Patent Publication Number: US-2007115685-A1

Title: Flat lighting source, luminance correcting circuit, luminance correcting method and liquid crystal display

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a flat lighting source, luminance correcting circuit, and luminance correcting method to be used in the flat lighting source and liquid crystal display and particularly to the flat lighting source suitably used when illuminating light with uniform luminance has to be provided to an entire displaying region of a display panel, for example, in the case of a liquid crystal display device where illuminating light is provided by a backlight, and to the luminance correcting circuit and the luminance correcting method to be used in the flat lighting source.  
      The present application claims priority of Japanese Patent Application No. 2005-337871 filed on Nov. 22, 2005, which is hereby incorporated by reference.  
      2. Description of the Related Art  
      Conventionally, as a backlight for a liquid crystal display device, a CCFL (Cold Cathode Fluorescent Lamp) has been used in many cases. However, in recent years, an LED (Light Emitting Diode) is also employed increasingly. In the LED backlight, ordinarily, a plurality of LEDs is connected in series and is driven at a constant current. Therefore, variations in current-luminance character-istic of each LED are reflected straightly as variations in luminance in a display region of a liquid crystal display device. Here, the “variations” represent secular changes in luminance characteristic to currents and temperatures. Conventional technologies to correct variations of these types are disclosed, for example, in the following references.  
       FIG. 12  is a diagram showing electrical configurations of main components of an illuminating device disclosed in Patent Reference 1 [(Japanese Patent Application Laid-open No. 2004-221158, Abstract, FIGS. 1 and 4)]. The disclosed illuminating device, as shown in  FIG. 12 , includes an LED printed circuit board (PCB)  10 , a constant current source  20 , and a temperature compensating circuit  30 . Alarge number of LED  11 , . . . ,  11  are formed on the LED PCB  10 . The constant current source  20  has a resistor  21 , a transistor  22 , an amplifying circuit  23 , and a comparison voltage generating circuit  24 . The temperature compensating circuit  30  includes an FET (Field Effect Transistor)  31 , an LED  32 , an amplifying circuit  33 , and a photo-detecting device  34 .  
      In the disclosed illuminating device, the LED  32  having the same temperature characteristic as each of the LED  11 , . . . ,  11  emits light which is converted by the photo-detecting device  34  into electrical signals. The electrical signals are input into the comparison voltage generating circuit  24 . Signals output from the comparison voltage generating circuit  24  are input into the amplifying circuit  23  and currents determined by a specified reference value through the transistor  22  based on signals output from the amplifying circuit  23  are fed to each of the LEDs  11 , . . . ,  11  and changes in the temperature characteristic of each of the LEDs  11 , . . . ,  11  are compensated for. In this situation, if luminance of the LED  32  is changed by changes in the temperature (atmospheric temperature) as shown in the characteristic diagram G 2  in  FIG. 13 , currents output from the constant current source  20  are changed by the comparison voltage generating circuit  24 , amplifying circuit  23 , transistor  22  as shown in the characteristic diagram H 1  and the luminance of light emitted from the LED PCB  10  is corrected. As a result, in contrast to the state shown in the characteristic diagram G 1  where the luminance changes with temperature, a characteristic appears that the luminance does not change with temperature as shown in the characteristic diagram G 3 .  
       FIG. 14  is a diagram showing one example of configurations of a conventional LED backlight. Colors emitted from the LEDs  41 , . . . ,  41  making up the LED backlight are a combination of R (Red), G(Green), and B(Blue) or white. The LEDs  41 , . . . ,  41  are mounted on the PCB  42  and every specified number of the LEDs  41 , . . . ,  41  are connected serially. In this state, the LEDs emitting white color having the number corresponding to each level of supply power are connected serially and, when the LEDs emitting the white color can not be contained in a row, a plurality of rows each including the LEDs emitting the white color is arranged and connected in parallel among rows in some cases. Also, when the LEDs emitting each of the R, G, and B is used, a specified number of LEDs in every color are serially connected. In this LED backlight, part of light components having given luminance emitted from the entire LEDs  41 , . . . ,  41  is converted into electrical signals by the photo-detecting device  43 . In this case, by arranging the photo-detecting device  43  and the color filter  44  for each of R, G, and B, or by controlling the photo-detecting device  43  so that its light sensing characteristic has dependence on a wavelength, light having given luminance for each of R, G, and B is converted into an electrical signal. A driving current and a driving voltage to be fed to the LEDs  41 , . . . ,  41  are adjusted based on the electrical signal generated in proportion to the luminance and are controlled so that luminance of light received by the photo-detecting device  43  becomes constant.  
