BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a display device with an imaging function and a method for driving the display device. In particular, the present invention relates to a display device with an imaging function that irradiates an object to be detected with light emitted therefrom and detects light reflected from the object, and a method for driving the display device. 
         [0003]    2. Description of the Related Art 
         [0004]    Display devices in which display on a screen can be operated when a user touches the screen (i.e., touch panels) have been developed in recent years. For example, Patent Document 1 discloses a display device (a touch panel) in which adverse effect of external light on image capture of an object to be detected can be reduced. Specifically, the display device disclosed in Patent Document 1 can capture an image of an object to be detected with less effect of external light by capturing an image in a state where a display surface emits light and an image in a state where the display surface does not emit light to detect a difference between the two images. 
       REFERENCE 
       [0005]    Patent Document 1: Japanese Published Patent Application No. 2006-276223 
       SUMMARY OF THE INVENTION 
       [0006]    In the display device disclosed in Patent Document 1, light used for display is utilized for image capture. In other words, an image is captured by detecting light reflected from an object to be detected. For that reason, an object to be captured is affected by display on the display device. Specifically, for example, it is difficult to capture an image of an object in a region where black is expressed in the display device. On the other hand, when the entire display surface emits light to capture an image, it is difficult to perform desired display, for example. 
         [0007]    In view of the foregoing problems, an object of one embodiment of the present invention is to achieve desired display on a display surface and accurate image capture. Another object of one embodiment of the present invention is to realize a display device having the above feature with a simple structure. Note that one embodiment of the present invention aims to achieve at least one of the above objects. 
         [0008]    The above object can be achieved by separately providing a light source for display and a light source for image capture, which does not adversely affect display when it is on, in a display device. Here, in order to prevent the light source for image capture in the on state from adversely affecting display, it is necessary that the area irradated with light emitted from the light source for image capture is smaller than the display surface, and that a lighting period of the light source for image capture is short. The main point of one embodiment of the present invention is to use a light-emitting element (e.g., an organic electroluminescent element (also referred to as an organic EL element) as the light source for image capture that satisfies such conditions. Note that in a display device including the light-emitting elements as a light source for display and a light source for image capture, the light source for image capture with approximately the same size as the light source for display provided in a display element can be provided, and thus the area of the light source for image capture can be smaller than that in a liquid crystal display device including LEDs as a backlight. Moreover, it is easy to shorten the lighting period because such a display device has no limitation such as the response speed of liquid crystal in a liquid crystal display device. 
         [0009]    As for the light-emitting elements included in a display device according to one embodiment of the present invention, a light-emitting layer that can emit white light is shared between the light source for display and the light source for image capture. In other words, the light source for display and the light source for image capture are formed by appropriately providing electrodes included in the light source for display and the light source for image capture for one light-emitting layer. Consequently, a step of forming the light-emitting layer into a desired shape is not needed, and the light source for display and the light source for image capture can be simply formed. Note that in the display device, white light emitted from the light source for display is changed into chromatic light by a color filter, and display is performed using the chromatic light. 
         [0010]    Specifically, one embodiment of the present invention is a display device that includes a first light-emitting element and a second light-emitting element emitting white light, a color filter capable of absorbing light with a wavelength in a first specific range included in the white light and capable of transmitting light with a wavelength in a second specific range included in the white light, and a photodiode generating a photocurrent by being irradiated with the white light or the light with a specific wavelength included in the white light. The light with the wavelength in the second specific range is chromatic light, and display is performed using the chromatic light based on the light emitted from the first light-emitting element. An image is captured by irradiating an object to be detected with the light emitted from the second light-emitting element or the light with the specific wavelength included in the light emitted from the second light-emitting element, and irradiating the photodiode with light reflected from the object. 
