Abstract:
Reduction of luminance dispersion of a plurality of light-emitting panels combined into one light-emitting device is achieved by the use of a new light-emitting device which has a photosensor, a plurality of light-emitting panels, DC/DC converters connected to their respective light-emitting panels, and a control circuit configured to control output currents of the DC/DC converters in accordance with illuminance data acquired with the photosensor. The control circuit successively turns on the plurality of light-emitting panels, and controls the output currents of the DC/DC converters in accordance with differences of the illuminance data acquired with the photosensor when the light-emitting panels are turned on.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a light-emitting device, particularly to a light-emitting device applicable to lighting. The present invention also relates to a method of driving a light-emitting device. 
         [0003]    2. Description of the Related Art 
         [0004]    Light-emitting devices utilizing electroluminescent (EL) elements are being expected to find wider applications in lighting because they have low power consumption and can emit light uniformly from a planar surface. 
         [0005]    A light-emitting device employing an EL element for lighting is disclosed in Patent Document 1. Patent Document 1 discloses a structure in which light adjustment can be controlled in accordance with the environment or the place where a plurality of light-emitting panels is used in combination. 
       REFERENCE  
       [0000]    
       
         Patent Document 1: Japanese Published Patent Application No. 2010-177048 
       
     
       SUMMARY OF THE INVENTION 
       [0007]    A problem of a light-emitting device using an EL element is that luminance differs among a plurality of light-emitting panels combined into one light-emitting device. Such luminance dispersion results from the difference in characteristics among the light-emitting panels which are generated during the manufacturing process; thus, the degree of the dispersion also varies among the light-emitting devices. Therefore in the case where a plurality of light-emitting panels is combined into one light-emitting device, it is difficult to estimate deviation of luminance of the light-emitting panels in advance. 
         [0008]    In view of the foregoing, an object of one embodiment of the present invention is to provide a light-emitting device which includes a plurality of combined light-emitting panels having a small luminance dispersion. 
         [0009]    One embodiment of the present invention is a light-emitting device which includes a photosensor, a plurality of light-emitting panels, a plurality of DC/DC converters each connected to a corresponding one of the plurality of light-emitting panels, and a power control circuit configured to control output currents of the plurality of DC/DC converters in accordance with illuminances acquired with the photosensor. In the light-emitting device, the power control circuit successively turns on the plurality of light-emitting panels, and controls the output currents of the plurality of DC/DC converters in accordance with a dispersion of the illuminances acquired with the photosensor when the plurality of light-emitting panels is turned on. 
         [0010]    One embodiment of the present invention is a light-emitting device which includes a photosensor, a plurality of light-emitting panels, a plurality of DC/DC converters each connected to one of the plurality of light-emitting panels, and a power control circuit configured to control output currents of the plurality of DC/DC converters in accordance with illuminances acquired with the photosensor. In the light-emitting device, the power control circuit acquires an external light illuminance, successively turns on the plurality of light-emitting panels in accordance with the external light illuminance, and controls the output currents of the plurality of DC/DC converters in accordance with a dispersion of the illuminances acquired with the photosensor when the plurality of light-emitting panels is turned on. 
         [0011]    In one embodiment of the present invention, the light-emitting device preferably includes a plurality of photosensors. 
         [0012]    In one embodiment of the present invention, each of the light-emitting panels in the light-emitting device preferably includes an EL element. 
         [0013]    One embodiment of the present invention is a method of driving a light-emitting device, which includes the following successive steps: generating a reference current with a power control circuit in accordance with an external environment; supplying the reference current from any one of a plurality of DC/DC converters to a corresponding one of a plurality of light-emitting panels; acquiring an illuminance with a photosensor when light is emitted from each light-emitting panel supplied with the reference current; and successively controlling, with a current control circuit, output of a corrected current that is obtained in accordance with the illuminance acquired with the photosensor and to be supplied from any one of the DC/DC converters to the light-emitting panel electrically connected to the DC/DC converter. 
