Patent Publication Number: US-2011050870-A1

Title: Organic el display device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2009-203897, filed Sep. 3, 2009; the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments described herein relate generally to an organic electroluminescence (EL) display device. 
     BACKGROUND 
     In recent years, display devices using organic electroluminescence (EL) elements have vigorously been developed, which have features of self-emission, a high response speed, a wide viewing angle and a high contrast, and which can realize further reduction in thickness and weight. The organic EL element is configured to include a thin film which tends to easily degrade due to the influence of moisture. 
     For example, there is known an electronic device including display means on which a 2D (two-dimensional) image and a 3D (three-dimensional) image are selectively displayed by switching. This electronic device has a display function of forcibly effecting switching to 2D image display at a time of 3D image display. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an exemplary plan view which schematically shows the structure of an organic EL display device according to an embodiment, which adopts an active matrix driving method; 
         FIG. 2  is an exemplary system configuration diagram of the organic EL display device according to the embodiment, which is configured such that a first display mode of displaying a two-dimensional (2D) image and a second display mode of displaying a three-dimensional (3D) image can be switched; 
         FIG. 3  is an exemplary block diagram showing the structure of a control circuit according to the embodiment; 
         FIG. 4  is an exemplary timing chart for explaining a luminance switching method, according to the embodiment, for switching the first display mode and the second display mode by the control circuit shown in  FIG. 3 ; 
         FIG. 5  is an exemplary timing chart for explaining another luminance switching method, according to the embodiment, for switching the first display and the second display mode by the control circuit shown in  FIG. 3 ; 
         FIG. 6  is an exemplary circuit diagram showing a pixel circuit which is of a voltage write type, according to the embodiment; 
         FIG. 7  is an exemplary timing chart for explaining the basic operation in the pixel circuit shown in  FIG. 6 , according to the embodiment; 
         FIG. 8  is an exemplary view for explaining the relationship between a duty ratio and an emission luminance of an organic EL element, according to the embodiment; 
         FIG. 9  is an exemplary view for explaining the relationship between a duty ratio in a case where the first display mode is selected and a duty ratio in a case where the second display mode is selected, according to the embodiment; 
         FIG. 10  is an exemplary view for explaining the relationship between a driving voltage, which is supplied to the pixel circuit via a video signal line, and an emission luminance of the organic EL element, according to the embodiment; 
         FIG. 11  is an exemplary view for explaining a relationship between a gradation and a driving voltage, according to the embodiment; 
         FIG. 12  is an exemplary view showing the structure of the control circuit according to the embodiment; 
         FIG. 13  is an exemplary view for describing another relationship between the gradation and the driving voltage, according to the embodiment; 
         FIG. 14  is an exemplary circuit diagram showing a pixel circuit which is of a current write type, according to the embodiment; 
         FIG. 15  is an exemplary timing chart for explaining the basic operation in the pixel circuit shown in  FIG. 14 , according to the embodiment; 
         FIG. 16  is an exemplary view for explaining the relationship between a driving current, which is supplied to the pixel circuit via a video signal line, and an emission luminance of the organic EL element, according to the embodiment; 
         FIG. 17  is an exemplary view for explaining a relationship between the gradation and the driving current, according to the embodiment; 
         FIG. 18  is an exemplary view showing the structure of a control circuit according to the embodiment; 
         FIG. 19  is an exemplary plan view which schematically shows the structure of an organic EL display device according to the embodiment, which adopts an active matrix driving method; and 
         FIGS. 20A ,  20 B,  20 C,  20 D, and  20 E are exemplary diagrams for explaining methods of controlling the luminance in cases of displaying a video signal of a two-dimensional image corresponding to a first operational frequency (60 Hz) and a video signal of a three-dimensional image corresponding to a second operational frequency (120 Hz). 
     
    
    
     DETAILED DESCRIPTION 
     In general, according to one embodiment, there is provided an organic EL display device comprising: an organic EL element; a pixel circuit configured to include an output switch which controls emission/non-emission of the organic EL element; and a luminance switching module configured to output to the output switch a first control signal which causes the organic EL element to emit light with a first total emission time per unit time, when a first display mode of displaying a two-dimensional image is selected, and to output to the output switch a second control signal which causes the organic EL element to emit light with a second total emission time, which is longer than the first total emission time, per unit time, when a second display mode of displaying a three-dimensional image is selected. 
     First Embodiment  
     An embodiment of the invention will now be described with reference to the accompanying drawings. The structural elements having the same or similar functions in the respective drawings are denoted by like reference numerals, and an overlapping description is omitted. 
       FIG. 1  is an exemplary plan view which schematically shows the structure of an organic EL display device according to an embodiment, which adopts an active matrix driving method. 
     The organic EL display device includes a display panel  1 . The display panel  1  includes an array substrate  100  and a sealing substrate  200 . The array substrate  100  includes a substantially rectangular active area  102  which displays an image. The active area  102  comprises an (m×n) number of pixels PX which are arranged in a matrix (m=a positive integer, n=a positive integer). Specifically, an m-number of pixels PX, which are arranged in a first direction D 1 , constitute one line of the active area  102 . The active area  102  is composed of an n-number of lines which are arranged in a second direction D 2 . 
     The array substrate  100  includes an insulative substrate SUB such as a glass substrate, and organic EL elements OLED and pixel circuits CT, which are disposed in the respective pixels PX above the insulative substrate SUB. The organic EL element OLED may be constructed as a top emission type organic EL element which emits generated light from the sealing substrate  200  side, or a bottom emission type organic EL element which emits generated light from the array substrate  100  side. 
