Abstract:
A backlight device for a transmissive liquid crystal display device includes a plurality of light sources, including a cold cathode fluorescent lamp and an LED, a liquid crystal panel, and a light guide plate. The light guide plate causes light entering into it through one surface thereof to emerge out of another surface thereof toward the liquid crystal panel. A controller for the backlight device selects at least one of the cold cathode fluorescent lamp and LED, depending on brightness required for the liquid crystal display device and determines, in accordance with the required brightness, the brightness of the selected light source to operate the light source accordingly.

Description:
FIELD OF THE INVENTION 
   This invention relates to a liquid crystal display device and an electronic apparatus with a liquid crystal display device. More particularly, this invention relates to a structure and arrangement of a backlight device with improved power efficiency for use in a liquid crystal display device for electronic apparatuses. 
   BACKGROUND OF THE INVENTION 
   Maruyama et al., in their Japanese Patent Application Publication No. HEI 11-38410 A laid open for public inspection on Feb. 12, 1999, disclose use of a semi-transmissive liquid crystal display device in order to reduce liquid crystal display device power dissipation or consumption. The liquid crystal display device of Maruyama et al. is operated as a transmissive liquid crystal display device by the use of a cold-cathode fluorescent lamp (CCFL) as a backlight source, when the liquid crystal display device is operated in a dark environment. In a light environment, it does not use the cold cathode fluorescent lamp, but uses a white reflective plate to reflect environmental light so that the liquid crystal display device can be operated as a reflective liquid crystal display device. 
   In order to reduce power dissipation, Kurumizawa discloses in his Japanese Patent Application Publication No. HEI 11-101980 A laid open for public inspection on Apr. 13, 1999, a liquid crystal display device using a cold cathode fluorescent lamp and chemiluminescence. The liquid crystal display device of Kurumizawa uses a cold cathode fluorescent lamp as a backlight source when an electronic apparatus which employs the liquid crystal display device is operated from an AC power supply, while it uses a bag containing a chemiluminescent mixture solution as a backlight source when the electronic apparatus is operated from a DC battery. 
   The semi-transmissive liquid crystal display device disclosed in Japanese Patent Application Publication No. HEI 11-38410 A can use a DC power supply battery for a longer time when it is operated as a reflective liquid crystal display device in a light place. The semi-transmissive liquid crystal display device uses a cold cathode fluorescent lamp when it is used in a dark environment and, therefore, requires higher brightness. However, its display is less bright than and, therefore, inferior in quality to an ordinary transmissive liquid crystal display device when it is operated from the same power supply level as the ordinary transmissive liquid crystal display device, because light transmission is restricted due to its semi-transmissive nature. Accordingly, the liquid crystal display device of Maruyama et al. requires higher power to provide the same brightness as the ordinary transmissive liquid crystal display device. 
   The liquid crystal display device employing a cold cathode fluorescent lamp and chemiluminescence disclosed in Japanese Patent Application Publication No. HEI 11-101980 A requires a bag containing chemically luminescent mixture solution to be inserted into the liquid crystal display device. This liquid crystal display device is not economical because, once the bag starts emitting light, the light emission cannot be interrupted. In addition, a user of the liquid crystal display device must take a chemiluminescent bag or bags with him or her, and must take a trouble of disposing the used bag. 
   The inventors have recognized that power dissipation of a liquid crystal display device and an electronic apparatus with the liquid crystal display device can be reduced by selectively using a cold cathode fluorescent lamp and light-emitting diodes as a backlight source for the liquid crystal display device depending on brightness required for the liquid crystal display device. 
   An object of the present invention is to provide a liquid crystal display device with power efficient backlight sources selectively useable depending on desired brightness. 
   Another object of the present invention is to prolong the life of a battery used to operate an electronic apparatus through selective use of power efficient backlight sources for a liquid crystal display device used with the electronic apparatus depending on brightness required for the liquid crystal display device. 
   SUMMARY OF THE INVENTION 
   In accordance with one aspect of the present invention, an electronic apparatus includes a liquid crystal display device which includes a plurality of light sources including a cold cathode fluorescent lamp and a light-emitting diode (LED) and a liquid crystal unit. The electronic apparatus further includes a controller for selecting and operating at least one of the light sources in accordance with brightness required for the liquid crystal display device. 
   In an embodiment, the liquid crystal display device may further include at least one light guide plate which causes light from at least one of the plurality of light sources entering into the light guide plate through at least one surface thereof to be projected toward the liquid crystal unit. 
   The liquid crystal display device may further include a light guide member for causing light entering into it through one surface thereof to be scattered and projected through another surface thereof, and a light guide plate which causes the scattered light entering into it through one side surface thereof to be projected toward the liquid crystal unit. 
   The liquid crystal display device may further include at least one light guide plate for causing light entering into it through a side surface thereof from at least one of the light sources to be scattered and projected toward the liquid crystal unit. 
   In accordance with another aspect of the present invention, a liquid crystal display device includes a plurality of light sources including at least one cold cathode fluorescent lamp and at least one LED, a liquid crystal panel, a light guide plate for causing light from at least one of the plurality of light sources entering into the light guide plate through a surface thereof to be projected toward the liquid crystal panel, and a controller for selecting at least one of the plurality of light sources in accordance with required brightness and determining the brightness of the selected light source to operate the selected light source. 
