Abstract:
A back light is provided with light sources, lighting control circuits for lighting the light sources, respectively, and a brightness control circuit for outputting light volume adjusting signals to the lighting control circuits, respectively. The brightness control circuit receives a luminance adjusting signal, adjusts only a part of the light volume adjusting signals, and fixes output of the rest of the light volume adjusting signals. The lighting control circuits apply voltages corresponding to the light volume adjusting signals to the light sources, thereby adjusting the light sources in luminance. The light volume adjusting signals can be adjusted one by one, which facilitates luminance adjustment. Thus, by lighting only one light source at the minimum luminance, the screen brightness of the liquid crystal display having this back light can be lowered to an appropriate level when it is used in dark places.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a liquid crystal display and a back light to be arranged behind a liquid crystal panel in the liquid crystal display.  
           [0003]    2. Description of the Related Art  
           [0004]    Generally, liquid crystal displays have a liquid crystal panel and a back light for irradiating the backside of the liquid crystal panel with light. In recent years, liquid crystal displays have improved and approached CRTs (Cathode Ray Tubes) in terms of performance. Liquid crystal displays thus have found an increasing range of applications, including navigation systems to be mounted on motor vehicles.  
           [0005]    In motor vehicles, the interior brightness varies greatly between daytime and nighttime hours. Cars are dark inside in the nighttime, so that cars&#39; navigation systems must be lowered sufficiently in screen brightness in the nighttime. That is, liquid crystal displays for use in the navigation systems need to have a smaller minimum luminance for the sake of nighttime use.  
           [0006]    [0006]FIG. 1 shows a block diagram of a control circuit in a back light  10  to be used for this type of liquid crystal display. In the diagram, the back light  10  includes an oscillating circuit  12 , lighting circuits  14   a,    14   b,  and  14   c,  a light volume adjusting circuit  16 , and fluorescent tubes  18   a,    18   b,  and  18   c.    
           [0007]    The light volume adjusting circuit  16  receives brightness adjusting input which is generated in accordance with a luminance adjusting signal from exterior, and outputs, to the oscillating circuit  12 , a light volume adjusting signal for adjusting the fluorescent tubes  18   a,    18   b,  and  18   c  in brightness. The oscillating circuit  12  generates an alternating voltage corresponding to the light volume adjusting signal out of the power supplied from a power source, and outputs the generated alternating voltage to the lighting circuits  14   a,    14   b,  and  14   c.  The lighting circuits  14   a,    14   b,  and  14   c  boost the alternating voltage output from the oscillating circuit  12 , and supply the boosted voltages to the fluorescent tubes  18   a,    18   b,  and  18   c.  The fluorescent tubes  18   a,    18   b,  and  18   c  light up at luminances corresponding to the voltage waveforms supplied.  
           [0008]    [0008]FIG. 2 shows an overview of structure of a light emitting part  10   a  in the back light  10 . The light emitting part  10   a  has a reflector  20  for accommodating the fluorescent tubes  18   a,    18   b,  and  18   c.    
           [0009]    The inner surface of the reflector  20  is given a reflecting coat of metal. The fluorescent tubes  18   a,    18   b,  and  18   c  are arranged in parallel inside the reflector  20 . The light emitted from the fluorescent tubes  18   a,    18   b,  and  18   c  radiates out directly or after reflected from the inner surface of the reflector  20 .  
           [0010]    In the back light  10  shown in FIG. 1, the single light volume adjusting circuit  16  adjusts the plurality of fluorescent tubes  18   a,    18   b,  and  18   c  in luminance. On this account, when the back light  10  produces an output of the minimum luminance, the fluorescent tubes  18   a,    18   b,  and  18   c  are lit at their respective minimum luminances. Consequently, the minimum luminance possible for the back light  10  to output is the sum of the minimum luminances of the individual fluorescent tubes  18   a,    18   b,  and  18   c.  When liquid crystal displays having such a back light are applied to the navigation systems, the screen brightness cannot be lowered to an appropriate brightness in nighttime use.  
           [0011]    According to the structure of the light emitting part  10   a  of the back light  10  shown in FIG. 2, the central fluorescent tube  18   b  faces a smaller area of the reflector  20  and the outer fluorescent tubes  18   a  and  18   b  face greater areas of the reflector  20 . The parasitic capacitance occurring between the fluorescent tube  18   b  and the reflector  20  is therefore smaller than the parasitic capacitance occurring between the fluorescent tube  18   a  and the reflector  20 , and the parasitic capacitance occurring between the fluorescent tube  18   c  and the reflector  20 .  
           [0012]    Therefore, the current to flow through the fluorescent tube  18   b  is greater than the currents to flow the fluorescent tubes  18   a  and  18   c.  This shortens the life of the fluorescent tube  18   b  more than the lives of the fluorescent tubes  18   a  and  18   c.  In general, fluorescent tubes of a back light cannot be replaced separately. Thus, the entire back light must be replaced when any one of the fluorescent tubes no longer works. That is, the life of a back light becomes shorter depending on the fluorescent tube of the shortest life.  
           [0013]    Recently, parts of the liquid crystal displays tend to get smaller in size due to a growing demand for liquid crystal panels of larger size. The reflectors accommodating the fluorescent tubes of the back lights also have the inclination to shrink in size. This results in reducing interior spaces of the reflectors and easy trapping of heat within the reflectors. Consequently, if a plurality of fluorescent tubes is used, there is a possibility that concentrate generated heat therein may hamper sufficient heat dissipation.  
         SUMMARY OF THE INVENTION  
         [0014]    It is an object of the present invention to provide a back light capable of further reducing the minimum value of luminance and a liquid crystal display equipped with this back light.  
           [0015]    Another object of the present invention is to prolong the life of the fluorescent tubes constituting the back light.  
           [0016]    Another object of the present invention is to dissipate heat produced by the light sources constituting the back light with higher efficiency.  
           [0017]    According to one of the aspects of the back light and the liquid crystal display of the present invention, a brightness control circuit selects at least one of light volume adjusting signals and fixes output of the light volume adjusting signals other than the selected light volume adjusting signal. On this account, the light sources other than the light source corresponding to the selected light volume adjusting signal will not change in luminance. The light sources are arranged, for example, behind a liquid crystal panel.  
           [0018]    When a luminance adjusting signal changes, the brightness control circuit adjusts the selected light volume adjusting signal and outputs the same to a lighting control circuit so that the luminance of the back light (the total luminance of the light sources) approaches a luminance indicated by the luminance adjusting signal. The lighting control circuit receiving the adjusted light volume adjusting signal generates a voltage having a waveform corresponding to the received light volume adjusting signal and applies the same to the light source. The light source is on at a luminance corresponding to the waveform of the voltage applied from the lighting control circuit. Then, the light source corresponding to the selected light volume adjusting signal is adjusted in luminance, so that the back light changes in luminance. Subsequently, the brightness control circuit fixes the output of the selected light volume adjusting signal and selects one or more of the light volume adjusting signals according to need. Then, the same luminance adjustment as described above is performed.  
           [0019]    The back light of the present invention can adjust the light volume adjusting signals one by one to change the luminances of the light sources, whereby the luminance adjustment control of the back light is facilitated. Since the plurality of light sources can be successively adjusted in luminance, minimum light sources alone can be turned on at their respective minimum luminances with the rest of the light sources turned off. For example, turning only a single light source on at its minimum luminance enables the minimum luminance of a single light source to be the minimum luminance of the back light. Consequently, when the back light and the liquid crystal display of the present invention are used in dark places, the screen brightness can be lowered to an appropriate brightness.  