     The array substrate  100  includes an extension portion  110  which extends outward from an end portion  200 E of the sealing substrate  200  on the outside of the active area  102 . A driving IC  120  is mounted on the extension portion  110 . The driving IC  120  supplies the pixel circuit CT with power and various control signals which are necessary for driving the organic EL element OLED. A flexible printed circuit board (hereinafter referred to as “FPC board”)  130  is connected to the extension portion  110 . Further, a module board  140  is connected to the FPC board  130 . 
     The sealing substrate  200  faces the organic EL elements OLED of the array substrate  100  in the active area  102 . The sealing substrate  200  is an insulative substrate such as a glass substrate. 
     The array substrate  100  and the sealing substrate  200  are attached by a sealant  300  which is disposed in a frame shape in a manner to surround the active area  102 . The sealant  300  is formed of a resin material or frit glass. 
     In the above-described organic EL display device, image data, which is successively transmitted, is accumulated for one line and then successively written in the pixels PX. Thereby, the organic EL elements OLED of the respective pixels PX are turned on, and an image for one frame is displayed during one frame period by the organic EL elements OLED of the active area  102 . 
       FIG. 2  is an exemplary system structure configuration diagram of the organic EL display device according to the embodiment, which is configured such that a first display mode of displaying a two-dimensional (2D) image and a second display mode of displaying a three-dimensional (3D) image can be switched. 
     This example of the system configuration includes the display panel  1  having the above-described structure, glasses  2  which are provided with optical shutters on the right-eye side and left-eye side, and a control circuit  10 . The display panel  1  and glasses  2  are connected to the control circuit  10 . The control circuit  10  is supplied with video data of a 2D image or a 3D image, a 3D determination signal for determining that a 3D image is to be displayed, and a glasses operation determination signal for determining that the shutters of the glasses  2  have operated. The control circuit  10  outputs a driving signal, which is necessary for displaying a 2D image or a 3D image, to the display panel  1 . 
     The control circuit  10  may be mounted on the display panel  1 , or may be mounted on the above-described FPC board  130  or module board  140 . Besides, the components constituting the control circuit  10  may be disposed in a distributed fashion on the display panel  1 , FPC board  130  and module board  140 . The display panel  1  includes, at least, an input terminal IT to which the driving signal, which is output from the control circuit  10 , is input. 
     In the first display mode of displaying a 2D image, the display panel  1  displays an image during one frame period, based on the driving signal that is supplied from the control circuit  10 . In the first display mode, the display panel  1  operates with a first operational frequency (e.g. 60 Hz). 
     In the second display mode of displaying a 3D image, the display panel  1  displays a right-eye image and a left-eye image by switching them during one frame period. Specifically, in the second display mode, each of the right-eye image and the left-eye image is switched with a second operational frequency (e.g. 120 Hz) which is double the first operational frequency. 
     In the second display mode, the right-eye and left-eye shutters of the glasses  2  are switched in sync with an image which is displayed on the display panel  1 . Specifically, when a right-eye image is displayed on the display panel  1 , the right-eye shutter of the glasses  2  is opened and the left-eye shutter is closed. On the other hand, when a left-eye image is displayed on the display panel  1 , the left-eye shutter of the glasses  2  is opened and the right-eye shutter is closed. Thereby, the display of a 3D image corresponding to left-and-right parallax is realized. 
     When a 2D image is displayed, the 2D image that is displayed in one frame period can be observed by both eyes. When a 3D image is displayed, a right-eye image, which is displayed during about ½ of one frame period, is observed by the right eye, and a left-eye image, which is displayed during about ½ of one frame period, is observed by the left eye. Consequently, when the 3D image is displayed, it is felt that the displayed 3D image looks darker than in the case where the 2D image is displayed. 
     Thus, in the embodiment, the luminance of the organic EL element OLED in the case where the 3D image is displayed is set to be higher than the luminance of the organic EL element OLED in the case where the 2D image is displayed. Thereby, the difference in brightness of the displayed image, which is felt by the user when the 2D image and 3D image are switched, is decreased. An example of the concrete method therefor is described below. 
       FIG. 3  an exemplary block diagram showing the structure of the control circuit according to the embodiment. The control circuit  10  includes a mode select module  11  and a luminance switching module  12 . 
     When the above-described glasses operation determination signal from the glasses  2  or the 3D determination signal is in the OFF state, the mode select module  11  selects the first display mode of displaying the 2D image. In this case, the mode select module  11  may output a first select signal (e.g. “Low” level), which selects the first display mode, to the luminance switching module  12 , or may not output a special select signal. 
     On the other hand, for example, when the glasses operation determination signal or the 3D determination signal, which is in the ON state, is received, the mode select module  11  selects the second display mode of displaying the 3D image. In this case, the mode select module  11  outputs a second select signal (e.g. “High” level), which selects the second display mode, to the luminance switching module  12 . 
     The luminance switching module  12  outputs to the display panel  1  a driving signal corresponding to the selected display mode, based on the input video data. For example, when the first select signal is input from the mode select module  11  or when no select signal is input, the luminance switching module  12  outputs a first driving signal corresponding to the first display mode. On the other hand, when the second select signal is input from the mode select module  11 , the luminance switching module  12  outputs a second driving signal corresponding to the second display mode. The first driving signal or second driving signal, which is output from the luminance switching module  12 , is supplied to the input terminal IT of the display panel  1 . 
     In the first display mode, the luminance switching module  12  outputs to the display panel  1  such a first driving signal that, for example, the emission luminance of the organic EL element OLED corresponding to each gray level of input video data may have a first gamma. On the other hand, in the second display mode, the luminance switching module  12  outputs to the display panel  1  such a second driving signal that, for example, the emission luminance of the organic EL element OLED corresponding to each gray level of input video data may have a second gamma which is greater than the first gamma. In short, the luminance range of the emission luminance of the organic EL element OLED in the second display mode is wider than the luminance range of the emission luminance of the organic EL element OLED in the first display mode. 