      As a result, the conventional LED backlight shown in  FIG. 14  has a problem. That is, by keeping smooth track of a change in luminance of the light emitted from the LEDs  41 , . . . ,  41 , the current and voltage to be fed to the LEDs  41 , . . . ,  41  existing in the vicinity of the photo-detecting device  43  are well controlled, however, it is difficult to keep track of a change in luminance of the light emitted from the LEDs  41 , . . . ,  41  existing at a location some distance from the photo-detecting device  43 . Another problem is that, since radiation is difficult in a central portion of the PCB  42 , temperatures are likely to rise and, therefore, due to place-to-place variations of a temperature in areas surrounding the LEDs  41 , . . . ,  41  and due to dependence of the LEDs  41 , . . . ,  41  upon temperatures, it is made difficult to make uniform the distribution of luminance. Moreover, when variations occur in luminance of light emitted from the LEDs  41 , . . . ,  41  due to secular changes in the LEDs  41 , . . . ,  41 , only the luminance characteristic of the LED  41  existing in the vicinity of the photo-detecting device  43  is adjusted and variations still occur in the characteristic of luminance distribution of the entire LEDs  41 , . . . ,  41 . To solve these problems, an LED backlight is disclosed in Patent Reference 2 [(Japanese Patent Application Laid-open No. 2005-115372 (Abstract, FIG. 7A, 7B, 8B, FIG. 10)) in which photo-detecting devices are mounted not in one place but in a plurality of places.  
       FIG. 15  is a diagram showing configurations of LEDs used as the conventional backlight disclosed in the Patent Reference 2. In the disclosed backlight, as shown in  FIG. 15 , four rows of the LEDs  51  each row having 31 pieces are arranged on the PCB  50  and are connected in series.  
       FIG. 16  is a diagram showing another configurations of LEDs used as the conventional backlight disclosed in the Patent Reference 2. In the disclosed backlight, as shown in  FIG. 16 , a plurality of rows of LEDs each having three to seven LEDs connected serially to one another is arranged.  
       FIG. 17  is a cross-sectional view of the conventional backlight in which the LEDs  51  shown in  FIG. 15  or  FIG. 16  are mounted. In this backlight  52 , as shown in  FIG. 17 , each photo-detecting device  53  is mounted between the LEDs  51  and a diffusing body  54  and an LCD (Liquid Crystal Display) panel  55  are mounted in a direction in which light is emitted from each of the LEDs  51 .  
       FIG. 18  is a block diagram showing configurations of a correcting circuit to correct luminance characteristic of the LEDs  51  shown in  FIG. 15  or  FIG. 16 . In the correcting circuit, a signal “a” output from the photo-detecting device  53  is detected by a detector  61  and, based on a detected signal “b”, a luminance characteristic of the LEDs  51  is corrected by the controlling unit  62 .  
      In a backlight controlling unit of a liquid crystal display device disclosed in Patent Reference 3 (Japanese Patent Application Laid-open No. 2003-215534), an amount of light emitted from LEDs mounted as a backlight on a rear surface side of the liquid crystal display device is controlled according to brightness in a place surrounding the liquid crystal panel and even if a temperature which the LEDs use changes, the amount of emitted light is controlled so as to be a specified value.  
      However, the above conventional technologies have the following problems. In the illuminating device disclosed in the Patent Reference 1 and the backlight disclosed in the Patent Reference 2, it is impossible to detect and correct luminance of light emitted from an individual one out of a plurality of LEDs making up the LED light source. As a result, a problem occurs in that the occurrence of changes in luminance due to secular change in the LEDs causes a change in luminance distribution in the liquid crystal panel. Another problem is that, though total changes in luminance of light emitted from the LEDs caused by a change in temperatures can be corrected, a change in luminance of light emitted from an individual one of the LEDs cannot be corrected, as a result, causing a change in luminance distribution in the liquid crystal panel.  
