Patent Publication Number: US-2010128051-A1

Title: Backlight device, backlight control method, and liquid crystal display device

Description:
TECHNICAL FIELD 
     The present invention relates to a backlight device, a backlight control method, and a liquid crystal display device, and in particular, to a backlight device, a backlight control method, and a liquid crystal display device that allow light-emission brightness or chromaticity to be corrected with high accuracy and low cost. 
     BACKGROUND ART 
     A liquid crystal display device (LCD (Liquid crystal display)) is constituted by a liquid crystal panel including a color filter substrate having colors of red, green, and blue, a liquid crystal layer, and the like, a backlight placed on the back surface of the liquid crystal panel, and the like. 
     In a liquid crystal display device, the twist of liquid crystal molecules in a liquid crystal layer is controlled by changing voltage, and light (white light) from a backlight transmitted through the liquid crystal layer in accordance with the twist of the liquid crystal molecules becomes red, green, or blue light by passing through the color filter substrate having colors of red, green, or blue. Accordingly, an image is displayed. 
     Note that, in the following description, controlling the twist of liquid crystal molecules by changing voltage so that the transmittance of light can be changed, is called control of a liquid crystal aperture ratio. In addition, the brightness of light emitted from a backlight, which is a light source, is called “light-emission brightness”, and the brightness of light emitted from the front surface of a liquid crystal panel, which is the brightness of light perceived by a viewer who visually recognizes a displayed image, is called “display brightness”. 
     In liquid crystal display devices, control has been performed in such a manner that a necessary display brightness can be achieved in each pixel of the screen by illuminating, with a backlight, the entire screen of a liquid crystal panel at a uniform and maximum (substantially maximum) brightness and controlling only the aperture ratio of each pixel of the liquid crystal panel. Thus, for example, a problem has occurred in which a large amount of power is consumed even when a dark image is displayed since a backlight emits light at the maximum backlight brightness. 
     With respect to this problem, for example, techniques for realizing reduced power consumption and an extended dynamic range of display brightness by dividing a screen into a plurality of blocks and changing the backlight brightness for each divided block in accordance with an input image signal, have been suggested (see, for example, Patent Documents 1 and 2.) 
     In order to perform control in such a manner that the backlight brightness is changed for each divided block in accordance with an input image signal, it is necessary to correct, for each divided block, the light-emission brightness and chromaticity when a backlight is turned on. 
     As a method for correcting the light-emission brightness and chromaticity for each block, feedback control is generally performed, in which a predetermined number of sensors for detecting light-emission brightness or chromaticity are provided for a light-emission area and correction is performed in accordance with the light-emission brightness or chromaticity detected by each of the sensors.
     Patent Document 1: Japanese Unexamined Patent Application Publication No. 2005-17324   Patent Document 2: Japanese Unexamined Patent Application Publication No. 11-109317   

     DISCLOSURE OF INVENTION 
     Technical Problem 
     In such feedback control, how many sensors are to be provided within a light-emission area is an issue. That is, when a large number of sensors are provided for a light-emission area so that a range within which a single sensor performs detection can become as small as possible, the measurement accuracy increases, and more accurate control of light-emission brightness or chromaticity can be achieved. However, the cost of the device increases. 
     Meanwhile, when a small number of sensors, such as one or two sensors, are provided for the entire light-emission area, although correction for the entire light-emission area can be performed, correction in units of blocks becomes difficult. Thus, irregularity of light-emission brightness or chromaticity within the light-emission area occurs. 
     The present invention has been made in view of the situation described above, and allows light-emission brightness or chromaticity to be corrected with high accuracy and low cost. 
     Technical Solution 
     A backlight device according to a first aspect of the present invention that has a light-emission area in which N (≧1) small areas each including one or more blocks and serving as units for which light-emission brightness or chromaticity is corrected are provided and in which M (≧2) areas constituted by the N small areas are adjacent to each other and that is capable of controlling the light-emission brightness for each block, includes light-emission control means for causing processing to be sequentially performed for all the M areas, the processing including setting one of the M areas as a correction area and causing light emission in a detection area, which is a small area within the correction area, and light emission in small areas which are located in (M-1) areas other than the correction area and whose positions in the areas correspond to the detection area to be sequentially performed for all the small areas in the correction area; and detecting means for detecting the light-emission brightness or chromaticity of the detection area, the detecting means being provided in the M areas on a one-to-one basis. 
     The light-emission control means can cause the light emission in the detection area and the light emission in the corresponding small areas within the areas other than the correction area to be performed during a sensing period provided prior to or subsequent to light-emission brightness control based on an input image signal. 
     The small areas can each include one block. The backlight device can further include current control means for controlling a current value to be supplied to a light-emitting element in the block. The current control means can cause, to a light-emitting element in a block for which the detecting means cannot perform detection with the same current value as a current value supplied at the time of the light-emission brightness control based on the input image signal, a current value greater than the current value supplied at the time of the light-emission brightness control to be supplied. 
     The light emission in each of the small areas can be caused to be performed at a frequency of 60 Hz or more. 
     A backlight control method according to a first aspect of the present invention for a backlight device that has a light-emission area in which N (≧1) small areas each including one or more blocks and serving as units for which light-emission brightness or chromaticity is corrected are provided and in which M (≧2) areas constituted by the N small areas are adjacent to each other, that includes detecting means for detecting the light-emission brightness or chromaticity, the detecting means being provided in the M areas on a one-to-one basis, and that is capable of controlling the light-emission brightness for each block, includes the step of causing processing to be sequentially performed for all the M areas, the processing including setting one of the M areas as a correction area and causing light emission in a detection area, which is a small area within the correction area, and light emission in small areas which are located in (M-1) areas other than the correction area and whose positions in the areas correspond to the detection area to be sequentially performed for all the small areas in the correction area, and detecting the light-emission brightness or chromaticity of the detection area. 
