Abstract:
The present invention provides a backlight device that has a heat-generating light source, such as a fluorescent tube. This backlight device includes; fluorescent tubes that emit light when power is supplied to the fluorescent tube electrodes; and a light guide plate that guides the light emitted from the fluorescent tubes to a liquid crystal panel. In this backlight device, heat release members for releasing the heat from the fluorescent tube electrodes are provided at the corners of the light guide plate that face the fluorescent tube electrodes of the fluorescent tubes. With these heat release members, the light guide plate is prevented from melting.

Description:
BACKGROUND OF THE INVENTION 
     The present invention generally relates to backlight devices, and, more particularly, to a backlight device that has a heat-generating light source such as a fluorescent tube. 
     Conventional display devices normally include CRTs (cathode-ray tubes), but, these CRT displays are being rapidly replaced by flat panel displays that employ liquid crystal panels. These liquid crystal displays have higher display quality today, and therefore are expected to become larger in size and have a higher luminance for use in television sets. Accordingly, backlight devices for illuminating the liquid crystal panels are also expected to have higher luminance. 
     Conventional liquid crystal displays are used mainly for notebook type personal computers, land the screen size is normally limited to 13 inches. The resolution is XGA at the highest, and the display luminance is only as high as 150 cd/m 2 . FIGS. 1A,  1 B, and  2  illustrate a liquid crystal display  1 A of this type. 
     The liquid crystal display  1 A includes a liquid crystal panel  2 A, a housing  5 , and a backlight device  10 A. The housing  5  holds the liquid crystal panel  2 A and the backlight device  10 A with a resin frame  6  and a backboard  9 . The backlight device  10 A illuminates the liquid crystal panel  2 A from the back, and gives a predetermined luminance to the display screen of the liquid crystal panel  2 A. 
     The backlight device  10 A includes a fluorescent tube  3  that serves as a light source, a rubber holder  12  (shown in FIG. 5) that holds the fluorescent tube  3 , and a light guide plate  4  that guides the light from the fluorescent tube  3  to the liquid crystal panel  2 A. The fluorescent tube  3  contains Ar gas or Ne gas in which mercury is sealed, and the tube wall of the fluorescent tube  3  is coated with a fluorescent material. The mercury gas radiates ultraviolet rays during electric discharge, and the ultraviolet rays then strike the fluorescent material to generate visible rays. 
     The light guide plate  4  is an acrylic resin plate that cooperates with a provided optical sheet  8  to illuminate the entire area of the liquid crystal panel  2 A with the light guided from the fluorescent tube  3 . In the liquid crystal display  1 A that has a small screen size (approximately 13 inches, as shown in FIG. 1A) and is not required to have a high resolution and a high screen luminance, the backlight device  10 A is provided only by one side of the light guide plate  4 , and only the single fluorescent tube  3  is employed. 
     On the other hand, a liquid crystal display  1 B for monitoring, which is shown in FIGS. 3A through 5C, normally has a display size of 15 inches, and is required to have a SXGA resolution and a screen luminance of approximately 250 cd/m 2 . For this reason, two backlight devices  10 B are incorporated into the liquid crystal display  1 B, with the light guide plate  4  being interposed in between. Further, two fluorescent tubes  3  are provided for each of the backlight devices  10 B. As DVD drives for personal computers have been widely spread, however, liquid crystal displays for monitoring are expected to have a larger and brighter screen, so that users can enjoy movies on the screen of the liquid crystal display. 
     The problem with the liquid crystal display  1 B is that the fluorescent tubes  3  provided for each of the backlight devices  10 B generate heat as well as light. A temperature rise in the vicinity of the fluorescent tube electrode  11  of each of the fluorescent tubes  3  is particularly large. When the supply current is increased to obtain a higher luminance, the temperature becomes as high as 120° C. or even higher. The fluorescent tube electrodes  11  are located at both ends of each of the fluorescent tubes  3 . To accommodate each fluorescent tube electrode  11 , a heat-conductive rubber holder  12  is provided at both ends of each of the fluorescent tubes  3 . Each rubber holder  12  is engaged with the corresponding holder  7 , so that both ends of each of the fluorescent tubes  3 , where the temperature rises by the greatest degree, can be cooled down. 