         [0014]    One embodiment of the present invention is a method of driving a light-emitting device, which includes the following successive steps: estimating an external light illuminance, with a photosensor; generating a reference current with a power control circuit in accordance with the external light illuminance; supplying the reference current from any one of a plurality of DC/DC converters to a corresponding one of a plurality of light-emitting panels; acquiring an illuminance with a photosensor when light is emitted from each light-emitting panel supplied with the reference current; and successively controlling, with a current control circuit, output of a corrected current that is obtained in accordance with the illuminance acquired with the photosensor and to be supplied from any one of the DC/DC converters to the light-emitting panel electrically connected to the DC/DC converter. 
         [0015]    In accordance with one embodiment of the present invention, luminance dispersion among the plurality of light-emitting panels combined into one light-emitting device can be reduced. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    In the accompanying drawings: 
           [0017]      FIG. 1  is a block diagram illustrating a configuration in Embodiment 1; 
           [0018]      FIG. 2  is a flow chart illustrating a configuration in Embodiment 1; 
           [0019]      FIGS. 3A to 3C  are block diagrams illustrating a configuration in Embodiment 1; 
           [0020]      FIGS. 4A and 4B  are block diagrams illustrating a configuration in Embodiment 1; 
           [0021]      FIGS. 5A and 5B  are block diagrams illustrating a configuration in Embodiment 1; 
           [0022]      FIG. 6  is a block diagram illustrating a configuration in Embodiment 1; 
           [0023]      FIG. 7  is a circuit diagram illustrating a configuration in Embodiment 2; 
           [0024]      FIG. 8  is a schematic view illustrating a structure in Embodiment 3; 
           [0025]      FIGS. 9A to 9C  are schematic views illustrating structures in Embodiment 4; and 
           [0026]      FIGS. 10A and 10B  are views illustrating structures in Embodiment 5. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0027]    Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. However, the present invention can be carried out in many different modes, and it is easily understood by those skilled in the art that modes and details of the present invention can be modified in various ways without departing from the purpose and the scope of the present invention. Therefore the present invention should not be construed as being limited to the following description of the embodiments. Note that in structures of the present invention described below, reference numerals denoting the same portions are used in common in different drawings. 
         [0028]    Note that the size of components and the thickness of layers illustrated in the drawings of the embodiments and the like are exaggerated in some cases for simplicity. Therefore, the scale is not necessarily limited to that illustrated in the drawings and the like. 
         [0029]    Note that in this specification, the terms “first”, “second”, “third”, and “N-th” (N is a natural number) are used in order to avoid confusion between components and thus do not limit the number of the components. 
       Embodiment 1 
       [0030]    In this embodiment, a light-emitting device and a method of driving a light-emitting device in accordance with one embodiment of the present invention are described. 
         [0031]    A block diagram of a light-emitting device  10  is shown in  FIG. 1 . In  FIG. 1 , the light-emitting device  10  includes a light emission control unit  101  and a light-emitting unit  102 . The light emission control unit  101  of the light-emitting device  10  is connected to a power supply unit  100  including an AC power supply  103 . 
         [0032]    The power supply unit  100  includes a rectifier circuit  104  and an AC/DC converter  105  in addition to the AC power supply  103 . Note that the rectifier circuit  104  and the AC/DC converter  105  are present outside the light-emitting device  10  in the example in  FIG. 1  but may be included in the light-emitting device  10 . When a DC power supply is used instead of the AC power supply  103 , the rectifier circuit  104  and the AC/DC converter  105  are not necessary in the power supply unit  100 . 
         [0033]    The light emission control unit  101  includes a power control circuit  106  and a first DC/DC converter  107 _ 1  to an Nth DC/DC converter  107 _N (N is a natural number greater than or equal to 2). 
         [0034]    The light-emitting unit  102  includes a photosensor  108  and a first light-emitting panel  109 _ 1  to an Nth light-emitting panel  109 _N. 
         [0035]    The rectifier circuit  104  is a circuit for rectifying an AC voltage output from the AC power supply  103  to give a DC voltage. The rectifier circuit  104  is formed using a diode element, for example. When formed using a diode element, the rectifier circuit may be a full-wave rectifier circuit, a half-wave rectifier circuit, a circuit using a diode bridge, a full-wave rectifier circuit using a transformer, or the like. 