   The present invention makes it possible to choose a backlight source having high power efficiency in accordance with required brightness in a liquid crystal display device, whereby the life of a battery for operating the backlight sources can be long. Also, an electronic apparatus with such a liquid crystal display device can be realized. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1A ,  1 B,  1 C and  1 D illustrate a liquid crystal display device with a backlight device disposed on the rear surface of a transmissive liquid crystal panel, in accordance with an embodiment of the present invention; 
       FIGS. 2A and 2B  illustrate a liquid crystal display device with a backlight device, in accordance with another embodiment of the invention; 
       FIGS. 3A and 3B  illustrate a liquid crystal display device with a backlight device, in accordance with a further embodiment of the invention; 
       FIGS. 4A and 4B  illustrate a liquid crystal display device with a backlight device, in accordance with a still further embodiment of the invention; 
       FIGS. 5A and 5B  illustrate a liquid crystal display device with a backlight device, in accordance with a still further embodiment of the invention; 
       FIGS. 6A ,  6 B and  6 C illustrate a liquid crystal display device with a backlight device, in accordance with a still further embodiment of the invention; 
       FIGS. 7A ,  7 B and  7 C illustrate a liquid crystal display device with a backlight device, in accordance with a still further embodiment of the invention; 
       FIGS. 8A ,  8 B and  8 C illustrate a liquid crystal display device with a backlight device, in accordance with a still further embodiment of the invention; 
       FIGS. 9A ,  9 B and  9 C illustrate a liquid crystal display device with a backlight device, in accordance with a still further embodiment of the invention; 
       FIGS. 10A and 10B  illustrate a liquid crystal display device with a backlight device, in accordance with a still further embodiment of the invention; 
       FIGS. 11A and 11B  illustrate a liquid crystal display device with a backlight device, in accordance with a still further embodiment of the invention; and 
       FIGS. 12A ,  12 B and  12 C illustrate a liquid crystal display device with a backlight device, in accordance with a still further embodiment of the invention. 
   

   DESCRIPTION OF PREFERRED EMBODIMENTS 
   In accordance with the present invention a cold cathode tube or fluorescent lamp and a light emitting diode (LED) are used as light sources providing backlight for a transmissive liquid crystal display device (LCD) for use in a portable or mobile electronic apparatus, such as a notebook personal computer, a handheld personal computer or a personal digital assistant (PDA). For a range of low brightness, an LED has a higher power efficiency than a cold cathode fluorescent lamp. The inventors have recognized that, by the use of ten LED&#39;s for providing low display brightness obtainable by a cold cathode fluorescent lamp on a liquid crystal display screen having a display area of about 200 cm 2 , dissipated power can be reduced by an amount of up to about 40% to about 60% (about 300 mW to about 400 mW) of the power which would be dissipated if the cold cathode fluorescent lamp was used. 
   In a transmissive liquid crystal display device in accordance with the present invention and an electronic apparatus with such a liquid crystal display device, a cold cathode fluorescent lamp is used for desired display brightness of, for example, 23 cd/m 2  higher than a threshold value of, for example, 20 cd/m 2  to thereby ensure satisfactory display quality. On the other hand, if a desired display brightness is equal to or lower than the threshold value, for example, 5 cd/m 2  or 20 cd/m 2 , an LED is used to save the power dissipation to prolong the life of a battery. Also, the life of the cold cathode fluorescent lamp can be prolonged by using the LED as frequently as possible. For that purpose, switching control between the light sources is provided for the electronic apparatus. 
   Alternatively, when an external AC power supply is used for a transmissive liquid crystal display device and an electronic apparatus with such a liquid crystal display device, a cold cathode fluorescent lamp may be used as a light source to ensure satisfactory display quality. On the other hand, when a DC battery source is used, an LED may be used as a light source to save power dissipation so that the battery can be used longer. 
   Alternatively, when an external AC power supply is used or when desired display brightness is set to a value of, for example, 25 cd/m 2 , which is higher than a threshold value of, for example, 20 cd/m 2 , for a transmissive liquid crystal display device and an electronic apparatus with such a liquid crystal display device, a cold cathode fluorescent lamp may be used as a light source to thereby ensure satisfactory display quality. On the other hand, when a DC battery is employed as a power supply with desired brightness of, for example, 5 cd/m 2  or 20 cd/m 2 , which is equal to or lower than the threshold value, an LED may be used. 
   Now, preferred embodiments are described with reference to the accompanying drawings. Throughout the drawings, similar or same elements and functions are provided with the same reference numerals. 
     FIGS. 1A ,  1 B and  1 C illustrate a liquid crystal display device  5  including a transmissive liquid crystal panel  54  with a backlight device  100  disposed on the rear surface of the panel  54 , in accordance with one embodiment of the present invention.  FIG. 1A  shows a front view of the liquid crystal display device  5  including the backlight device  100 , and a light source switching control unit  72 , a cold cathode fluorescent lamp driving unit  74  and an LED driving unit  76  which are associated with the liquid crystal display device  5 . In  FIG. 1A , the liquid crystal panel  54  is shown with its part removed. (Similarly, in  FIGS. 2A ,  3 A,  4 A,  5 A,  6 A,  7 A,  8 A,  9 A,  10 A,  11 A and  12 A, the liquid crystal panel  54  is shown with its part removed.)  FIG. 1B  is a left side view of the liquid crystal display device  5  shown in  FIG. 1A , and  FIG. 1C  is a bottom view of the liquid crystal display device  5 .  FIG. 1D  is useful for explaining the structure of a reflecting sheet or reflecting plate  53 . As indicated in  FIGS. 1A ,  1 B and  1 C, the vertical direction is defined as an X direction, the horizontal direction is defined as a Y direction, and the direction perpendicular to both of the X and Y directions is defined as a Z direction. 
   The cold cathode fluorescent lamp driving unit  74  is coupled to an external AC power supply (not shown) and to a DC battery (not shown). The LED driving unit  76  is coupled to the DC battery. The LED driving unit  76  may be additionally coupled to the external AC power supply. The light source switching control unit  72  activates selectively the cold cathode fluorescent lamp driving unit  74  and the LED driving unit  76  in response to an instruction IS from a microprocessor or microcontroller  70  of an electronic apparatus (not shown). The microprocessor provides the instruction IS in accordance with display brightness set by a user. 