     The rubber holders  12  are situated near the light guide plate  4 , and directly face the light guide plate  4 . Because of this arrangement, the heat generated by each fluorescent tube electrode  11  is transferred to the light guide plate  4  via the corresponding rubber holder  12 . 
     Due to the heat generated by the fluorescent tube electrodes  11 , there is always a risk of melting the part (indicated by the arrow B in FIG. 5C) of the resin light guide plate  4 , which faces the fluorescent tube electrodes  11  in each of the conventional backlight devices  10 B. When melted in this manner, the light guide plate  4  is deformed and deteriorates. A deformed light guide plate  4  cannot properly guide the light from the fluorescent tubes  3  to the liquid crystal panel  2 B, resulting in decreases in the luminance and resolution of the display screen. If the liquid crystal display  1 B becomes larger in size and generates a greater amount of heat from the fluorescent tube electrodes  11 , this problem will be aggravated even further. 
     SUMMARY OF THE INVENTION 
     A general object of the present invention is to provide backlight devices in which the above disadvantages are eliminated. 
     A more specific object of the present invention is to provide a backlight device that can prevent the light guide plate from melting, even when the temperature rises due to the heat generated by the light source. 
     The above objects of the present invention are achieved by a backlight device that includes: a light source that emits light when power is supplied to electrodes thereof; a light guide plate for guiding the light emitted from the light source to a liquid crystal panel; and heat release members for releasing heat generated by the electrodes, the heat release members being located at least either at the electrodes of the light source or at the corners of the light guide plate facing the electrodes. 
     In this backlight device, the heat release members are located at the positions between the light guide plate and the electrodes, where the heat generated by the light source is most likely to build up. To release the heat, the heat release members are placed by the side of each electrode of the light source, or at the corners of the light guide plate facing the electrodes. Alternatively, the heat release members may be placed both by the side of each electrode and at the corners of the light guide plate. 
     In this structure, the heat generated by the light source can be released through the heat release members placed between the light guide plate and the electrodes. As a result, the light guide plate can be prevented from melting due to heat generation, even when the light source has a higher luminance and a larger amount of heat is generated by the light source. Thus, the deformation and deterioration of the light guide plate can be avoided. 
     The above objects of the present invention are also achieved by a liquid crystal display that includes: a liquid crystal panel; the backlight device of the present invention; and a light guide plate that is provided at the light-entering surface side of the backlight device, and guides the light emitted from the backlight device to the liquid crystal panel. 
     The above and other objects and features of the present invention will become more apparent from the following description taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A and 1B illustrate a liquid crystal display having a small-sized screen and a conventional backlight device; 
     FIG. 2 is an enlarged view of the part indicated by the arrow A 1  in FIG. 1B; 
     FIGS. 3A and 3B illustrate a liquid crystal display having a large-sized screen and conventional backlight devices; 
     FIG. 4 is an enlarged view of the part indicated by the arrow A 2  in FIG. 3B; 
     FIGS. 5A through 5C illustrate the problem with the conventional backlight devices; 
     FIG. 6A is a plan view of a backlight device that is a first embodiment of the present invention; 
     FIG. 6B is an enlarged view of the part indicated by the arrow A 4  in FIG. 6A; 
     FIG. 6C is an exploded perspective view of the part indicated by the arrow A 4  in FIG. 6A; 
     FIG. 7A is a plan view of a backlight device that is a second embodiment of the present invention; 
     FIG. 7B is an enlarged view of the part indicated by the arrow A 5  in FIG. 7A; 
     FIG. 7C is an exploded perspective view of the part indicated by the arrow A 5  in FIG. 7A; 
     FIG. 8A is a plan view of a backlight device that is a third embodiment of the present invention; 
     FIG. 8B is an enlarged view of the part indicated by the arrow A 6  in FIG. 8A; 
     FIG. 8C is an exploded perspective view of the part indicated by the arrow A 6  in FIG. 8A; 
     FIG. 9A is a plan view of a backlight device that is a fourth embodiment of the present invention; 
     FIG. 9B is an enlarged view of the part indicated by the arrow A 7  in FIG. 9A; and 
     FIG. 9C is an exploded perspective view of the part indicated by the arrow A 7  in FIG.  9 A. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following is a description of embodiments of the present invention, with reference to the accompanying drawings. 