           [0020]    According to another aspect of the back light of the present invention, an adjusting part adjusts the selected light volume adjusting signal to lower its corresponding light source in luminance (luminance-down operation) when the luminance indicated by the luminance adjusting signal is smaller than the actual total luminance of the light sources. A detecting part outputs a detecting signal to the adjusting part when the luminance of the light source corresponding to the selected light volume adjusting signal is minimum.  
           [0021]    On receiving the detecting signal, the adjusting part fixes the selected light volume adjusting signal to a turn-off level to turn off the corresponding light source (turn-off operation), and selects anew at least one of the rest of the light volume adjusting signals corresponding to a lit light source (selecting operation). In this state, when the luminance indicated by the luminance adjusting signal is smaller than the actual luminance of the back light, the adjusting part adjusts the newly selected light volume adjusting signal to lower the corresponding light source in luminance.  
           [0022]    The adjusting part thus performs the luminance-down operation, turn-off operation, and selecting operation in succession to adjust the luminance of the back light. Since the reception of the detecting signal can facilitate detecting that the light source has reached its minimum luminance, the control of lowering the luminance of the back light can be simplified.  
           [0023]    According to another aspect of the back light of the present invention, the adjusting part temporarily adjusts the light volume adjusting signal corresponding to at least one of the lit light sources in synchronization with the selected light volume adjusting signal&#39;s turning to the turn-off level, thereby enhancing the corresponding light source in luminance temporarily. Consequently, the back light can be prevented from dropping in luminance when a light source lit at its minimum luminance is turned off.  
           [0024]    According to another aspect of the back light of the present invention, the detecting part includes a monitoring circuit for directly detecting currents supplied to the light sources. The detecting part can thus detect the luminances of the light sources with higher precision. That is, the detecting part can detect more precisely that light sources are lit at the minimum luminances. This allows the detecting part to output the detecting signal to the adjusting part with more accurate timing. As a result, the detecting part can perform precise control of lowering the luminance of the back light.  
           [0025]    According to another aspect of the back light of the present invention, a fluorescent tube causing a smaller parasitic capacitance with the reflector is subjected to an effective voltage relatively lower than that applied to the rest of the fluorescent tubes. Consequently, the fluorescent tube with smaller parasitic capacitance is supplied with a current smaller than the rest of the fluorescent tubes are. The fluorescent tube with smaller parasitic capacitance leaks a current smaller than the rest of the fluorescent tubes do, resulting in equalizing the currents which contribute to light emission of the fluorescent tubes. The lives of fluorescent tubes can thus be made equal to each other. As a result, the life of the back light can be prolonged.  
           [0026]    According to another aspect of the back light of the present invention, a fluorescent tube causing a smaller parasitic capacitance with the reflector is smaller in diameter than the rest of the fluorescent tubes, and this is the smaller the diameter, the higher the impedance. Consequently, given that the same effective voltage is applied to all the fluorescent tubes, the fluorescent tube with higher impedance is supplied with a current smaller than the rest of the fluorescent tubes are. Here, the fluorescent tube with higher impedance leaks a current smaller than the rest of the fluorescent tubes do, so that the amounts of the currents contributing to light emission of the fluorescent tubes become equal to each other. This enables the fluorescent tubes to be identical in life. The life of the back light can be prolonged accordingly.  
           [0027]    According to another aspect of the back light of the present invention, the fluorescent tube causing a smaller parasitic capacitance with the reflector is higher in internal gas pressure than the rest of the fluorescent tubes, and this is the smaller the internal gas pressure, the higher the impedance. Consequently, given that the same effective voltage is applied to all the fluorescent tubes, the fluorescent tube with higher impedance is supplied with a current smaller than the rest of the fluorescent tubes are. Here, the fluorescent tube with higher impedance leaks a current smaller than the rest of the fluorescent tubes do so that the amounts of the currents contributing to light emission of the fluorescent tubes become equal to each other. This enables the lives of the fluorescent tubes to be identical. The life of the back light can be prolonged accordingly.  
           [0028]    According to another aspect of the back light of the present invention, each reflector accommodates a single light source. This allows dissipation of heat produced by the light sources with higher efficiency.  
           [0029]    In addiction, the parasitic capacitances occurring between the light sources and the respective reflectors can be substantially equalized so that the amounts of the current leakage from the light sources to the respective reflectors can be also equalized. Accordingly, the amounts of the currents contributing to light emission of the light sources become equal to each other, whereby the light sources can be made identical in life. As a result, the life of the back light can be prolonged. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0030]    The nature, principle, and utility of the invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings in which like parts are designated by identical reference numbers, in which:  
         [0031]    [0031]FIG. 1 is a block diagram of the control circuit in a conventional back light;  
         [0032]    [0032]FIG. 2 is an explanatory diagram showing an overview of the structure of a light emitting part in the conventional back light;  
         [0033]    [0033]FIG. 3 is a block diagram showing a structure of the liquid crystal display according to a first embodiment of the present invention;  
         [0034]    [0034]FIG. 4 is a block diagram showing the details of control in the back light of FIG. 3;  
         [0035]    [0035]FIG. 5 is an explanatory diagram showing luminance variations of the individual fluorescent tubes and the back light of the first embodiment;  
         [0036]    [0036]FIG. 6 is a block diagram showing a structure of the liquid crystal display according to a second embodiment of the present invention;  
         [0037]    [0037]FIG. 7 is a block diagram showing the details of control in the back light of FIG. 6;  
         [0038]    [0038]FIG. 8 is an explanatory diagram showing luminance variations of the individual fluorescent tubes and the back light of the second embodiment;  
         [0039]    [0039]FIG. 9 is a block diagram showing a structure of the liquid crystal display according to a third embodiment of the present invention;  
         [0040]    [0040]FIG. 10 is a block diagram showing the details of control in the back light of FIG. 9;  
         [0041]    [0041]FIG. 11 is an explanatory diagram showing luminance variations of the individual fluorescent tubes and the back light of the third embodiment;  
         [0042]    [0042]FIG. 12 is a block diagram showing a fourth embodiment of the liquid crystal display of the present invention; and  
         [0043]    [0043]FIG. 13 is a block diagram showing a structural example of the back light in which the fluorescent tubes are disposed on only one side of a light guide plate.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0044]    Hereinafter, embodiments of the present invention will be described with reference to the drawings.  
         [0045]    [0045]FIG. 3 shows a first embodiment of the liquid crystal display of the present invention. In the diagram, a liquid crystal display  28  includes a liquid crystal panel  30 , a back light  32 , and a case  34  for accommodating the liquid crystal panel  30  and the back light  32 .  
         [0046]    The liquid crystal panel  30  has a structure that liquid crystal is sealed between two transparent glass plates which are opposed to each other with a predetermined spacing. The liquid crystal panel  30  is of TN (Twisted Nematic) type, for example. The liquid crystal panel  30  is not limited to TN type but may be of, e.g., VA (Vertically Aligned) type.  
         [0047]    The back light  32  includes a prism sheet  36 , a diffusion sheet  38 , a light guide plate  40  (light pipe), a reflecting sheet  42 , and light emitting parts  44 A arranged on both sides of the light guide plate  40 , respectively, as well as a brightness control circuit  48  and lighting control circuits  50   a,    50   b,  and  50   c  which are shown in FIG. 4 to be seen later. The light emitting parts  44 A contain fluorescent tubes  46   a,    46   b,  and  46   c  each.  
