  119  protrudes from the lamp housing. Second, the heat sink  120  is arranged at one end of the unit  101   d  in the Y-axis direction. The fifth embodiment achieves advantages similar to those of the fourth embodiment. 
       FIG. 8  is a top plan view showing a backlight unit  101   e  according to a sixth embodiment of the present invention, as viewed in the Z-axis direction. The sixth embodiment is similar to the fifth embodiment except that the heat-radiation member  119  serves as a return substrate  114   a  and a return cable  114   b  for a power source. In the sixth embodiment the heat-radiation member  119  is made of electrically conductive material. The lamps  112  are connected at one of the terminals to an inverter  113 , and at the other terminal to the ground through the heat-radiation member  119 . Therefore, neither a return substrate  114   a  nor a return cable  114   b  needs to be provided for the power source. In other words, the six embodiment is advantageous in that the number of components can be reduced. 
       FIG. 9  is an exploded perspective view showing a backlight unit  101   f  according to a seventh embodiment of the present invention. The seventh embodiment is similar to the first embodiment except that it is a one-side backlight unit that emits light from a single light-emitting surface. The structure shown in the Z-axis direction is similar to that of the first embodiment shown in  FIG. 3 . In the seventh embodiment, the heat from the lamps  112  is absorbed mainly in the heat-radiation member  119  and is then radiated from the lamp housing. Thus, in the backlight unit  101   f , the heat is radiated from the reflecting plate  111  opposing the diffusion plate  116  and optical sheet  117  in a smaller amount than in the case where the heat-radiation member  119  is not provided. 
     In most LCD devices having a one-side backlight unit, the X-direction drive circuit  204  and Y-direction drive circuit  205  are arranged at the rear side of the reflecting plate  111 , as is illustrated in  FIG. 18 . In the seventh embodiment i.e., backlight unit  101   f , the heat radiated from the reflecting plate  111  can be reduced. Thus, the LCD device that incorporates therein the backlight unit  101   f  does not involve a problem in that the X-direction drive circuit  204  and Y-direction drive circuit  205  are heated. In addition, since the heat radiated from the optical sheet  117  is also reduced, a malfunction involved with the heating of the liquid crystal panel  202  can be avoided. 
     The heat radiated from the lamp housing via the heat-radiation member  119  can be released into the atmosphere, as will be described hereinafter. In the case of the backlight unit that has the structure of  FIG. 5 , the heat sinks  120  provided at both the ends of the heat-radiation member  119  in the X-axis direction are exposed outside the backlight chassis  118  or housing  121  of the LCD device as is illustrated in  FIG. 10 . The housing  121  may be defined by a shield front  203  ( FIG. 16 ) of the LCD device. In this configuration, the heat sink  120  is exposed to the atmosphere, and the heat in the lamp housing can be radiated from the backlight unit  101   g.    
     Alternatively, the housing  121  of the LCD device may have ventilation holes  122  as shown in  FIG. 11 . Then, the air entering through the ventilation holes  122  can be applied to the heat sink  120 . If the backlight unit has the structure of  FIG. 7  and does not have the heat sink  120 , a contact surface  123  may be provided between the housing  121  of the LCD device and the heat-radiation member  119  as is illustrated in  FIG. 12 . Heat can then be radiated from the lamp housing into the atmosphere through the contacting surface  123 . In this case, it is desired that the housing  121  be made of a material having a higher thermal conductivity, such as aluminum, SUS, iron or copper. Having no heat sinks  120 , the backlight unit may be composed of fewer parts than otherwise. 
       FIG. 13  is a top plan view of a backlight unit  101   j , as observed in the Z-axis direction. The heat-radiation member  119  of the backlight unit  101   j  is of a ring shape and filled with liquid. As shown in  FIG. 13 , a device for circulating the liquid, such as a pump  124 , is provided on an appropriate portion of the heat-radiation member  119 . Thus, the liquid is forced to circulate in the heat-radiation member  119 . The heat in the lamp housing can therefore be conducted to the heat sink at a higher efficiency. 
     The embodiments described above may be combined in various ways. For instance, the one-side backlight unit, i.e., the seventh embodiment, may include a heat-radiation member  119  having a structure similar to those in the first to sixth embodiments. Further, the third embodiment and the sixth embodiment may be combined to provide a backlight unit shown in  FIG. 14 , in which air enters through ventilation holes  122  and is applied to the heat sinks  120 . It is to be noted that the backlight unit is not limited to such a backlight unit for use in a LCD device, and may be used as a lighting apparatus in an advertising board, for example.