     FIGS. 6A through 6C illustrate a backlight device  20 A that is the first embodiment of the present invention. This backlight device  20 A is incorporated into a liquid crystal display device. It should be noted that the backlight device  20 A of this embodiment is provided in the same arrangement as shown in FIGS. 3A through 4. In FIGS. 6A through 6B, the same components as in FIGS. 3A through 4 are denoted by the same reference numerals as in FIGS. 3A through 4. 
     The backlight device  20 A includes fluorescent tubes  23 , a light guide plate  24 , holders  27 , rubber holders  32 , and heat release members  40 A. 
     The fluorescent tubes  23  are cold cathode tubes, and the light guide plate  24  is interposed between the two fluorescent tubes  23 , as shown in FIG.  6 A. In each of the fluorescent tubes  23 , Ar gas and Ne gas, as well as mercury, are sealed in a glass tube, and the tube wall is coated with a fluorescent material. 
     A fluorescent tube electrode  31  is internally provided at both ends of each of the fluorescent tubes  23 . A wire  33  that is connected to each fluorescent tube electrode  31  extends outward. When the wire  33  is energized, the corresponding fluorescent tube electrode  11  discharges, and the mercury gas then radiates ultraviolet rays. The ultraviolet rays strike the fluorescent material to radiate visible rays. 
     A rubber holder  32  is provided at both ends of each of the fluorescent tubes  23 , so that each of the fluorescent tubes  23  is fixed to each corresponding holder  27  with the rubber holder  32 . 
     Each of the holders  27  is made of a metal material, such as SUS, steel, or aluminum, and extends along each corresponding fluorescent tube  23 . A silver deposition layer or a white reflecting layer is formed on the surface of each holder  27  facing the fluorescent tubes  23 , so that the light emitted from the fluorescent tubes  23  can be efficiently reflected. By doing so, the light emitted from the fluorescent tubes  23  can be prevented from dispersing and be efficiently guided to the light guide plate  24 . 
     Each of the rubber holders  32  has highly conductive metal particles as filler contained in silicone rubber, for instance, and has high heat conductivity. The rubber holders  32  are located at the four corners of the light guide plate  24 . Because of this, each of the rubber holders  32  has an L-shaped step part  32   a . The step parts  32   a  are engaged with the corner parts  24   a  of the light guide plate  24 , so that the fluorescent tubes  23  are positioned with respect to the light guide plate  24 . 
     The light guide plate  24  is made of a resin material having a high transparency, such as acrylic. The light guide plate  24  faces the back of the liquid crystal panel  2 B. Each of the fluorescent tubes  23  faces the corresponding light-entering side  25  of the light guide plate  24 , as shown in FIGS. 6B and 6C. 
     The optical sheet  8  is placed at the front side of the light guide plate  24 . This optical sheet  8  gathers and disperses the light that is emitted from the light guide plate  24 , so that the light can be efficiently guided to the liquid crystal panel  2 B. Also, a reflecting sheet (not shown) is provided at the back of the light guide plate  24 . This reflecting sheet reflects light that leaks from the light guide plate  24 , so that the leaking light is returned into the light guide plate  24 . 
     The heat release members  40 A are the essential parts of the present invention. As shown in FIGS. 6B and 6C, each of the heat release members  40 A is an L-shaped metal plate. More specifically, each of the heat release members  40 A is formed by an aluminum plate that has a high heat release rate, and consists of a first heat release part  41  and a second heat release part  42  that are integrally formed and are perpendicular to each other. 
     Each of the heat release members  40 A is bonded to the corresponding corner part  24   a  of the light guide plate  24  with double-faced adhesive tape (not shown) that has a highly heat-conductive adhesive agent applied to both sides. Being fixed to the corner parts  24   a , the first heat release parts  41 , which are longer than the second heat release parts  42 , are located on the sides  26  of the light guide plate  24 , while the second heat release parts  42 , which are shorter than the first heat release parts  41 , are located on the light-entering sides  25  of the light guide plate  24 . 
