Abstract:
A method for controlling a backlight for illuminating a transmissive display panel includes measuring an amount of current supplied to the transmissive display panel, and controlling brightness caused by the backlight source on the basis of the amount of the current. Further the method includes storing data of a threshold value to be compared with the amount of the current, and controlling the brightness to decrease when the transmissive panel is a normally white type and the amount of the current is smaller than the threshold value data or when the transmissive panel is a normally black type and the amount of the current is larger than the threshold value data.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2008-236761, filed on Sep. 16, 2008, the entire contents of which are incorporated herein by reference. 
       FIELD 
       [0002]    The embodiments discussed herein are related to a control method of a backlight for a transmissive type display panel and a display device. 
       BACKGROUND 
       [0003]    A flat screen display device using a liquid crystal panel (LCD panel) and the like is widespread as a display device of a computer and a television. In general, a transmissive-type LCD panel is used and brightness of an image is controlled with a backlight. 
         [0004]    The display device using the backlight requires a higher luminance in recent years. A higher luminance is preferable for clear viewing when watching a moving image on a television program, while an image with a higher luminance gives a dazzling feeling upon creating and editing a document by using a computer in many cases. 
         [0005]    For adjusting a luminance or brightness of screen, Japanese Laid-open Patent Publication 2005-122187 has proposed a method for adjusting the luminance of the display device in accordance with intensity of ambient light. Japanese Laid-open Patent Publication 2006-164842 has proposed the use of a cold-cathode tube (FL tube) or LED used for adjusting a luminance of the screen. 
         [0006]    Further it is also known that a method enables analyzing distribution of luminance of the image for adjusting a luminance or brightness of screen. 
         [0007]    That is,  FIG. 11  shows the structure of a conventional scaler  80  for displaying and driving a liquid crystal panel.  FIG. 11  illustrates one of a scaler  80  for use of brightness control of a liquid crystal display panel. The scaler  80  includes: a video converter EH; a look-up table  84 ; a Y-data analyzer  85 ; a brightness changing unit  86 ; a PMW controller  87 ; and the like. The video converting section EH comprises: an RGB/YUV converter  81 ; a YUV/RGB converter unit  82 ; and a Y-data extractor  83 . 
         [0008]    The RGB/YUV converter  81  converts an inputted RGB video signal S 1  into YUV format as a color-spatial conversion. The Y-data extractor  83  extracts Y data (luminance data) from the video signal of YUV format. The Y-data analyzer  85  obtains the Y data distribution from the inputted Y data. On the basis of the Y data distribution the brightness changing unit  86  generates a signal for changing the brightness of the backlight. In this case, when the Y-data is concentrated near the maximal value thereof, the brightness changing unit  86  generates a signal for reducing the brightness. When the Y-data is not concentrated near the maximal value, the brightness changing unit  86  generates a signal for keeping the current situation. 
         [0009]    The PMW controller  87  generates a signal S 11  for a pulse width modulation (PWM) on the basis of a signal outputted from the brightness changing unit  86 . The signal S 11  is inputted to a brightness controller  88  of an inverter  13   j  in which a voltage is varied according to the signal  11  and applied to the cold-cathode tube, thereby controlling the luminance of the cold-cathode tube. 
         [0010]    The YUV/RGB converter unit  82  converts the video signal in YUV format into the video signal in RGB format as the color space conversion. Then, the video signals S 2  in RGB format are converted based on the look-up table  84  and are outputted. The video signals S 2  are inputted to a liquid crystal panel, thereby driving a liquid crystal panel via a driver circuit. 
       SUMMARY 
       [0011]    According to an aspect of the invention, a method for controlling a backlight for illuminating a transmissive display panel includes measuring an amount of current supplied to the transmissive display panel and controlling brightness caused by the backlight source on the basis of the amount of the current. 
         [0012]    According to another aspect of the invention, a display device including a transmissive type panel and a backlight includes a detector for detecting an amount of current supplied to the transmissive panel and a brightness controller for control brightness caused by the backlight source on the basis of the amount of the current. 