         [0011]    In a display device according to one embodiment of the present invention, a light source for display (a first light-emitting element) and a light source for image capture (a second light-emitting element) that does not adversely affect display when it is on are separately provided. In the display device, an image can be appropriately captured using the light source for image capture in a period during which display is performed using the light source for display. Consequently, desired display on a display surface and accurate image capture can be achieved in the display device. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    In the accompanying drawings: 
           [0013]      FIG. 1  illustrates a structure example of a display device; 
           [0014]      FIGS. 2A to 2C  are circuit diagrams illustrating structure examples of a display element, a light source element for image capture, and an optical sensor, respectively; 
           [0015]      FIG. 3  is a cross-sectional view illustrating a structure example of a display device; 
           [0016]      FIGS. 4A and 4B  illustrate examples of an image capture method for a display device; 
           [0017]      FIG. 5  is a cross-sectional view illustrating a structure example of a display device; and 
           [0018]      FIGS. 6A to 6F  illustrate examples of electronic devices. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]    Embodiments of the present invention will be hereinafter described in detail with reference to the accompanying drawings. Note that the present invention is not limited to the description below, and it is easily understood by those skilled in the art that a variety of changes and modifications can be made without departing from the spirit and scope of the present invention. Therefore, the present invention should not be limited to the descriptions of the embodiments below. 
       &lt;Structure Example of Display Device&gt; 
       [0020]    First, an example of the structure of a display device according to one embodiment of the present invention will be described with reference to  FIG. 1 ,  FIGS. 2A to 2C , and  FIG. 3 . 
         [0021]      FIG. 1  illustrates a structure example of a display device according to one embodiment of the present invention. The display device illustrated in  FIG. 1  includes a pixel portion  10 , a power source circuit  11 , a signal line driver circuit  12 , a scan line driver circuit  13 , an imaging light source driver circuit  14 , an optical sensor driver circuit  15 , and an imaging-signal read circuit  16 . Further, the pixel portion  10  includes a plurality of display elements  20  arranged in matrix and imaging elements  25  arranged in matrix. The imaging element  25  includes an imaging light source element  21  and an optical sensor  22 . 
         [0022]      FIG. 2A  is a circuit diagram illustrating an example of the configuration of the display element  20  illustrated in  FIG. 1 . The display element  20  in  FIG. 2A  includes an n-channel transistor  201 , a p-channel transistor  202 , a capacitor  203 , and a light-emitting element  204 . A gate of the n-channel transistor  201  is electrically connected to a scan line  130 . One of a source and a drain of the n-channel transistor  201  is electrically connected to a signal line  120 . A gate of the p-channel transistor  202  is electrically connected to the other of the source and the drain of the n-channel transistor  201 . One of a source and a drain of the p-channel transistor  202  is electrically connected to a wiring  111  that supplies a high power supply potential (VDD) (hereinafter referred to as the high power supply potential line  111 ). One electrode of the capacitor  203  is electrically connected to the other of the source and the drain of the n-channel transistor  201  and the gate of the p-channel transistor  202 . The other electrode of the capacitor  203  is electrically connected to the high power supply potential line  111 . An anode of the light-emitting element  204  is electrically connected to the other of the source and the drain of the p-channel transistor  202 . A cathode of the light-emitting element  204  is electrically connected to a wiring  112  that supplies a low power supply potential (VSS) (hereinafter referred to as the low power supply potential line  112 ). 
         [0023]    The scan line  130  is a wiring whose potential is controlled by the scan line driver circuit  13 . The signal line  120  is a wiring whose potential is controlled by the signal line driver circuit  12 . The high power supply potential line  111  is a wiring to which the high power supply potential (VDD), which is a fixed potential, is supplied from the power source circuit  11 . The low power supply potential line  112  is a wiring to which the low power supply potential (VSS), which is a fixed potential, is supplied from the power source circuit  11 . In addition, the scan line  130  is provided for every row of the plurality of display elements  20  arranged in matrix in the pixel portion  10 . The potentials of the plurality of scan lines  130  provided for every row of the display elements  20  are independently controlled by the scan line driver circuit  13 . Similarly, the signal line  120  is provided for every column of the plurality of display elements  20  arranged in matrix in the pixel portion  10 . The potentials of the plurality of signal lines  120  provided for every column of the display elements  20  are independently controlled by the signal line driver circuit  12 . 