         [0036]    The AC/DC converter  105  is a circuit for converting the AC voltage rectified by the rectifier circuit  104  into a DC voltage. The AC/DC converter  105  is formed using a switching element or a capacitor, for example. 
         [0037]    The power control circuit  106  is a circuit for individually controlling currents output from the first DC/DC converter  107 _ 1  to the Nth DC/DC converter  107 _N, in accordance with signals from the photosensor  108 . The power control circuit  106  is formed using a micro processing unit (MPU), for example. 
         [0038]    The first DC/DC converter  107 _ 1  to the Nth DC/DC converter  107 _N are circuits that can supply a different current to each of the first light-emitting panel  109 _ 1  to the Nth light-emitting panel  109 _N under the control of the power control circuit  106 . The first DC/DC converter  107 _ 1  to the Nth DC/DC converter  107 _N are each formed using a non-isolated or isolated type DC/DC converter, for example. 
         [0039]    The photosensor  108  is a circuit for measuring the external light illuminance or the illuminance in the vicinity of the light-emitting device when light is emitted from the first light-emitting panel  109 _ 1  to the Nth light-emitting panel  109 _N, by absorbing visible light. The photosensor  108  is formed using an element with an amorphous silicon p-i-n junction, for example. 
         [0040]    The first light-emitting panel  109 _ 1  to the Nth light-emitting panel  109 _N are each a panel including a light-emitting layer between an anode and a cathode. A current flowing from the anode side to the cathode side causes the light-emitting layer to emit light. Note that the anode, the cathode, and the light-emitting layer are included in an EL element, in which a hole-injection layer, a hole-transport layer, the light-emitting layer, an electron-transport layer, an electron-injection layer, and the like can be stacked between the anode and the cathode. Alternatively, each of the first light-emitting panel  109 _ 1  to the Nth light-emitting panel  109 _N may include a plurality of EL elements having a light-emitting layer between an anode and a cathode. 
         [0041]      FIG. 2  is a flow chart of a method of driving the light-emitting device  10  illustrated in  FIG. 1 . In addition,  FIGS. 3A to 3C ,  FIGS. 4A and 4B ,  FIGS. 5A and 5B , and  FIG. 6  are schematic views illustrating specific operations of the light-emitting device  10  which are described with reference to the flow chart in  FIG. 2 . Note that the same components are commonly denoted by the same reference numerals in  FIG. 1 ,  FIGS. 3A to 3C ,  FIGS. 4A and 4B ,  FIGS. 5A and 5B , and  FIG. 6 . 
         [0042]    First, in a step  201  in  FIG. 2 , a reference current Iref is set in accordance with the external environment. A specific operation is as follows: the photosensor  108  measures an illuminance Ls in the vicinity of the light-emitting unit  102 ; data of the illuminance Ls obtained with the photosensor  108  is input to the power control circuit  106 ; and in accordance with the level of the illuminance Ls, the power control circuit  106  sets the reference current Iref, which is to be supplied to each of the first light-emitting panel  109 _ 1  to the Nth light-emitting panel  109 _N from the corresponding one of the first DC/DC converter  107 _ 1  to the Nth DC/DC converter  107 _N. This operation is schematically depicted in  FIG. 3A . 
         [0043]    The reference current Iref is set in accordance with the illuminance Ls and can also be set with another sensor used in combination. For example, the reference current Iref may be set with a device such as a timer. With a timer, the operations of the light-emitting device can be combined with light adjustment in accordance with scenes in the morning, evening, and night, for example. 
         [0044]    Next, in a step  202  in  FIG. 2 , under the control of the power control circuit  106 , the reference current Iref is supplied from any one of the first DC/DC converter  107 _ 1  to the Nth DC/DC converter  107 _N to the corresponding one of the first light-emitting panel  109 _ 1  to the Nth light-emitting panel  109 _N. Here, a case is exemplified in which the reference current Iref is first supplied from the first DC/DC converter  107 _ 1  to the first light-emitting panel  109 _ 1 . The first light-emitting panel  109 _ 1  emits light as a result of being supplied with the reference current Iref from the first DC/DC converter  107 _ 1 . The photosensor  108  measures an illuminance L 1  when this light emission occurs from the first light-emitting panel  109 _ 1  (EL 1 ). This operation is schematically shown in  FIG. 3B . 