   Referring to  FIGS. 1A ,  1 B and  1 C, the backlight device  100  includes a cold cathode fluorescent lamp  10 , a plurality of LED&#39;s  30 , a light guide bar or rod  40 , and a generally rectangular light guide plate  50 . Typically, the light guides  40  and  50  are made of acrylic resin. The light guide plate  50  is disposed behind the transmissive liquid crystal panel  54  in parallel therewith. The light guide plate  50  has a flat and rectangular surface facing the liquid crystal panel  54 , as shown in  FIG. 1A , and has a downward tapered profile in the X-Z plane as shown in FIG.  1 B. In other words, the rear surface of the light guide plate  50  tapers downward in the X direction and forward in the Z direction. The light guide plate  50  has the largest thickness of about 2 mm at the top and the smallest thickness of about 1 mm at the bottom. 
   The light guide bar  40  has a tapered or wedge-shaped profile which is the same as the profile of the light guide plate  50 . 
   The rear surface of the light guide plate  50  is provided with a plurality of grooves  51  extending in the X direction so that a succession of a plurality of prismatic portions extending in the X direction and arranged in the Y direction can be formed, as shown in FIG.  1 C. The prismatic portions formed by the grooves  51  in the light guide plate  50  scatter light entering in the Y direction from the LED&#39;s  30  through the light guide bar  40  within the light guide plate  50  to direct it forward in the Z direction. In  FIG. 1A , parts of base lines and ridges of some of the prisms or the grooves  51 , which extend in parallel with each other in the X direction, are shown by broken lines  52 . 
   The rear surface of the light guide plate  50  is covered with a known reflecting sheet or plate  53 . As shown in  FIG. 1D , a number of protuberances having spherical surfaces or convex lens-shaped protuberances for scattering light are formed on the surface of the reflecting sheet  53  facing the light guide plate  50 . 
   The cold cathode fluorescent lamp  10 , which projects light toward the light guide plate  50  in the X direction, is disposed on the top surface of the light guide plate  50 . Thus, the cold cathode fluorescent lamp  10  functions as a side light for the liquid crystal display device  5 . 
   As described above, the LED&#39;s  30  are arranged on the left side surface of the light guide plate  50 . The LED&#39;s  30  emit light through the elongated light guide bar  40  to the light guide plate  50 . Thus, the LED&#39;s  30  also function as a side light of the liquid crystal display device  5 . Similarly to the rear surface of the light guide plate  50 , a plurality of grooves  41  extending in the Z direction are arranged in the X direction on the surface of the light guide bar  40  facing the LED&#39;s  30  so that prismatic portions can be formed. The grooves  41  or prismatic portions function to scatter light within the light guide bar  40 . Base lines and ridges of some of the prismatic portions are indicated by broken lines  42  in FIG.  1 B. Preferably, the LED&#39;s  30  are of the type emitting light which is white or approximately white. 
   The cold cathode fluorescent lamp  10  is enclosed in a cover formed by reflecting plates  16 , which opens toward the top surface of the light guide plate  50 . The LED&#39;s  30  and the light guide bar  40  are enclosed in a cover formed by reflecting plates  36 , which opens toward the light guide plate  50 . Typically, the reflecting plates  16  and  36  are made of aluminum and provided with a mirror surface film applied over their inner surfaces. Reflecting sheets  58  cover the bottom and right side surfaces of the light guide plate  50 , as shown in FIG.  1 A. Throughout the drawings, except  FIG. 1D , the portions of the reflecting plates and sheets  16 ,  36  and  58  and other elements located on the viewer&#39;s side are not shown to facilitate understanding of the structure of the backlight device  100 . 
   In operation, in the electronic apparatus including the liquid crystal display device  5  shown in  FIGS. 1A ,  1 B and  1 C with the backlight device  100 , when the desired brightness set by the user is higher than a threshold value of, for example, 20 cd/m 2 , the processor  70  supplies an instruction IS for selecting the cold cathode fluorescent lamp and designating the magnitude of the display brightness to the light source switching control unit  72 . In response to the instruction IS from the microprocessor  70 , the light source switching control unit  72  supplies a control signal CTRL to activate the cold cathode fluorescent lamp driving unit  74  which powers the cold cathode fluorescent lamp  10 , and also causes the cold cathode fluorescent lamp driving unit  74  to control the brightness of the cold cathode fluorescent lamp  10  in accordance with the desired brightness. 
   When the desired display brightness set by the user is equal to or lower than the threshold value of 20 cd/m 2 , the processor  70  supplies the instruction IS for selecting the LED&#39;s and designating the magnitude of the display brightness to the light source switching control unit  72 . In response to this instruction IS, the light source switching control unit  72  provides a control signal CTRL to activate the LED driving unit  76  which powers the LED&#39;s  30 , and also causes the LED driving unit  76  to control the brightness of the LED&#39;s  30  in accordance with the desired display brightness. 
   In an alternative arrangement, when the electronic apparatus is operated from an AC power supply, the processor  70  may supply the light source switching control unit  72  with an instruction IS for causing the cold cathode fluorescent lamp to be selected and for designating the magnitude of the display brightness. In response to this instruction IS, the light source switching control unit  72  provides a control signal CTRL to activate the cold cathode fluorescent lamp driving unit  74  which powers the cold cathode fluorescent lamp  10 , and also causes the cold cathode fluorescent lamp driving unit  74  to control the brightness of the cold cathode fluorescent lamp  10  for providing a desired display brightness in a relatively high brightness range of, for example, 15 cd/m 2  and higher. On the other hand, when the electronic apparatus is operated from a DC battery, the processor  70  supplies the light source switching control unit  72  with an instruction IS for causing the LED&#39;s to be selected and for designating the magnitude of the display brightness. In response to this instruction IS, the light source switching control unit  72  provides a control signal CTRL to activate the LED driving unit  76  which powers the LED&#39;s  30 , and also causes the LED driving unit  76  to control the brightness of the LED&#39;s  30  for providing a desired display brightness in a relatively low brightness range of, for example, from 5 cd/m 2  to 20 cd/m 2 . 