         [0024]      FIG. 2B  is a circuit diagram illustrating an example of the configuration of the imaging light source element  21  included in the imaging element  25  illustrated in  FIG. 1 . The imaging light source element  21  in  FIG. 2B  includes an n-channel transistor  211 , a p-channel transistor  212 , a capacitor  213 , and a light-emitting element  214 . The circuit configuration of the imaging light source element  21  in  FIG. 2B  is the same as the display element  20  in  FIG. 2A ; therefore, the detailed description is not repeated. Note that the imaging light source element  21  in  FIG. 2B  differs from the display element  20  in  FIG. 2A  in that a gate of the n-channel transistor  211  is electrically connected to a lighting-period control signal line  141  and one of a source and a drain thereof is electrically connected to an on/off control signal line  140 . Furthermore, it is preferable that the light-emitting element  214  included in the imaging light source element  21  can emit more instantaneous and intense light than the light-emitting element  204  included in the display element  20 . Therefore, the current drive capability of the p-channel transistor  212  is preferably higher than that of the p-channel transistor  202 . For example, the W/L of the p-channel transistor  212  is preferably higher than that of the p-channel transistor  202 . Here, W represents the channel width and L represents the channel length. 
         [0025]    The lighting-period control signal line  141  is provided for every row of the plurality of imaging elements  25  arranged in matrix in the pixel portion  10 . In addition, the plurality of lighting-period control signal lines  141  provided for every row of the imaging elements  25  are electrically connected to the imaging light source driver circuit  14  through a common wiring. That is, the potentials of the plurality of lighting-period control signal lines  141  are collectively controlled by the imaging light source driver circuit  14 . Similarly, the on/off control signal line  140  is provided for every column of the plurality of imaging elements  25  arranged in matrix in the pixel portion  10 . Moreover, the plurality of on/off control signal lines  140  provided for every column of the imaging elements  25  are electrically connected to the imaging light source driver circuit  14  through a common wiring. That is, the potentials of the plurality of on/off control signal lines  140  are collectively controlled by the imaging light source driver circuit  14 . 
         [0026]      FIG. 2C  is a circuit diagram illustrating an example of the configuration of the optical sensor  22  included in the imaging element  25  illustrated in  FIG. 1 . The optical sensor  22  in  FIG. 2C  includes a photodiode  221 , an n-channel transistor  222 , an n-channel transistor  223 , and an n-channel transistor  224 . An anode of the photodiode  221  is electrically connected to a reset signal line  151 . A gate of the n-channel transistor  222  is electrically connected to an imaging-period control signal line  152 . One of a source and a drain of the n-channel transistor  222  is electrically connected to a cathode of the photodiode  221 . A gate of the n-channel transistor  223  is electrically connected to the other of the source and the drain of the n-channel transistor  222 . One of a source and a drain of the n-channel transistor  223  is electrically connected to a wiring  113  that supplies a reference potential (Vref) (hereinafter referred to as the reference potential line  113 ). A gate of the n-channel transistor  224  is electrically connected to a reading-period control signal line  153 . One of a source and a drain of the n-channel transistor  224  is electrically connected to the other of the source and the drain of the n-channel transistor  223 . The other of the source and the drain of the n-channel transistor  224  is electrically connected to a read signal line  160 . 