         [0048]    The prism sheet  36  is arranged behind the liquid crystal panel  30 . The diffusion sheet  38  is arranged behind the prism sheet  36 . The light guide plate  40  is arranged behind the diffusion sheet  38  with its light emitting side toward the diffusion sheet  38 . The light guide plate  40  is made of transparent synthetic resin such as acrylic, and has a generally flat shape. The reflecting sheet  42  is arranged on the back of the light guide plate  40 .  
         [0049]    The light emitting parts  44 A have a reflector  45  of U-shaped section. The light emitting parts  44 A are arranged with the apertures of the respective reflectors  45  toward sides of the light guide plate  40 . The inner surfaces of the reflectors  45  are provided with a reflecting part  52  (reflecting coat) for reflecting light emitted from the fluorescent tubes  46   a,    46   b,  and  46   c.  Incidentally, the reflectors  45  are not limited to the foregoing configuration. For example, reflectors of semicircular section may be used.  
         [0050]    The fluorescent tubes  46   a,    46   b,  and  46   c  are identical to one another. The fluorescent tubes  46   a,    46   b,  and  46   c  are arranged at regular intervals in the respective reflectors  45  so as to face the sides of the light guide plate  40 . The fluorescent tubes  46   a,    46   b,  and  46   c  are subjected to voltages from the lighting control circuits  50   a,    50   b,  and  50   c  to be described later, and light up at luminances corresponding to the waveforms of these voltages. The minimum luminances possible for the fluorescent tubes  46   a,    46   b,  and  46   c  to light at with stability are approximately 20% the maximum luminances.  
         [0051]    In the present embodiment, luminance adjustment control of the fluorescent tubes  46   a,    46   b,  and  46   c  accommodated in one of the light emitting parts  44 A and of the fluorescent tubes  46   a,    46   b,  and  46   c  accommodated in the other light emitting part  44 A are performed in synchronization with each other. For this reason, the following description will exclusively deal with the luminance adjustment control of the fluorescent tubes  46   a,    46   b,  and  46   c  accommodated in one of the light emitting parts  44 A.  
         [0052]    [0052]FIG. 4 shows the details of control in the back light  32 . The back light  32  has the brightness control circuit  48 , the lighting control circuits  50   a,    50   b,  and  50   c,  an oscillating circuit  54 , and the fluorescent tubes  46   a,    46   b,  and  46   c.  The brightness control circuit  48  is composed of a detecting part  56  and an adjusting part  58 .  
         [0053]    The detecting part  56  includes a monitoring circuit  60 . The monitoring circuit  60  detects the currents supplied to the fluorescent tubes  46   a,    46   b,  and  46   c,  and outputs the luminance of the back light  32  (the total luminance of the fluorescent tubes  46   a,    46   b,  and  46   c ) determined from the detected currents to the adjusting part  58  as a luminance detecting signal VD. The monitoring circuit  60  outputs a detecting signal DET 1  to the adjusting part  58  when it detects that a fluorescent tube (any of  46   a,    46   b,  and  46   c ) reaches a minimum luminance.  
         [0054]    The adjusting part  58  has a comparing unit  62 , a selecting unit  64 , and signal generating units  66   a,    66   b,  and  66   c.  The comparing unit  62  receives a luminance adjusting signal LUM for adjusting the luminance of the back light  32  from exterior. The comparing unit  62  also receives the luminance detecting signal VD which indicates the luminance of the back light  32 . The comparing unit  62  outputs a gap between the luminance indicated by the luminance adjusting signal LUM and the luminance of the back light  32  indicated by the luminance gap signal GAP. The comparing unit  62  also outputs a detecting signal DET 2  to the selecting unit  64  in synchronization with the detecting signal DET 1 .  
         [0055]    The selecting unit  64  outputs any one of selecting signals OPTa, OPTb, and OPTc all the time (always keeps any one of them at high level). The selecting signal OPTa is output to the signal generating unit  66   a.  The selecting signal OPTb is output to the signal generating unit  66   b.  The selecting signal OPTc is output to the signal generating unit  66   c.    
         [0056]    The selecting unit  64  outputs the selecting signal OPTa when the back light  32  is at the maximum in luminance. Upon receiving the detecting signal DET 2  while outputting the selecting signal OPTa, the selecting unit  64  stops outputting the selecting signal OPTa (turns the selecting signal OPTa to low level) in synchronization with the reception and outputs the selecting signal OPTb (turns the selecting signal OPTb to high level). Upon receiving the detecting signal DET 2  while outputting the selecting signal OPTb, the selecting unit  64  stops outputting the selecting signal OPTb (turns the selecting signal OPTb to low level) in synchronization with the reception and outputs the selecting signal OPTc (turns the selecting signal OPTc to high level). Upon receiving the detecting signal DET 2  while outputting the selecting signal OPTc, the selecting unit  64  keeps outputting the selecting signal OPTc without any change in output.  
         [0057]    The signal generating unit  66   a,  while receiving the selecting signal OPTa of high level, operates to adjust the fluorescent tube  46   a  in luminance. More specifically, the signal generating unit  66   a  outputs, to the lighting control circuit  50   a,  a light volume adjusting signal ADJa for adjusting the luminance of the fluorescent tube  46   a  so that the luminance gap signal GAP decreases. Moreover, in synchronization with the falling edge of the selecting signal OPTa, the signal generating unit  66   a  fixes the light volume adjusting signal ADJa to a turn-off level for turning off the fluorescent tube  46   a.    
         [0058]    The signal generating unit  66   b,  while receiving the selecting signal OPTb of high level, operates to adjust the fluorescent tube  46   b  in luminance. More specifically, the signal generating unit  66   b  outputs, to the lighting control circuit  50   b,  a light volume adjusting signal ADJb for adjusting the luminance of the fluorescent tube  46   b  so that the luminance gap signal GAP decreases. Moreover, in synchronization with the falling edge of the selecting signal OPTb, the signal generating unit  66   b  fixes the light volume adjusting signal ADJb to a turn-off level for turning off the fluorescent tube  46   b.    
         [0059]    The signal generating unit  66   c,  while receiving the selecting signal OPTc of high level, operates to adjust the fluorescent tube  46   c  in luminance. More specifically, the signal generating unit  66   c  outputs, to the lighting control circuit  50   c,  a light volume adjusting signal ADJc for adjusting the luminance of the fluorescent tube  46   c  so that the luminance gap signal GAP decreases.  
         [0060]    The signal generating units  66   a,    66   b,  and  66   c  fix the output of the respective light volume adjusting signals ADJa, ADJb, and ADJc unless receiving the selecting signals OPTa, OPTb, and OPTc (while the selecting signals OPTa, OPTb, and OPTc are at low level).  
         [0061]    Now, the light volume adjusting signal ADJb to be output from the signal generating unit  66   b  is set to be relatively smaller than the light volume adjusting signal ADJa and the light volume adjusting signal ADJc. On this account, the effective voltage that the lighting control circuit  50   b  applies to the fluorescent tube  46   b  is relatively lower than the effective voltage that the lighting control circuit  50   a  applies to the fluorescent tube  46   a  and the effective voltage that the lighting control circuit  50  applies to the fluorescent tube  46   c.    