     Accordingly, in the first display mode, the luminance switching module  12  outputs the first driving signal which causes the organic EL element OLED to emit light with a first luminance (maximum luminance in the first display mode) in accordance with the maximum gray level. On the other hand, in the second display mode, the luminance switching module  12  outputs the second driving signal which causes the organic EL element OLED to emit light with a second luminance (maximum luminance in the second display mode), which is higher than the first luminance, in accordance with the maximum gray level. In the case where the second operational frequency in the second display mode is double the first operational frequency in the first display mode, it is desirable that the second luminance be double the first luminance. 
       FIG. 4  is an exemplary timing chart for explaining a luminance switching method, according to the embodiment, for switching the first display mode and the second display mode by the control circuit shown in  FIG. 3 . 
     In this example, when the first select signal of “Low” level is input from the mode select module  11  to the luminance switching module  12 , the luminance switching module  12  outputs to the display panel  1  the first driving signal corresponding to the first operational frequency (60 Hz), based on the input video data of the 2D image. At this time, in the display panel  1 , the organic EL element OLED emits light with a luminance in a relative luminance range between “0” and “1”, based on the first driving signal which is supplied from the luminance switching module  12 . Specifically, the maximum luminance (first luminance) of the organic EL element OLED, which corresponds to the maximum gray level in the first display mode, is “1”. 
     The mode select module  11  outputs the second select signal of “High” level to the luminance switching module  12 , when the glasses operation determination signal has been rendered “ON” and the 3D determination signal has been rendered “ON”. Based on the input of the second select signal, the luminance switching module  12  outputs, on the basis of the input video data of the 3D image, to the display panel  1  the second driving signal of the second operational frequency (120 Hz) corresponding to the second display mode, that is, the second driving signal including a driving signal for displaying a right-eye image and a driving signal for displaying a left-eye image. At this time, in the display panel  1 , the organic EL element OLED emits light with a luminance in a relative luminance range between “0” and “2”, based on the second driving signal which is supplied from the luminance switching module  12 . Specifically, the maximum luminance (second luminance) of the organic EL element OLED, which corresponds to the maximum gray level in the second display mode, is “2”, which corresponds to double the maximum luminance in the first display mode. As regards the emission luminance of the organic EL element OLED which corresponds to not only the maximum gray level but also each of gray levels, the emission luminance in the second display mode is double the emission luminance in the first display mode. 
     According to the above-described embodiment, each of the display period for displaying the right-eye image and the display period for displaying the left-eye image in one frame period in the second display mode is ½ of the display period for displaying the 2D image in one frame period in the first display mode. However, the emission luminance of the organic EL element OLED, which forms each of the right-eye image and left-eye image, is double the emission luminance of the organic EL element OLED at the time of forming the 2D image. Therefore, even if the display mode is switched between the first display mode of displaying the 2D image and the second display mode of displaying the 3D image, the variation in luminance of displayed images is reduced, and the 2D image and 3D image with good display quality can be displayed. 
       FIG. 5  is an exemplary timing chart for explaining another luminance switching method, according to the embodiment, for switching the first display and the second display mode by the control circuit shown in  FIG. 3 . In this example of the luminance switching method, when the first display mode of displaying the 2D image is switched to the second display mode of displaying the 3D image, the luminance is varied stepwise. 
     In the display panel  1 , the organic EL element OLED emits light with a luminance in a relative luminance range between “0” and “1”, based on the output of the first driving signal corresponding to the first display mode from the luminance switching module  12 . Specifically, the maximum luminance of the organic EL element OLED, which corresponds to the maximum gray level in the first display mode, is “1”. 
     At a timing immediately after the switching from the first display mode to the second display mode, the luminance switching module  12  sets the maximum luminance of the organic EL element OLED at a level higher than “1” and lower than “2”. In the meantime, the maximum luminance may be varied in a plurality of steps, until the maximum luminance of the organic EL element OLED is set at “2”. 
     In the example shown in  FIG. 5 , while the maximum luminance varies from “1” to “2”, the maximum luminance varies in two steps. At first, the maximum luminance of the organic EL element OLED is set at “1.3”. In other words, the organic EL element OLED emits light with a luminance in a relative luminance range between “0” and “1.3”. At a subsequent timing, the maximum luminance of the organic EL element OLED is set at “1.6”. In other words, the organic EL element OLED emits light with a luminance in a relative luminance range between “0” and “1.6”. In the example shown in  FIG. 5 , although the hold time of each step is not indicated, each step may be varied with a time of about 0.2 to 1.0 sec. In addition, the number of steps may be increased so that the maximum luminance may be gradually varied. 
     In the above-described example, when the maximum luminance in the first display mode is set at “1”, the maximum luminance in the second display mode is “2”. However, the setting of the maximum luminance is not limited to this example. The maximum luminance in the second display mode may be selected in a range of about 1.5 times to 2.5 times the maximum luminance in the first display mode, and it is desirable that the maximum luminance be adjusted such that the decrease in display quality of the 3D image due to the decrease in luminance may be tolerable. 
     If the switching of the display mode occurs frequently, there may occur such a case that an unpleasant feeling is given to the user. Taking this into account, it is effective to provide a so-called hysteresis which keeps a state for a predetermined time. In the examples of  FIG. 4  and  FIG. 5 , the select signal for switching the first display mode to the second display mode is output at the same time as the two determination signals, namely, the glasses operation determination signal and 3D determination signal, have been rendered “ON”. However, in an alternative setting, for example, with a hold period of about 1 sec after the two determination signals are rendered “ON”, the first display mode may be switched to the second display mode. Similarly, in another alternative setting, if the “OFF” state of the two determination signals remains unchanged for a predetermined time period, the second display mode may be switched to the first display mode. 