      Moreover, the illuminating device disclosed in the Patent Reference 1 has still another problem. That is, in the disclosed conventional illuminating device, when a change in temperature of the LEDs or a secular change in the LEDs are detected, luminance of light emitted from LEDs different from the LEDs actually used as the light source is referenced, however, there occurs rare coincidence of temperature, temperature characteristic, secular change characteristic (life characteristic) between the LEDs actually used and the LEDs used as the reference, which makes it difficult to make exact correction to these characteristics. Also, in the backlight disclosed in the Patent Reference 2, a plurality of LEDs is connected to one another in series, however, it is impossible to correct the characteristic of luminance such as luminance of light emitted from an individual LED out of the plurality of LEDs. Actually, driving currents or a like for the entire group of a plurality of LEDs connected serially can be calibrated and luminance of light emitted from the group of the LEDs cannot be changed as a whole. Furthermore, in the backlight controlling device disclosed in the Patent Reference 3, luminance of light emitted from the LEDs is controlled according to brightness in a place surrounding the liquid crystal panel and, therefore, the object of the conventional invention is different from that of the present invention and thus the above problems cannot be solved by this conventional invention.  
     SUMMARY OF THE INVENTION  
      In view of the above, it is an object of the present invention to provide a flat lighting source which is capable of providing illuminating light with uniform luminance to an entire display region of a display panel of a liquid crystal display device or a like and a luminance correcting circuit to be used for the above.  
      According to a first aspect of the present invention, there is provided a flat lighting source having a plurality of light emitting devices so arranged as to form a planar surface and letting illuminating light come in a display region of a transmissive-type display panel from its rear, including:  
      a plurality of luminance correcting circuits each to set luminance of light to be emitted from each of the plurality of light emitting devices at a targeted value for every light emitting device and to detect an amount of deviation in luminance of light from the targeted value occurring when each of the plurality of light emitting devices is turned ON, and to make the luminance of light coincide with the targeted value, based on the detected amount of deviation.  
      In the foregoing first aspect, a preferable mode is one wherein the luminance correcting circuit includes:  
      a plurality of photo-detecting devices so mounted as to correspond to each of the plurality of light emitting devices in a one-to-one relationship which receives light emitted from each of the plurality of light emitting device and generates a luminance detecting signal having a level corresponding to luminance of light emitted from each of the plurality of light emitting devices; and  
      a plurality of driving/correcting circuits each being mounted so as to correspond to each of the plurality of light emitting devices in a one-to-one relationship, each of which supplies driving power to each of the plurality of light emitting devices and detects deviation in luminance of light emitted from each of the plurality of light emitting devices from a targeted value, based on a level of the luminance detecting signal, and corrects the driving power so that the deviation is compensated for.  
      Also, a preferable mode is one wherein each of the plurality of light emitting devices is arranged, in a one-to-one relationship, in a vicinity of each of the plurality of photo-detecting devices.  
      Also, a preferable mode is one wherein each of the plurality of light emitting devices and each of the plurality of photo-detecting devices are mounted in a same package.  
      According to a second aspect of the present invention, there is provided a luminance correcting circuit to be used in a flat lighting source having a plurality of light emitting devices so arranged as to form a planar surface and letting illuminating light come in a display region of a transmissive-type display panel from its rear, wherein each of the luminance correcting circuits sets luminance of light to be emitted from each of the plurality of light emitting devices at a targeted value for every light emitting device and detects an amount of deviation in luminance of light from the targeted value occurring when each of the plurality of light emitting devices is turned ON, and makes the luminance of light coincide with the targeted value based on the detected amount of deviation.  
      In the foregoing second aspect, a preferable mode is one that wherein includes:  
      a plurality of photo-detecting devices each being mounted so as to correspond to each of the plurality of light emitting devices in a one-to-one relationship and to receive light emitted from each of the plurality of light emitting devices; and  
      a plurality of driving/correcting circuits each being mounted so as to correspond to each of the plurality of light emitting devices in a one-to-one relationship and to supply driving power to each of the plurality of light emitting devices and to detect deviation in luminance of light emitted from each of the plurality of light emitting devices from a targeted value, based on a level of the luminance detecting signal, and to correct the driving power so that the deviation is compensated for.  
      According to a third aspect of the present invention, there is provided a luminance correcting method to be applied to a flat lighting source having a plurality of light emitting devices so arranged as to form a planar surface and letting illuminating light come in a display region of a transmissive-type display panel from its rear, including;  
      setting luminance of light to be emitted from each of the plurality of light emitting devices at a targeted value for every light emitting device;  
      detecting an amount of deviation in luminance of light from the targeted value occurring when each of the plurality of light emitting devices is turned ON; and  
      making the luminance of light coincide with the targeted value based on the detected amount of deviation.  