   In a still alternative arrangement, when the electronic apparatus is powered from an AC power supply, or when the electronic apparatus is powered from a DC battery and the desired brightness designated by the user is higher than a threshold value of, for example, 20 cd/m 2 , the processor  70  may supply the light source switching control unit  72  with an instruction IS for selecting the cold cathode fluorescent lamp and designating the magnitude of the display brightness. In response to this instruction IS, the light source switching control unit  72  supplies the cold cathode fluorescent lamp driving unit  74  with a control signal CTRL to activate the cold cathode fluorescent lamp driving unit  74 , and also causes the cold cathode fluorescent lamp driving unit  74  to control the brightness of the cold cathode fluorescent lamp  10  in accordance with the desired brightness designated by the user. On the other hand, when the electronic apparatus is operated from a DC battery and the desired brightness designated by the user is equal to or lower than the threshold value of 20 cd/m 2 , the processor  70  supplies the light source switching control unit  72  with an instruction IS for selecting the LED&#39;s and designating the magnitude of the display brightness. In response to this instruction IS, the light source switching control unit  72  supplies the LED driving unit  76  with a control signal CTRL to activate the LED driving unit  76 , and also causes the LED driving unit  76  to control the brightness of the LED&#39;s  30  in accordance with the desired brightness designated by the user. 
   Light is projected downward into the light guide plate  50  from the cold cathode fluorescent lamp  10  as represented by broken line arrows in  FIG. 1A , and scattered and reflected by the reflecting sheet  53  on the slanting rear surface of the light guide plate  50  and by the reflecting sheets  58  on the right side and bottom surfaces of the light guide plate  50  so that it can be directed to the liquid crystal panel  54  as indicated by broken line arrows in FIG.  1 B. Light emitted by the LED&#39;s  30 , represented by broken line arrows in  FIG. 1A , is projected rightward toward the light guide plate  50  through the light guide bar  40 . Because of the grooves  41  in the light guide bar  40 , the light is scattered in the light guide bar  40 , and the scattered light enters into the light guide plate  50 . The scattered light entering the light guide plate  50  is, then, scattered and reflected again by the prismatic portions formed by the grooves  51  in the rear surface of the light guide plate  50  and is directed to the liquid crystal panel  54  as represented by broken line arrows in FIG.  1 C. 
   As described, the use of the cold cathode fluorescent lamp  10  as the backlight source ensures good display quality, while the use of the LED&#39;s  30  as the backlight source can prolong the life of the DC battery used as the power source. 
     FIGS. 2A and 2B  show a liquid crystal display device with a backlight device  101  in accordance with another embodiment of the present invention.  FIG. 2A  is a front view of the liquid crystal display device including the backlight device  101 , and  FIG. 2B  is a right side view of the liquid crystal display device shown in FIG.  2 A. Similarly to the embodiment shown in  FIG. 1A , a cold cathode fluorescent lamp  10  of  FIG. 2A  is connected to a cold cathode fluorescent lamp driving unit  74  similar to the one shown in  FIG. 1A , and LED&#39;s  30  of  FIG. 2A  are connected to an LED driving unit  76  similar to the one shown  FIG. 1A , although the driving units  74  and  76  are not shown in FIG.  2 A. 
   The liquid crystal display device shown in  FIGS. 2A and 2B  include a rectangular light guide plate  502  having a uniform thickness of about 2 mm. The cold cathode fluorescent lamp  10  is disposed on the upper surface of the light guide plate  502 . A plurality of LED&#39;s  30 , which emit light directly into the light guide plate  502 , are disposed beneath the bottom surface of the light guide plate  502 . A liquid crystal panel  54  is disposed in front of the light guide plate  502 . The cold cathode fluorescent lamp  10  is enclosed in a cover formed by reflecting plates or sheets  16  which opens toward the light guide plate  502 , and the LED&#39;s  30  are enclosed in a cover formed by reflecting plates or sheets  36 , which opens toward the light guide plate  502 . The left and right side surfaces of the light guide plate  502  are covered with reflecting sheets  58 . 
   Light from the cold cathode fluorescent lamp  10  is projected downward into the light guide plate  502  as represented by broken line arrows, and scattered and reflected by the reflecting sheet  53  on the rear surface of the light guide plate  502  and also by the reflecting sheets  58  on the left and right side surfaces of the light guide plate  502 . Light from the cold cathode fluorescent lamp  10  is then directed toward the liquid crystal panel  54  as shown in FIG.  2 B. 
   Light from the LED&#39;s  30  is projected upward as represented by broken line arrows, scattered and reflected by the reflecting sheet  53  on the rear surface of the light guide plate  502 , and directed to the liquid crystal panel  54 , as shown in FIG.  2 B. In this embodiment, the light guide bar  40  used in the embodiment shown in  FIGS. 1A-1C  is not required, but, since an LED, in general, has directivity regarding light emission, causing light to diverge forward, there may be dark portions in the bottom of the light guide plate  502  at locations where no LED&#39;s  30  are disposed. Accordingly, as the LED&#39;s for this embodiment, low directivity LED&#39;s, which may be provided by appropriately designing mold resin for them, are preferred. 
     FIGS. 3A and 3B  illustrate a liquid crystal display device with a backlight device  103  in accordance with a further embodiment of the invention.  FIG. 3A  is a front view of the liquid crystal display device including the backlight device  103 , and  FIG. 3B  is a right side view of the liquid crystal display device of FIG.  3 A. Although not shown in  FIG. 3A , a cold cathode fluorescent lamp  10  of  FIGS. 3A and 3B  is connected to a cold cathode fluorescent lamp driving unit  74  similar to the one shown in  FIG. 1A , and LED&#39;s  32  and  34  is connected to an LED driving unit  76  similar to the one shown in FIG.  1 A. 