         [0027]    Note that the reset signal line  151 , the imaging-period control signal line  152 , and the reading-period control signal line  153  are wirings whose potentials are controlled by the optical sensor driver circuit  15 . The reference potential line  113  is a wiring to which the reference potential (Vref), which is a fixed potential, is supplied from the power source circuit  11 . The reset signal line  151 , the imaging-period control signal line  152 , and the reading-period control signal line  153  are provided for every row of the plurality of imaging elements  25  arranged in matrix in the pixel portion  10 . The potentials of the plurality of reset signal lines  151  provided for every row of the imaging elements  25  are controlled independently or collectively by the optical sensor driver circuit  15 . Further, the potentials of the plurality of imaging-period control signal lines  152  provided for every row of the imaging elements  25  are controlled independently or collectively by the optical sensor driver circuit  15 . The potentials of the plurality of reading-period control signal lines  153  provided for every row of the imaging elements  25  are controlled independently by the optical sensor driver circuit  15 . The read signal line  160  is provided for every column of the plurality of imaging elements  25  arranged in matrix in the pixel portion  10 . The potentials of the plurality of read signal lines  160  provided for every column of the imaging elements  25  are judged by the imaging-signal read circuit  16 . 
         [0028]      FIG. 3  is a cross-sectional view illustrating a structure example of part of the display device disclosed in this specification. Specifically,  FIG. 3  illustrates the p-channel transistor  202  and the light-emitting element  204  illustrated in  FIG. 2A , the p-channel transistor  212  and the light-emitting element  214  illustrated in  FIG. 2B , and the photodiode  221  illustrated in  FIG. 2C . 
         [0029]    The p-channel transistor  202  includes p-type impurity regions  2021  and  2022  and a channel formation region  2020  that are formed using single crystal silicon provided over a substrate  200  having an insulating surface, a gate insulating layer  2023  provided over the channel formation region  2020 , and a gate layer  2024  provided over the gate insulating layer  2023 . The p-channel transistor  212  can be a transistor having a structure similar to that of the p-channel transistor  202 . Note that the example where the p-channel transistors  202  and  212  are formed using single crystal silicon is shown here; alternatively, these transistors can be formed using polycrystalline silicon or amorphous silicon. In addition, the p-channel transistors  202  and  212  are top-gate transistors here; however, they are not limited to top-gate transistors and may be bottom-gate transistors or the like. 
         [0030]    The photodiode  221  can have any structure as long as it generates a photocurrent by being irradiated with light emitted from the light-emitting element  214  or light with a specific wavelength included in the light. Here, the photodiode  221  is a PIN photodiode including a p-type impurity region  2121 , an i-type region  2122 , and an n-type impurity region  2123  that are formed using single crystal silicon provided over the substrate  200  having an insulating surface. Alternatively, a PN diode can be applied to the photodiode  221 . Moreover, the structure of the photodiode  221  is preferably selected in accordance with the intended wavelength. For example, a photodiode formed using amorphous silicon is preferably used to detect light in the visible wavelength region, and a photodiode formed using single crystal silicon or polycrystalline silicon is preferably used to detect light in a wavelength region including infrared rays. 
         [0031]    An insulating layer  230  is provided over the p-channel transistors  202  and  212  and the photodiode  221 . The p-type or n-type impurity region included in the p-channel transistors  202  and  212  and the photodiode  221  is electrically connected to any of electrode layers  241  to  246  in an opening in the insulating layer  230 . Furthermore, an insulating layer  250  is provided over the electrode layers  241  to  246 . 
         [0032]    The light-emitting element  204  includes an electrode layer  261  connected to the electrode layer  242  in an opening in the insulating layer  250 , a light-emitting layer  270  provided over the electrode layer  261 , and an electrode layer  280  provided over the light-emitting layer  270 . The light-emitting element  214  includes an electrode layer  262  connected to the electrode layer  243  in an opening in the insulating layer  250 , the light-emitting layer  270  provided over the electrode layer  262 , and the electrode layer  280  provided over the light-emitting layer  270 . The light-emitting layer  270  and the electrode layer  280  are thus shared between the light-emitting elements  204  and  214  in  FIG. 3 . Consequently, a step of forming the light-emitting layer  270  into a desired shape is not needed, and the light source for display and the light source for image capture can be simply formed. Further, since the light-emitting elements  204  and  214  are formed without a step of processing the light-emitting layer  270  into a desired shape, the reliability and yield of the display device can be improved. 