         [0045]    Next, in a step  203  in  FIG. 2 , the power control circuit  106  acquires data based on the illuminance L 1  obtained when light is emitted from the first light-emitting panel  109 _ 1  in the step  202 . In sum, in the steps  202  and  203 , the reference current Iref is supplied from the first DC/DC converter  107 _ 1  to the first light-emitting panel  109 _ 1  to cause light emission of the first light-emitting panel  109 _ 1  (EL 1 ), and the data based on the illuminance L 1  obtained with the photosensor  108  is acquired with the power control circuit  106 . This operation is schematically shown in  FIG. 3C . 
         [0046]    A next step  204  in  FIG. 2  is to determine whether or not the power control circuit  106  has acquired the data based on the illuminances after all the individual light-emitting panels are supplied with the reference current Iref from the DC/DC converters and emit light. 
         [0047]    If it is determined that the power control circuit  106  has not completed the acquirement of the data based on the illuminances after all the light-emitting panels are supplied with the reference current Iref from the DC/DC converters and emit light, the operation returns to the step  202  in  FIG. 2 . Described here is an operation after the reference current Iref is supplied from the first DC/DC converter  107 _ 1 , the first light-emitting panel  109 _ 1  emits light, and the power control circuit  106  acquires the data based on the illuminance L 1 . 
         [0048]    In this case, the following operation is carried out in accordance with the step  202 . Specifically, under the control of the power control circuit  106 , the reference current Iref is supplied from the second DC/DC converter  107 _ 2  to the second light-emitting panel  109 _ 2 , for example. The second light-emitting panel  109 _ 2  emits light as a result of being supplied with the reference current Iref from the second DC/DC converter  107 _ 2 . The photosensor  108  measures an illuminance L 2  when this light emission occurs from the second light-emitting panel  109 _ 2  (EL 2 ). This operation is schematically shown in  FIG. 4A . 
         [0049]    Next, in the step  203 , the power control circuit  106  acquires the data based on the illuminance L 2  obtained when light is emitted from the second light-emitting panel  109 _ 2  in the step  202 . So far, the power control. circuit  106  has acquired the data based on the illuminance L 1 , which is obtained by the supply of the reference current Iref to the first light-emitting panel  109 _ 1 , and the illuminance L 2 , which is obtained by the supply of the reference current Iref to the second light-emitting panel  109 _ 2 . This operation is schematically shown in  FIG. 4B . 
         [0050]    When the acquisition of the illuminances with all the light-emitting panels is still not completed in the step  204 , the operation returns to the step  202  in  FIG. 2 . By repeating these operations, the reference current Iref is supplied from the first DC/DC converter  107 _ 1  to the Nth DC/DC converter  107 _N to the first light-emitting panel  109 _ 1  to the Nth light-emitting panel  109 _N. 
         [0051]    In the case where the reference current Iref is supplied from the Nth DC/DC converter  107 _N to the Nth light-emitting panel  109 _N, the following operation is also carried out in accordance with the step  202 . Specifically, under the control of the power control circuit  106 , the reference current Iref is supplied from the Nth DC/DC converter  107 _N to the Nth light-emitting panel  109 _N. The Nth light-emitting panel  109 _N emits light as a result of being supplied with the reference current Iref from the Nth DC/DC converter  107 _N. The photosensor  108  measures an illuminance LN when this light emission occurs from the second Nth light-emitting panel  109 _N (ELN). This operation is schematically shown in  FIG. 5A . 