         [0013]    The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
         [0014]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is a block diagram illustrating a display device using a liquid crystal display; 
           [0016]      FIG. 2  is a schematic circuit diagram illustrating a current detector; 
           [0017]      FIG. 3  is another schematic circuit diagram illustrating a current detector; 
           [0018]      FIGS. 4A and 4B  each illustrates a relationship between detected current data and brightness; 
           [0019]      FIG. 5  is a diagram explaining a determining whether a liquid crystal panel is a normally white or a normally black type; 
           [0020]      FIG. 6  is a schematic circuit diagram illustrating a liquid crystal panel; 
           [0021]      FIG. 7  is an operation chart illustrating a brightness control for the normally white panel; 
           [0022]      FIG. 8  is an operation chart illustrating a brightness control for the normally black panel; 
           [0023]      FIG. 9  is an operation chart illustrating determination whether a liquid crystal panel is a normally white or a normally black type; 
           [0024]      FIG. 10  is a diagram illustrating an information processing apparatus using one of the present embodiments; and 
           [0025]      FIG. 11  is a conventional scaler used in a liquid crystal display device. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0026]    The conventional control method of the backlight shown in  FIG. 11  uses complicated conversion in which the inputted RGB video signals S 1  are temporarily converted into YUV video signals, the Y-data is extracted from the converted signals, and the signal is thereafter converted into RGB video signals again. 
         [0027]    In order to accurately obtain the video signal S 2 , the video signals S 1  and S 2  have 8 bits for respective color signals, RGB. However, the video converting section EH performs processing with 10 bits for the respective colors. Therefore, a processing circuit is complicated, a circuit for this processing, such as an integrated circuit device, is increased, and the implementation space is also increased and costs also rise. 
         [0028]    The embodiments according to the present invention is devised in consideration of the problem, and it is an object of the embodiments to provide a control method of a backlight in which luminance of the backlight is controlled in accordance with a video signal with a simple circuit and low costs and a display device. 
         [0029]    As a method according to an embodiment of the present invention, the level of current supplied to the display panel is detected and the luminance of the backlight may be controlled on the basis of the detected level of the current which is applied to a transmissive-type display panel. 
         [0030]    Since the luminance of the backlight is controlled on the basis of the level of the current supplied to the display panel, the conventional complicated processing circuit may not need to be disposed and the luminance of the backlight may be controlled with a simple circuit. 
         [0031]    Further, the detected current is compared with a threshold. The luminance of the backlight may be controlled to be reduced when the detected current is lower than the threshold on the display panel in a normally white type panel or when the detected current is higher than the threshold on the display panel in a normally black type panel. 
         [0032]    Hereinbelow, a description of the embodiments will be given. 
         [0033]    According to the embodiment, the level of current I supplied to a display panel is detected and the luminance of the backlight is controlled on the basis of the level of the detected current I. Then, the detected current I is supplied to an inverter which controls lighting-on the backlight such as a cold-cathode tube in accordance with the detected current I. 
         [0034]    In the liquid crystal display panel of the normally white type, a white image in a full-screen requires a minimum current supplied to the panel. On the other hand, a white image in a full-screen requires a maximum current supplied to the panel when the panel is the normally black type. 
         [0035]    Therefore, the inverter may be controlled by monitoring the current I supplied to the display panel. 
         [0036]    In the case of normally white type of the panel, the inverter is controlled so as to set the brightness (luminance) of the backlight low when the current I is small. On the contrary, the inverter is controlled so as to the brightness high when the current is large. Further, it is preferable to control the inverter so that the brightness is adjusted linearly with the analogue current I value. 
         [0037]    The above-mentioned control may not need the RGB/YUV converter and the YUV/RGB converter that are conventionally required. Further, since an advanced image analyzing device is not required for controlling the brightness of screen, costs will be reduced and the space is saved. 
         [0038]    In other words, the detected current I is compared with a threshold “th” and the control is performed so that the luminance of the backlight is reduced when the detected current I is lower than the threshold th in the normally white type panel of the display panel or when the detected current I is higher than the threshold th in the normally black type panel of the display panel. 
         [0039]    Alternatively, analog control is performed so that the luminance of the backlight is reduced when the detected current I is lower in the normally white type panel of the display panel and when the detected current I is higher in the normally black type of the display panel. 
         [0040]    Hereinbelow, a specific description will be given. 