         [0033]    Note that here, the light-emitting layer  270  can emit white light (W) with a current generated between the electrode layer  261  or the electrode layer  262  and the electrode layer  280 , and the white light (W) at least has a wavelength of red light, a wavelength of green light, a wavelength of blue light, and a wavelength of yellow light. For example, the light-emitting elements  204  and  214  can be an organic electroluminescent element. In addition, the electrode layer  280  is formed using a light-transmitting conductive film here. As a result, the light-emitting elements  204  and  214  can emit white light (W) at least in the direction where the electrode layer  280  is provided. Further, partition layers  291  to  295  are provided at edges of the electrode layers  261  and  262  and in the openings in the insulating layer  250 . Note that the partition layer is formed from an organic insulator or an inorganic insulator. By providing the partition layers  291  to  295 , short circuit between the electrode layer  261  or the electrode layer  262  and the electrode layer  280  can be prevented and disconnection of the light-emitting layer  270  can be suppressed. Here, the light-emitting elements  204  and  214  are provided to overlap the respective p-channel transistors  202  and  212 . For that reason, the area of the light-emitting elements  204  and  214  (the aperture ratio of the display device) can be increased. 
         [0034]    In addition, the display device in  FIG. 3  includes a color filter  300  and a light-transmitting sealing substrate  310 . A region where the light-emitting elements  204  and  214  and the like are placed is sealed by the substrate  200  having an insulating surface and the sealing substrate  310 . Thus, moisture can be prevented from being mixed into the light-emitting elements  204  and  214  and the like, so that the reliability of the display device can be increased. The color filter  300  is provided directly above the region where the light-emitting element  204  is placed. The color filter  300  can absorb light with a wavelength in a specific range, which is included in white light (W) emitted from the light-emitting element  204 , and change it into chromatic light (C). Note that here, the chromatic color is red, green, blue, or yellow. 
         [0035]    The color filter is not provided directly above the light-emitting element  214  and the photodiode  221 . Consequently, in the display device in  FIG. 3 , an image of an object to be detected can be captured by irradiating the object with white light (W) emitted from the light-emitting element  214  and irradiating the photodiode  221  with white light (W) reflected from the object. 
       &lt;Example of Image Capture Method for Display Device&gt; 
       [0036]    Next, an example of an image capture method for the above-described display device will be described with reference to  FIGS. 4A and 4B . 
         [0037]      FIGS. 4A and 4B  illustrate an example of a method for capturing an image by the imaging element  25  illustrated in  FIG. 1  and  FIGS. 2B and 2C . Specifically,  FIG. 4A  illustrates an example of capturing an image while the light-emitting element  214  included in the imaging light source element  21  emits light.  FIG. 4B  illustrates an example of capturing an image while the light-emitting element  214  included in the imaging light source element  21  does not emit light. Note that in  FIGS. 4A and 4B , V( 141 ) is the potential of the lighting-period control signal line  141 ; V( 140 ) is the potential of the on/off control signal line  140 ; V( 151 ) is the potential of the reset signal line  151 ; V( 152 ) is the potential of the imaging-period control signal line  152 ; V( 153 ) is the potential of the reading-period control signal line  153 ; V(N) is the potential of a node where the other of the source and the drain of the n-channel transistor  222  and the gate of the n-channel transistor  223  are electrically connected to each other; and V( 160 ) is the potential of the read signal line  160 . 
         [0038]    In the example of the image capture method illustrated in  FIG. 4A , before a time A, the potential (V( 140 )) of the on/off control signal line  140  is set at a low-level potential and the potential (V( 141 )) of the lighting-period control signal line  141  is set at a high-level potential. Thus, the n-channel transistor  211  and the p-channel transistor  212  illustrated in  FIG. 2B  are turned on. Consequently, the light-emitting element  214  emits light. 
