Abstract:
Luminescent portions correspondent to each gradation are arranged point-symmetrically with one another around a prescribed position provided at the center, thereby forming a unit pixel element including a plurality of luminescent elements. Such a structure allows provision of a display device wherein a luminous center does not shift for each gradation. Accordingly, when the brightness of the displayed images is changed, unfavorable shifting of display positions does not take place. The present invention thus solves defects related to the picture quality, such as flickering of images, or an impression of unnatural display or fatigue caused to the viewer.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to a display device, especially to an improved technology of gradation display. 
     2. Description of the Related Art 
     TFT-ELDs, namely thin-film-transistor (TFT) driven electoroluminescent displays, which include electoroluminescent elements (EL elements) driven and controlled by thin film transistors, are considered as future potential displays due to their light weight, small size, high resolution, wide visual field, low electric consumption, etc. 
     FIG. 4 is a circuit diagram of a conventional TFT-ELD, and FIG. 5 is a cross section of such TFT-ELD. FIG. 4 shows a unit pixel  11  of the TFT-ELD, a scanning line  12 , a signal line  13 , current supplying line  14 , a retention capacitor  15 , a selective transistor  16 , a driving transistor  17 , and an EL element  15 . As shown in FIG. 5, the driving transistor  17  for adjusting light emission intensity (gradation) of the EL element  18  is formed on a glass substrate  10 . A drain electrode of the driving transistor  17  is connected to a cathode (transparent electrode)  21  of the EL element  18 , and a source electrode is connected to the analog signal supply line  14 . The EL element  18  is formed of the anode  21 , a luminescent layer  22 , and an cathode  23 . The EL element  18  may be a inorganic electroluminescent element, a low-molecular organic electroluminescent element, or a high-molecular organic electroluminescent element. 
     The selective transistor  16  includes a gate electrode connected to the scanning line  12 , a source electrode connected to a signal line  13 , and a drain electrode connected to a gate electrode of the driving transistor  17 . The retention capacitor  15  is provided between the analog signal supplying line  14  and the source electrode of the selective transistor  16 . 
     In order to cause the EL element  18  to emit light in the aforementioned structure, the scanning line  12  and the signal line  13  are set at level “H”, and current is conducted between the drain and the source of the selective transistor  16 , whereby the driving transistor  17  is on state. An analog signal supplied from the analog supplying line  14  in this condition is delivered to the retention capacitor  15  and alters the conductance of the driving transistor  17 . As a result, the EL element  18  emits light with light emission intensity pursuant to the analog signal, thereby accomplishing gradations of light emission intensity. 
     However, as a problem of the above-described. structure, resolution of the picture lowers due to the EL element  18  included in each pixel emitting light with unequal light emission intensity, especially in the middle gradation, because of the difference in the transistor properties of the driving transistor  17 . 
     In order to solve this problem, the applicant of the present invention suggested in Japanese Patent Laid-Open Publication No. HEI 11-73158 a technology of displaying respective gradations by controlling on/off states of light emission of EL elements and changing the luminous area for each gradation. FIG. 6 is a circuit diagram of the TFT-ELD disclosed in said Laid-Open Publication. FIG. 6 shows an EL element included in each pixel, which is formed of EL elements  18 - 1  and  18 - 2 . Such structure allows display of four gradations by controlling on/off states of EL elements  18 - 1  and  18 - 2  respectively via a 2-bit signal line formed of signal lines  13 - 1  and  13 - 2 . More specifically, there are: gradation “0”, where neither EL element  18 - 1  nor  18 - 2  emits light; gradation “1”, where only EL element  18 - 1  emits light; gradation “2”, where only EL element  19 - 2  emits light; and gradation “3”, where both EL elements  18 - 1  and  18 - 2  emit light. Luminous areas of EL element  18 - 1  and EL element  18 - 2  are in a ratio of 1:2. 
     As shown in FIG. 7, in the structure above, signals S, D 1 , and D 2  are respectively supplied to the scanning line  12 , signal line  13 - 1  and signal line  13 - 2 . When signal S is set at level “H”, current is conducted between the drain and the sources of selective transistors  16 - 1  and  16 - 2 . In FIG. 7, gradation “1” is obtained when signal S is set at level “H”, signal D 1  at level RHO, and signal D 2  at level “L”. As a consequence, driving transistor  17 - 1  is turned on, and transistor  17 - 2  is turned off, whereby only EL element  18 - 1  emits light. Furthermore, in order to realize gradation “2”, signal S should be set at level “H”, signal D 1  at level “L”, and signal D 2  at level “H”. By doing so, driving transistor  17 - 2  is turned on and transistor  17 - 1  is turned off, and consequently, only EL element  18 - 2  emits light. 
     In this method, driving transistors  17 - 1  and  17 - 2  are to be regarded as either almost completely on state or almost completely off state. When driving transistors  17 - 1  and  17 - 2  are on state,resistance is negligibly small compared to the resistance of driving transistors  18 - 1  and  18 - 2 , such that the amount of current conducted through driving transistors  17 - 1 ,  17 - 2 ,  18 - 1  and  18 - 2  depends substantially on the resistance of driving transistors  18 - 1  and  18 - 2  alone. Accordingly, light emission intensity is never uneven due to the difference in the transistor properties of driving transistors  18 - 1  and  18 - 2 . Furthermore, when driving transistors  17 - 1  and  17 - 2  are off state, the voltage applied to EL elements  18 - 1  and  18 - 2  will be smaller than the threshold voltage, and driving transistors  18 - 1  and  19 - 2 , will not emit light at all. Therefore, also in this case, the light emission intensity of EL elements  18 - 1  and  18 - 2  is never uneven by the difference in the transistor properties of driving transistors  18 - 1  and  18 - 2 . 