         [0041]      FIG. 1  is a block diagram illustrating a circuit of a display device  1  according to an embodiment of the present invention.  FIG. 2  is a diagram illustrating a circuit of a current detector  16 A as an example.  FIG. 3  is a diagram illustrating a circuit of a current detector  16 B as another example.  FIG. 4  is a diagram illustrating examples of relationships between detecting data SC 2  and the luminance.  FIG. 5  is a diagram for explaining a determining operation in a determining portion  27 .  FIG. 6  is a block diagram illustrating an example of a circuit of a liquid crystal panel unit  12 . 
         [0042]    Referring to  FIG. 1 , the display device  1  using a transmissive liquid crystal panel  33  is shown. The display device  1  comprises: a scaler  11 ; a liquid crystal panel unit  12 ; an inverter  13 ; a cold-cathode tube  14 ; a power unit  15 ; and a current detector  16 . 
         [0043]    The scaler  11  has a look-up table (LUT)  21 ; an A/D converter  22 ; an MPU unit  23 ; and a PWM controller  24 . 
         [0044]    The MPU unit  23  comprises: a brightness controller  25 ; a threshold memory  26 ; and a determining portion  27 . The MPU unit  23  further has a micro processing unit (MPU) or a central processing unit (CPU), a memory, and other electrical elements, and executes various processing, operation, or control by performing a program. The MPU processing realizes functions of the brightness controller  25  and the determining portion  27 . 
         [0045]    The RGB video signals (image signals) S 1  is inputted into the scaler  11  in which the signals S 1  are converted to RGB video signals S 2  by using the look-up table  21 . The look-up table  21  may be a conventional well-known one. 
         [0046]    The scaler  11  controls also the luminance (brightness) of the cold-cathode tube  14  as the backlight via the inverter  13  on the basis of the level of the current I detected by the current detector  16 . That is, the A/D converter  22  converts the detecting signal SC 1  corresponding to the current I into the detecting data SC 2  with 8 bits. The detecting signal SC 1  can be obtained in a form of a binary signal (detecting signal SC 1 A) indicating “H (high)” and “L (low)” or a form of an analog signal (detecting signal SC 1 B) that continuously changes from the minimal value to the maximal value. The each form of the signal SC 1  will be described later. 
         [0047]    The brightness controller  25  determines whether or not the luminance of the cold-cathode tube  14  needs to be changed on the basis of the detecting data SC 2 . When the luminance needs to be changed, the brightness controller  25  outputs a signal SC 3  necessary for the change to the PWM controller  24 . The PWM controller  24  controls the inverter  13  so that the luminance of the cold cathode ray tube  14  reaches the luminance corresponding to the signal SC 3 . In detail, the brightness controller  25  compares the detecting data SC 2  with the threshold th stored in the threshold memory  26 . 
         [0048]    In the normally white type panel of the display device  1 , when the detecting data SC 2  is lower than the threshold th, it is controlled to reduce the luminance of the backlight. In the normally black type panel of the display device  1 , when the detecting data SC 2  is higher than the threshold th, it is controlled to reduce the luminance of the backlight as shown by curves CV 1  to CV 3  (refer to  FIG. 4A ). 
         [0049]    In an example shown in  FIG. 4A , when the detecting data SC 2  is lower than the threshold th, the luminance is adjusted to 30% (curve CV 1 ), 50% (curve CV 2 ), and 60% (curve CV 3 ), respectively. When the detecting data SC 2  is the threshold th or more, the luminance is 100%. For example, a voltage of 2.5V is used as the threshold th. 
         [0050]    Thus, when the video signal S 1  indicates a video image (video image with high luminance) brighter than an image with a predetermined value, it is controlled to reduce the luminance of the backlight. 
         [0051]    The predetermined value is generally determined depending on the level of the threshold th. When the detecting signal SC 1  is a binary signal (detecting signal SC 1 A), the predetermined value is substantially determined depending on a threshold upon obtaining a binary signal in many cases. 