   The backlight device  103  includes an elongated light guide bar  44  extending along the bottom surface of a light guide plate  503 . The LED&#39;s  32  are arranged on the right end surface of the light guide bar  44 , and the LED&#39;s  34  are arranged on the left end surface of the light guide bar  44 . Similarly to the light guide bar  40  of  FIG. 1A , the light guide bar  44  includes a plurality of grooves  41  extending in the Z direction which are arranged in the Y direction such that a succession of prismatic portions are formed in the bottom portion of the light guide bar  44 . Also, as shown in  FIG. 3B , the rear surface of the light guide plate  503  is provided with a plurality of grooves  51  which extend in the horizontal direction Y such that a succession of prismatic portions arranged in the X direction can be formed in the rear portion of the light guide plate  503 . By virtue of the grooves  51 , light propagating in the X direction in the light guide plate  503  is scattered and reflected so that it is projected forward in the Z direction to a liquid crystal panel  54 . The front, rear and bottom surfaces of the light guide bar  44  are covered with reflecting sheets  58 . The remainder of the structure of the backlight device  103  is similar to the backlight device  101  shown in  FIGS. 2A and 2B , and is not described again. 
   As represented by broken line arrows in  FIGS. 3A and 3B , light emitted by the LED&#39;s  32  and  34  enters into the light guide bar  44 , where it is scattered by the prismatic portions formed by the grooves  41 , and the scattered light is directed upward into the light guide plate  503 . The scattered light entering into the light guide plate  503  is scattered by the prismatic portions formed by the grooves  51  and reflected by the reflecting sheet  53  to be projected toward the liquid crystal panel  54  as represented by broken line arrows in FIG.  3 B. In this manner, the light guide bar  44  produces a uniform brightness over the entire light guide plate  503 . 
   As represented by broken line arrows, the cold cathode fluorescent lamp  10  projects light into the light guide plate  503 , as in the embodiment shown in  FIGS. 2A and 2B . The light entering into the light guide plate  503  is scattered by the prismatic portions in the rear surface of the plate  503  and reflected by the reflecting sheet  53  to be projected toward the liquid crystal panel  54 . 
     FIGS. 4A and 4B  illustrate a liquid crystal display device with a backlight device  105  in accordance with a still further embodiment of the invention.  FIG. 4A  is a front view of the liquid crystal display device with the backlight device  105 , and  FIG. 4B  is a right side view of the liquid crystal display device of FIG.  4 A. 
   In  FIGS. 4A-4B ,  5 A- 5 B,  6 A- 6 C,  7 A- 7 C,  8 A- 8 C,  9 A- 9 C,  10 A- 10 B ,  11 A- 11 B and  12 A- 12 C, although not shown, a cold cathode fluorescent lamp driving unit  74  similar to the one shown in  FIG. 1A  is connected to a cold cathode fluorescent lamp  10 , and an LED driving unit  76  similar to the one shown in  FIG. 1A  is connected to LED&#39;s  32 ,  34  and the like. 
   The backlight device  105  includes a light guide plate  506  which is similar to the light guide plate  50  shown in  FIGS. 1A through 1C  and, therefore, tapered or wedged downward. The thickest, top portion has a thickness of about 2 mm, and the thinnest, bottom portion has a thickness of about 1 mm. The rear surface of the light guide plate  506  can be planar. The light guide bar  44  for scattering light is disposed between the upper surface of the tapered light guide plate  506  and the cold cathode fluorescent lamp  10 , and the LED  32  is disposed at the right end of the light guide bar  44 , and the LED  34  is disposed at the left end of the light guide bar  44 . Similarly to the light guide bar  44  of  FIG. 3A , a plurality of grooves  41  extending in the Z direction and arranged in the Y direction are formed in the upper surface of the light guide bar  44 . Similarly to the embodiment shown in  FIGS. 3A and 3B , the cold cathode fluorescent lamp  10  is enclosed in a cover formed of reflecting plates  16 , and the LED&#39;s  32  and  34  are enclosed in covers formed of reflecting plates  36 . The rear surface of the light guide plate  506  is covered with a reflecting sheet  53 . Also, the right and left side surfaces and the bottom surface of the light guide plate  506  are covered with reflecting sheets  58 . Further, although not shown, the front and rear surfaces of the light guide bar  44  are covered with the reflecting sheets  58 . In this embodiment, since the light guide plate  506  is tapered or wedged, the size and weight of the liquid crystal display device can be reduced. 
   Light from the cold cathode fluorescent lamp  10 , as represented by broken line arrows, passes through the light guide bar  44  into the light guide plate  506 , and is scattered and reflected by the reflecting sheet  53  on the rear surface of the light guide plate  506  to be directed to the liquid crystal panel  54 , as shown in FIG.  4 B. Light from the LED&#39;s  32  and  34 , as represented by broken line arrows, passes in the Y direction into the light guide bar  44  and is scattered by the prismatic portions formed in the light guide bar  44  by the grooves  41 . The scattered light is directed downward into the light guide plate  506 , further scattered and reflected by the reflecting sheet  53  on the rear surface of the light guide plate  506 , and directed to the liquid crystal panel  54 . 
     FIGS. 5A and 5B  illustrate a liquid crystal display device with a backlight device  107  in accordance with a still further embodiment of the invention.  FIG. 5A  is a front view of the liquid crystal display device including the backlight device  107 .  FIG. 5B  is a right side view of the liquid crystal display device of FIG.  5 A.  FIG. 5A  shows the liquid crystal display device with parts of a cold cathode fluorescent lamp  10  and a reflecting sheet  16  removed in order to show a portion of a light guide bar  44  disposed behind the cold cathode fluorescent lamp  10 . 