         [0052]    Next, in the step  203 , the power control circuit  106  acquires the data based on the illuminance LN obtained when light is emitted from the Nth light-emitting panel  109 _N in the step  202 . So far, the power control circuit  106  has acquired the data based on the illuminances L 1  to LN, which is obtained by the supply of the reference current Iref to all of the first light-emitting panel  109 _ 1  to the Nth light-emitting panel  109 _N. This operation is schematically shown in  FIG. 5B . 
         [0053]    When the power control circuit  106  has acquired the data based on the illuminances after all the light-emitting panels are supplied with the reference current Iref from the DC/DC converters and emit light, the operation proceeds to the step  205  in  FIG. 2 . Thus, before the step  205 , the power control circuit  106  acquires the data based on the illuminances L 1  to LN, which are obtained when all of the first light-emitting panel  109 _ 1  to the Nth light-emitting panel  109 _N are individually supplied with the same reference current Iref. 
         [0054]    By being supplied with the same reference current Iref, the first light-emitting panel  109 _ 1  to the Nth light-emitting panel  109 _N are supposed to exhibit the same luminance and enable the same illuminances L 1  to LN to be acquired as long as the light-emitting panels have the same current-luminance characteristics. However, a plurality of light-emitting panels has significantly different current-luminance characteristics when they each have a large size and employs an EL element. When such light-emitting panels each including an EL element are combined into one light-emitting device, the difference in luminance of the panels is conspicuous due to the significant differences of current-luminance characteristics. Such a luminance dispersion is reflected in differences of the illuminances L 1  to LN obtained when the reference current Iref is supplied to all of the first light-emitting panel  109 _ 1  to the Nth light-emitting panel  109 _N in the above-described steps  201  to  204 . 
         [0055]    Therefore in the step  205 , corrected currents Ic are estimated from the already acquired illuminances L 1  to LN with the light-emitting panels, and are supplied from the first DC/DC converter  107 _ 1  to the Nth DC/DC converter  107 _N to the first light-emitting panel  109 _ 1  to the Nth light-emitting panel  109 _N. Specifically, a corrected current Ic lower than the reference current Iref can be supplied to a light-emitting panel that provides a higher illuminance than another light-emitting panel, and a corrected current Ic higher than the reference current Iref is supplied to a light-emitting panel that provides a lower illuminance than another light-emitting panel. This operation is schematically shown in  FIG. 6 . 
         [0056]    In the example described with reference to  FIGS. 3A to 3C ,  FIGS. 4A and 4B ,  FIGS. 5A and 5B , and  FIG. 6 , a corrected current Id, which is corrected to enable the light-emitting panels to provide the same illuminance, is supplied from the first DC/DC converter  107 _ 1  to the first light-emitting panel  109 _ 1  under the control of the power control circuit  106 . Further, a corrected current Ic 2 , which is corrected so as to make the light-emitting panels provide a uniform illuminance, is supplied from the second DC/DC converter  107 _ 2  to the second light-emitting panel  109 _ 2  under the control of the power control circuit  106 . In a similar way, a corrected current IcN, which is corrected so as to make the light-emitting panels provide a uniform illuminance, is supplied from the Nth DC/DC converters  107 _N to the Nth light-emitting panel  109 _N under the control of the power control circuit  106 . Consequently, the same illuminance Lc can be obtained with the first light-emitting panel  109 _ 1  to the Nth light-emitting panel  109 _N. In other words, the first light-emitting panel  109 _ 1  to the Nth light-emitting panel  109 _N can emit light with the same luminance. 
         [0057]    The above-described series of operations in the steps  201  to  205  can be started at the time when a light-emitting panel is turned on or at certain periodic intervals. In the series of operations in the steps  201  to  205 , the light-emitting panels are preferably switched on and off at such speed that humans cannot perceive these operations. For example, the light-emitting panels are preferably switched on and off at 60 Hz or more, so that the illuminances are measured. 
         [0058]    In accordance with one embodiment of the present invention, luminance dispersion of the plurality of light-emitting panels combined into one light-emitting device can be reduced. 
         [0059]    This embodiment can be implemented as appropriate in combination with any of the structures described in the other embodiments. 