     The above-described glasses operation determination signal for determining that the shutters of the glasses  2  have operated may be configured to interlock with an operation switch (not shown) of the glasses  2 , or may be output on the basis of a detection signal from a detection device which is provided on a temple or a bridge of the glasses  2  to determine the state in which the glasses  2  are worn. It is desirable that the glasses operation determination signal be produced as a signal reflecting the actual state of use of the glasses  2 . 
     In the above-described luminance switching method, it is possible to apply a method of switching the total emission time per unit time of the organic EL element OLED (in this example, the total emission time corresponds to the ratio (T 2 /T 1 ) of an emission time T 2  of light emission of the organic EL element OLED to a display time T 1 , and this total emission time is referred to as “duty ratio”) between the first display mode and the second display mode, or to apply a method of switching a video signal necessary for driving the organic EL element OLED (a driving voltage in the case where the pixel circuit CT is of a voltage write type, or a driving current in the case where the pixel circuit CT is of a current write type) between the first display mode and the second display mode. 
     Next, a description is given of a more concrete luminance switching method in the case where the pixel circuit CT of the voltage write type is applied. 
       FIG. 6  is an exemplary circuit diagram showing the pixel circuit which is of the voltage write type, according to the embodiment. 
     The pixel circuit CT comprises a driving transistor DRT which controls the driving of the organic EL element OLED, three switches SW 1 , SW 2  and SW 3 , and two storage capacitance elements CS 1  and CS 2 . The three switches SW 1 , SW 2  and SW 3  and the driving transistor DRT are composed of p-channel thin-film transistors. 
     The gate electrode of the switch SW 1  is connected to a first gate line GL 1 , and the source electrode thereof is connected to a video signal line SG. A control signal  1 , which controls ON/OFF of the switch SW 1 , is supplied to the first gate line GL 1 . A video signal is supplied to the video signal line SG. The gate electrode of the driving transistor DRT is connected to the drain electrode of the switch SW 1  via the storage capacitance element CS 1 . The source electrode of the driving transistor DRT is connected to a power line P, and the drain electrode thereof is connected to the switch SW 3 . The storage capacitance element CS 2  is formed between the gate electrode and source electrode of the driving transistor DRT. 
     The switch SW 2  is connected between the gate electrode and drain electrode of the driving transistor DRT, and the gate electrode of the switch SW 2  is connected to a second gate line GL 2 . A control signal  2 , which controls ON/OFF of the switch SW 2 , is supplied to the second gate line GL 2 . The gate electrode of the switch SW 3  is connected to a third gate line GL 3 . A control signal  3 , which controls ON/OFF of the switch SW 3 , is supplied to the third gate line GL 3 . The source electrode of the switch SW 3  is connected to the driving transistor DRT, and the drain electrode thereof is connected to the organic EL element OLED. The switch SW 3  corresponds to an output switch which controls emission/non-emission of the organic EL element OLED. 
     Next, a description is given of a luminance switching method of switching the duty ratio between the first display mode and the second display mode, as a luminance switching method which is applicable to the above-described pixel circuit CT. 
       FIG. 7  is an exemplary timing chart for explaining the basic operation in the pixel circuit shown in  FIG. 6 , according to the embodiment. One horizontal period includes a reset period, a cancel period which follows the reset period, and a write period which comes after the cancel period. 
     The reset period corresponds to a period in which the control signal  1  supplied to the first gate line GL 1  is “OFF”, the control signal  2  supplied to the second gate line GL 2  is “ON” and the control signal  3  supplied to the third gate line GL 3  is “ON” (i.e. the period in which the switch SW 1  is “OFF” and the switch SW 2  and switch SW 3  are “ON”). 
     The cancel period corresponds to a period in which the control signal  1  is “OFF”, the control signal  3  is “OFF” and the control signal  2  is “ON” (i.e. the period in which the switch SW 1  and switch SW 3  are “OFF” and the switch SW 2  is “ON”). The write period corresponds to a period in which the control signal  2  and control signal  3  are “OFF” and the control signal  1  is “ON” (i.e. the period in which the switch SW 2  and switch SW 3  are “OFF” and the switch SW 1  is “ON”), and in this write period a video signal is written in the pixel circuit CT. Subsequently, the control signal  1 , control signal  2  and control signal  3  are rendered “OFF”, and the video signal written in the pixel circuit CT is retained for a predetermined time period. 
     Thereafter, in a period in which the control signal  1  and control signal  2  are “OFF” and the control signal  3  is rendered “ON”, electric current, which is controlled by the driving transistor DRT, is supplied to the organic EL element OLED, and the organic EL element OLED emits light with a predetermined luminance. In the example shown in  FIG. 7 , in the display period T 1 , the control signal  3  is constantly “ON” and the organic EL element OLED constantly emits light. In other words, the display period T 1  is equal to the emission period T 2  in which the organic EL element OLED emits light, and this corresponds to the case in which the duty ratio is 100%. 
     In this manner, the duty ratio can be controlled by the time (i.e. emission time T 2 ) in which the control signal  3  is “ON” in the display period T 1 . Specifically, since the control signal  3  corresponds to the signal that controls the emission time T 2 , the effective luminance of the organic EL element OLED can be adjusted by adjusting the emission time T 2  of the organic EL element OLED in the display period T 1  by the ON/OFF of the control signal  3 . The control signal  3  and the video signal, which is written in the pixel circuit CT, are output from the luminance switching module  12  to the display panel  1  as a driving signal corresponding to the display mode. 