   The backlight device  107  includes a light guide plate  508  tapered or wedged, similarly to the light guide plate  506  shown in  FIGS. 4A and 4B . The top portion of the light guide plate  508  has the largest thickness of about 3 mm, and the thinnest, bottom portion has a thickness of about 1.5 mm. On top of the light guide plate  508 , light bar guide  44  is disposed and the light scattering, light guide bar  44  is also disposed behind the lamp  10 . An LED  32  is disposed at the right end of the cold cathode fluorescent lamp  10 , and an LED  34  is disposed at the left end of the light bar guide  44 . In the upper surface of the light guide bar  44 , a plurality of grooves  41  similar to the grooves  41  in the bar  44  shown in  FIGS. 4A and 4B , extending in the Z direction are arranged in the Y direction. The cold cathode fluorescent lamp  10  is enclosed in a cover formed of reflecting plates  16  which is similar to the cover shown in  FIGS. 4A and 4B , and the LED&#39;s  32  and  34  are enclosed in covers formed of reflecting plates  36  like the ones shown in  FIGS. 4A and 4B . The left and right side and bottom surfaces of the light guide plate  508  are covered with reflecting sheets  58 . The upper, front and rear surfaces of the light guide bar  44  are also covered with the reflecting sheets  58 . 
   Light from the cold cathode fluorescent lamp  10  propagates downward and enters directly into the light guide plate  508 , as indicated by broken line arrows, and is scattered and reflected by the reflecting sheet  53  disposed on the rear surface of the light guide plate  508  to be projected toward the liquid crystal panel  54 , as represented by broken line arrows in FIG.  3 B. As in the embodiment shown in  FIGS. 4A and 4B , light from the LED&#39;s  32  and  34  is emitted in the horizontal Y direction and is scattered by the prismatic portions formed by the grooves  41  in the light guide bar  44 . The scattered light is projected downward into the light guide plate  508  and is further scattered and reflected by the reflecting sheet  53  on the rear surface of the light guide plate  508  to be projected toward the liquid crystal panel  54 . 
   Since light from the cold cathode fluorescent lamp  10  enters directly into the light guide plate  508 , it is attenuated less than in the embodiment of  FIGS. 4A and 4B . When the cold cathode fluorescent lamp  10  is energized, the upper portion of the liquid crystal panel  54  may be darker than the rest because of the thickness in the upper portion of the light guide plate  508 . If such occurs, the LED&#39;s  32  and  34  as well as the cold cathode fluorescent lamp  10  can be energized to supplement the low brightness provided by the cold cathode fluorescent lamp  10  in the upper portion of the liquid crystal panel  54 . 
     FIGS. 6A ,  6 B and  6 C illustrate a liquid crystal display device with a backlight device  109  in accordance with a still further embodiment of the invention.  FIG. 6A  is a front view of the liquid crystal display device including the backlight device  109 .  FIGS. 6B and 6C  are right side and bottom views, respectively, of the liquid crystal display device of FIG.  6 A. 
   The backlight device  109  includes a tapered or wedge-shaped light guide plate  510  similar to the light guide plate  50  of the embodiment shown in  FIGS. 1A ,  1 B and  1 C, with a thickest, top portion having a thickness of about 2 mm and a thinnest, bottom portion having a thickness of about 1 mm. The light guide plate  510  is provided with a plurality of grooves  51  extending in the vertical X direction. A cold cathode fluorescent lamp  10  is disposed on top of the light guide plate  510 , which emits light toward the light guide plate  510 . On and along the right side surface of the light guide plate  510 , there is disposed an elongated, wedge-shaped light guide bar  46 . Also an elongated, wedge-shaped light guide bar  47  is disposed on and along the left side surface of the light guide plate  510 . The front surfaces of the light guide bars  46  and  47  are in parallel with a liquid crystal panel  54 , and the left and right side surfaces of the light guide bars  46  and  47  are in parallel with the right and left side surfaces of the light guide plate  510 . The rear surfaces of the light guide bars  46  and  47  slant downward, as the light guide plate  510 . A plurality of grooves  41  extending in the Z direction and arranged in the X direction are formed in each of the right and left side surfaces, i.e. outer surfaces, of the light guide bars  46  and  47 , respectively, similarly to the light guide bar  44  shown in FIG.  3 A. On the top and bottom end surfaces of the light guide bar  46 , LED&#39;s  32  and  34  are disposed, respectively. Similarly, on the top and bottom end surfaces of the light guide bar  47 , LED&#39;s  33  and  35  are disposed, respectively. The cold cathode fluorescent lamp  10  is covered with reflecting plates  16 , leaving the lower side open, and the LED&#39;s  32 ,  33 ,  34  and  35  are covered with reflecting plates  36 , leaving the sides facing the light guide bars  46  and  47  open. The rear and bottom surfaces of the light guide plate  510  are covered with a reflecting sheet  53  and a reflecting sheet  58 , respectively. The front, right side and rear side surfaces of the light guide bar  46  are covered with the reflecting sheets  58 , and the front, left side and rear surfaces of the light guide bar  47  are also covered with the reflecting sheets  58 . 
   As represented by broken line arrows, light from the cold cathode fluorescent lamp  10  is projected downward into the light guide plate  510  and scattered and reflected by the reflecting sheet  53  disposed on the slanting rear surface of the light guide plate  510  to be projected toward a liquid crystal panel  54 , as represented by broken line arrows in FIG.  6 B. The LED&#39;s  32  and  33  emit light vertically downward into the light guide bars  46  and  47 , respectively, and the LED&#39;s  34  and  35  emit light vertically upward into the light guide bars  46  and  47 , respectively. The light from the LED&#39;s  32 ,  33 ,  34  and  35  is then scattered and reflected by the prismatic portions formed by the grooves  41  in the light guide bars  46  and  47  and directed horizontally into the light guide plate  510 . The light entering into the light guide plate  510  is then scattered by the prismatic portions formed by the grooves  51  and directed toward the liquid crystal panel  54 . In this embodiment, the LED&#39;s  32 ,  33 ,  34  and  35  are disposed, being spaced from each other. Accordingly, a more uniform distribution of brightness can be realized over the liquid crystal panel  54 , whereby the brightness can be increased efficiently. 