       Embodiment 2 
       [0060]    This embodiment shows an example of a circuit configuration of the first DC/DC converter  107 _ 1  to the Nth DC/DC converter  107 _N, which are described above in Embodiment 1. A circuit configuration of a DC/DC converter  107  and a periphery thereof is specifically illustrated in  FIG. 7 . 
         [0061]    The DC/DC converter  107  illustrated in  FIG. 7  includes a D/A converter  301 , an error amplifier  302 , a triangular-wave generating circuit  303 , a comparator  304 , a buffer  305 , a transistor  306 , an inverter  307 , a transistor  308 , and a coil  309 .  FIG. 7  illustrates an equivalent circuit of a light-emitting panel  109  as the circuit configuration of the first light-emitting panel  109 _ 1  to the Nth light-emitting panel  109 _N, which are described above in Embodiment 1. The light-emitting panel  109  includes a light-emitting element  310  and a sensing resistor  311 .  FIG. 7  also illustrates the AC/DC converter  105  and the power control circuit  106  described above in Embodiment 1. 
         [0062]    In the DC/DC converter  107  in  FIG. 7 , conversion of a signal from the power control circuit  106  to an analog value is made by the D/A converter  301 , followed by input of the converted signal to a non-inverting input terminal of the error amplifier  302 . Further, a potential between the light-emitting element  310  and the sensing resistor  311  is input to an inverting input terminal of the error amplifier  302 . 
         [0063]    An output terminal of the error amplifier  302  is connected to a non-inverting input terminal of the comparator  304 . A triangular wave is input from the triangular-wave generating circuit  303  to an inverting input terminal of the comparator  304 . An output terminal of the comparator  304  is connected to the buffer  305  and the inverter  307 . Further, the buffer  305  controls the conducting state of the transistor  306 . The inverter  307  controls the switching of the transistor  308 . By the control of the switching of the transistors  306  and  308 , a current in accordance with the signal from the power control circuit  106  can be supplied from the AC/DC converter  105  to the light-emitting panel  109 . 
         [0064]    By adoption of the DC/DC converter  107  described in one embodiment of the present invention as the first DC/DC converter  107 _ 1  to the Nth DC/DC converter  107 _N described above in Embodiment 1, luminance dispersion of a plurality of light-emitting panels combined into one light-emitting device can be reduced. 
         [0065]    This embodiment can be implemented as appropriate in combination with any of the structures described in the other embodiments. 
       Embodiment 3 
       [0066]    In this embodiment, a simple schematic view of the light-emitting panel  109  described in Embodiment 2 is described with reference to  FIG. 8 . 
         [0067]    In the light-emitting panel  109  illustrated in  FIG. 8 , an anode  403 , a light-emitting layer  404 , and a cathode  405  are stacked between a first substrate  401  and a second substrate  402 . Upon application of a voltage between the anode  403  and the cathode  405  through the DC/DC converter  107 , holes injected from the anode  403  side and electrons injected from the cathode  405  side are transported and then recombined in the light-emitting layer  404  to excite a light-emitting substance and, when the light-emitting substance returns from the excited state to the ground state, light is emitted. The light-emitting layer  404  functions in this way. The light-emitting layer  404  can be used as a stack with a hole-injection layer, a hole-transport layer; a light-emitting layer, an electron-transport layer, an electron-injection layer, and the like. 
         [0068]    The light-emitting layer  404  deteriorates due to the atmosphere including moisture. Therefore the light-emitting layer  404  is preferably prevented from contacting the atmosphere including moisture, with use of the first substrate  401 , the second substrate  402 , a sealant  406 , or the like. 
         [0069]    By adoption of the light-emitting panel  109  described in one embodiment of the present invention as the first light-emitting panel  109 _ 1  to the Nth light-emitting panel  109 _N described above in Embodiment 1, luminance dispersion of a plurality of light-emitting panels combined into one light-emitting device can be reduced. 
         [0070]    This embodiment can be implemented as appropriate in combination with any of the structures described in the other embodiments. 
       Embodiment 4 
       [0071]    In this embodiment, arrangement of the photosensor  108  and the first light-emitting panel  109 _ 1  to the Nth light-emitting panel  109 _N in the light-emitting unit  102 , which are described in Embodiment 1, is described with reference to  FIGS. 9A to 9C . 