     A liquid crystal display device according to a second aspect of the present invention including a backlight that has a light-emission area in which N (≧1) small areas each including one or more blocks and serving as units for which light-emission brightness or chromaticity is corrected are provided and in which M (≧2) areas constituted by the N small areas are adjacent to each other and that is capable of controlling the light-emission brightness for each block, includes light-emission control means for causing processing to be sequentially performed for all the M areas, the processing including setting one of the M areas as a correction area and causing light emission in a detection area, which is a small area within the correction area, and light emission in small areas which are located in (M-1) areas other than the correction area and whose positions in the areas correspond to the detection area to be sequentially performed for all the small areas in the correction area; and detecting means for detecting the light-emission brightness or chromaticity of the detection area, the detecting means being provided in the M areas on a one-to-one basis. 
     The light-emission control means can cause the light emission in the detection area and the light emission in the corresponding small areas within the areas other than the correction area to be performed during a sensing period provided prior to or subsequent to light-emission brightness control based on an input image signal. 
     The small areas can each include one block. The backlight device can further include current control means for controlling a current value to be supplied to a light-emitting element in the block. The current control means can cause, to a light-emitting element in a block for which the detecting means cannot perform detection with the same current value as a current value supplied at the time of the light-emission brightness control based on the input image signal, a current value greater than the current value supplied at the time of the light-emission brightness control to be supplied. 
     The light emission in each of the small areas can be caused to be performed at a frequency of 60 Hz or more. 
     In the first and second aspects of the present invention, processing is caused to be performed for all the M areas. The processing includes setting one of the M areas as a correction area and causing light emission in a detection area, which is a small area within the correction area, and light emission in small areas which are located in (M-1) areas other than the correction area and whose positions in the areas correspond to the detection area to be sequentially performed for all the small areas in the correction area. In this processing, the light-emission brightness or chromaticity of the detection area is detected. 
     Advantageous Effects 
     According to the present invention, correction of light-emission brightness or chromaticity can be performed with high accuracy and low cost. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is an illustration showing an example of the configuration of an embodiment of a liquid crystal display device to which the present invention is applied. 
         FIG. 2  is an illustration showing the detailed configuration of a backlight. 
         FIG. 3  is an illustration showing the detailed configuration of a correction unit area of the backlight. 
         FIG. 4  is an illustration for explaining the position of a sensing period in a 4-frame time period. 
         FIG. 5  is an illustration for explaining the order of lighting of blocks at the time of brightness correction. 
         FIG. 6  is an illustration for explaining the details of the sensing period. 
         FIG. 7  is an illustration showing lighting of individual blocks at the time of brightness correction. 
         FIG. 8  is an illustration showing lighting of individual blocks at the time of brightness correction. 
         FIG. 9  is a functional block diagram of a backlight and a light source controller. 
         FIG. 10  is a flowchart for explaining a backlight control process. 
         FIG. 11  is an illustration for explaining a reduction in the level of a light reception signal in accordance with the distance from a sensor. 
         FIG. 12  is an illustration for explaining a reduction in the level of a light reception signal in accordance with the distance from a sensor. 
         FIG. 13  is an illustration for explaining a change in the current value supplied to a block distant from the sensor. 
         FIG. 14  is an illustration for explaining extension of a correction area in a case where the supplied current value is changed. 
         FIG. 15  is an illustration for explaining a change in an LED caused by a deterioration with the lapse of time. 
         FIG. 16  is an illustration for explaining a change in an LED caused by a deterioration with the lapse of time. 
         FIG. 17  is an illustration for explaining a change in an LED caused by a deterioration with the lapse of time. 
     
    
    
     EXPLANATION OF REFERENCE NUMERALS 
       1  liquid crystal display device,  12  backlight,  13  control unit,  32  light source controller,  51  control part,  61  calculator,  62  timing controller, SR sensor, B block, SA area, LA correction unit area 
     BEST MODES FOR CARRYING OUT THE INVENTION 
     Hereinafter, embodiments of the present invention will be explained with reference to the drawings. 
       FIG. 1  shows an example of the configuration of an embodiment of a liquid crystal display device to which the present invention is applied. 
     A liquid crystal display device  1  shown in  FIG. 1  is constituted by a liquid crystal panel  11  including a color filter substrate having colors of red, green, and blue, a liquid crystal layer, and the like; a backlight  12  placed on the back surface of the liquid crystal panel  11 ; a control unit  13  that controls the liquid crystal panel  11  and the backlight  12 ; and a power supply unit  14 . 
     The liquid crystal display device  1  displays an original image corresponding to an image signal in a predetermined display area (an area corresponding to a display section  21  of the liquid crystal panel  11 ). Note that an input image signal input to the liquid crystal display device  1  corresponds to, for example, an image with a frame rate of 60 Hz (hereinafter, referred to as a frame image), and in the following description, 1/60 seconds is called a 1-frame time period. 
     The liquid crystal panel  11  is constituted by the display section  21  in which a plurality of apertures through which white light from the backlight  12  is transmitted are arranged, and a source driver  22  and a gate driver  23  that output driving signals to transistors (TFTs: Thin Film Transistors), which are not illustrated, provided for individual apertures in the display section  21 . 