         [0039]    At the time A, the potential (V( 151 )) of the reset signal line  151  and the potential (V( 152 )) of the imaging-period control signal line  152  are set at a high-level potential (a reset operation starts). At this time, the potential (V(N)) of the node is set at a reset potential. 
         [0040]    At a time B, the potential (V( 151 )) of the reset signal line  151  is set at a low-level potential (the reset operation ends and a storage operation starts). At that time, the rate of change in the potential (V(N)) of the node varies depending on the amount of light with which the photodiode  221  is irradiated. Here, the potential of the node is decreased more rapidly as the amount of light emitted to the photodiode  221  is larger. 
         [0041]    At a time C, the potential (V( 152 )) of the imaging-period control signal line  152  is set at a low-level potential (the storage operation ends). After that, the potential (V(N)) of the node has a fixed value. Note that the larger the amount of light emitted to the photodiode  221  in the storage operation is, the lower the potential (V(N)) of the node becomes. 
         [0042]    At a time D, the potential (V( 153 )) of the reading-period control signal line  153  is set at a high-level potential (a selection operation starts). At this time, the potential (V( 160 )) of the read signal line  160  is changed in accordance with the potential (V(N)) of the node, that is, the gate voltage of the n-channel transistor  223  illustrated in  FIG. 2C . Here, when the potential (V( 160 )) of the read signal line  160  is set at a high-level potential in advance, the potential (V( 160 )) of the read signal line  160  becomes lower as the gate voltage of the n-channel transistor  223  is higher. Here, the gate voltage of the n-channel transistor  223  becomes lower as the amount of light emitted to the photodiode  221  is larger; therefore, the potential (V( 160 )) of the read signal line  160  becomes higher as the amount of light emitted to the photodiode  221  is larger. 
         [0043]    At a time E, the potential (V( 153 )) of the reading-period control signal line  153  is set at a low-level potential (the selection operation ends). Thus, the potential (V( 160 )) of the read signal line  160  becomes a fixed value. After that, the potential (V( 160 )) of the read signal line  160  is judged by the imaging-signal read circuit  16 . 
         [0044]    In the example of the image capture method illustrated in  FIG. 4B , before a time F, the potential (V( 140 )) of the on/off control signal line  140  is set at a high-level potential and the potential (V( 141 )) of the lighting-period control signal line  141  is set at a high-level potential. Thus, the n-channel transistor  211  illustrated in  FIG. 2B  is turned on and the p-channel transistor  212  is turned off. As a result, the light-emitting element  214  does not emit light. 
         [0045]    At the time F, the potential (V( 151 )) of the reset signal line  151  and the potential (V( 152 )) of the imaging-period control signal line  152  are set at a high-level potential (a reset operation starts). At this time, the potential (V(N)) of the node is set at a reset potential. 
         [0046]    At a time G, the potential (V( 151 )) of the reset signal line  151  is set at a low-level potential (the reset operation ends and a storage operation starts). At that time, the rate of change in the potential (V(N)) of the node varies depending on the amount of light emitted to the photodiode  221 . Since the light-emitting element  214  does not emit light in the image capture method illustrated in  FIG. 4B , the amount of light emitted to the photodiode  221  is smaller than that in the image capture method illustrated in  FIG. 4A . Consequently, the potential of the node in the image capture method illustrated in  FIG. 4B  is decreased more slowly than in the image capture method illustrated in  FIG. 4A . 
         [0047]    At a time H, the potential (V( 152 )) of the imaging-period control signal line  152  is set at a low-level potential (the storage operation ends). After that, the potential (V(N)) of the node has a fixed value. 