         [0072]    Schematic views of the light-emitting unit  102  in  FIGS. 9A to 9C  show examples of the arrangement of the photosensor  108  and the first light-emitting panel  109 _ 1  to the fourth light-emitting panel  109 _ 4 . 
         [0073]    As described above in Embodiment 1, the photosensor  108  selects any one of the first light-emitting panel  109 _ 1  to the fourth light-emitting panel  109 _ 4  and measures the illuminance corresponding to the luminance of the light-emitting panel. Thus, preferably, small dispersion arises in the measured illuminances depending on the locations of the photosensor and the light-emitting panels. 
         [0074]    For example, as illustrated in  FIG. 9A , the photosensor  108  may be provided at equal distances from the light-emitting panels, the first light-emitting panel  109 _ 1  to the fourth light-emitting panel  109 _ 4 . Note that the number of the light-emitting panels in the example in  FIG. 9A  is four but not limited as long as the photosensor  108  is provided at equal distances from the plurality of light-emitting panels. 
         [0075]    In another structure illustrated in  FIG. 9B , for example, a plurality of photosensors  108  may be arranged at equal distances from each of the first light-emitting panel  109 _ 1  to the fourth light-emitting panel  109 _ 4 , in which case the sum of the illuminances obtained with the plurality of photosensors  108  may be used for the operations described in Embodiment 1. Note that although two photosensors are arranged at equal distances from one light-emitting panel in  FIG. 9B , there is no limitation as long as the plurality of photosensors  108  is provided at equal distances from the light-emitting panel. 
         [0076]    In another structure illustrated in  FIG. 9C , for example, the photosensor  108  may be located at an edge portion of the light-emitting unit  102  where the first light-emitting panel  109 _ 1  to the fourth light-emitting panel  109 _ 4  are provided. In this case, the distance to the photosensor  108  differs among the light-emitting panels, which affects the illuminances obtained with the photosensor  108 . Hence in the structure in  FIG. 9C , the observed illuminances may be corrected in accordance with the distance differences, and the operations described in Embodiment 1 are performed. 
         [0077]    By adoption of the light-emitting panel  109  described in one embodiment of the present invention as the first light-emitting panel  109 _ 1  to the Nth light-emitting panel  109 _N described above in Embodiment 1, luminance dispersion of a plurality of light-emitting panels combined into one light-emitting device can be reduced. 
         [0078]    This embodiment can be implemented as appropriate in combination with any of the structures described in the other embodiments. 
       Embodiment 5 
       [0079]    In this embodiment, application examples of the light-emitting device of one embodiment of the present invention are described. 
         [0080]      FIG. 10A  illustrates an example in which the light-emitting device of one embodiment of the present invention is used as an indoor lighting device  1301 . 
         [0081]    Since the light-emitting device of one embodiment of the present invention has a planar light source, it requires fewer components than a lighting device using a point light source (e.g., a light-reflecting plate can be omitted), and generates less heat than an incandescent lamp, for example. Thus the light-emitting device of one embodiment of the present invention is preferred as an indoor lighting device. 
         [0082]      FIG. 10B  illustrates an example in which the light-emitting device of one embodiment of the present invention is applied to an outdoor lighting device. 
         [0083]    An example of an outdoor lighting device is a street lamp. For example, a street light can include a support  1601  and a lighting device  1602 , as illustrated in  FIG. 10B . For the lighting device  1602 , a plurality of light-emitting devices of one embodiment of the present invention can be used. As illustrated in  FIG. 10B , for example, the street light can be provided along a road so as to uniformly illuminate the surroundings with the lighting device  1602 , so that the visibility of the surroundings including the road can be increased. 
         [0084]    This embodiment can be implemented as appropriate in combination with any of the structures described in the other embodiments. 
         [0085]    This application is based on Japanese Patent Application serial no. 2011-269719 filed with the Japan Patent Office on Dec. 9, 2011, the entire contents of which are hereby incorporated by reference.