       FIG. 8  is an exemplary view for explaining the relationship between the duty ratio and the emission luminance of the organic EL element, according to the embodiment. The abscissa in  FIG. 8  indicates the duty ratio (%) and the ordinate indicates a relative luminance at a time when the emission luminance of the organic EL element OLED in the case where the duty ratio is 50% is set at 1. As shown in  FIG. 8 , the duty ratio and the luminance have a substantially proportional relationship. When the duty ratio is 100%, the relative luminance of the organic EL element OLED is 2. When the duty ratio is 25%, the relative luminance of the organic EL element OLED is 0.5. When the duty ratio is 75%, the relative luminance of the organic EL element OLED is 1.5. 
       FIG. 9  is an exemplary view for explaining the relationship between the duty ratio in a case where the first display mode is selected and the duty ratio in a case where the second display mode is selected, according to the embodiment. In  FIG. 9 , it is assumed that the emission luminance of the organic EL element OLED in the case where the duty ratio is 50% is the emission luminance in the first display mode, and the emission luminance of the organic EL element OLED in the case where the duty ratio is 100% is the emission luminance in the second display mode. 
     When the first display mode is selected, the luminance switching module  12  outputs from the third gate line GL 3  to the switch SW 3  the control signal  3  (first control signal) which controls the emission/non-emission of the organic EL element OLED so that the total emission time, which is the total of the emission time T 2  in which the organic EL element OLED emits light (i.e. the “ON” period of control signal  3 ), may become ½ of the display period T 1 , as shown in Example 1 or Example 2. 
     When the second display mode is selected, the luminance switching module  12  outputs the control signal  3  (second control signal), which is constantly “ON” in the display period T 1 , to the switch SW 3  from the third gate line GL 3 . 
     In  FIG. 9 , Example 3 and Example 4 correspond to the case in which the duty ratio is 75%. Specifically, in the case of applying the luminance switching method which has been described with reference to  FIG. 5 , when the display mode is switched between the first display mode with the duty ratio of 50% and the second display mode with the duty ratio of 100%, the duty ratio is temporarily set at 75%. Thereby, the luminance can be varied stepwise. 
     Next, a description is given of a luminance switching method of switching a driving voltage, which is supplied to the video signal line SG as a video signal necessary for driving the organic EL element OLED, between the first display mode and the second display mode, as a luminance switching method which is applicable to the above-described pixel circuit CT. 
       FIG. 10  is an exemplary view for explaining the relationship between the driving voltage, which is supplied to the pixel circuit via the video signal line, and the emission luminance of the organic EL element, according to the embodiment. In  FIG. 10 , the abscissa indicates the driving voltage, and the ordinate indicates the emission luminance of the organic EL element OLED. In the case where the pixel circuit CT is of the voltage write type, the pixel circuit CT executes voltage-current conversion, and, as a result, the driving voltage and emission luminance have a substantially proportional relationship. 
     When the driving voltage is V 0 , the emission luminance of the organic EL element OLED is L 0 . When the driving voltage is V 1  (=2*V 0 ), the emission luminance of the organic EL element OLED is L 1  (=2*L 0 ). In this case, for example, the emission luminance of the organic EL element OLED, which corresponds to the maximum gray level in the case where the first display mode is selected, is set at L 0 , and the emission luminance of the organic EL element OLED, which corresponds to the maximum gray level in the case where the second display mode is selected, is set at L 1 . 
       FIG. 11  is an exemplary view for explaining a relationship between a gradation and a driving voltage, according to the embodiment. In  FIG. 11 , the abscissa indicates the gradation, and the ordinate indicates the driving voltage. The driving voltage at the minimum gray level is a low-potential voltage VSS or a ground potential GND.  FIG. 11  shows a first tone curve A having a gamma with which the driving voltage at the maximum gray level is V 0 , and a second tone curve B having a gamma with which the driving voltage at the maximum gray level is V 1 . In this case, the maximum gray level in the first tone curve A is equal to the maximum gray level in the second tone curve B. 
     When the first display mode is selected, use can be made of a first voltage range between the driving voltage VSS or GND at the minimum gray level and the driving voltage V 0  at the maximum gray level. The driving voltage in the first voltage range is output to the driving transistor DRT in accordance with each of the gray levels. When the second display mode is selected, use can be made of a second voltage range between the driving voltage VSS or GND at the minimum gray level and the driving voltage V 1  at the maximum gray level. The driving voltage in the second voltage range is output to the driving transistor DRT in accordance with each of the gray levels. 
       FIG. 12  is an exemplary view showing the structure of the control circuit according to the embodiment. 
     The control circuit  10 , as described above, is configured to include the mode select module  11  and luminance switching module  12 . The luminance switching module  12  comprises a D/A converter  21  which converts input digital-format video data to an analog-format voltage, a voltage division circuit  22 , an amplifier  23 , a first reference voltage source PV 0  and a second reference voltage source PV 1 . The first reference voltage source PV 0  supplies a first reference voltage V 0  of a higher potential than the low-potential voltage VSS or ground potential GND. The second reference voltage source PV 1  supplies a second reference voltage V 1  of a higher potential than the first reference voltage V 0 . The voltage division circuit  22  divides a voltage between the low-potential voltage VSS or ground potential GND and the first reference voltage V 0  or second reference voltage V 1 , which has a higher potential than the low-potential voltage VSS or ground potential GND. 
     When the first display mode is selected by the mode select module  11 , this luminance switching module  12  selects the first reference voltage V 0  as the driving voltage at the maximum gray level. Thereby, the first voltage range between the low-potential voltage VSS or ground potential GND and the first reference voltage V 0  can be used, and the luminance switching module  12  outputs a driving voltage in the first voltage range, which corresponds to the gradation of video data, as a first video signal to the driving transistor DRT via the video signal line SG. When the driving voltage V 0  corresponding to the maximum gray level is output to the driving transistor DRT, the organic EL element OLED emits light with the emission luminance L 0 . 