         [0048]    At a time I, the potential (V( 153 )) of the reading-period control signal line  153  is set at a high-level potential (a selection operation starts). At this time, the potential (V( 160 )) of the read signal line  160  is changed in accordance with the potential (V(N)) of the node, that is, the gate voltage of the n-channel transistor  223  illustrated in  FIG. 2C . Here, the potential (V(N)) of the node is higher in the image capture method illustrated in  FIG. 4B  than in the image capture method illustrated in  FIG. 4A . As a result, the potential (V( 160 )) of the read signal line  160  is lower in the image capture method illustrated in  FIG. 4B  than in the image capture method illustrated in  FIG. 4A . 
         [0049]    At a time J, the potential (V( 153 )) of the reading-period control signal line  153  is set at a low-level potential (the selection operation ends). Thus, the potential (V( 160 )) of the read signal line  160  becomes a fixed value. After that, the potential (V( 160 )) of the read signal line  160  is judged by the imaging-signal read circuit  16 . 
         [0050]    In the display device disclosed in this specification, it is possible to detect a difference between data judged by the image capture method illustrated in  FIG. 4A  and data judged by the image capture method illustrated in  FIG. 4B . Detection of the difference can reduce noise due to external light at the time of image capture, which leads to improvement in the accuracy of detecting an object. 
       &lt;Variation of Display Device&gt; 
       [0051]    The above-described display device is an embodiment of the present invention, and the present invention also includes a display device different from the above display device. 
         [0052]    For example, the display device has the structure where an image is captured using white light (W) emitted from the light-emitting element  214  (see  FIG. 3 ); alternatively, it is possible to employ a structure where an image is captured using invisible light (IR) including light with a wavelength in the infrared region (see  FIG. 5 ).  FIG. 5  is a cross-sectional view of a display device obtained by adding color filters  301  and  302  for absorbing light in the same wavelength region as that of light the color filter  300  absorbs and color filters  321  and  322  for absorbing light in a wavelength region different from that of light the color filter  300  absorbs, to the display device illustrated in  FIG. 3 . Here, white light (W) emitted from the light-emitting element  214  illustrated in  FIG. 5  includes light with a wavelength in the infrared region. On the sealing substrate  310 , the color filters  301  and  321  are stacked and the color filters  302  and  322  are stacked. The stack of the color filters  301  and  321  is provided directly above a region provided with the light-emitting element  214 . The stack of the color filters  302  and  322  is provided directly above a region provided with the photodiode  221 . 
         [0053]    By using invisible light (IR) including light with a wavelength in the infrared region as described above, an image can be captured without adversely affecting display in the display device. Moreover, the intensity of invisible light (IR) can be increased without consideration of adverse effect on display, so that adverse effect of external light on the imaging element at the time of image capture can be reduced and the detection accuracy can be improved. 
       EXAMPLE 1 
       [0054]    In this example, examples of electronic devices each including the above display device will be described with reference to  FIGS. 6A to 6F . 
         [0055]      FIG. 6A  illustrates a personal digital assistant. The personal digital assistant in  FIG. 6A  includes at least a display-and-imaging unit  601 . In the personal digital assistant in  FIG. 6A , for example, the display-and-imaging unit  601  can be provided with an operation unit  602 . By using the display device for the display-and-imaging unit  601 , operation of the personal digital assistant or input of data to the personal digital assistant can be performed with a finger or a pen, for example. 
         [0056]      FIG. 6B  illustrates an information guide terminal including an automotive navigation system. The information guide terminal in  FIG. 6B  includes a display-and-imaging unit  611 , operation buttons  612 , and an external input terminal  613 . By using the display device for the display-and-imaging unit  611 , operation of the information guide terminal or input of data to the information guide terminal can be performed with a finger or a pen, for example. 
         [0057]      FIG. 6C  illustrates a laptop personal computer. The laptop personal computer in  FIG. 6C  includes a housing  621 , a display-and-imaging unit  622 , a speaker  623 , an LED lamp  624 , a pointing device  625 , a connection terminal  626 , and a keyboard  627 . By using the display device for the display-and-imaging unit  622 , operation of the laptop personal computer or input of data to the laptop personal computer can be performed with a finger or a pen, for example. 