     White light transmitted through an aperture of the display section  21  is converted, with a color filter formed on the color filter substrate, which is not illustrated, into red, green, or blur light. A set of three apertures through which red, green, and blue light beams are emitted corresponds to one pixel of the display section  21 . 
     The backlight  12  emits white light in a light-emission area corresponding to the display section  21 . The light-emission area of the backlight  12  is divided into a plurality of blocks (areas) and lighting is controlled for individual divided blocks, as described later with reference to  FIG. 2 . 
     The control unit  13  is constituted by a display brightness calculator  31 , a light source controller  32 , and a liquid crystal panel controller  33 . 
     An image signal corresponding to each frame image is supplied from an external device to the display brightness calculator  31 . The display brightness calculator  31  calculates the distribution of brightnesses of a frame image from the supplied image signal, and calculates necessary display brightness for each block in accordance with the distribution of brightnesses of the frame image. The calculated display brightness is supplied to the light source controller  32  and the liquid crystal panel controller  33 . 
     The light source controller  32  calculates the backlight brightness of each block in accordance with the display brightness of the block supplied from the display brightness calculator  31 . Then, by performing PWM (Pulse Width Modulation) control, the light source controller  32  controls each block of the backlight  12  so that the calculated backlight brightness can be obtained. Controlling the light-emission brightness (backlight brightness) of the backlight  12  in accordance with an input image signal will be called normal PWM control. 
     In addition, the light source controller  32  also performs light-emission control (hereinafter, referred to as sensing control, in an appropriate manner) for correcting light-emission brightness or chromaticity in accordance with the light-emission brightness or chromaticity of each block detected by a sensor SR ( FIG. 2 ) provided in the backlight  12 . 
     Here, the sensor SR is an illuminance sensor or a color sensor. Note that in the following description, for simple explanation, an example in which sensors SR provided in the backlight  12  are illuminance sensors and the light-emission brightness of individual blocks is corrected by sensing control will be explained. However, similar processing can be performed for a case where the chromaticity of each bock is corrected. In addition, both light-emission brightness and chromaticity may be corrected. 
     The backlight brightness of each block calculated by the light source controller  32  is supplied to the liquid crystal panel controller  33 . 
     The liquid crystal panel controller  33  calculates the liquid crystal aperture ratio of each pixel in the display section  21  in accordance with the display brightness of each block supplied from the display brightness calculator  31  and the backlight brightness of each block supplied from the light source controller  32 . Then, the liquid crystal panel controller  33  supplies a driving signal to the source driver  22  and the gate driver  23  of the liquid crystal panel  11  so that the calculated liquid crystal aperture ratio can be achieved, and performs driving control of a TFT in each pixel of the display section  21 . 
     The power supply unit  14  supplies predetermined power to each unit of the liquid crystal display device  1 . 
       FIG. 2  shows the detailed configuration of the backlight  12 . Note that  FIG. 2  illustrates only part of the light-emission area of the backlight  12 . In addition, outside numbers in  FIG. 2  are illustrated for explanation, and those numbers are not part of the backlight  12 . 
     The smallest square grids shown in  FIG. 2  represent blocks B, which are control units of the light-emission brightness of the backlight  12 . In each block B, one or more set of LEDs (Light Emitting Diodes) serving as light-emitting elements which emit red, green, and blue lights are provided. 
     Note that blocks B are obtained by virtually diving the light-emission area of the backlight  12 , not by physically dividing the light-emission area using partition boards or the like. Thus, light emitted from a light-emitting element provided in a block B is diffused by a diffusion plate, which is not illustrated, and is applied not only to the front side of the block B but also to the front side of blocks near the block B. 
     In the backlight  12 , an area SA is constituted by four blocks in the horizontal direction (lateral direction in the drawing) and four blocks in the vertical direction (longitudinal direction in the drawing), that is, 4×4, sixteen blocks B. In  FIG. 2 , individual areas SA are illustrated using different patterns. Furthermore, a correction unit area LA is formed of an area in which 2×2 areas SA are arranged in the horizontal and vertical directions. Thus, in the light-emission area of the backlight  12 , the areas SA and the correction unit areas LA are arranged in a repeated manner in the horizontal and vertical directions. 
     The sensors SR are provided in the areas SA on the one-to-one basis. An area SA is the largest area for which a sensor SR can perform detection with the same current value as a current value supplied when normal PWM control is performed, that is, when light-emission brightness is controlled in accordance with an input image signal. A sensor SR is placed at the center of an area SA. 
     The light source controller  32  performs the same sensing control for individual correction unit areas LA in a parallel manner. Hereinafter, sensing control for a single correction unit area LA will be explained. Obviously, normal PWM control for controlling light-emission brightness in accordance with an input image signal is control for each block B. 
       FIG. 3  is an illustration showing the detailed configuration of a correction unit area LA. 
     A correction unit area LA includes 2×2 areas SA, as described above. In a case where individual areas SA within the correction unit area LA need to be distinguished from each other, an area SA located in an upper left portion of the correction unit area LA is called an area SA-a, an area SA located in an upper right portion of the correction unit area LA is called an area SA-b, an area SA located in a lower left portion of the correction unit area LA is called an area SA-c, and an area SA located in a lower right portion of the correction unit area LA is called an area SA-d. Similarly, in a case where sensors SR provided at the center of the areas SA-a, SA-b, SA-c, and SA-d need to be distinguished from each other, they are called sensors SR-a, SR-b, SR-c, and SR-d. 