      According to a fourth aspect of the present invention, there is provided a flat lighting source including:  
      a plurality of light emitting devices;  
      a plurality of luminance detecting devices, each of which is provided, in a one-to-one relationship with each of the plurality of the light emitting devices, to receive light emitted from the corresponding light emitting device; and  
      a plurality of luminance correcting circuits, each of which is provided, in a one-to-one relationship with each of the plurality of the light emitting devices, to correct luminance of light emitted from the corresponding light emitting devices, based on a difference between a predetermined desirable luminance value and a detected luminance value obtained from the corresponding luminance detecting device, so as to maintain uniformity of the luminance of each of the light emitting devices.  
      In the foregoing fourth aspect, a preferable mode is one wherein the light emitting devices each include a light emitting diode.  
      According to a fifth aspect of the present invention, there is provided a liquid crystal display provided with a flat lighting source for backlighting,  
      the flat lighting source including:  
      a plurality of light emitting devices;  
      a plurality of luminance detecting devices, each of which is provided, in a one-to-one relationship with each of the plurality of the light emitting devices, to receive light emitted from the corresponding light emitting device; and  
      a plurality of luminance correcting circuits, each of which is provided, in a one-to-one relationship with each of the plurality of the light emitting devices, to correct luminance of light emitted from the corresponding light emitting devices, based on a difference between a predetermined desirable luminance value and a detected luminance value obtained from the corresponding luminance detecting device, so as to maintain uniformity of the luminance of each of the light emitting devices.  
      In the foregoing fifth aspect, a preferable mode is one wherein the light emitting devices each include a light emitting diode.  
      With the above configurations, there is provided a plurality of luminance correcting circuits each of which sets luminance of light to be emitted from each of the light emitting devices at a specified targeted value and detects an amount of deviation of luminance of light emitted from each of the light emitting devices from the specified targeted value and makes the luminance of light emitted from each of the light emitting devices coincide with the specified targeted value based on the amount of deviation and, therefore, luminance of light emitted from each of the light emitting devices and brightness of an entire display region of a display panel can be made uniform. Light having given luminance emitted from each of the light emitting devices is converted into a luminance detecting signal by each of the photo-detecting devices in a one-to-one relationship and the luminance detecting signal is fed back to each of the driving/correcting circuits and, therefore, variations and changes in luminance of light emitted from each of the light emitting devices can be automatically compensated for. Light having given luminance emitted from each of the light emitting devices is fed back and a process of extracting only an amount of change in temperature for feeding back is not required and, therefore, each of the temperature correcting circuits for each of the light emitting devices is made unnecessary. Each of the light emitting devices and each of the photo-detecting devices are mounted in the same package and in the vicinity of each other in a one-to-one relationship and, therefore, even if there are variations in luminance characteristic among the light emitting devices and the photo-detecting devices, luminance of light to be emitted from each of the light emitting devices can be made uniform by appropriately setting a targeted value of luminance. Each of the light emitting devices is mounted so as to correspond to each of the photo-detecting devices in a one-to-one relationship and, therefore, whichever color light out of R, G, and B is emitted by each of the light emitting devices, changes in color balance can be compensated for without using a color filter. As a result, when the flat lighting source is used as a backlight for a transmissive-type liquid crystal panel, it is made possible to provide illuminating light with uniform luminance to an entire display region of the liquid crystal panel. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The above and other objects, advantages, and features of the present invention will be more apparent from the following description taken in conjunction with the accompanying drawings in which:  
       FIG. 1  is a block diagram showing electrical configurations of main components of a flat lighting source according to a first embodiment of the present invention;  
       FIG. 2  is a circuit diagram showing one example of electrical configurations of LED driving/correcting circuits and driving/detecting circuits of  FIG. 1 ;  
       FIG. 3  is a diagram showing one example of layout of the LEDs, photodiodes, set of LED driving/correcting circuits and driving/detecting circuits of  FIG. 1 ;  
       FIG. 4  is a diagram showing another example of layout of the LEDs, photodiodes, LED driving/correcting circuits formed integrally with the driving/detecting circuits of  FIG. 1 ;  
       FIG. 5  is a cross-sectional view showing layout of main components of the flat lighting source of the second embodiment of the present invention;  
       FIG. 6  is a cross-sectional view showing layout of main components of the flat lighting source of the third embodiment of the present invention;  
       FIG. 7  is a cross-sectional view showing layout of main components of the flat lighting source of the fourth embodiment of the present invention;  
       FIG. 8  is a cross-sectional view showing layout of main components of the flat lighting source of the fifth embodiment of the present invention;  
       FIG. 9  is a cross-sectional view showing layout of main components of the flat lighting source of the sixth embodiment of the present invention;  
       FIG. 10  is a cross-sectional view showing layout of main components of the flat lighting source of the seventh embodiment of the present invention;  
       FIG. 11  is a circuit diagram showing electrical configurations of luminance correcting circuits to be used in the flat lighting source of the eighth embodiment of the present invention;  
       FIG. 12  is a diagram showing electrical configurations of main components of an illuminating device disclosed in Patent Reference 1.  