         [0052]    Therefore, upon using the current detector  16 A shown in  FIG. 2 , as will be described later, a state of the detecting signal SC 1 , i.e., “L” (0V) or “H” (5V) is determined depending on the threshold adjusted by resistance values of resistors R 1  to R 3 . Therefore, the threshold of the current detector  16 A may be a threshold according to the embodiment. In this case, the threshold th stored in the threshold memory  26  may be not required and the brightness controller  25  may control the PWM controller  24  on the basis of the detecting data SC 2 . In the example, the luminance is adjusted to two states. However, the adjustment of the luminance may have three states, four states, or more. 
         [0053]    Alternatively, the brightness controller  25  may be structured to perform the following operation. 
         [0054]    That is, in the normally white type panel of the display device  1 , as the detecting data SC 2  is lower (detecting signal SC 1  is lower), the luminance of the backlight is controlled to be reduced. In the normally black type panel of the display device  1 , as the detecting data SC 2  is higher (detecting signal SC 1  is higher), the luminance of the backlight is controlled to be reduced (refer to  FIG. 4B ). When the detecting signal SC 1  is an analog signal (detecting signal SC 1 B) that continuously changes, the control may be effective. 
         [0055]    In an example shown in  FIG. 4B , the luminance is individually adjusted to 0 to 100% (curve CV 4 ), 30 to 100% (curve CV 5 ), and 60 to 100% (curve CV 6 ) depending on the level of the detecting data SC 2 . Each curve shown in  FIG. 4B  is a straight line in the range of 0 V to 5 V, it is preferable to vary the brightness along a quadratic, a cubic, or a logarithmic curves in a predetermined range of SC 2 . 
         [0056]    Incidentally, the normally white means that a white image (bright image) is displayed when a voltage is not applied to a liquid crystal material of the liquid crystal panel  33 . The normally black means that a black image (dark image) is displayed when a voltage is not applied. 
         [0057]    The determining portion  27  determines whether the liquid crystal panel  33  is a normally white or a normally black type. Assuming that the current detector  16  detects a current Iw at displaying a fully white image on the screen of the liquid crystal panel  33  and a current Ik at displaying a fully black image, then Iw is smaller that Ik, that is Iw&lt;Ik, then the liquid crystal panel  33  is determined as the normally white type by the determining portion  27 . In the scaler  11 , the fully white and black images may be generated by a computer (not shown) or by using data, which stored for example in a memory provided in the MPU unit  23 , corresponding to each of the images. 
         [0058]    Data of the above described fully white and black images may be generated by a video signal STS with RGB signals having “0” for all pixels and a video signal STD with RGB signals having “255” for all pixels are created and which of the video signal STS or STD corresponds to the video signal generating the fully white or black image is depend on the type of the panel. The created signals are outputted as the video signals S 2 , and are displayed on the liquid crystal panel  33 . The current detector  16  detects the current I when the displayed signals are displayed. When the liquid crystal display panel  33  is viewed and the video image has intensive highlight or a fully white image, the current I is set as the current Iw. When the video image has deep shadow or a fully black image, the current I is set as the current Ik. 
         [0059]      FIG. 5  illustrates schematically the relationship between the detected current such as SC 1  the current detector  16  and the brightness of the image according to the detected SC 1 . As shown in  FIG. 5 , a relation of Iw&lt;Ik is obtained in the normally white type panel. In the normally black type panel, a relation of Iw&gt;Ik is obtained. Therefore, as described above, the two-type current Iw and Ik is compared with each other, thereby determining whether the image is in the normally white type panel or normally black type panel. This determination may be performed by the determination portion  23 . 
         [0060]    The liquid crystal panel unit  12  comprises: a drive power unit  31 ; a video controller (LVDS)  32 ; and a liquid crystal panel  33 . The drive power unit  31  generates a voltage necessary for an operation of units in the panel  12  on the basis of a DC voltage PDC 1   a  to be inputted. The video controller  32  is a control circuit for controlling the liquid crystal panel  33  on the basis of the video signal S 2  outputted from the scaler  11 , thereby displaying the video image. The liquid crystal panel  33  is an active-matrix type one in the embodiment, while other types of a liquid crystal panel may be used. 