     In addition, in a case where sixteen blocks B within the area SA-a are distinguished from each other, they are called blocks SA-a( 1 ) to SA-a( 16 ). Similarly, in a case where blocks B in the areas SA-b, SA-c, and SA-d are distinguished from each other, they are called blocks SA-b( 1 ) to SA-b( 16 ), blocks SA-c( 1 ) to SA-c( 16 ), and blocks SA-d( 1 ) to SA-d( 16 ). 
     Note that in  FIG. 3 , individual block numbers of the blocks SA-a( 1 ) to SA-a( 16 ), the blocks SA-b( 1 ) to SA-b( 16 ), the blocks SA-c( 1 ) to SA-c( 16 ), and the blocks SA-d( 1 ) to SA-d( 16 ) are illustrated as encircled numbers (numbers surrounded by circles) in the corresponding blocks B. The same applies to  FIGS. 7 and 8 , which will be described later. 
     The light source controller  32  performs a single sensing control operation for the correction unit area LA within a 4-frame time period. 
     Thus, as shown in  FIG. 4 , the light source controller  32  performs sensing control for the area SA-a within the first 1-frame time period of a 4-frame time period, performs sensing control for the area SA-b within the next 1-frame time period, performs sensing control for the area SA-c within the next 1-frame time period, and performs sensing control for the area SA-d within the last 1-frame time period. 
     A 1-frame time period is constituted by sixteen sub-frame time periods. For example, within the first 1-frame time period, the light source controller  32  sequentially performs sensing control for the sixteen blocks SA-a( 1 ) to SA-a( 16 ), each for a 1-sub-frame time period. Thus, the length of a 1-sub-frame time period is one-sixteenth the length of a 1-frame time period ( 1/60 seconds), that is, 1/960 seconds. 
     Sensing control is performed between normal PWM control operations. For example, after a period during which normal PWM control is performed (hereinafter, referred to as a normal PWM period, in an appropriate manner) within a 1-sub-frame time period, a period during which sensing control is performed (hereinafter, referred to as a sensing period, in an appropriate manner) is provided. Note that the sensing period may be provided before the normal PWM period. 
     Thus, in the correction unit area LA, the order of blocks B for which light-emission brightness is corrected is as shown in  FIG. 5 . 
     Light-emission brightness is corrected in the order of the blocks SA-a( 1 ) to SA-a( 16 ), the blocks SA-b( 1 ) to SA-b( 16 ), the blocks SA-c( 1 ) to SA-c( 16 ), and the blocks SA-d( 1 ) to SA-d( 16 ). After correction for the block SA-d( 16 ) is completed, correction for the block SA-a( 1 ) is performed again. Here, a time period during which blocks B arranged in a single line in  FIG. 5  are processed corresponds to a 1-frame time period. 
       FIG. 6  shows the detailed configuration of the first 1-sub-frame time period within a 4-frame time period, that is, a sub-frame time period during which the light-emission brightness of the block SA- 1 ( 1 ) is corrected. 
     During a sub-frame time period corresponding to the block SA-a( 1 ), in a sensing period, light emission in the block SA-a( 1 ) to be corrected and light emission in the blocks SA-b( 1 ), SA-c( 1 ), and SA-d( 1 ) which are located in the three areas SA-b, SA-c, and SA-d other than the area SA-a in the correction unit area LA and whose positions in the areas SA-b, SA-c, and SA-d correspond to the block SA-a( 1 ) are sequentially performed. 
     Note that although an example in which light emission in the blocks SA-b( 1 ), SA-c( 1 ), and SA-d( 1 ) is performed prior to light emission in the block SA-a( 1 ) is shown in  FIG. 6 , the order of light emission may be reversed. 
     A period (time) during which light emission in the blocks SA-b( 1 ), SA-c( 1 ), and SA-d( 1 ) is performed is a so-called dummy light-emission period during which a value (sensor value) is not acquired using the SR-a although lighting is performed. The subsequent period during which light emission in the block SA-a( 1 ) is performed is a light-emission period for sensor value acquisition for acquiring a sensor value using the sensor SR-a. 
     In  FIG. 6 , a period provided prior to the dummy light-emission period and a period provided prior to the light-emission period for sensor value acquisition, the periods being represented by oblique lines, are blank periods provided for eliminating the influence of previous light emission. 
     Each of the dummy light-emission period and the light-emission period for sensor value acquisition is set to be as short as possible within a range capable of acquiring a sufficiently stable sensor value. It is desirable that, for example, a time period shorter than or equal to 5% of a 1-sub-frame time period is set. This is because when the dummy light-emission period and the light-emission period for sensor value acquisition are set to be longer, the proportion of the sensing period in a 1-sub-frame time period increases, and the average light-emission brightness of the entire backlight  12  reduces. 
     Thus, by setting the dummy light-emission period and the period for sensor value acquisition to be as short as possible within a range capable of acquiring a sufficiently stable sensor value, a reduction in the average light-emission brightness of the entire backlight  12  can be suppressed. In other words, even in a case where light-emission brightness due to normal PWM control is extremely low, an increase in the light-emission brightness due to light emission in sensing control can be minimized. 