         [0062]    To put it in details, the fluorescent tube  46   b  arranged in the center of the reflector  45  shown in FIG. 3 faces a smaller area of the reflecting part  52  than the outer fluorescent tubes  46   a  and  46   c  do. That is, the parasitic capacitance occurring between the fluorescent tube  46   b  and the reflector  45  is smaller than the parasitic capacitance occurring between the fluorescent tube  46   a  and the reflector  45  and the parasitic capacitance occurring between the fluorescent tube  46   c  and the reflector  45 . This makes the leakage current from the fluorescent tube  46   b  to the reflector  45  smaller than the leakage currents from the fluorescent tubes  46   a  and  46   c  to the reflector  45 . When the effective voltage to be applied to the fluorescent tube  46   b  is made relatively lower than the effective voltages to be applied to the fluorescent tubes  46   a  and  46   c  as mentioned above, the current supplied to the fluorescent tube  46   b  becomes smaller than the currents supplied to the fluorescent tubes  46   a  and  46   c.  Here, the current leaking from the fluorescent tube  46   b  of smaller parasitic capacitance to the reflector  45  is smaller than the currents leaking from the fluorescent tubes  46   a  and  46   c.  Then, the fluorescent tubes  46   a,    46   b,  and  46   c  turn out to be identical in the current that contributes to light emission.  
         [0063]    The lighting control circuit  50   a  is formed for the fluorescent tube  46   a,  and has a light volume adjusting circuit  70   a  and a lighting circuit  72   a.  The lighting control circuit  50   b  is formed for the fluorescent tube  46   b,  and has a light volume adjusting circuit  70   b  and a lighting circuit  72   b.  The lighting control circuit  50   c  is formed for the fluorescent tube  46   c,  and has a light volume adjusting circuit  70   c  and a lighting circuit  72   c.    
         [0064]    The light volume adjusting circuits  70   a,    70   b,  and  70   c  are supplied with an alternating voltage from the oscillating circuit  54 . The light volume adjusting circuit  70   a  receives the light volume adjusting signal ADJa, adjusts the alternating voltage in amplitude according to the luminance indicated by the light volume adjusting signal ADJa, and outputs the adjusted alternating voltage to the lighting circuit  72   a.  The light volume adjusting circuit  70   b  receives the light volume adjusting signal ADJb, adjusts the alternating voltage in amplitude according to the luminance indicated by the light volume adjusting signal ADJb, and outputs the adjusted alternating voltage to the lighting circuit  72   b.  The light volume adjusting circuit  70   c  receives the light volume adjusting signal ADJc, adjusts the alternating voltage in amplitude according to the luminance indicated by the light volume adjusting signal ADJc, and outputs the adjusted alternating voltage to the lighting circuit  72   c.    
         [0065]    Each of the lighting circuits  72   a,    72   b,  and  72   c  boosts the alternating voltage supplied from the light volume adjusting circuit and applies the boosted voltage to the corresponding fluorescent tube (any of  46   a,    46   b,  and  46   c ).  
         [0066]    Incidentally, in the present embodiment, the alternative voltages are adjusted in amplitude (effective value) to adjust the luminances of the respective fluorescent tubes  46   a,    46   b,  and  46   c.  Otherwise, the alternative voltages to be applied to the fluorescent tubes may be adjusted in period (duty ratio control) to adjust the luminances of the respective fluorescent tubes.  
         [0067]    Now, description will be given of the luminance adjustment of the back light  32  described above.  
         [0068]    [0068]FIG. 5 shows luminance variations of the fluorescent tubes  46   a,    46   b,  and  46   c,  and the back light  32 . In this example, in an initial state (at time t 0 ), the signal generating units  66   a,    66   b,  and  66   c  shown in FIG. 4 output light volume adjusting signals ADJa, ADJb, and ADJc for lighting the corresponding fluorescent tubes  46   a,    46   b,  and  46   c  at the maximum luminances, respectively. The selecting unit  64  outputs the selecting signal OPTa to select the signal generating unit  66   a.  The fluorescent tubes  46   a,    46   b,  and  46   c  are lit at their respective maximum luminances. That is, at time t 0 , the back light  32  is at the maximum in luminance.  
         [0069]    In this example, the comparing unit  62  receives, at time t 1 , a luminance adjusting signal LUM indicating that the back light  32  is to be minimized in luminance. Here, the luminance indicated by the luminance adjusting signal LUM is smaller than the luminance indicated by the luminance detecting signal VD. The comparing unit  62  thus outputs a luminance gap signal GAP for lowering the luminance of the back light. The signal generating unit  66   a  is in a luminance adjustable state, receiving the selecting signal OPTa of high level. The signal generating unit  66   a  outputs to the light volume adjusting circuit  70   a  a light volume adjusting signal ADJa for lowering the luminance of the fluorescent tube  46   a  (luminance-down operation).  
         [0070]    Consequently, the fluorescent tube  46   a  gradually decreases in luminance (FIG. 5( a )). With the decreasing luminance of the fluorescent tube  46   a,  the back light  32  also decreases in luminance (FIG. 5( b )). Since the output of the signal generating units  66   b  and  66   c  is fixed, the fluorescent tubes  46   b  and  46   c  have no change in luminance (FIG. 5( c )).  
         [0071]    At time t 2 , the luminance of the fluorescent tube  46   a  reaches the minimum luminance, and the detecting part  56  outputs the detecting signal DET 1  (FIG. 5( d )). In synchronization with the reception of the detecting signal DET 1 , the comparing unit  62  outputs the detecting signal DET 2  (FIG. 5( d )). In response to the detecting signal DET 2 , the selecting unit  64  turns the selecting signal OPTa to low level and turns the selecting signal OPTb to high level. In synchronization with the falling edge of the selecting signal OPTa, the signal generating unit  66   a  fixes the light volume adjusting signal ADJa to the turn-off level for turning off the fluorescent tube  46   a  (turn-off operation). Then, the fluorescent tube  46   a  goes out (FIG. 5( e )). The signal generating unit  66   b  receives the selecting signal OPTb of high level and enters a luminance adjustable state. The signal generating unit  66   b  outputs to the light volume adjusting circuit  70   b  a light volume adjusting signal ADJb for lowering the luminance of the fluorescent tube  46   b  (luminance-down operation). Consequently, the fluorescent tube  46   b  decreases in luminance (FIG. 5( f )). With the decreasing luminance of the fluorescent tube  46   b,  the back light  32  also decreases in luminance (FIG. 5( g )).  
         [0072]    At time t 3 , the luminance of the fluorescent tube  46   b  reaches the minimum luminance, and the detecting part  56  outputs the detecting signal DET 1  (FIG. 5( h )). In synchronization with the detecting signal DET 1 , the comparing unit  62  outputs the detecting signal DET 2  (FIG. 5( h )). In response to the detecting signal DET 2 , the selecting unit  64  turns the selecting signal OPTb to low level and turns the selecting signal OPTc to high level. In synchronization with the falling edge of the selecting signal OPTb, the signal generating unit  66   b  fixes the light volume adjusting signal ADJb to the turn-off level for turning off the fluorescent tube  46   b  (turn-off operation). Then, the fluorescent tube  46   b  goes out (FIG. 5( i )). The signal generating unit  66   c  receives the selecting signal OPTc of high level and enters a luminance adjustable state. The signal generating unit  66   c  outputs to the light volume adjusting circuit  70   c  a light volume adjusting signal ADJc for lowering the luminance of the fluorescent tube  46   c  (luminance-down operation). Consequently, the fluorescent tube  46   c  decreases in luminance (FIG. 5( j )). With the decreasing luminance of the fluorescent tube  46   c,  the back light  32  also decreases in luminance (FIG. 5( k )).  