         [0061]    The inverter  13  includes a brightness controller  51  and an inverter circuit  52 . The brightness controller  51  controls the inverter circuit  52  in response to a PWM control signal from the PWM controller  24 . The inverter circuit  52  supplies a high AC voltage to the cold-cathode tube  14  so as to light-on the cold-cathode tube  14 . The brightness controller  51  controls the current flowing to the cold-cathode tube  14  by the inverter circuit  52 , thereby controlling the luminance of the cold-cathode tube  14 . It is possible to use a well-known inverter applicable to the cold-cathode tube  14  as the backlight. The number of the cold-cathode tubes  13  may be one or more depending the screen size of the liquid crystal panel  33  or the required quality of the image on the screen. 
         [0062]    The power unit  15  comprises: an AC/DC converter  41 ; an output circuit  42  for panel driving; and an output circuit  43  for inverter driving. The AC/DC converter  41  converts an AC voltage PAC for commercial use to be inputted into a DC voltage PDC. The output circuit  42  for panel driving outputs a DC voltage PDC 1  for driving the liquid crystal panel unit  12 . The output circuit  43  for inverter driving outputs a DC voltage PDC 2  for driving the inverter  13 . 
         [0063]    The current detector  16  detects the value or level of current I supplied to the liquid crystal panel unit  12 . A method for detecting the current I by the current detector  16  includes a first current-detecting method and a second current-detecting method according to the embodiment. 
         [0064]    With the first current-detecting method, the current detector  16  outputs the binary detecting signal SC 1  depending on the value or level of the current I supplied to the liquid crystal panel unit  12 . 
         [0065]    Referring to  FIG. 2 , the current detector  16 A comprises resistors R 1 , R 2  and R 3  and a transistor Q 1 . When the current I is low and a voltage drop through the resistor R 1  is equal to or less than the forward voltage drop of the transistor Q 1 , the transistor Q 1  is OFF-state and the detecting signal SC 1 A is 0 volts. When a voltage drop through the resistor R 1  is equal to or more than the forward voltage drop, the transistor Q 1  is ON-state and the detecting signal SC 1 A has a similar voltage to the DC voltage PDC 1 . If the DC voltage PDC 1  is, e.g., 5V, the detecting signal SC 1 A is 0V (i.e., a low state or “L” state) or 5V (i.e., a high state or “H” state). The adjustment of resistances of the resistors R 1  to R 3  enables the threshold discriminating L and H states to be adjusted, because the relationship between the value of current I and the levels of the detecting signal SC 1 A varies by the values of resistors R 1  to R 2 . 
         [0066]    With the second current-detecting method, the current detector  16  outputs the detecting signal SC 1  of a voltage value depending on the level of the current I supplied to the liquid crystal panel unit  12 . Referring to  FIG. 3 , the current detector  16 B comprises a diode DD 1 , a transistor Q 2  and resistors R 4  to R 7 , where the diode DD 1  and the resistor R 1  are serially connected between the liquid crystal panel unit  12  and the input terminal for receiving the direct voltage PDC 1 . In this embodiment, the detecting signal SC 1 B corresponds to a voltage value amplified by the transistor Q 2  depending on the ratio of the value of resistor  6  to the value of resistor  7  with respect to the current I flowing to the resistor R 4 . That is, the detecting signal SC 1 B has a voltage value in proportional to the current I flowing to the resistor R 4 . The adjustment of the resistances of the resistors R 4  to R 7  enables the adjustment of a relationship between the level of the current I and the level of the detecting signal SC 1 B, i.e., voltage values and or inclination. 
         [0067]    The use of the detecting signal SC 1 B with the second current-detecting method enables each of control operations shown in  FIGS. 4A and 4B . That is, the second current-detecting method enables the control of luminance on the basis of the detecting signal SC 1 B with two steps. Further, such a control operation is possible that the luminance of the backlight continuously changes depending on the detecting signal SC 1 B indicating the voltage value. 
       [Liquid Crystal Panel Unit According to Another Embodiment] 
       [0068]    Next, a description will be given of the liquid crystal panel unit  12 B according to another embodiment referring to  FIG. 6 . 
         [0069]    With the display device  1  according to the embodiment previously described, the current detector  16  detects the current I flowing to the entire liquid crystal panel unit  12 . In this embodiment, the detection is performed to detect the current supplied to a source driver of the liquid crystal panel  33  in the liquid crystal panel unit  12 B not to the current I flowing to the entire liquid crystal panel unit  12  (in  FIG. 1 ). 