     During the light-emission period for sensor value acquisition within the sensing period shown in  FIG. 6 , light emission is performed only in the block SA-a( 1 ) in the correction unit area LA. Light-emission is performed only in the block SA-a( 1 ) in order to eliminate the influence of light emission in peripheral blocks B and to obtain an accurate light-emission brightness of the block SA-a( 1 ) since each block B obtained by dividing the backlight  12  is not obtained by physical division using a partition board or the like, as described above. 
     In addition, during the dummy light-emission period, light emission is performed only in the blocks SA-b( 1 ), SA-c( 1 ), and SA-d( 1 ) within the correction unit area LA. Light emission in the blocks SA-b( 1 ), SA-c( 1 ), and SA-d( 1 ) is performed in order to prevent human eyes from recognizing light emission for brightness correction as flicker, as described later. 
       FIGS. 7 and 8  are illustrations showing lighting of individual blocks B within a correction unit area LA in a case where only a sensing period is focused on. 
     First, the area SA-a of the correction unit area LA is set as an area to be corrected (hereinafter, referred to as a correction area, in an appropriate manner). As described above with reference to  FIG. 6 , dummy light-emission is performed in the blocks SA-b( 1 ), SA-c( 1 ), and SA-d( 1 ), and then, light emission for sensor value acquisition is performed in the block SA-a( 1 ). The sensor SR-a within the correction area SA-a receives light emitted from the block SA-a( 1 ). Next, dummy light-emission is performed in the blocks SA-b( 2 ), SA-c( 2 ), and SA-d( 2 ). Then, light emission for sensor value acquisition is performed in the block SA-a( 2 ), and the sensor SR-a receives light emitted from the block SA-a( 2 ). 
     Similarly, light emission is sequentially performed. Light emission for sensor value acquisition is performed until the block SA-a( 16 ), and the sensor SR-a receives light emitted from the block SA-a( 16 ). 
     Next, the area SA-b within the correction unit area LA is set as a correction area. As shown in  FIG. 8 , dummy light emission is performed in the blocks SA-a( 1 ), SA-c( 1 ), and SA-d( 1 ). Then, light emission for sensor value acquisition is performed in the block SA-b( 1 ). The sensor SR-b within the correction area SA-b receives light emitted from the block SA-b( 1 ). Next, dummy light-emission is performed in the blocks SA-a( 2 ), SA-c( 2 ), and SA-d( 2 ). Then, light emission for sensor value acquisition is performed in the block SA-b( 2 ), and the sensor SR-b receives light emitted from the block SA-b( 2 ). 
     Similarly, light emission is sequentially performed. Light emission for sensor value acquisition is performed until the block SA-b( 16 ), and the sensor SR-b receives light emitted from the block SA-b( 16 ). 
     Next, the area SA-c and the area SA-d are sequentially set as correction areas, and similar dummy light emission and light emission for sensor value acquisition are performed. 
     Thus, for example, the number of times lighting is performed in the block SA-a( 1 ) for correction of light-emission brightness within a 4-frame time period is four in total, one light emission operation for sensor value acquisition and three dummy light emission operations. That is, the frequency of lighting when control other than normal PWM control is performed for the block SA-a( 1 ) is ( 4/60 seconds)/4= 1/60(seconds/times)=60 Hz since four light emission operations are performed during a 4-frame time period ( 4/60 seconds), and human eyes do not recognize light emission for brightness correction as flicker. 
       FIG. 9  is a functional block diagram of the backlight  12  and the light source controller  32  in a case where correction of light-emission brightness is performed for the block SA-a( 1 ). 
     In the block SA-a( 1 ) of the backlight  12 , LEDs  41  serving as light-emitting elements that emit red, green, and blue light are provided. One end (anode side) of the LEDs  41  is connected to a driving power supply part  54  of the light source controller  32 , and the other end (cathode side) of the LEDs  41  is connected to a switching element  42  constituted by, for example, an FET (Field Effect Transistor) or the like. 
     Similarly, in the block SA-b( 1 ) of the backlight  12 , LEDs  43  serving as light-emitting elements that emit red, green, and blue light are provided. One end (anode side) of the LEDs  43  is connected to the driving power supply part  54  of the light source controller  32 , and the other end (cathode side) of the LEDs  43  is connected to a switching element  44 . Since the blocks SA-c( 1 ) and SA-d( 1 ) are similar to the block SA-b( 1 ), illustration is omitted. 
     The switching element  42  or  44  functions as a switch for causing a current to flow to the LEDs  41  or  43  when a signal (pulse signal) at a predetermined level is supplied from a pulse generation part  52 . When a current is supplied to the LEDs  41  or  43 , the LEDs  41  or  43  emit light. The sensor SR-a converts (A-D converts) the amount of light received from the LEDs  41  of the block SA-a( 1 ) into a digital signal, and supplies the converted light reception signal to a sampling part  53 . 
     The light source controller  32  is constituted by a control part  51 , the pulse generation part  52 , the sampling part  53 , the driving power supply part  54 , and a memory  55 . 
     The control part  51  includes a calculator  61  and a timing controller  62 . The calculator  61  calculates the backlight brightness of the block SA-a( 1 ) based on display brightness supplied from the display brightness calculator  31 , and supplies the calculated backlight brightness to the timing controller  62 . In addition, the calculator  61  supplies, to the driving power supply part  54 , a power supply control signal for controlling the values of currents supplied to the LEDs  41  and the LEDs  43 . In the calculator  61 , the values of currents supplied to the LEDs  41  and the LEDs  43  are corrected when necessary in accordance with a light reception signal supplied from the sampling part  53 . That is, in the calculator  61 , feedback control of backlight brightness corresponding to changes in light-emission brightness, such as time-lapse deterioration and temperature change, is performed. Note that correction for a brightness change may be performed by changing the pulse width of PWM, changing the number of pulses of PWM, or the like, instead of changing the value of a supplied current. 