     The rubber holders  32  are bonded to the heat release members  40 A fitted to the corners  24   a  of the light guide plate  24  in the above described manner. As described above, each of the rubber holders  32  has the step part  32   a , and the corresponding heat release member  40 A is bonded to the step part  32   a . Accordingly, the light guide plate  24  and the rubber holders  32  are bonded with the heat release members  40 A. As shown in FIG. 6B, with the rubber holders  32  being fixed to the light guide plate  24 , the end of each rubber holder  32  substantially meets the end of the corresponding second heat release part  42  at the side of the corresponding fluorescent tube  23 . 
     Although the rubber holders  32  are bonded to the light guide plate  24  after the heat release members  40 A are bonded to the light guide plate  24  in this embodiment, the rubber holders  32  may be bonded to the heat release members  40 A before the light guide plate  24  is bonded to the heat release members  40 A. Also, the heat release members  40 A are not necessarily mechanically fixed to, or directly in contact with the light guide plate  24 , as long as the heat release members  40 A are thermally connected to the light guide plate  24 . The same applies to the bonding between the heat release members  40 A and the rubber holders  32 . 
     In the backlight device  20 A of this embodiment, the heat release members  40 A are located at the positions between the light guide plate  24  and fluorescent tube electrodes  11 , where the heat generated by the fluorescent tubes  23  tends to build up. In this arrangement, the heat generated by the fluorescent tubes  23  (more particularly, the heat generated by the fluorescent tube electrodes  11 ) reaches the heat release members  40 A via the rubber holders  32 , and is then released through the heat release members  40 A, as indicated by the arrows in FIG.  6 B. 
     In this structure, even when the amount of heat generated by the fluorescent tubes  23  increases as the luminance of the fluorescent tubes  23  becomes higher with the increasing size of the liquid crystal panel  2 B, the light guide plate  24  can be prevented from melting due to the heat generated by the fluorescent tubes  23 . Thus, the deformation and deterioration of the light guide plate  24  can be avoided. As a result, decreases in the luminance and the resolution of the display screen due to the heat generated by the backlight device  20 A can be attenuated. 
     Since each of the heat release members  40 A has an L-shape in this embodiment, the release members  40 A cover the corner parts  24   a  of the light guide plate  24 , so that the heat generated by the fluorescent tube electrodes  31  cannot reach the light guide plate  24 . 
     In an experiment carried out by the inventor of the present invention, the temperature at the corner parts  24   a  of the light guide plate  24  was decreased to 55° C. in the backlight device  20 A of this embodiment. In a conventional backlight device, by comparison, the temperature at the corner parts of the light guide plate was approximately 100° C. 
     Referring now to FIGS. 7A through 7C, the second embodiment of the present invention will be described. FIGS. 7A through 7C show a backlight device  20 B that is the second embodiment of the present invention. In FIGS. 7A through 7C, the same components as in the structure of the first embodiment shown in FIGS. 6A through 6C are denoted by the same reference numerals as in FIGS. 6A through 6C, and explanation of those components is omitted. This applies as well to FIGS. 8A through 9C illustrating third and fourth embodiments of the present invention. 
     In the backlight device  20 A of the first embodiment, the heat release members  40 A are placed at the corner parts  24   a  of the flat-panel type light guide plate  24 . In such a structure, the first heat release part  41  and the second heat release part  42  of each of the heat release members  40 A protrude from the side  26  and the light-entering side  25  of the light guide plate  24  by a distance equivalent to the thickness of each of the heat release members  40 A. 
     With the second heat release parts  42  protruding from the light-entering sides  25 , however, a gap that is equivalent to the thickness of each second heat release part  42  is formed between each light-entering side  25  and the corresponding fluorescent tubes  23 . As a result, the amount of light entering from the fluorescent tubes  23  into the light guide plate  24  decreases, and so does the luminance. 
     To solve this problem, a concave part  24   b  of a shape corresponding to the shape of the heat release member  40 B is formed at each corner part  24   a  of the light guide plate  24  in the backlight device  20 B of this embodiment. In such a structure, each of the heat release members  40 B is fitted into the corresponding concave part  24   b  of the light guide plate  24 . Here, the outer face of each first heat release part  41  lies in the same plane as the corresponding side  26 , and the outer face of each second heat release part  42  lies in the same plane as the corresponding light-entering side  25 . 