         [0070]    The liquid crystal panel unit  12 B comprises a drive power unit  31 , a video controller (LDS)  32 , a liquid crystal panel  33 , and a current detector  34 . The drive power unit  31  and the video controller  32  may be identical or similar to the ones previously described. The liquid crystal panel  33  is a TFT-active-matrix-type one and includes a gate driver  33 G and a source driver  33 S to drive a plurality of elements of the liquid crystal panel  33  arrayed in a row and a column of a matrix. 
         [0071]    The drive power unit  31  supplies power to the video controller  32  and the liquid crystal panel  33 . Power is separately supplied to the gate driver  33 G and the source driver  33 S in the liquid crystal panel  33 . 
         [0072]    The video controller  32  outputs a gate driver driving signal to the gate driver  33 G, and outputs a source driver driving signal (data for driving source driver) to the source driver  33 S. 
         [0073]    The current detector  34  detects current Is supplied to the source driver  33 S from the drive power unit  31 . The current detector  34  has the same structure as that of the current detector  16 . When the liquid crystal panel  12 B has the current detector  34  as mentioned above, the current detector  16  is not required. 
         [0074]    That is, the detecting signal SC 1  is outputted depending on the current Is detected by the current detector  34 . Similarly to the foregoing, the scaler  11  performs processing and operation for controlling the luminance of the cold-cathode tube  14  on the basis of the detecting signal SC 1 . 
         [0075]    When the current detector  34  detects the current Is, it is possible to determine whether or not the luminance of the cold-cathode tube  14  needs to be changed and how the luminance is set after the change, more properly than the case of detecting the current I of the entire liquid crystal panel  33  by the current detector  16 . It is preferable to control the luminance of the backlight according to an angle of twist of liquid crystal molecules in the liquid crystal panel  33 . Since the current Is is more relative to the angle than the current I which flows into the whole of the liquid crystal panel unit  12  ( FIG. 1 ), the control of the luminance of the backlight depending on the current Is may be more preferable than the control the current I. 
         [0076]    Next, a description will be given of the operation of the display device  1  with reference to an operation chart.  FIG. 7  is an operation chart illustrating a luminance control in the normally white type panel.  FIG. 8  is an operation chart illustrating a luminance control in the normally black type panel.  FIG. 9  is an operation chart illustrating processing for determining the panel in the determining portion  27 . 
         [0077]    Referring to  FIG. 7 , the current I flowing to the liquid crystal panel unit  12  is detected (S 11 ). The detected current I is converted into a voltage and is further set as the detecting signal SC 1  (S 12 ). The detecting signal SC 1  is converted into the detecting data SC 2  as digital data (S 13 ). The detecting data SC 2  is compared with the threshold th (S 14 ). 
         [0078]    When the detecting data SC 2  is lower than the threshold th (YES in S 14 ), the brightness controller  25  issues an instruction to reduce the brightness (S 15 ). In accordance therewith, the PWM controller  24  performs PWM control to reduce the duty ratio in ON-time (S 16 ). In accordance therewith, the inverter  13  performs a control operation to reduce the current flowing to the cold-cathode tube  14  (S 17 ). As a result, the luminance of the display operation of the liquid crystal panel unit  12  (S 18 ). When the detecting data SC 2  is higher than the threshold th (NO in S 14 ), the brightness controller  25  keeps the current situation (S 19 ). 
         [0079]    Referring to  FIG. 8 , the current I flowing to the liquid crystal panel unit  12  is detected (S 21 ). The detected current I is converted into a voltage and is further set as the detecting signal SC 1  (S 22 ). The detecting signal SC 1  is converted into the detecting data SC 2  as digital data (S 23 ). The detecting data SC 2  is compared with the threshold th (S 24 ). 
         [0080]    When the detecting data SC 2  is higher than the threshold th (YES in S 24 ), an instruction to reduce the brightness is issued (S 25 ). The PWM control is performed to reduce the duty ratio in the ON-time (S 26 ). The current flowing to the cold-cathode tube  14  is reduced (S 27 ). As a consequence, the luminance of the display operation in the liquid crystal panel unit  12  is reduced (S 28 ). When the detecting data SC 2  is lower than the threshold th (NO in S 24 ), the brightness controller  25  keeps the current situation (S 29 ). 