     FIGS. 7A ,  7 B and  7 C illustrate a liquid crystal display device with a backlight device  111  hi accordance with a still further embodiment of the invention.  FIG. 7A  is a font view of the liquid crystal display device including the backlight device  111 .  FIGS. 7B and 7C  are right side and bottom views, respectively, of the liquid crystal display device of FIG.  7 A. 
   The backlight device  111  includes a light guide plate  512  having a downward tapered or wedge-shaped profile like the light guide plate  50  of the embodiment shown in  FIGS. 1A-1C  , and elongated light guide bars  48  and  49  disposed on and along the right and left side surfaces of the light guide plate  512 . Each of the light guide bars  48  and  49  has parallel top and bottom surfaces like the light guide bars of the embodiments of  FIGS. 1A-1C  and  FIGS. 6A-6C  . The top surface has a larger size of about 2 mm×about 2 mm than the bottom surface which has a size of about 1 mm×about 1 mm. The side surfaces of the light guide bars  48  and  49  adjacent to the light guide plate  512  are in parallel with the side surfaces of the plate  512 , and the front surfaces of the light guide bars  48  and  49  are in line with the front surface of the light guide plate  512 . Thus, the light guide bar  48  tapers downward with the rear surface slanting forward and with the right side surface slanting leftward. Similarly, the light guide bar  49  tapers downward with the rear surface slanting forward and with the left side surface slanting rightward. 
   An LED  32  is disposed on the top surface of the light guide bar  48 , and an LED  33  is disposed on the top surface of the light guide bar  49 . The remainder of the structure of the backlight device  111  is the same as the backlight device  109  shown in  FIGS. 6A-6C , and is not described again. 
   Since the light guide bars  48  and  49  have their side surfaces tapered in addition to their rear surfaces, the size and weight of the liquid crystal display device can be reduced. 
   Light from the cold cathode fluorescent lamp  10 , as represented by broken line arrows, is projected downward into the light guide plate  512  and scattered and reflected by a reflecting sheet  53  on the rear surface of the light guide plate  512  to be projected toward a liquid crystal panel  54 , as represented by broken line arrows in FIG.  6 B. The LED&#39;s  32  and  33  emit light downward into the light guide bars  48  and  49 , respectively, as represented by broken line arrows, and the light is reflected by reflecting sheets  58  on their slanting side surfaces to enter horizontally into the light guide plate  512  where it is scattered by the prismatic portions formed by grooves  51  in the rear surface of the light guide plate  512  and directed toward the liquid crystal panel  54 . 
     FIGS. 8A ,  8 B and  8 C illustrate a liquid crystal display device with a backlight device  113  in accordance with a still further embodiment of the invention.  FIG. 8A  is a front view of the liquid crystal display device including the backlight device  113 .  FIGS. 8B and 8C  are right side and bottom views, respectively, of the liquid crystal display device of  FIG. 8A. A  liquid crystal panel  54  disposed in front of the backlight device  113  is not shown in FIG.  8 C. 
   The backlight device  113  includes a modified wedge-shaped light guide plate  514 . The front surface of the light guide plate  514  is in parallel with the liquid crystal panel  54 . The light guide plate  514  has symmetrical right and left halves with respect to a vertical center line CL. The light guide plate  514  tapers from the left and right sides toward the center line CL so that it is thinnest along the center line CL. The light guide plate  514  tapers also from the top toward the bottom. The rear surface of the light guide plate  514  is covered with a reflecting sheet  53 . The remainder of the structure of the backlight device  113 , including light guide bars  48  and  49  disposed along the side surfaces of the light guide plate  514 , is the same as the backlight device  111  shown in  FIGS. 7A ,  7 B and  7 C, and is not described again. 
   By making the light guide plate  514  thinnest along the vertical center line CL, light emitted by the LED&#39;s  32  and  33  entering inward into the light guide plate  514  can be efficiently directed toward the liquid crystal panel  54 . 
   As represented by broken line arrows, light from a cold cathode fluorescent lamp  10  enters downward into the light guide plate  514  and is scattered and reflected by the reflecting sheet  53  to propagate toward the liquid crystal panel  54  as indicated by broken line arrows in FIG.  8 B. LED&#39;s  32  and  33  emit downward directed light into the light guide bars  48  and  49 , respectively. The light is, then, reflected by a reflecting sheet  58  on each of the outer side surfaces and enters horizontally in the Y direction into the light guide plate  514  where it is scattered and reflected by the reflecting sheet  53  to propagate toward the liquid crystal panel  54 , as shown in FIG.  8 B. 
     FIGS. 9A ,  9 B and  9 C illustrate a liquid crystal display device with a backlight device  115  in accordance with a still further embodiment of the invention.  FIG. 9A  is a front view of the liquid crystal display device including the backlight device  115 .  FIGS. 9B and 9C  are right side and bottom views, respectively, of the liquid crystal display device of FIG.  9 A. In  FIG. 9C , a liquid crystal panel  54  disposed in front of the backlight device  115  is not shown. 
   The structure and design of the backlight device  115  is the same as the right half of the backlight device  113  shown in  FIGS. 8A-8C . The backlight device  115  requires only one LED  32  and only one light guide bar  48 , but an LED that can provide higher brightness may have to be used as the LED  32 . 
     FIGS. 10A and 10B  illustrate a liquid crystal display device with a backlight device  117  in accordance with a still further embodiment of the invention.  FIG. 10A  is a front view of the liquid crystal display device including the backlight device  117 .  FIG. 10B  is a right side view of the liquid crystal display device of FIG.  10 A. 