       FIG. 13  is a diagram explaining operations of the illuminating device of  FIG. 12 ;  
       FIG. 14  is a diagram showing one example of configurations of a conventional LED backlight.  
       FIG. 15  is a diagram showing configurations of LEDs used as a conventional backlight disclosed in Patent Reference 2;  
       FIG. 16  is a diagram showing another configurations of LEDs used as a conventional backlight disclosed in the Patent Reference 2;  
       FIG. 17  is a cross-sectional view of a backlight in which the LEDs shown in  FIG. 15  or  FIG. 16  are mounted; and  
       FIG. 18  is a block diagram showing configurations of a correcting circuit to correct luminance characteristic of the LEDs shown in  FIG. 15  or  FIG. 16 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      Best modes of carrying out the present invention will be described in further detail using various embodiments with reference to the accompanying drawings. A flat lighting source is provided in which light having given luminance emitted from each LED used as a light emitting device is converted, in a one-to-one relationship, into a luminance detecting signal which is fed back to each luminance correcting circuit and luminance correcting circuits to be used in the flat lighting source are provided.  
     First Embodiment  
       FIG. 1  is a block diagram showing electrical configurations of main components of a flat lighting source according to the first embodiment of the present invention. The flat lighting source of the embodiment includes is used as a backlight of a transmissive-type liquid crystal panel mounted in a liquid crystal display panel and, as shown in  FIG. 1 , LEDs  71 , photodiodes  72  as a photo-detecting device (luminance detecting device), LED driving/correcting circuits  73 , and driving/detecting circuits  74 . In  FIG. 1 , only one LED  71  is shown, however, the flat lighting source is made up of a plurality of LEDs serving as a backlight and arranged in a manner to form a planar surface. Each of the photodiodes  72  is mounted, in a one-to-one relationship, for every LED  71  and receives light emitted from each of the LEDs  71  and generates a luminance detecting voltage “a” having a level corresponding to the luminance of the received light. Each of the driving/detecting circuits  74  is mounted, in a one-to-one relationship, for every photodiode  72  and supplies power to each of the photodiodes  72  and sends out the luminance detecting voltage “a” generated by each of the photodiodes  72  as a luminance detecting voltage V 2  to each of the LED driving/correcting circuits  73 .  
      Each of the LED driving/correcting circuits  73  is mounted, in a one-to-one relationship, for every LED  71  and supplies driving power “c” to each of the LEDs  71  and detects, based on the luminance detecting voltage V 2  fed from each of the driving/detecting circuits  74 , a deviation from each targeted value (value corresponding to a luminance setting voltage V 1 ) of luminance of light emitted from each of the LED and corrects the driving power “c” so as to compensate for the deviation from each of the targeted luminance value. In this embodiment in particular, each of the LED driving/correcting circuits  73  increases a current to be fed, for example, as luminance of light emitted from each of the LED  71  decreases. Each of the photodiodes  72 , LED driving/correcting circuits  73 , and driving/detecting circuits  74  make up each luminance correcting circuit. Each of the luminance correcting circuits sets luminance of light to be emitted from each of the LEDs  71  at a targeted value and detects an amount of deviation from the targeted value of the luminance of light emitted from each of the LEDs  71  and then makes the luminance of light emitted from each of the LEDs  71  coincide with the targeted value, based on the amount of deviation from the targeted value.  
       FIG. 2  is a circuit diagram showing one example of electrical configurations of each of the LED driving/correcting circuits  73  and driving/detecting circuits  74  of  FIG. 1 . Each of the LED driving/correcting circuits  73 , as shown in  FIG. 2 , includes resistors  81 ,  82 ,  83 , and  84 , an operating amplifier  85 , resistors  86 ,  87 , and  88 , an operating amplifier  89 , a resistor  90 , and an operating amplifier  91 , and a resistor  92 . Each of the driving/detecting circuits  74  includes a constant current circuit  93  and an operating amplifier  94 .  