         [0073]    At time t 4 , the luminance of the fluorescent tube  46   c  reaches the minimum luminance, and the detecting part  56  outputs the detecting signal DET 1  (FIG. 5( l )). In synchronization with the detecting signal DET 1 , the comparing unit  62  outputs the detecting signal DET 2  (FIG. 5( l )). Fluorescent tubes lit here are the fluorescent tube  46   c  alone. That is, at time t 4 , the back light  32  becomes minimum in luminance. Here, the luminance indicated by the luminance detecting signal VD becomes equal to the luminance indicated by the luminance adjusting signal LUM, and the comparing unit  62  outputs a luminance gap signal GAP of 0 in magnitude. When the fluorescent tube  46   c  lit to the end becomes minimum in luminance, the brightness control circuit  48  makes no turn-off operation and keeps the fluorescent tube  46   c  lit at the minimum luminance.  
         [0074]    In a case where the fluorescent tubes  46   a,    46   b,  and  46   c  are lit at their maximum luminances, the luminance of the back light  32  is maximum. Also, when only a single fluorescent tube ( 46   c,  in this example) in each light emitting part  44 A is lit at its minimum luminance, the luminance of the back light  32  is minimum. Hence, the minimum luminance of the back light  32  is one-third that of conventional back lights in which all the fluorescent tubes are lit together at their minimum luminances.  
         [0075]    As has been described, according to the present embodiment, the adjusting part  58  has only to receive the detecting signal DET 1  to easily detect that any one of the fluorescent tubes  46   a,    46   b,  and  46   c  reaches its minimum luminance. Thus, the adjusting part  58  can easily switch the fluorescent tube to adjust (any one of  46   a,    46   b,  and  46   c ). This can facilitate the control in lowering the luminance of the back light  32 .  
         [0076]    Since the detecting part  56  directly detects the currents supplied to the fluorescent tubes  46   a,    46   b,  and  46   c  by using the monitoring circuit  60 , it can easily detect that any one of the fluorescent tubes  46   a,    46   b,  and  46   c  reaches the minimum luminance.  
         [0077]    The fluorescent tube  46   b,  which causes a smaller parasitic capacitance with the reflector  45 , is subjected to an effective voltage relatively lower than the effective voltages applied to the other fluorescent tubes  46   a  and  46   c.  This can make the florescent tubes  46   a,    46   b,  and  46   c  identical in the current that contributes to light emission. The fluorescent tubes  46   a,    46   b,  and  46   c  can thus be made identical in life. The result is that the fluorescent tube  46   b,  causing a smaller parasitic capacitance with the reflector  45 , is prolonged in life, allowing extended life of the back light  32 .  
         [0078]    [0078]FIG. 6 shows a second embodiment of the liquid crystal display of the present invention. The same elements as those of the first embodiment will be designated by identical reference numbers or symbols. Detailed description will be omitted of these elements. A liquid crystal display  28   b  includes a liquid crystal panel  30 , a back light  32   b,  and a case  34  for accommodating the liquid crystal panel  30  and the back light  32   b.    
         [0079]    The back light  32   b  includes a prism sheet  36 , a diffusion sheet  38 , a light guide plate  40 , a reflecting sheet  42 , light emitting parts  44 B arranged on both sides of the light guide plate  40 , a brightness control circuit  48   b,  and lighting control circuits  50   a,    50   b,  and  50   c  which are shown in FIG. 5 and to be described later.  
         [0080]    The back light  32   b  has the same configuration as that of the first embodiment except that the diameters of fluorescent tubes  46   d  arranged in the centers of reflectors  45  of the light emitting parts  44 B are smaller than those of fluorescent tubes  46   a  and  46   c  arranged on both sides of the respective fluorescent tubes  46   d,  and that signal generating units  66   d  and  66   e  (shown in FIG. 7 to be seen later) are formed instead of the signal generating units  66   b  and  66   c  of the first embodiment.  
         [0081]    The fluorescent tubes  46   d  are smaller in diameter than the fluorescent tubes  46   a  and  46   c,  and thus are higher in impedance than the fluorescent tubes  46   a  and  46   c.  Accordingly, given that the same effective voltage is applied to all the fluorescent tubes  46   a,    46   d,  and  46   c,  the currents supplied to the fluorescent tubes  46   d  of higher impedance become smaller than the currents supplied to the other fluorescent tubes  46   a  and  46   c.  Moreover, as in the first embodiment, the fluorescent tubes  46   d  arranged in the centers of the reflectors  45  are less prone to leak currents to the reflectors  45  than the fluorescent tubes  46   a  and  46   c  do. Consequently, the currents leaking from the fluorescent tubes  46   d  of higher impedance to the reflector  45  become smaller than the currents leaking from the fluorescent tubes  46   a  and  46   c.  Then, the fluorescent tubes  46   a,    46   d,  and  46   c  turn out to be identical in the current that contributes to light emission. Thus, as in the first embodiment, the fluorescent tubes  46   a,    46   d,  and  46   c  become identical in life.  
         [0082]    In the present embodiment, the lighting control circuits  50   a,    50   b,  and  50   c  shown in FIG. 5 to be seen later apply relatively the same effective voltages to the respective fluorescent tubes  46   a,    46   d,  and  46   c.    
         [0083]    Now, in the present embodiment, luminance adjustment control of the fluorescent tubes  46   a,    46   d,  and  46   c  accommodated in one of the light emitting parts  44 B and luminance adjustment control of the fluorescent tubes  46   a,    46   d,  and  46   c  accommodated in the other light emitting part  44 B are performed in synchronization with each other. For this reason, the following description will exclusively deal with the luminance adjustment control of the fluorescent tubes  46   a,    46   d,  and  46   c  accommodated in one of the light emitting parts  44 B.  
         [0084]    [0084]FIG. 7 shows the details of control of the back light  32   b.    
         [0085]    The signal generating unit  66   d  receives a luminance gap signal GAP and a selecting signal OPTb, and outputs to the lighting control circuit  50   b  a light volume adjusting signal ADJd for adjusting the luminance of the fluorescent tube  46   d.  In response to the rising edge of the selecting signal OPTb, the signal generating unit  66   d  enhances the light volume adjusting signal ADJd steeply so that the fluorescent tube  46   d  increases in luminance as much as the minimum luminance of the fluorescent tube  46   a.  Subsequently, while receiving the selecting signal OPTb of high level, the signal generating unit  66   d  outputs to the lighting control circuit  50   b  a light volume adjusting signal ADJd for adjusting the luminance of the fluorescent tube  46   d  so that the luminance gap signal GAP decreases. Besides, in synchronization with the falling edge of the selecting signal OPTb, the signal generating unit  66   d  fixes the light volume adjusting signal ADJd to a turn-off level for turning off the fluorescent tube  46   d.    
         [0086]    The signal generating unit  66   e  receives the luminance gap signal GAP and a selecting signal OPTc, and outputs to the lighting control circuit  50   c  a light volume adjusting signal ADJe for adjusting the luminance of the fluorescent tube  46   c.  In response to the rising edge of the selecting signal OPTc, the signal generating unit  66   e  enhances the light volume adjusting signal ADJe steeply so that the fluorescent tube  46   c  increases in luminance as much as the minimum luminance of the fluorescent tube  46   d.  Subsequently, while receiving the selecting signal OPTc of high level, the signal generating unit  66   e  outputs to the lighting control circuit  50   c  a light volume adjusting signal ADJe for adjusting the luminance of the fluorescent tube  46   c  so that the luminance gap signal GAP decreases.  
         [0087]    Incidentally, the signal generating units  66   d  and  66   e  fix the output of the light volume adjusting signals ADJd and ADJe while the selecting signals OPTb and OPTc are at low level.  
         [0088]    Now, description will be given of the luminance adjustment of the back light  32   b  described above.  