     The timing controller  62  supplies, to the pulse generation part  52 , a pulse control signal for controlling the pulse width (duty ratio) of a pulse signal, the pulse interval, and the like in accordance with the backlight brightness calculated by the calculator  61 . In addition, the timing controller  62  supplies, to the sampling part  53 , a timing signal representing a timing at which a light reception signal is acquired (sampled) from the sensor SR-a. 
     The pulse generation part  52  generates a pulse signal based on the pulse control signal, and supplies the generated pulse signal to the switching elements  42  and  44 . The sampling part  53  performs sampling in accordance with the timing signal, and supplies a light reception signal, which is obtained by sampling, to the calculator  61 . The driving power supply part  54  supplies a predetermined current value to the LEDs  41  and  43  in accordance with the power supply control signal supplied from the calculator  61 . The power of the driving power supply part  54  is supplied from the power supply unit  14  of  FIG. 1 . The memory  55  stores predetermined data necessary for control. 
     Next, a backlight control process by the light source controller  32  for a single correction unit area LA will be explained with reference to a flowchart of  FIG. 10 . This process starts when the display brightness of each block B is supplied from the display brightness calculator  31  to the light source controller  32 . 
     First, in step S 11 , the control part  51  substitutes  1  for the area number m (m=1, 2, . . . , M), which is a variable for determining a correction area from among four areas SA in the correction unit area LA. In the correction unit area LA, m=1 corresponds to the area SA-a, m=2 corresponds to the area SA-b, m=3 corresponds to the area SA-c, and m=4 corresponds to the area SA-d. Thus, in the correction unit area LA, the area SA-a is first set as a correction area. 
     In step S 12 , the control part  51  substitutes  1  for the block number n (n=1, 2, . . . , N), which is a variable for distinguishing individual blocks B constituting each area SA in the correction unit area LA from each other. 
     In step S 13 , the control part  51  causes pulse light emission corresponding to an input image signal to be performed in all the blocks B in all the areas SA (that is, the areas SA-a, SA-b, SA-c, and SA-d). That is, this processing is processing performed during a normal PWM period within a 1-sub-frame time period. 
     In step S 14 , the control part  51  causes dummy light emission to be performed in the nth bock in the correction area. For example, in a case where the correction area is the area SA-a, the control part  51  causes dummy light emission to be performed in the blocks SA-b(n), SA-c(n), and SA-d(n). This processing is processing performed during a dummy light-emission period within a 1-sub-frame time period. 
     In step S 15 , the control part  51  causes light emission for sensor value acquisition to be performed in the nth bock in the correction area. For example, in a case where the correction area is the area SA-a, the control part  51  causes light emission for sensor value acquisition to be performed in the block SA-a(n). Then, the sensor SR-a shares, with the sampling part  53 , a light reception signal when light emission for sensor value acquisition in the block SA-a(n) is received. This processing is processing performed during a light-emission period for sensor value acquisition within a 1-sub-frame time period. 
     In step S 16 , the control part  51  calculates the amount of correction of light-emission brightness of the nth block in the correction area in accordance with the light reception signal from the sensor SR. For example, in a case where the correction area is the area SA-a, the control part  51  calculates a difference from a desired value of the light-emission brightness of the block SA-a(n) in accordance with the light reception signal supplied from the sampling part  53 , and calculates the correction amount corresponding to the calculated difference. The calculated correction amount is stored in the memory  55  and fed back when light emission control is performed for the block SA-a(n) next time. Note that the desired value of the light-emission brightness of the block SA-a(n) is also stored in advance in the memory  55 . 
     In step S 17 , the control part  51  determines whether or not the block number n is the same as the number N (=16) of blocks in the area SA. 
     In a case where it is determined in step S 17  that the block number n is not the same as the number N of blocks in the area SA, that is, the block number n is smaller than the number N of blocks, the process proceeds to step S 18 . In step S 18 , the block number n is incremented by one by the control part  51 , and the process returns to step S 13 . 
     Meanwhile, in a case where it is determined in step S 17  that the block number n is the same as the number N of blocks in the area SA, that is, light emission for sensor value acquisition has been performed for all the blocks B in the current correction area, the process proceeds to step S 19 . In step S 19 , the control part  51  determines whether or not the area number m is the same as the number M (=4) of areas in the correction unit area LA. 
     In a case where it is determined in step S 19  that the area number m is not the same as the number M of areas in the correction unit area LA, that is, light emission for sensor value acquisition has not been performed for all the areas SA-a to SA-d in the correction unit area LA, the process proceeds to step S 20 . In step S 20 , the area number m is incremented by one by the control part  51 , and the process returns to step S 12 . Accordingly, the next area SA is set as a correction area. 
     Meanwhile, in a case where it is determined in step S 19  that the area number m is the same as the number M of areas in the correction unit area LA, that is, light emission for sensor value acquisition has been performed for all the areas SA-a to SA-d in the correction unit area LA, the process returns to step S 11 . Then, the processing of steps S 11  to S 20  is performed again. 
     The process of  FIG. 10  is repeatedly performed until supply of an input image signal from an external device to the liquid crystal display device  1  is completed. 