       FIG. 3  is a diagram showing one example of layout of each of the LEDs  71 , photodiodes  72 , set of LED driving/correcting circuits  73  and driving/detecting circuits  74  of  FIG. 1 . As shown in  FIG. 3 , On the PCB  75  is arranged one each of the LEDs  71 , photodiodes  72 , set of LED driving/correcting circuits  73  and driving/detecting circuits  74 . In this state, each of the LEDs  71  has various shapes, that is, each of the LEDs  71  may be sealed hermetically or may have a shape of a bare chip. In the case of the bare chip, part or all of a portion of the PCB  75  in which each of the LEDs  71  is mounted is preferably sealed hermetically with a resin or a like. In the embodiment in particular, each of the LEDs  71  is mounted, in a one-to-one relationship, in the vicinity of each of the photodiodes  72  and each of the LED driving/correcting circuits  73  is integrally formed with each of the driving/detecting circuits  74 .  
       FIG. 4  is a diagram showing another example of layout of the LEDs  71 , photodiodes  72 , and LED driving/correcting circuits  73  formed integrally with the driving/detecting circuits  74  of  FIG. 1 . As shown in  FIG. 4 , on the PCB  75  are arranged  20  sets of the LEDs  71 , photodiodes  72 , and LED driving/correcting circuits  73  formed integrally with driving/detecting circuits  74 . In this case, if any one of the photodiodes  72  in any one of the arrangement sets receives light from any one the LEDs  71  contained in an adjacent arrangement set, the photodiode  72  received light is affected by light emitted from any one the LEDs other than the LED  71  that originally has to provide light and, therefore, it is preferable that a structural measure to shield the photodiodes  72  from light emitted from other LEDs  71  is taken.  
      According to the luminance correcting method to be used for the flat lighting source of the present invention, luminance of light emitted from each of the LEDs  71  is set at a specified targeted value and an amount of deviation from the targeted value occurring when each of the LEDs  71  is turned ON is detected and, based on the amount of the deviation, the luminance of light emitted from each of the LEDs  71  is corrected so as to coincide with the targeted value already set. At this time point, a current or voltage is supplied by each of the LED driving/correcting circuits  73  shown in  FIG. 4  to each of the LEDs  71 . In this case, either of constant current driving or constant voltage driving may be applied as a driving method. Part of light components with given luminance emitted from each of the LEDs  71  is received by each of the photodiode  72  and is converted into electrical signals and a resistance value of each of the photodiodes  72  decreases in proportion to the luminance of received light accordingly. That is, if luminance of light emitted from each of the LEDs  71  increases, a resistance value of each of the photodiodes  72  decreases while the luminance of light emitted from each of the LEDs  71  decreases, the resistance value of each of the photodiodes  71  increases. The resistance value is detected by each of the driving/detecting circuits  74  and the resistance value is fed back to each of the LED driving/correcting circuits  73 . Each of the LED driving/correcting circuits  73  changes a driving current or driving voltage so that luminance of light emitted from each of the LEDs  71  becomes a level corresponding to a luminance setting voltage V 1 .  
      As shown in  FIG. 2 , a luminance setting voltage V 1  is input to determine a driving current of each of the LEDs  71  and luminance of light emitted from each of the LEDs  71  is set according to the luminance setting voltage V 1 . Then, a current I 0  is fed from each of the LED driving/correcting circuit  73  to each of the LEDs  71 . The current I 0  is shown by the following equation (1): 
 
 I 0 =V 3 /RSC    (1) 
 
 where “RSC” denotes a resistance value of the resistor  90 . The voltage V 3  that determines the current I 0  is a voltage output from an adding and subtracting circuit made up of the operational amplifiers  85  and is given by the following equation (2): 
 
 V 3=(− R 2 /R 1)× V 1+( R 4 /R 3)× V 2   (2) 
 
 where “R1” denotes a resistance value of the resistor  81 , “R2” a resistance value of the resistor  82 , “R3” a resistance value of the resistor  83 , and “R4” a resistance value of the resistor  84 . 
 
      The luminance detecting voltage V 2  is the driving voltage of each of the photodiodes  72  that has already been fed through each of the operational amplifiers  94  serving as a voltage follower to each of the LED driving/correcting circuit  73 . Each of the photodiodes  72 , since it is driven by each constant current circuit  93 , has a resistance value corresponding to luminance of light emitted from each of the LEDs  71 . That is, when luminance of light emitted from each of the LEDs  71  decreases, a resistance value of each of the photodiodes  72  increases and a driving voltage of each of the photodiodes  72  increases, thus causing an increase in the luminance detecting voltage V 2 . When the luminance voltage V 2  increases, the voltage V 3  is increased to become a voltage being R 4 /R 3  times larger than the luminance detecting voltage V 2  according to the equation (2). An increase in the voltage V 3  causes an increase in a driving current of each of the LEDs  71  and further an increase in the luminance of light emitted from each of the LEDs  71 . Therefore, by selecting the resistance values R 1 , R 2 , R 3 , and R 4  and a current Id of each of the constant current circuits  93 , a voltage is fed back from each of the photodiode  72  and, as a result, a change in luminance of light emitted from each of the LEDs  71  is compensated for.  