         [0089]    [0089]FIG. 8 shows luminance variations of the fluorescent tubes  46   a,    46   d,  and  46   c,  and the back light  32   b.  In this example, as with the first embodiment, the fluorescent tubes  46   a,    46   d,  and  46   c  are lit at their respective maximum luminances in an initial state (at time t 0 ). In addition, the selecting unit  64  outputs the selecting signal OPTa of high level and keeps the selecting signals OPTb and OPTc at low level.  
         [0090]    At time t 1 , a luminance adjusting signal LUM indicating that the back light  32   b  is to be minimized in luminance is supplied, in which case the fluorescent tube  46   a  gradually decreases in luminance (FIG. 8( a )). With the decreasing luminance of the fluorescent tube  46   a,  the back light  32   b  also decreases in luminance (FIG. 8( b )). Since the output of the signal generating units  66   d  and  66   e  is fixed, the fluorescent tubes  46   d  and  46   c  have no change in luminance (FIG. 8( c )).  
         [0091]    At time t 2 , the luminance of the fluorescent tube  46   a  reaches the minimum luminance, and the detecting part  56  outputs the detecting signal DET 1  (FIG. 8( d )). In synchronization with the detecting signal DET 1 , the comparing unit  62  outputs the detecting signal DET 2  (FIG. 8( d )). In response to the detecting signal DET 2 , the selecting unit  64  turns the selecting signal OPTa to low level and turns the selecting signal OPTb to high level. In synchronization with the falling edge of the selecting signal OPTa, the signal generating unit  66   a  fixes the light volume adjusting signal ADJa to the turn-off level for turning off the fluorescent tube  46   a  (turn-off operation). Then, the fluorescent tube  46   a  goes out (FIG. 8( e )). In response to the rising edge of the selecting signal OPTb, the signal generating unit  66   d  enhances the light volume adjusting signal ADJd steeply so that the fluorescent tube  46   d  increases in luminance as much as the minimum luminance of the fluorescent tube  46   a  (FIG. 8( f )). This prevents the back light  32   b  from dropping in luminance due to the turning-off of the fluorescent tube  46   a.  That is, the back light  32   b  makes little variation in luminance when the fluorescent tube  46   a  is turned off.  
         [0092]    Subsequently, while receiving the selecting signal OPTb of high level, the signal generating unit  66   d  outputs to the lighting control circuit  50   b  a light volume adjusting signal ADJd for adjusting the luminance of the fluorescent tube  46   d  so that the luminance gap signal GAP decreases. Consequently, the fluorescent tube  46   d  decreases in luminance (FIG. 8( g )). With the decreasing luminance of the fluorescent tube  46   d,  the back light  32   b  also decreases in luminance (FIG. 8( h )).  
         [0093]    At time t 3 , the luminance of the fluorescent tube  46   d  reaches the minimum luminance, and the detecting part  56  outputs the detecting signal DET 1  (FIG. 8( i )). In synchronization with the detecting signal DET 1 , the comparing unit  62  outputs the detecting signal DET 2  (FIG. 8( i )). In response to the detecting signal DET 2 , the selecting unit  64  turns the selecting signal OPTb to low level and turns the selecting signal OPTc to high level. In synchronization with the falling edge of the selecting signal OPTb, the signal generating unit  66   d  fixes the light volume adjusting signal ADJd to the turn-off level for turning off the fluorescent tube  46   d  (turn-off operation). Then, the fluorescent tube  46   d  goes out (FIG. 8( j )). In response to the rising edge of the selecting signal OPTc, the signal generating unit  66   e  enhances the light volume adjusting signal ADJe steeply so that the fluorescent tube  46   c  increases in luminance as much as the minimum luminance of the fluorescent tube  46   d  (FIG. 8( k )). This prevents the back light  32   b  from dropping in luminance due to the turning-off of the fluorescent tube  46   d.  That is, the back light  32   b  makes little variation in luminance when the fluorescent tube  46   d  is turned off.  
         [0094]    Subsequently, while receiving the selecting signal OPTc of high level, the signal generating unit  66   e  outputs to the lighting control circuit  50   c  a light volume adjusting signal ADJe for adjusting the luminance of the fluorescent tube  46   c  so that the luminance gap signal GAP decreases. Consequently, the fluorescent tube  46   c  decreases in luminance (FIG. 8( l )). With the decreasing luminance of the fluorescent tube  46   c,  the back light  32   b  also decreases in luminance (FIG. 8( m )).  
         [0095]    At time t 4 , the luminance of the fluorescent tube  46   c  reaches the minimum luminance, and the detecting part  56  outputs the detecting signal DET 1  (FIG. 8( n )). In synchronization with the detecting signal DET 1 , the comparing unit  62  outputs the detecting signal DET 2  (FIG. 8( n )). Here, the back light  32   b  is at the minimum in luminance, so that the luminance indicated by the luminance detecting signal VD and the luminance indicated by the luminance adjusting signal LUM become equal. Then, the comparing unit  62  outputs a luminance gap signal GAP of 0 in magnitude. When the fluorescent tube  46   c  lit to the end becomes minimum in luminance, the brightness control circuit  48   b  makes no turn-off operation and keeps the fluorescent tube  46   c  lit at the minimum luminance.  
         [0096]    As has been described, the liquid crystal display  28   b  of the present embodiment can offer the same effects as those of the foregoing first embodiment. Besides, in the present embodiment, the adjusting part  58   b  enhances the luminances of the fluorescent tubes  46   d  and  46   c  steeply in turning off the fluorescent tubes  46   a  and  46   d,  respectively. The back light  32   b  can thus be prevented from changing in luminance due to the turning-off of the fluorescent tubes  46   a  and  46   d  that are lit at the minimum luminances.  
         [0097]    The fluorescent tubes  46   d  arranged in the centers of the reflectors  45  have a diameter smaller than the diameter of the outer fluorescent tubes  46   a  and  46   c.  Hence, the fluorescent tubes  46   d  of smaller parasitic capacitance can be increased in impedance. Consequently, even if relatively the same effective voltages are applied to all the fluorescent tubes  46   a,    46   d,  and  46   c,  the fluorescent tubes  46   a,    46   d,  and  46   c  can be made identical in the current that contributes to light emission. As a result, the fluorescent tubes  46   a,    46   d,  and  46   c  can be made identical in life, allowing extended life of the back light  32   b.    
         [0098]    In addition, since relatively the same effective voltages may be applied to the fluorescent tubes  46   a,    46   d,  and  46   c,  the signal generating units  66   a,    66   d,  and  66   e  can be made common in circuitry.  
         [0099]    [0099]FIG. 9 shows a third embodiment of the liquid crystal display of the present invention. The same elements as those of the first and second embodiments will be designated by identical reference numbers or symbols. Detailed description will be omitted of these elements. A liquid crystal display  28   c  includes a liquid crystal panel  30 , a back light  32   c,  and a case  34  for accommodating the liquid crystal panel  30  and the back light  32   c.    
         [0100]    The back light  32   c  includes a prism sheet  36 , a diffusion sheet  38 , a light guide plate  40 , a reflecting sheet  42 , and light emitting parts  44 C arranged on both sides of the light guide plate  40 , respectively.  
         [0101]    The light emitting parts  44 C have a reflector  45  each. The light emitting part  44 C on the right of the light guide plate  40  in FIG. 9 accommodates fluorescent tubes  46   a,    46   g,  and  46   c.  The light emitting part  44 C on the left of the light guide plate  40  in FIG. 9 accommodates fluorescent tubes  46   a,    46   h,  and  46   c.    