   The backlight device  117  includes a downward tapering light guide plate  519  which a typical liquid crystal display device employs, and an additional light guide plate  518  disposed between the light guide plate  519  and a liquid crystal panel  54 . The light guide plate  518  has substantially parallel front and rear surfaces, parallel top and bottom surfaces and parallel side surfaces, and is provided with a plurality of grooves  51  extending in the Y direction in the rear surface. The grooves  51  are arranged in succession in the X direction, whereby a succession of prismatic portions are formed in the rear portion of the light guide plate  518 . The prismatic portions scatters light entering into the light guide plate  518  in the X direction so that the light can propagate in the Z direction. An elongated light guide bar  44  for scattering light is disposed to extend on and along the top surface of the light guide plate  518 . Similarly to the light guide bar  44  shown in  FIGS. 4A and 4B , a plurality of grooves  41  extending in the Z direction are formed in the top surface of the light guide bar  44 . The grooves  41  are arranged in succession along the Y direction. LED&#39;s  32  and  34  are disposed adjacent to the right and left ends of the light guide bar  44 . A cold cathode fluorescent lamp  10  is disposed on top of the wedge-shaped light guide plate  519 . 
   The upper, front and rear sides of the cold cathode fluorescent lamp  10  are covered with reflecting plates  16 . The LED&#39;s  32  and  34  are covered with reflecting plates  36 , except the inward facing sides. The left and right side surfaces and the bottom surfaces of the light guide plates  518  and  519  are covered with reflecting sheets  58 . The rear surface of the light guide plate  519  is also covered with a reflecting sheet  53 . The top, front and rear surfaces of the light guide bar  44  are covered with the reflecting sheets  58 . 
   Light from the cold cathode fluorescent lamp  10  is directed from the top surface of the light guide plate  519  downward into it, where it is scattered and reflected by the reflecting sheet  53  on the rear surface of the light guide plate  519  and directed toward the liquid crystal panel  54 , as represented by broken line arrows in  FIGS. 10A and 10B . 
   The LED&#39;s  32  and  34  emit light, as represented by broken line arrows, in the horizontal Y direction into the light guide bar  44  through its right and left end surfaces. The light is, then, scattered by the prismatic portions in the top portion of the light guide bar  44  and directed downward into the light guide plate  518 , where it is further scattered by the prismatic portions provided by the horizontally extending grooves  51  and is directed toward the liquid crystal panel  54 , as shown in FIG.  10 B. Part of light entering through the rear surface of the light guide plate  518  into the light guide plate  519  is reflected by the reflecting sheet  53  back into the light guide plate  518  and propagates toward the liquid crystal panel  54 , as shown in FIG.  10 B. 
     FIGS. 11A and 11B  illustrate a liquid crystal display device with a backlight device  119  in accordance with a still further embodiment of the invention.  FIG. 11A  is a front view of the liquid crystal display device including the backlight device  119 .  FIG. 11B  is a right side view of the liquid crystal display device of FIG.  11 A. The backlight device  119  is the same as the backlight device  105  shown in  FIGS. 4A and 4B , except that the backlight device  119  includes a plurality of LED&#39;s  12  disposed at the right end of the light guide bar  44  and a plurality of LED&#39;s  34  disposed at the left end of the light guide bar  44 . The backlight device  119  can provide increased brightness by the use of plural LED&#39;s  32  and  34 . 
     FIGS. 12A ,  12 B and  12 C illustrate a liquid crystal display device with a backlight device  121  in accordance with a still further embodiment of the invention.  FIG. 12A  is a front view of the liquid crystal display device including the backlight device  121 .  FIGS. 12B and 12C  are right side and bottom views, respectively, of the liquid crystal display device of FIG.  12 A. The backlight device  121  is similar to the backlight device  109  shown in  FIGS. 6A ,  6 B and  6 C, except that a combination of LED&#39;s  32  and  34  and a light guide bar  44  is disposed on top of the light guide plate  510  in place of the cold cathode fluorescent lamp  10  and that two cold cathode fluorescent lamps  10  and  11  are disposed on the right and left side surfaces, respectively, of the light guide plate  510  in place of the combination of the LED&#39;s  32  and  34  with the light guide bar  44  and the combination of the LED&#39;s  33  and  35  with the light guide bar  47 . 
   The LED&#39;s  32  and  34  are disposed on the right and left end surfaces, respectively, of the light guide bar  44  disposed on top of the light guide plate  510 . The light guide bar  44  has a plurality of grooves extending in the Z direction arranged in the Y direction. 
   Although the two cold cathode fluorescent lamps  10  and  11  are used in this embodiment, only one cold cathode fluorescent lamp may be used. 
   Each of the cold cathode fluorescent lamps  10  and  11  is surrounded by reflecting plates  16 , except the side facing the light guide plate  510 . Each of the LED&#39;s  32  and  34  is also surrounded by reflecting plates  36 , except the side facing the light guide bar  44 . Also, the light guide bar  44  is surrounded by reflecting sheets  58 , except the side facing the light guide plate  510 . 
   The wedge-shaped light guide plate  510  has the same shape and configuration as the light guide plate  510  shown in  FIGS. 6A ,  6 B and  6 C, and is not described again. 
   Light from the cold cathode fluorescent lamps  10  and  11  enters in the horizontal Y direction into the light guide plate  510 , as represented by broken line arrows in  FIG. 12A , where it is scattered by the prismatic portions formed in the rear portion of the light guide plate  510  by the grooves  51 , so that it can propagate toward a liquid crystal panel  54 , as shown in FIG.  12 C. 
   Light from the LED&#39;s  32  and  34  enters into the light guide bar  44  in the horizontal Y direction, as represented by broken line arrows in  FIG. 12A , where it is scattered by the prismatic portions formed by the grooves  41  and directed downward into the light guide plate  510 . The light from the light guide bar  44 , then, is scattered and reflected by the reflecting sheet  53  on the rear surface of the light guide plate  510  so as to be directed toward the liquid crystal panel  54 , as represented by broken line arrows in FIG.  12 B. 
   The above-described embodiments are only typical examples, and a person skilled in the art may readily modify the illustrated embodiments to realize the objects of the present invention based on the principle of the present invention without departing the scope of the invention as defined by the accompanying claims, by, for example, appropriately combining the elements of the embodiments.