     As described above, in the liquid crystal display device  1  in  FIG. 1 , in the case of correcting the light-emission brightness of a predetermined block B in a correction area SA within a correction unit area LA, in a state where only the block B to be corrected in the correction unit area LA is lit and the other blocks B are not lit, light is received at a sensor SR and the correction amount of light-emission brightness is calculated in accordance with the amount of received light. Thus, the light-emission brightness of the lit block B can be measured with high accuracy and corrected. 
     In addition, the sensor SR is provided for each area SA, which is the largest area for which detection can be performed with the same current value as a current value supplied when light-emission brightness is controlled in accordance with an input image signal. Accordingly, the minimum necessary number of sensors SR can be provided. Thus, the production cost of the backlight  12  (the liquid crystal display device  1 ) can be reduced. 
     That is, according to the liquid crystal display device  1 , correction of light-emission brightness can be performed with high accuracy and low cost. 
     Furthermore, since the lighting frequency of each block B at the time of correcting brightness is set to 60 Hz, light emission for brightness correction is prevented from being recognized as flicker by human eyes. 
     A method for correcting light-emission brightness by performing correction of light-emission or chromaticity only when a display image is dark at a scene change or the like and thus reducing the influence of light emission for brightness correction on the display image, has been available. However, in this method, there is a problem in which it is difficult to correct chromaticity changing in several seconds due to a temperature change or the like. 
     Obviously, in the backlight control process described above, by using a color sensor not an illuminance sensor as the sensor SR, correction of chromaticity can also be performed with high accuracy and high efficiency. Since an operation for correcting the chromaticity of each block B can be performed for each 4-frame time period ( 4/60 seconds), correction of chromaticity changing in several seconds can also be performed. 
     The sensor SR is provided for each area SA, which is the largest area for which detection can be performed with the same current value as a current value supplied when light-emission brightness is controlled in accordance with an input image signal. The amount of light received at the sensor SR is inversely proportional to distance. Thus, for example, as shown in  FIG. 11 , although a light reception signal at high level can be acquired in the blocks SA-a( 7 ) and SA-a( 11 ), which are close to the sensor SR-a of the correction area SA-a, the signal level in the blocks SA-a( 4 ) and SA-a( 16 ), which are distant from the sensor SR-a, is reduced even if light is emitted at the same light-emission brightness as the blocks SA-a( 7 ) and SA-a( 11 ). 
     More detailed explanation will be given with reference to  FIG. 12 . 
       FIG. 12  shows driving waveforms (waveforms of current values) supplied to LEDs in the block SA-a( 7 ) near the sensor SR-a and in the block SA-a( 16 ) distant from the sensor SR-a within the correction area SA-a, a driving waveform supplied to LEDs in the block SA-d( 1 ) outside the correction area SA-a, which is further distant from the sensor SR-a than the block SA-a( 16 ), and the output waveform of the sensor SR-a. 
     In  FIG. 12 , the lateral direction represents a time axis, and the longitudinal direction represents the level of a waveform (signal). 
     Note that, originally, a light-emission timing during a sensing period is the same throughout the blocks SA-a( 7 ), SA-a( 16 ), and SA-d( 1 ), as shown in  FIG. 4 . However, in  FIG. 12 , for simple explanation by comparison, the light-emission timings for these blocks are different. The same applies to  FIG. 13 , which will be described later. 
     In the backlight control process described above, a current value X a7  supplied to the LEDs in the block SA-a( 7 ), a current value X a16  supplied to the LEDs in the block SA-a( 16 ), and a current value X d1  supplied to the LEDs in the block SA-d( 1 ) are the same current value I 0 . 
     In addition, the level of the output waveform of the sensor SR-a when light is received from the block SA-a( 7 ) near the sensor SR-a exhibits a value y a7 , and the level of the output waveform of the sensor SR-a when light is received from the block SA-a( 16 ), which is distant from the sensor SR-a, exhibits a value y a16 , which is lower than the value y a7  and equal to or higher than the lowest level y L  necessary for performing correction. 
     Meanwhile, the level of the output waveform of the sensor SR-a when light is received from the block SA-d( 1 ) outside the correction area SA-a exhibits a value y d1 , which is lower than the lowest level y L . Thus, the light-emission brightness of the block SA-d( 1 ) outside the correction area SA-a cannot be corrected using the sensor value of the sensor SR-a, and the sensor SR-d is used for the block SA-d( 1 ). 
     As shown by providing oblique lines in  FIG. 13 , the light source controller  32  of the liquid crystal display device  1  sets the current value X d1  supplied to the LEDs in the block SA-d( 1 ) to a current value I 1 , which is greater than the current value I 0  supplied to the LEDs in the blocks SA-a( 7 ) and SA-a( 16 ). In this case, the level of the output waveform of the sensor SR-a when light is received from the block SA-d( 1 ) exhibits a value y d1 ′, which is equal to or higher than the lowest level y L . Thus, the amount of received light necessary for correction can be acquired by the sensor SR-a. 
     As described above, by supplying, to LEDs in a block B for which the level of a light reception signal with the current value I 0  at the time of normal PWM control is lower than or equal to the lowest level y L  since the block B is distant from the sensor SR-a, the current value I 1  which is greater than the current value I 0  supplied to LEDs in a block B near the sensor SR-a, an area SA for which a sensor SR performs detection for brightness correction can be extended, for example, to include 6×6 blocks, that is, 36 blocks, as shown in  FIG. 14 . Accordingly, the number of sensors SR in the entire backlight  12  can be reduced. Thus, correction of light-emission brightness or chromaticity can be performed with lower cost and more efficiency. Alternatively, if the number of blocks B for which one sensor SR is provided is the same, an inexpensive sensor SR having a smaller light reception area can be used. Thus, correction of light-emission brightness or chromaticity can be performed with lower cost and more efficiency. 