     When the second display mode is selected by the mode select module  11 , the luminance switching module  12  selects the second reference voltage V 1  as the driving voltage at the maximum gray level. Thereby, the second voltage range between the low-potential voltage VSS or ground potential GND and the second reference voltage V 1  can be used, and the luminance switching module  12  outputs a driving voltage in the second voltage range, which corresponds to the gradation of video data, as a second video signal to the driving transistor DRT via the video signal line SG. When the driving voltage V 1  corresponding to the maximum gray level is output to the driving transistor DRT, the organic EL element OLED emits light with the emission luminance L 1 . In this manner, by adjusting the maximum driving voltage, the compression and expansion of the dynamic range of the driving voltage are enabled. 
     In the case of applying the luminance switching method which has been described with reference to  FIG. 5 , when the display mode is switched between the first display mode in which the driving voltage corresponding to the maximum gray level is V 0  and the second display mode in which the driving voltage corresponding to the maximum gray level is V 1 , the driving voltage corresponding to the maximum gray level is varied stepwise between V 0  and V 1 . 
       FIG. 13  is an exemplary view for describing another relationship between the gradation and the driving voltage, according to the embodiment. In  FIG. 13 , the abscissa indicates the gradation, and the ordinate indicates the driving voltage.  FIG. 13  shows a first tone curve A having a gamma with which the driving voltage at a maximum gray level t 0  is V 0 , and a third tone curve C having a gamma with which the driving voltage at a maximum gray level t 1 , which is higher than t 0 , is V 1  which is higher than V 0 . In the case where the maximum gray level t 1  of the third tone curve C corresponds to double the maximum gray level t 0  of the first tone curve A, the driving voltage V 1  is almost double the driving voltage V 0 . If the steps of the gradation of the third tone curve C are equal to those of the gradation of the first tone curve A, the third tone curve C is identical to the second tone curve B shown in  FIG. 11 . 
     As indicated by the third tone curve C, by increasing the number of gray levels in accordance with the expansion of the dynamic range of the driving voltage, an image with a higher image quality can be displayed. 
     Next, a description is given of a more concrete luminance switching method in the case where the pixel circuit CT of the current write type is applied. An overlapping description with the above-described case of the voltage write type is omitted here. 
       FIG. 14  is an exemplary circuit diagram showing the pixel circuit which is of the current write type, according to the embodiment. 
     The pixel circuit CT comprises a driving transistor DRT which controls the driving of the organic EL element OLED, three switches SW 1 , SW 2  and SW 3 , and a storage capacitance element CS. The three switches SW 1 , SW 2  and SW 3  and the driving transistor DRT are composed of p-channel thin-film transistors. 
     The gate electrode of the switch SW 1  is connected to a first gate line GL 1 , and the source electrode thereof is connected to a video signal line SG. A control signal  1 , which controls ON/OFF of the switch SW 1 , is supplied to the first gate line GL 1 . A video signal is supplied to the video signal line SG. The source electrode of the driving transistor DRT is connected to a power line P, and the drain electrode thereof is connected to the switch SW 3 . The storage capacitance element CS is formed between the gate electrode and source electrode of the driving transistor DRT. 
     The switch SW 2  is connected between the switch SW 1  and the gate electrode of the driving transistor DRT. The gate electrode of the switch SW 2  is connected to the first gate line GL 1 . The gate electrode of the switch SW 3  is connected to a third gate line GL 3 . A control signal  3 , which controls ON/OFF of the switch SW 3 , is supplied to the third gate line GL 3 . The source electrode of the switch SW 3  is connected to the driving transistor DRT, and the drain electrode thereof is connected to the organic EL element OLED. The switch SW 3  corresponds to an output switch which controls emission/non-emission of the organic EL element OLED. 
     Next, a description is given of a luminance switching method of switching the duty ratio between the first display mode and the second display mode, as a luminance switching method which is applicable to the above-described pixel circuit CT. 
       FIG. 15  is an exemplary timing chart for explaining the basic operation in the pixel circuit shown in  FIG. 14 , according to the embodiment. One horizontal period includes a write period. The write period corresponds to a period in which the control signal  3 , which is supplied to the third gate line GL 3 , is “OFF” and the control signal  1 , which is supplied to the first gate line GL 1 , is “ON” (i.e. the period in which the switch SW 3  is “OFF” and the switch SW 1  and switch SW 2  are “ON”), and in this write period a video signal is written in the pixel circuit CT. Subsequently, the control signal  1  and control signal  3  are rendered “OFF”, and the video signal written in the pixel circuit CT is retained for a predetermined time period. 
     Thereafter, in a period in which the control signal  1  is “OFF” and the control signal  3  is rendered “ON”, electric current, which is controlled by the driving transistor DRT, is supplied to the organic EL element OLED, and the organic EL element OLED emits light with a predetermined luminance. In the example shown in  FIG. 15 , in the display period T 1 , the control signal  3  is constantly “ON” and the organic EL element OLED constantly emits light. In other words, the display period T 1  is equal to the emission period T 2  in which the organic EL element OLED emits light, and this corresponds to the case in which the duty ratio is 100%. 
     As has been described above, the duty ratio and the emission luminance of the organic EL element OLED have a substantially proportional relationship. Thus, the same luminance switching as in the case of the voltage driving method can be performed by using such setting that the emission luminance of the organic EL element OLED in the case where the duty ratio is 50% is the emission luminance in the first display mode, and the emission luminance of the organic EL element OLED in the case where the duty ratio is 100% is the emission luminance in the second display mode. 