      Thus, in the flat lighting source of the first embodiment, since light having given luminance emitted from each of the LEDs  71  is converted by each of the photodiodes  72 , in a one-to-one relationship, into a luminance detecting voltage V 2  and the luminance detecting voltage V 2  is fed back to each of the driving/detecting circuit  74 , variations and changes in luminance of light emitted from each of the LEDs  71  are automatically compensated for. Moreover, since light having the given luminance emitted from each of the LEDs  71  is fed back, a process of extracting only an amount of temperature change for feeding-back is not required, there is no need for mounting a temperature correcting circuit for each of the LEDs  71 . Also, since each of the LEDs  71  is mounted, in a one-to-one relationship, in the vicinity of each of the photodiodes  72 , luminance of light emitted from each of the LEDs  71  is individually corrected and, therefore, even if there are variations in luminance characteristic among the LEDs  71  and among the photodiodes  72 , luminance of light emitted from each of the LEDs  71  is made uniform by appropriately setting the luminance setting voltage V 1 . Furthermore, each of the LEDs  71  corresponds to each of the photodiodes  72  and, therefore, whichever color out of R (Red), G (Green), and B (Blue) is emitted from each of the LEDs  71 , changes in color balance are compensated for without using a color filter.  
     Second Embodiment  
       FIG. 5  is a cross-sectional view showing layout of main components of the flat lighting source of the second embodiment of the present invention and the same reference numbers are assigned to components having the same function as those of the first embodiment shown in  FIG. 4 . In the flat lighting source of the second embodiment, as shown in  FIG. 5 , each of the LED  71 , photodiodes  72 , LED driving/correcting circuits  73 , and driving/detecting circuits  74  is mounted in the same package  76 . In the packet  76 , terminals or a like (not shown) to be used fro connection to external components are mounted. According to the flat lighting source of the second embodiment, each of the LEDs  71  is arranged, in a one-to-one relationship, in the vicinity of each of the photodiode  72  in the same package  76  and each of the LED driving/correcting circuits  73  and each of the driving/detecting circuits  74  are integrally formed and, therefore, the second embodiment has the same advantage as the first embodiment.  
     Third Embodiment  
       FIG. 6  is a cross-sectional view showing layout of main components of the flat lighting source of the third embodiment of the present invention and the same reference numbers are assigned to components having the same function as those of the second embodiment shown in  FIG. 5 . In the lighting source of the third embodiment, as shown in  FIG. 6 , each of the photodiode  72 , each of the LED driving/correcting circuit  73  and each of the driving/detecting circuits  74  are integrally formed to operate as one IC (Integrated Circuit)  77 . As in the case of the first embodiment, each of the LEDs  71  is arranged, in a one-to-one relationship, in the vicinity of the photodiode  72  and, therefore, the same advantage as obtained in the first embodiment can be achieved in the third embodiment as well.  
     Fourth Embodiment  
       FIG. 7  is a cross-sectional view showing layout of main components of the flat lighting source of the fourth embodiment of the present invention. In the flat lighting source of the fourth embodiment, as shown in  FIG. 7 , each of the LED driving/correcting circuit  73  and each of the driving/detecting circuit  74  are mounted outside the package  76  and on the PCB  75  together with the package  76 . The PCB  75  is made of organic materials or inorganic materials. According to the flat lighting source of the fourth embodiment, as in the first embodiment, each of the LEDs  71  is arranged, in a one-to-one relationship, in the vicinity of the photodiode  72  and, therefore, the same advantage as obtained in the first embodiment can be achieved.  