         [0058]      FIG. 6D  illustrates a portable game machine. The portable game machine in  FIG. 6D  includes a display-and-imaging unit  631 , a display-and-imaging unit  632 , a speaker  633 , a connection terminal  634 , an LED lamp  635 , a microphone  636 , a recording medium read portion  637 , operation buttons  638 , and a sensor  639 . By using the display device for the display-and-imaging unit  631  and/or the display-and-imaging unit  632 , operation of the portable game machine or input of data to the portable game machine can be performed with a finger or a pen, for example. 
         [0059]      FIG. 6E  illustrates an e-book reader. The e-book reader in  FIG. 6E  includes at least a housing  641 , a housing  642 , a display-and-imaging unit  643 , a display-and-imaging unit  644 , and a hinge  645 . 
         [0060]    The housing  641  and the housing  642  are connected to each other with the hinge  645  so that the e-book reader in  FIG. 6E  can be opened and closed with the hinge  645  as an axis. With such a structure, the e-book reader can be handled like a paper book. The display-and-imaging unit  643  and the display-and-imaging unit  644  are incorporated in the housing  641  and the housing  642 , respectively. The display-and-imaging unit  643  and the display-and-imaging unit  644  can display different images; alternatively, one image can be displayed across both the display-and-imaging units. In the case where different images are displayed on the display-and-imaging unit  643  and the display-and-imaging unit  644 , for example, text can be displayed on the display-and-imaging unit on the right side (the display-and-imaging unit  643  in  FIG. 6E ) and graphics can be displayed on the display-and-imaging unit on the left side (the display-and-imaging unit  644  in  FIG. 6E ). 
         [0061]    In the e-book reader in  FIG. 6E , the housing  641  or the housing  642  may be provided with an operation unit or the like. For example, the e-book reader in  FIG. 6E  can include a power button  646 , operation keys  647 , and a speaker  648 . In the e-book reader in  FIG. 6E , the pages of an image can be turned with the operation keys  647 . Furthermore, in the e-book reader in  FIG. 6E , a keyboard, a pointing device, or the like may be provided in at least one of the display-and-imaging unit  643  and the display-and-imaging unit  644 . Moreover, an external connection terminal (e.g., an earphone terminal, a USB terminal, or a terminal connectable to an AC adapter or a variety of cables such as a USB cable), a recording medium insertion portion, and the like may be provided on a back surface or a side surface of the housing  641  and the housing  642  of the e-book reader in  FIG. 6E . In addition, a function of an electronic dictionary may be added to the e-book reader in  FIG. 6E . 
         [0062]    By using the display device for the display-and-imaging unit  643  and/or the display-and-imaging unit  644 , operation of the e-book reader or input of data to the e-book reader can be performed with a finger or a pen, for example. 
         [0063]      FIG. 6F  illustrates a display. The display in  FIG. 6F  includes a housing  651 , a display-and-imaging unit  652 , a speaker  653 , an LED lamp  654 , operation buttons  655 , a connection terminal  656 , a sensor  657 , a microphone  658 , and a supporting base  659 . By using the display device for the display-and-imaging unit  652 , operation of the display or input of data to the display can be performed with a finger or a pen, for example. 
         [0064]    This application is based on Japanese Patent Application serial No. 2010-248010 filed with Japan Patent Office on Nov. 5, 2010, the entire contents of which are hereby incorporated by reference.
To achieve desired display on a display surface and accurate image capture in a display device with an imaging function. A light source for display (a first light-emitting element) and a light source for image capture (a second light-emitting element) that does not adversely affect display when it is on are separately provided in the display device with an imaging function. In the display device, an image can be appropriately captured using the light source for image capture in a period during which display is performed using the light source for display. Consequently, desired display on the display surface and accurate image capture can be achieved in the display device.