     Next, a description is given of a luminance switching method of switching a driving current, which is supplied to the video signal line SG as a video signal necessary for driving the organic EL element OLED, between the first display mode and the second display mode, as a luminance switching method which is applicable to the above-described pixel circuit CT. 
       FIG. 16  is an exemplary view for explaining the relationship between the driving current, which is supplied to the pixel circuit via the video signal line, and the emission luminance of the organic EL element, according to the embodiment. In  FIG. 16 , the abscissa indicates the driving current, and the ordinate indicates the emission luminance of the organic EL element OLED. In the case where the pixel circuit CT is of the current write type, the driving current and emission luminance have a substantially proportional relationship. 
     When the driving current is I 0 , the emission luminance of the organic EL element OLED is L 0 . When the driving current is I 1  (=2*I 0 ), the emission luminance of the organic EL element OLED is L 1  (=2*L 0 ). In this case, for example, the emission luminance of the organic EL element OLED, which corresponds to the maximum gray level in the case where the first display mode is selected, is set at L 0 , and the emission luminance of the organic EL element OLED, which corresponds to the maximum gray level in the case where the second display mode is selected, is set at L 1 . 
       FIG. 17  is an exemplary view for explaining the relationship between the gradation and the driving current, according to the embodiment. In  FIG. 17 , the abscissa indicates the gradation, and the ordinate indicates the driving current.  FIG. 17  shows a fourth tone curve D having a gamma with which the driving current at the maximum gray level is I 0 , and a fifth tone curve E having a gamma with which the driving current at the maximum gray level is I 1 . In this case, the maximum gray level in the fourth tone curve D is equal to the maximum gray level in the fifth tone curve E. 
     When the first display mode is selected, use can be made of a first current range of up to the driving current I 0  at the maximum gray level, and the driving current in the first current range is output to the driving transistor DRT in accordance with each of the gray levels. When the second display mode is selected, use can be made of a second current range of up to the driving current I 1  at the maximum gray level, and the driving current in the second current range is output to the driving transistor DRT in accordance with each of the gray levels. 
       FIG. 18  is an exemplary view showing the structure of the control circuit according to the embodiment. 
     The control circuit  10 , as described above, is configured to include the mode select module  11  and luminance switching module  12 . The luminance switching module  12  comprises a D/A converter  21  which converts input digital-format video data to an analog-format voltage, a voltage division circuit  22 , an amplifier  23 , a first reference current source PI 0  and a second reference current source PI 1 . The first reference current source PI 0  supplies a first reference current I 0 . The second reference current source PI 1  supplies a second reference current I 1  which is higher than the first reference current I 0 . The voltage division circuit  22  divides a voltage in a range between the low-potential voltage VSS or ground potential GND and a high-potential voltage VDD. 
     When the first display mode is selected by the mode select module  11 , this luminance switching module  12  selects the first reference current source PI 0 . Thereby, the first current range of up to the maximum current I 0  of the first reference current source PI 0  can be used, and the luminance switching module  12  outputs a driving current in the first current range, which corresponds to the gradation of video data, as a first video signal to the driving transistor DRT via the video signal line SG. When the driving current I 0  corresponding to the maximum gray level is output to the driving transistor DRT, the organic EL element OLED emits light with the emission luminance L 0 . 
     When the second display mode is selected by the mode select module  11 , the luminance switching module  12  selects the second reference current source PI 1 . Thereby, the second current range of up to the maximum current I 1  of the second reference current source PI 1  can be used, and the luminance switching module  12  outputs a driving current in the second current range, which corresponds to the gradation of video data, as a second video signal to the driving transistor DRT via the video signal line SG. When the driving current I 1  corresponding to the maximum gray level is output to the driving transistor DRT, the organic EL element OLED emits light with the emission luminance L 1 . In this manner, by adjusting the maximum driving current, the compression and expansion of the dynamic range of the driving current are enabled. 
     In the case of applying the luminance switching method which has been described with reference to  FIG. 5 , when the display mode is switched between the first display mode in which the driving current corresponding to the maximum gray level is I 0  and the second display mode in which the driving current corresponding to the maximum gray level is I 1 , the driving current corresponding to the maximum gray level is varied stepwise between I 0  and I 1 . Thereby, a sharp variation in luminance can be relaxed. 
     As has been described with reference to  FIG. 13 , the number of gray levels may be increased in accordance with the expansion of the dynamic range of the driving current. 
     Second Embodiment  
     In a second embodiment, the structure of the display panel of the organic EL display device is disclosed in greater detail, and the duty ratio is switched by a luminance switching method which is different from the luminance switching method which has been described in the first embodiment. The same parts as in the first embodiment are denoted by like reference numerals, and a detailed description thereof is omitted here. 
       FIG. 19  is an exemplary plan view which schematically shows the structure of an organic EL display device according to the embodiment, which adopts an active matrix driving method. 
     The organic EL display device comprises a display panel  1  and a controller  3  which controls the display operation of the display panel  1 . 
     The display panel  1  comprises a plurality of pixels PX, a plurality of scanning signal lines GL 1   a  to GLMa, GL 1   b  to GLMb, and GL 1   c  to GLMc, a plurality of video signal lines SG 1  to SGN, a scanning signal line driver YDR, and a video signal line driver XDR. 
     The pixels PX are arranged in a matrix of M×N on a light-transmissive, insulative support substrate such as a glass plate. Each pixel PX includes an organic EL element OLED which is a self-luminous element, and includes a pixel circuit CT. The pixel circuit CT shown in  FIG. 19  is a pixel circuit of a voltage write type. Since the details of the operation of the pixel circuit CT have already been described, an overlapping description is omitted here. 