     Fifth Embodiment  
       FIG. 8  is a cross-sectional view showing layout of main components of the flat lighting source of the fifth embodiment of the present invention. In the flat lighting source of the fifth embodiment, as shown in  FIG. 8 , the same package  76  as shown in  FIG. 8  is mounted on the PCB  75  and each correcting device  78  is newly mounted on the PCB  75 . Each of the correcting devices  78  is made up of, for example, a variable resistor, a thick film printed resistance device to be trimmed by laser, a Zener zapping device consisting of a resistor and a Zener diode, a memory device that can write data corresponding to a targeted value of luminance of each of LEDs, or a like and is configured so as to provide a luminance setting voltage V 1  to each of the LED driving/correcting circuit  73 . Therefore, even if there are variations in luminance characteristic among the LEDs  71  and among the photodiodes  72 , luminance of light emitted from each of the LEDs  71  can be controlled to be uniform by appropriately setting the luminance setting voltage V 1  using each of the correcting devices  78 .  
     Sixth Embodiment  
       FIG. 9  is a cross-sectional view showing layout of main components of the flat lighting source of the sixth embodiment of the present invention and the same reference numbers are assigned to components having the same function as those of the fourth embodiment shown in  FIG. 7 . In the flat lighting source of the sixth embodiment, as shown in  FIG. 9 , each of the correcting devices  78  shown in  FIG. 8  is mounted on the PCB  75  shown in  FIG. 7 . Therefore, even if there are variations in luminance characteristic among the LEDs  71  and among the photodiodes  72 , luminance of light emitted from each of the LEDs  71  can be controlled to be uniform by appropriately setting the luminance setting voltage V 1  using each of the correcting devices  78 .  
     Seventh Embodiment  
       FIG. 10  is a cross-sectional view showing layout of main components of the flat lighting source of the seventh embodiment of the present invention and the same reference numbers are assigned to components having the same function as those of the sixth embodiment shown in  FIG. 9 . In the flat lighting source of the seventh embodiment, as shown in  FIG. 10 , each of the LED driving/correcting circuits  73 , driving/detecting circuits  74 , and correcting devices  78  are integrally formed to operate as one IC  79 . Therefore, as in the case of the sixth embodiment, even if there are variations in luminance characteristic among the LEDs  71  and among the photodiodes  72 , luminance of light emitted from each of the LEDs  71  can be controlled to be uniform by appropriately setting the luminance setting voltage V 1  using each of the correcting devices  78 .  
     Eighth Embodiment  
       FIG. 11  is a circuit diagram showing electrical configurations of a luminance correcting circuit to be used in the flat lighting source of the eighth embodiment of the present invention and the same reference numbers are assigned to components having the same function as those of the first embodiment shown in  FIG. 2 . Each of the luminance correcting circuits of the eighth embodiment has, as shown in  FIG. 11 , the LED driving/correcting circuit  73 A different from the LED driving/correcting circuit  73  shown in  FIG. 2 . Each of the LED driving/correcting circuits  73 A includes resistors  81 ,  82 ,  83 , and  84 , an operational amplifier  85 , an operational amplifier  95 , an n-channel type MOSFET (Metal Oxide Semiconductor Field Effect Transistor (nMOS)]  96 , and a variable resistor  97 .  
      In this luminance correcting circuit, a current I 0  flowing through the LED  71  is shown by the following equation (3). 
 
 I 0 =V 4 /RICC    (3) 
 
 where “RICC” denotes a resistance value of the variable resistor  97 . 
 
      Each of the luminance correcting circuit operates so that a voltage V 3  of a non-inverted input terminal (+) of the operational amplifier  95  is equal to a voltage of its inverted input terminal (−) and, as a result, the voltage V 3  is almost the same as the voltage V 4 . The current I 0  flowing through each of the LEDs  71  is determined by the voltage V 4  and the resistance value RICC of the variable resistor  97 . By adjusting the resistance value RICC, a desired current is made to flow through each of the LEDs  71  and each of the LEDs  71  emits light with a desired luminance. Moreover, the operational amplifier  85  makes up an adding/subtracting circuit as in the first embodiment and, therefore, by changing a luminance setting voltage V 1 , the voltage V 3  changes. Thus, in the luminance correcting circuit having configurations different from those in the first embodiment, the same advantage as obtained in the first embodiment can be achieved.  
      It is apparent that the present invention is not limited to the above embodiments but may be changed and modified without departing from the scope and spirit of the invention. For example, the luminance correcting circuit shown in  FIG. 2  or  FIG. 11  may have any configuration so long as it has the same functions as the configurations shown in  FIG. 2  or  FIG. 11  have.  
      Moreover, the present invention can be applied widely to cases where illuminating light with uniform luminance should be provided to an entire displaying region of a display panel, for example, of a liquid crystal display device in which illuminating light is supplied by a backlight.