         [0102]    The fluorescent tube  46   g  and the fluorescent tube  46   h  are identical to each other. The fluorescent tubes  46   g  and  46   h  are arranged in the centers of the respective reflectors  45 . All the minimum luminances and maximum luminances possible for the respective fluorescent tubes  46   a,    46   g,    46   h,  and  46   c  to light at with stability are approximately the same.  
         [0103]    The fluorescent tubes  46   g  and  46   h  have an internal gas pressure higher than that of the fluorescent tubes  46   a  and  46   c.  On this account, the fluorescent tubes  46   g  and  46   h  are higher in impedance than the fluorescent tubes  46   a  and  46   c.  Moreover, the fluorescent tubes  46   g  and  46   h,  arranged in the centers of the reflectors  45 , cause smaller parasitic capacitances with the reflectors  45  as in the second embodiment. Thus, the fluorescent tubes  46   g  and  46   h  are less prone to leak currents to the reflectors  45  than the fluorescent tubes  46   a  and  46   c  do.  
         [0104]    Consequently, as in the second embodiment, the fluorescent tubes  46   a,    46   g,    46   h,  and  46   c  become identical in the current that contributes to light emission, provided that the same effective voltage is applied to all the fluorescent tubes  46   a,    46   g,    46   h,  and  46   c.  The result is that the fluorescent tubes  46   a,    46   g,    46   h,  and  46   c  become identical in life as with the first embodiment.  
         [0105]    [0105]FIG. 10 shows the details of control of the back light  32   c.  The back light  32   c  has a brightness control circuit  48   c,  two lighting control circuits  50   a  corresponding to the fluorescent tubes  46   a,  respectively, two lighting control circuits  50   b  corresponding to the fluorescent tubes  46   g  and  46   h,  respectively, two lighting control circuits  50   c  corresponding to the fluorescent tubes  46   c,  respectively, and an oscillating circuit  54 .  
         [0106]    The lighting control circuits  50   a,    50   b,  and  50   c  apply relatively the same effective voltages to the fluorescent tubes  46   a,    46   g,    46   h,  and  46   c.    
         [0107]    The brightness control circuit  48   c  is composed of a detecting part  56  and an adjusting part  58   c.  The adjusting part  58   c  has a comparing unit  62 , a selecting unit  64   c,  and signal generating units  66   a,    66   g,    66   h,  and  66   j.    
         [0108]    The selecting unit  64   c  always outputs (keeps at high level) at least any one of selecting signals OPTa, OPTg, OPTh, and OPTc. The selecting signal OPTa is output to the signal generating unit  66   a.  The selecting signal OPTg is output to the signal generating unit  66   g  and the signal generating unit  66   h.  The selecting signal OPTh is output to the signal generating unit  66   h.  The selecting signal OPTc is output to the signal generating unit  66   j.    
         [0109]    Now, description will be given of the luminance adjustment of the back light  32 c described above.  
         [0110]    [0110]FIG. 11 shows luminance variations of the fluorescent tubes  46   a,    46   g,    46   h,  and  46   c,  and the back light  32   c.  In this example, as with the first embodiment, the fluorescent tubes  46   a,    46   g,    46   h,  and  46   c  are lit at their respective maximum luminances in an initial state (at time t 0 ). Besides, the selecting unit  64   c  outputs the selecting signal OPTa (keeps the selecting signal OPTa at high level).  
         [0111]    At time t 1 , a luminance adjusting signal LUM indicating that the back light  32   c  is to be minimized in luminance is supplied, in which case the two fluorescent tube  46   a  gradually decrease in luminance (FIG. 11( a )). With the decreasing luminances of the fluorescent tubes  46   a,  the back light  32   c  also decreases in luminance (FIG. 11( b )). Since the output of the signal generating units  66   g,    66   h,  and  66   j  is fixed, the fluorescent tubes  46   g,    46   h,  and  46   c  have no change in luminance (FIG. 11( c )).  
         [0112]    At time t 2 , the luminances of the fluorescent tubes  46   a  reach the minimum luminances, and the detecting part  56  outputs the detecting signal DET 1  (FIG. 11( d )). In synchronization therewith, the comparing unit  62  outputs the detecting signal DET 2  (FIG. 11( d )). In response to the detecting signal DET 2 , the selecting unit  64   c  turns the selecting signal OPTa to low level and turns the selecting signal OPTc to high level. In synchronization with the falling edge of the selecting signal OPTa, the signal generating unit  66   a  fixes the light volume adjusting signal ADJa to a turn-off level for turning off the fluorescent tubes  46   a  (turn-off operation). Then, the two fluorescent tubes  46   a  go out (FIG. 11( e )). In response to the rising edge of the selecting signal OPTc, the signal generating unit  66   j  enhances a light volume adjusting signal ADJj steeply so that the two fluorescent tubes  46   c  increase in luminance as much as the minimum luminances of the two fluorescent tubes  46   a  (FIG. 11( f )). This prevents the back light  32   c  from dropping in luminance due to the turning-off of the two fluorescent tubes  46   a.  That is, the back light  32   c  makes little variation in luminance when the two fluorescent tubes  46   a  are turned off.  
         [0113]    Subsequently, while receiving the selecting signal OPTc of high level, the signal generating unit  66   j  outputs a light volume adjusting signal ADJj for adjusting the luminances of the fluorescent tubes  46   c  so that the luminance gap signal GAP decreases. Consequently, the two fluorescent tubes  46   c  decrease in luminance (FIG. 11( g )). With the decreasing luminances of the fluorescent tubes  46   c,  the back light  32   c  also decreases in luminance (FIG. 11( h )).  
         [0114]    At time t 3 , the luminances of the two fluorescent tubes  46   c  reach the minimum luminances, and the detecting part  56  outputs the detecting signal DET 1  (FIG. 11( i )). In synchronization therewith, the comparing unit  62  outputs the detecting signal DET 2  (FIG. 11( i )). In response to the detecting signal DET 2 , the selecting unit  64   c  turns the selecting signal OPTc to low level and turns the selecting signals OPTg and OPTh to high level. In synchronization with the falling edge of the selecting signal OPTc, the signal generating unit  66   j  fixes the light volume adjusting signal ADJj to a turn-off level for turning off the fluorescent tubes  46   c  (turn-off operation). Then, the two fluorescent tubes  46   c  go out (FIG. 11( j )).  
         [0115]    In response to the rising edge of the selecting signal OPTg, the signal generating unit  66   g  enhances a light volume adjusting signal ADJg steeply so that the fluorescent tube  46   g  increases in luminance as much as the minimum luminance of a single fluorescent tube  46   c  (FIG. 11( k )). In response to the rising edge of the selecting signal OPTh, the signal generating unit  66   h  enhances a light volume adjusting signal ADJh steeply so that the fluorescent tube  46   h  increases in luminance as much as the minimum luminance of a single fluorescent tube  46   c  (FIG. 11( l )). This prevents the back light  32   c  from dropping in luminance due to the turning-off of the two fluorescent tubes  46   c.  That is, the back light  32   c  makes little variation in luminance when the two fluorescent tubes  46   c  are turned off.  
         [0116]    Subsequently, while receiving the selecting signal OPTg of high level, the signal generating unit  66   g  outputs a light volume adjusting signal ADJg for adjusting the luminance of the fluorescent tube  46   g  so that the luminance gap signal GAP decreases. Consequently, the fluorescent tube  46   g  decreases in luminance (FIG. 11( m )). In the meantime, the signal generating unit  66   h,  while receiving the selecting signal OPTh of high level, outputs a light volume adjusting signal ADJh for adjusting the luminance of the fluorescent tube  46   h  so that the luminance gap signal GAP decreases. Consequently, the fluorescent tube  46   h  decreases in luminance (FIG. 11( n )). With the decreasing luminances of the fluorescent tubes  46   g  and  46   h,  the back light  32   c  also decreases in luminance (FIG. 11( o )).  