     In this structure, the heat release members  40 B are fitted into the light guide plate  24  so as to not protrude from the light-entering sides  25  and the sides  26  of the light guide plate  24 . Accordingly, the gap between each fluorescent tube  23  and the corresponding light-entering side  25  of the light guide plate  24  becomes smaller than in the first embodiment. 
     In this structure, the incident rate of the light emitted from the fluorescent tubes  23  into the light guide plate  24  increases, and so does the luminance of the display screen. Meanwhile, the heat release members  40 B have the same heat release effect as the heat release members  40 A of the first embodiment, so that the light guide plate  24  can be prevented from melting. 
     Referring now to FIGS. 8A through 8C, the third embodiment of the present invention will be described. FIGS. 8A through 8C illustrate a backlight device  20 C that is the third embodiment of the present invention. 
     As described above, since the heat release members  40 A protrude from the light guide plate  24  in the first embodiment, there is risk of decreasing the amount of light entering into the light guide plate  24  from the fluorescent tubes  23  in the backlight device  20 A. In the backlight device  20 B of the second embodiment, on the other hand, a decrease of the amount of light entering into the light guide plate  24  can be prevented, but the production costs of the light guide plate  24  increase because of the addition of the concave parts  24   b  to the light guide plate  24 . 
     To solve these problems, the heat release members  40 C are formed by thin-film type metallic tape in the backlight device  20 C of this embodiment. The thin-film type metallic tape used for the heat release members  40 C may be formed by metallic foil made of a metallic material such as aluminum, or by laminating a thin film on a tape material that is made mainly of aluminum. 
     As the heat release members  40 C are formed by the thin-film type metallic tape, the protrusion of the heat release members  40  from the light guide plate  24  can be reduced (compared to the first embodiment) in the backlight device  20 C of this embodiment. In this structure, the gap between each fluorescent tube  23  and the corresponding light entering side  25  of the light guide plate  24  becomes smaller as in the backlight device  20 B of the second embodiment. As a result, the incident rate of the light entering into the light guide plate  24  from the fluorescent tubes  23  increases accordingly. 
     Meanwhile, as the heat release members  40 C have the same heat release effect as the heat release members  40 A of the first embodiment, the light guide plate  24  can be prevented from melting. Furthermore, there is no need to form the concave parts  24   b  in the light guide plate  24  as in the second embodiment, and a widely used material can be used as the thin-film type metallic tape. Because of these facts, the production costs of the backlight device  20 C of this embodiment are lower than the production costs of the backlight device  20 B of the second embodiment. 
     Referring now to FIGS. 9A through 9C, the fourth embodiment of the present invention will be described. FIGS. 9A through 9C illustrate a backlight device  20 D that is the fourth embodiment of the present invention. 
     In the backlight device  20 A of the first embodiment, each of the heat release members  40 A has an L-shaped structure, consisting of the first heat release part  41  and the second heat release part  42 . In the backlight device  20 D of this embodiment, on the other hand, each heat release member  40 D takes the form of a flat plate. Because of this simple form, the heat release members  40 D can contribute to reducing the production costs. 
     Also, each of the heat release members  40 D of the flat-plate type is placed at the corresponding side  26  of the light guide plate  24 , i.e., at the corresponding outer peripheral side of the light guide plate  24  not facing the fluorescent tubes  23 . As the heat release members  40 D do not exist at positions between the light guide plate  24  and the fluorescent tubes  23 , the gap between each fluorescent tube  23  and the corresponding light-entering side  25  becomes smaller compared to embodiments 1 and 3. The incident rate of the light entering into the light guide plate  24  from the fluorescent tubes  23  increases accordingly. 
     Although the light guide plate  24  is in direct contact with the fluorescent tube electrodes  31  at the corners  24   a  in this embodiment, the heat entering into the light guide plate  24  from the fluorescent tube electrodes  31  is immediately transferred to the heat release members  40 D, through which the heat is released. In this manner, the light guide plate  24  is prevented from melting. 
     It should be noted that the present invention is not limited to the embodiments specifically disclosed above, but other variations and modifications may be made without departing from the scope of the present invention.