         [0081]    Referring to  FIG. 9 , the current I flowing to the liquid crystal panel unit  12  is detected in the case of a video image with intensive highlight (a video image with a white image on a whole screen) and is then converted into a voltage Vw (S 31 ). The voltage Vw is converted into digital data (S 32 ), and the digital data is stored to the memory (S 33 ). The current I flowing to the liquid crystal panel unit  12  is detected in the case of a video image with intensive highlight and is then converted into a voltage Vk (S 34 ). The voltage Vk is converted into digital data (S 35 ), and the digital data is stored to the memory (S 36 ). 
         [0082]    The voltage Vw is compared with the voltage Vk as the digital data (S 37 ). When the voltage Vw is lower than the voltage Vk (YES in S 37 ), it is determined that the image is in the normally white type panel and the result is stored to the memory (S 38 ). When the voltage Vw is higher than the voltage Vk (NO in S 37 ), it is determined that the image is in the normally black type panel and the result is then stored to the memory (S 39 ). 
         [0083]    According to the embodiment, the scaler  11  and the inverter  13  correspond to luminance control means according to the present invention. Further, the current detector  16  and the current detector  34  correspond to current detecting means according to the present invention. 
         [0084]      FIG. 10  is a diagram illustrating an example of the structure of an image processing device JS using the display device  1 . Referring to  FIG. 10 , the information processing device JS comprises the display device  1 , a processing device  3 , and an input device  4 . The processing device  3  has a CPU, a RAM, a ROM, another peripheral element, a hard disk device, a DVD device, another storage device or medium drive device, and an interface device for connection to a network as needed. The CPU executes a program (computer program) stored to the RAM or ROM, thereby performing various processing operations. The processing device  3  outputs a video signal S 1  to display a video image on the display device  1 . 
         [0085]    The input device  4  is a keyboard, mouse, or another pointing device. The processing device  3  performs processing operation in accordance with an input instruction from the input device  4 , and the display device  1  displays the video image (image) as a processing result. Further, the processing device  3  can output a TV video image and captured photo image and video image, and can also display them on the display device  1 . 
         [0086]    As the display device (or monitor)  1 , the display device described above according to the embodiment is used. Commercial AC voltages PAC are individually supplied to the display device  1  and the processing device  3 . However, the commercial AC voltage PAC may be externally supplied to only the processing device  3  and the AC voltage PAC may be supplied to the display device  1  via the processing device  3 . 
         [0087]    As the processing device  3  and the input device  4 , a personal computer can be used. 
         [0088]    With the display device  1  described above according to the embodiment, the luminance of the backlight may be controlled in accordance with the video signal with low costs by using a simple circuit. Therefore, the brightness can be automatically adjusted in accordance with display contents of a screen (video image) on the liquid crystal panel unit  12 . The dazzle on a screen with a large white area may be suppressed and the power consumption can be reduced. 
         [0089]    Irrespective of the type of the display portion such as the liquid crystal panel unit  12 , i.e., of the normally white or normally black, the type may be automatically detected and it is possible to correspond to a multi-panel. 
         [0090]    According to the embodiment, since an advanced image processing device is not used, the implementing area may be small and the space of a product may be reduced. Further, costs of the device and development costs may be reduced. 
         [0091]    According to the embodiment, the case of using the cold-cathode tube  14  for the backlight is described above. A light-emitting diode (LED) is preferable instead of the cold-cathode tube  14 . In the case of using the LED, an LED driver is used in place of the inverter  13  for the cathode ray tube and controls the current to the LED. Further, as the display device  1 , a touch panel may be attached. 
         [0092]    In addition, it is possible to properly modify the components, structure, circuit, shape, the number of parts, function, control contents, and order of the entire or part of the scaler  11 , liquid crystal panel portions  12  and  12 B, inverter  13 , power unit  15 , current detecting units  16  and  34 , display device  1 , and information processing device JS according to the essential of the present invention. 
         [0093]    All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a illustrating of the superiority and inferiority of the invention. Although the embodiments of the present inventions have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.