         [0117]    At time t 4 , the luminances of the fluorescent tubes  46   g  and  46   h  reach the minimum luminances, and the detecting part  56  outputs the detecting signal DET 1  (FIG.  11 ( p )). In synchronization therewith, the comparing unit  62  outputs the detecting signal DET 2  (FIG. 11( p )). In response to the detecting signal DET 2 , the selecting unit  64   c  turns the selecting signal OPTg to low level. Here, the selecting signal OPTh is kept at high level. In synchronization with the falling edge of the selecting signal OPTg, the signal generating unit  66   g  fixes the light volume adjusting signal ADJg to a turn-off level for turning off the fluorescent tube  46   g  (turn-off operation). Then, the fluorescent tube  46   g  goes out (FIG. 11( q )).  
         [0118]    In response to the falling edge of the selecting signal OPTg, the signal generating unit  66   h  enhances the light volume adjusting signal ADJh steeply so that the fluorescent tube  46   h  increases in luminance as much as the minimum luminance of the fluorescent tube  46   g  (FIG. 11( r )). This prevents the back light  32   c  from dropping in luminance due to the turning-off of the fluorescent tube  46   g.  That is, the back light  32   c  makes little variation in luminance when the fluorescent tube  46   g  is turned off.  
         [0119]    At time t 5 , the luminance of the fluorescent tube  46   h  reaches the minimum luminance, and the detecting part  56  outputs the detecting signal DET 1  (FIG. 11( s )). In synchronization therewith, the comparing unit  62  outputs the detecting signal DET 2  (FIG. 11( s )). Here, the back light  32   c  is at the minimum in luminance, so that the luminance indicated by the luminance detecting signal VD and the luminance indicated by the luminance adjusting signal LUM become equal. Then, the comparing unit  62  outputs a luminance gap signal GAP of 0 in magnitude. When the fluorescent tube  46   h  lit to the end becomes minimum in luminance, the brightness control circuit  48   c  makes no turn-off operation and keeps the fluorescent tube  46   h  lit at the minimum luminance.  
         [0120]    As has been described, the liquid crystal display  28   c  of the present embodiment can offer the same effects as those of the foregoing first and second embodiments. Besides, in the present embodiment, when the luminance adjusting signal LUM for minimizing the luminance of the back light is received, only a single fluorescent tube  46   h  is lit at its minimum luminance while the other fluorescent tubes  46   a,    46   c,  and  46   g  are turned off. The maximum luminance possible for the back light  32   c  to output is for situations where the six fluorescent tubes  46   a,    46   g,    46   h,  and  46   c  are lit at their maximum luminances. On this account, the minimum luminance of the back light  32   c  can be rendered one-sixth that of conventional back lights in which all the fluorescent tubes are lit together at the minimum luminances.  
         [0121]    The fluorescent tubs  46   g  and  46   h  arranged in the centers of the reflectors  45  have an internal gas pressure higher than the internal gas pressure of the outer fluorescent tubes  46   a  and  46   c.  The fluorescent tubes  46   g  and  46   h  of smaller parasitic capacitance can thus be increased in impedance. Consequently, the fluorescent tubes  46   a,    46   g,    46   h,  and  46   c  can be made identical in the current that contributes to light emission, provided that relatively the same effective voltages are applied to all the fluorescent tubes  46   a,    46   g,    46   h,  and  46   c.  As a result, the fluorescent tubes  46   a,    46   g,    46   h,  and  46   c  can be made identical in life, allowing extended life of the back light  32   c.    
         [0122]    [0122]FIG. 12 shows a fourth embodiment of the liquid crystal display of the present invention. The same elements as those of the first and second embodiments will be designated by identical reference numbers or symbols. Detailed description will be omitted of these elements. A liquid crystal display  28   d  includes a liquid crystal panel  30 , a case  34 , and a back light  32   d  of the present embodiment.  
         [0123]    The back light  32   d  has light emitting parts  44 D arranged on both sides of a light guide plate  40 , respectively. In this embodiment, each light emitting part  44 D has reflectors  45   a,    45   b,  and  45   c  for accommodating fluorescent tubes  46   a,    46   b,  and  46   c,  respectively. The brightness control circuit and the lighting control circuits are the same as in the second embodiment.  
         [0124]    The reflectors  45   a,    45   b,  and  45   c  are identical to one another. The inner surfaces of the reflectors  45   a,    45   b,  and  45   c  are provided with reflecting parts  52   a,    52   b,  and  52   c  (reflecting coats) for reflecting the light emitted from the fluorescent tubes  46   a,    46   b,  and  46   c,  respectively. The reflectors  45   a,    45   b,  and  45   c  are arranged with their apertures toward the sides of the light guide plate  40 .  
         [0125]    The physical relationship between the fluorescent tube  46   a  and the reflector  45   a,  the physical relationship between the fluorescent tube  46   b  and the reflector  45   b,  and the physical relationship between the fluorescent tube  46   c  and the reflector  45   c  are identical. For this reason, all the parasitic capacitances occurring between the fluorescent tubes  46   a,    46   b,  and  46   c,  and the reflectors  45   a,    45   b,  and  45   c  accommodating the respective fluorescent tubes  46   a,    46   b,  and  46   c  are the same.  
         [0126]    The areas of parts of the reflectors  45   a,    45   b,  and  45   c  where there is no obstacle between the fluorescent tubes  46   a,    46   b,  and  46   c  and the reflectors  45   a,    45   b,  and  45   c,  and they face to each other, respectively, are greater than in the cases where a single reflector accommodates three fluorescent tubes  46   a,    46   b,  and  46   c.  This allows improved conductive efficiency of dissipating the heat produced by the lit fluorescent tubes  46   a,    46   b,  and  46   c  to the reflectors  45   a,    45   b,  and  45   c.    
         [0127]    As has been described, the liquid crystal display  28   d  of the present embodiment can offer the same effects as those of the foregoing second embodiment. In addition, the present embodiment provides the reflectors  45   a,    45   b,  and  45   c  for accommodating the fluorescent tubes  46   a,    46   b,  and  46   c,  respectively. The heat produced by the fluorescent tubes  46   a,    46   b,  and  46   c  can thus be dissipated with higher efficiency.  
         [0128]    Accommodating the fluorescent tubes  46   a,    46   b,  and  46   c  in the reflectors  45   a,    45   b,  and  45   c,  respectively, equalizes the parasitic capacitances between the fluorescent tubes  46   a,    46   b,  and  46   c  and the reflectors  45   a,    45   b,  and  45   c.  Consequently, the currents flowing through the respective fluorescent tubes  46   a,    46   b,  and  46   c  can be made identical so that the fluorescent tubes  46   a,    46   b,  and  46   c  become equal in life. As a result, the life of the back light  32   d  can be prolonged.  
         [0129]    Incidentally, the foregoing first to fourth embodiments have dealt with the cases where the light emitting parts (reflectors and fluorescent tubes) are arranged on both sides of the light guide plate  40 , respectively. However, the present invention is not limited to such embodiments. For example, a light emitting part may be arranged on either side of the light guide plate, as shown in FIG. 13.  
         [0130]    The invention is not limited to the above embodiments and various modifications may be made without departing from the spirit and scope of the invention. Any improvement may be made in part or all of the components.