     Note that when an area SA includes 36 block units, a 1-frame time period is divided into  36  sub-frame time periods. Thus, a 1-sub-frame time period in this case is different from a 1-sub-frame time period in a case where 16 blocks constitute an area SA. 
     In the example described above, an example in which a current value supplied only to a block B for which the level of a sensor SR is lower than or equal to the lowest level y L  is changed has been explained. However, since the level of the sensor SR is reduced in accordance with distance, a current value supplied at the time of brightness correction (during a sensing period) even to an LED in a block B for which the level of the sensor SR is equal to or higher than the lowest level y L  with the current value I 0  at the time of normal PWM control may be increased in accordance with the distance from the sensor SR. 
     In a case where a current value supplied to an LED is changed for each block B, it is necessary to acquire in advance the relationship between a supplied current value If and a light-emission brightness L, the relationship representing the light-emission brightness (level of an output waveform) when a certain current value is supplied to the LED, and to store the acquired relationship in the memory  55 . Then, the light source controller  32  performs comparison with the light-emission brightness in the initial state, which is stored in the memory  55 , and corrects the supplied current value I 0  during a normal PWM period. 
     By not only storing the relationship between the supplied current value If and the light-emission brightness L for each block but also storing the relationship between the supplied current value If and the applied voltage value Vf to an LED in the memory  55 , a factor that influences on a change in the light-emission brightness or chromaticity of the LED can be guessed to some extent. 
     More specifically, as shown in  FIG. 15 , the light-emission brightnesses L when the supplied current values I 0 , I 1 , I 2 , and the like are set for an LED in a predetermined block B are measured in advance. 
     In addition, as shown in  FIG. 16 , the applied voltage values Vf when the supplied current values I 0 , I 1 , I 2 , and the like are set for an LED in a predetermined block B are measured in advance. 
     Then, the relationship between the current value If and the light-emission brightness L and the relationship between the current value If and the applied voltage value Vf, which are represented by thick lines in  FIGS. 15 and 16 , are stored as the initial state in the memory  55 . 
     In general, an LED is regarded as an equivalent circuit constituted by an LED  71 , an equivalent parallel resistor  72  which is connected in parallel to the LED  71 , and an equivalent series resistor  73  which is connected in series to the LED  71 , as shown in  FIG. 17 . Here, the resistance of the equivalent parallel resistor  72  is denoted by Rp, and the resistance of the equivalent series resistor  73  is denoted by Rs. 
     In a case where both the light-emission brightness L and the applied voltage value Vf become lower than the initial state after a predetermined time has passed when the current values supplied to an LED are set to I 0 , I 1 , and I 2 , as shown in  FIGS. 15 and 16 , the resistance Rp of the equivalent parallel resistor  72  can be regarded as being reduced by deterioration with the lapse of time. 
     Meanwhile, in a case where the light-emission brightness L is not changed from the initial state and the applied voltage value Vf becomes higher than the initial state when the current values supplied to an LED are set to I 0 , I 1 , and I 2 , the resistance Rs of the equivalent series resistor  73  can be regarded as being increased by deterioration with the lapse of time. 
     In addition, in a case where the applied voltage value Vf is not changed from the initial state and the light-emission brightness L becomes lower than the initial state when the current values supplied to an LED are set to I 0 , I 1 , and I 2 , the influence of an external factor such as a lens can be guessed. 
     In actuality, since it is considered that the above-described three types of change are not independent and characteristics are established by the combination of these types of change, the ratio among “a change in the resistance Rp of the equivalent parallel resistor  72 ”, “a change in the resistance Rs of the equivalent series resistor  73 ”, and “an external factor” is estimated in accordance with the measured relationship between the current value If and the light-emission brightness L and relationship between the current value If and the applied voltage value Vf, and correction of the light-emission brightness can be performed in accordance with the ratio. That is, the optimal improvement measures against a change in the light-emission brightness caused by the deterioration with the lapse of time, such as a change in a supplied current value, a change in a pulse width, and exchange of LEDs, can be taken. 
     In the embodiment described above, an example in which brightness correction is performed for each block has been explained. However, brightness correction is not necessarily performed for each block. Brightness correction may be performed for each small area constituted by neighboring some blocks. Thus, the embodiment described above corresponds to an example of a case where one block constitutes a small area. However, for example, brightness correction may be performed for each small area constituted by four block units, such as the block SA-a( 1 ), the block SA-a( 2 ), the block SA-a( 5 ), and the block SA-a( 6 ) in the area SA-a in  FIG. 3 . 
     In addition, although an example in which the number N of blocks (small areas) is  16  (N=16) and the number M of blocks (small areas) constituting an area is 4 (M=4) has been explained in the above-described embodiment, the numbers of blocks are not limited to the above-described numbers. That is, any numbers can be set as long as the lighting frequency of each block B at the time of brightness correction is equal to or higher than 60 Hz. 
     In this description, the steps described in the flowchart include not only processing performed in time series in accordance with the written order but also processing performed in parallel or independently, the processing being not necessarily performed in time series. 
     Embodiments of the present invention are not limited to the embodiments described above. Various changes can be made without departing from the gist of the present invention.