     The scanning signal lines GL 1   a  to GLMa, GL 1   b  to GLMb, and GL 1   c  to GLMc extend along the rows of the pixels PX. The video signal lines SG 1  to SGN extend in a direction substantially perpendicular to the rows of the pixels PX. The scanning signal line driver YDR sequentially drives the scanning signal lines GL 1   a  to GLMa, GL 1   b  to GLMb, and GL 1   c  to GLMc. The video signal line driver XDR drives the video signal lines SG 1  to SGN. 
     The controller  3  is formed on a printed board which is disposed outside the display panel  1 , and controls the operations of the scanning signal line driver YDR and video signal line driver XDR. The controller  3  receives a digital video signal, a sync signal, a glasses operation determination signal and a 3D determination signal, which are supplied from the outside. The controller  3  generates, based on the sync signal, a vertical scan control signal which controls a vertical scan timing, and a horizontal scan control signal which controls a horizontal scan timing, and supplies the vertical scan control signal and horizontal scan control signal to the scanning signal line driver YDR and video signal line driver XDR. The controller  3  supplies the digital video signal to the video signal line driver XDR in sync with the horizontal and vertical scan timings. 
     The video signal line driver XDR converts, in each horizontal scanning period, the digital video signal to an analog-format signal under the control of the horizontal scan control signal, and supplies resultant video signals Vsig to the plural video signal lines SG 1  to SGN in parallel. The scanning signal driver YDR outputs scanning signals to the scanning signal lines GL 1   a  to GLMa, GL 1   b  to GLMb, and GL 1   c  to GLMc under the control of the vertical scan control signal. The scanning signal lines GL 1   a  to GLMa and GL 1   b  to GLMb are select scanning lines for selecting pixel circuits on a row-by-row basis. The scanning signal lines GL 1   c  to GLMc are light-control scanning lines for controlling emission periods of organic EL elements. 
     The controller  3 , video signal line driver XDR and scanning signal line driver YDR in the second embodiment correspond to the control circuit  10  in the first embodiment. 
       FIG. 20A  to  FIG. 20E  are exemplary diagrams for explaining methods of controlling the luminance in cases of displaying a video signal of a 2D image corresponding to a first operational frequency (60 Hz) and a video signal of a 3D image corresponding to a second operational frequency (120 Hz). In  FIG. 20A  to  FIG. 20E , hatched rectangular parts indicate emission periods in which the organic EL elements OLED emit light. Specifically, the emission period is a period in which the switch SW 3  of each pixel circuit CT is turned “ON” by the scanning signal which is output from the scanning signal line driver YDR via the scanning signal lines GL 1   c  to GLMc. 
       FIG. 20A  shows Example 1 in which the time interval of the scanning signal, which is output for light emission, is decreased. When a 2D image is displayed, two emission periods are provided in a display period (T 1 ) of 60 Hz. On the other hand, when a 3D image is displayed, two emission periods are provided in a display period (T 1 ) of 120 Hz. In order to equalize the total luminance in the respective display periods, the interval of emission start is varied, with a single emission time being the same. As a result, the duty ratio is 25% in the display of the 2D image, while the duty ratio is 50% in the display of the 3D image. 
       FIG. 20B  shows Example 2 in which the time interval of the scanning signal, which is output for light emission, is decreased. Example 2 differs from Example 1 in that the duty ratio in the display of the 2D image is 20%, while the duty ratio in the display of the 3D image is 40%. However, the luminance control method in Example 2 is the same as that in Example 1 shown in  FIG. 20A . 
       FIG. 20C  shows Example 3 in which the time interval of the scanning signal, which is output for light emission, is decreased. In Example 3, the duty ratio in the display of the 2D image is 20%, while the duty ratio in the display of the 3D image is 40%. This control of the duty ratio is the same as in Example 2. The method of the luminance control is the same as in Example 2 shown in  FIG. 20B . However, Example 3 differs from Example 2 in that four emission periods, in each of which the light emission time is decreased, are provided. 
       FIG. 20D  shows an example in which the light emission time of the scanning signal, which is output for light emission, is increased. In the display of the 2D image, four emission periods are provided and the duty ratio is set at 40%. In the display of the 3D image, two emission periods are provided, the light emission time in each emission period is doubled, and the duty ratio is set at 80%. 
       FIG. 20E  shows an example in which the light emission time and the interval of emission start of the scanning signal, which is output from for light emission, are varied. In the display of the 2D image, four emission periods are provided and the duty ratio is set at 20%. In the display of the 3D image, four emission periods are provided, the light emission time in each emission period is decreased, and the duty ratio is set at 30%. 
     As has been described above, in the second embodiment, the number of emission periods and the light emission time in the display period are controlled, and thereby a desired duty ratio is realized. The control of the number of emission periods and the light emission time can be realized by the cooperation between the controller  3  and the scanning signal line driver YDR. 
     In the second embodiment, the voltage write method is adopted in the pixel circuit CT. Alternatively, the current write method may be adopted in the pixel circuit CT. The structural elements of the organic EL display device disclosed in the first embodiment and the structural elements of the organic EL display device disclosed in the second embodiment may be combined, as needed. 
     According to the above-described embodiments, the switching of the luminance of the organic EL element OLED can be realized by a simpler structure, compared to the switching of the luminance of a liquid crystal display panel. Specifically, in the case of the liquid crystal display panel, it is necessary to establish synchronism with the backlight which illuminates the liquid crystal display panel, while considering the response speed of liquid crystal molecules and applying a driving method (e.g. black insertion driving) matching with the characteristics of the liquid crystal molecules. On the other hand, since the organic EL element OLED is self-luminous, an illumination unit, such as a backlight, is needless, and thus the synchronism with the illumination unit is needless. Moreover, the luminance of the organic EL element OLED can easily be adjusted by the duty ratio of the organic EL element OLED or the video signal (driving current or driving voltage) which is written in the pixel circuit CT. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.