     However, as a disadvantage of the aforementioned structure, the luminous center (the average position of the luminescent portion) shifts for each gradation and visibility is thereby decreased. Characteristics of such disadvantage will be explained with reference to FIGS. 8A-D. FIG. 8C, for example, shows a luminous center  40  of the unit pixel element  11 . The EL element  18 - 1  shown with oblique lines means that no light is emitted, and the EL element  18 - 2  shown in white means that light is emitted. In FIG. 8A, the EL elements  18 - 1 ,  18 - 2  do not emit light. In FIG. 8B, only EL element  18 - 1  emits light. In FIG. 8C, only EL element  18 - 2  emits light. Finally, in FIG. 8D, both EL elements  18 - 1 ,  18 - 2  emit light. It is clear from these drawings that the position of the luminous center  40  changes for each gradation. As a consequence, when the brightness of a displayed image is changed, the position of the image shifts unfavorably. Furthermore, if the displayed image is actually observed here, the displayed image will be seen to flicker, causing an impression of unnatural display or fatigue to the viewer. 
     SUMMARY OF THE INVENTION 
     Accordingly, the object of the present invention is to overcome such disadvantage and to provide a display device wherein a luminous center does not shift for each light emission gradation. 
     In the present invention, in order to achieve said object, a unit pixel is formed of multiple EL elements whose luminescent portions corresponding to each gradation are arranged point-symmetrically with one another with respect to a prescribed point. Such structure allows provision of a display device wherein the position of a luminous center does not change for each gradation. “Prescribed position” here means, for example, a luminous center of the EL element upon realizing the gradation of minimum luminance. 
     Furthermore, each electroluminescent element is preferably configured to have a state of “emission” or “non-emission”. By controlling on/off of the multiple Aluminescent elements, it is possible to prevent uneven aluminance caused by difference in the properties of luminescent elements. In order to achieve the structure above, electroluminescent elements may, for example, be used as luminescent elements, so that thin-film transistors may control the on/off states of light emission by the luminescent elements. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A-D are explanatory diagrams showing the emission state of the respective EL elements forming a unit pixel in the TFT-ELD according to embodiment 1; 
     FIGS. 2A-D are explanatory diagrams showing a unit pixel in the TFT-ELD according to embodiment 2; 
     FIGS. 3A-D are explanatory diagrams showing the emission state of the respective EL elements forming a unit pixel in the TFT-ELD according to embodiment 2; 
     FIG. 4 is a circuit diagram of a unit pixel in a conventional TFT-ELD. 
     FIG. 5 is a cross section of a unit pixel in a conventional TFT-ELD. 
     FIG. 6 is a circuit diagram of a unit pixel in a conventional TFT-ELD; 
     FIG. 7 is a timing chart indication a scanning line and a signal line of a conventional TFT-ELD; and 
     FIGS. 8A-D are explanatory diagrams showing the light emission state of the EL elements forming a unit pixel of a conventional TFT-ELD. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     (Embodiment 1) 
     FIGS. 1A-D show a unit pixel  11  included in a display device according to the present invention. Each unit pixel has EL elements  18 - 10 ,  18 - 21  and  18 - 22 , and is a 2 bit-4 gradation display. In FIG. 1A, for example, EL element  18 - 10  is an EL element for 0-bit display. The on/off states of EL elements  18 - 21  and  18 - 22  are simultaneously controlled by the same driving transistor, and EL element  18 - 21  is a first EL element for 1-bit display and EL element  18 - 22  is a second EL element for 1-bit display. Each EL element is driven and controlled by two scanning lines (for 0-bit and 1-bit display) which are not shown. 
     Furthermore, although FIGS. 1A-D show only the unit pixel element  11 , in reality the unit pixel elements  11  are arranged in a matrix over the entire screen of the display device. 
     FIG. 1A shows emission by none of the EL elements (gradation “0”); FIG.  1 B—emission only by EL element  18 - 10  (gradation “1”); FIG.  1 C—emission only by EL elements  18 - 21  and  18 - 22  (gradation “2”); and FIG.  1 D—emission by all EL elements  18 - 21 ,  18 - 10  and  18 - 22  (gradation “3”). 
     As shown in FIGS. 1A-D, the luminous center  40  for each gradation is located at the same position as the luminous center of the luminescent portion (EL element  18 - 10 ), and configured such that it does not shift for each gradation. In other words, the luminescent portion corresponding to gradation “2” is located point-symmetrically with respect to the luminescent portion corresponding to gradation “1”. Furthermore, the luminescent portion corresponding to gradation “3” is located point-symmetrically with respect to the luminescent portion corresponding to gradation “1”. By arranging the luminescent portions point-symmetrically around a prescribed point provided at the center, easily obtained is a structure which prevents shifting of the luminous center  40 . Accordingly, even when the brightness of a displayed image is changed, unfavorable shifting of the displayed position does not take place. Therefore, the present invention solves disadvantages related to the picture quality, such as flickering of images, or impression of unnatural display or fatigue caused to the viewer. 
     Furthermore, although respective EL elements are shaped in quadrilaterals (squares) in FIGS. 1A-D, they may be configured as circles or ovals. Moreover, by making the respective areas of EL elements  18 - 10 ,  18 - 21 , and  18 - 22  uniform, light emission intensity for respective gradations may be increased or decreased linearly. 
     (Embodiment 2) 
     FIGS. 2A-D show a unit pixel  11  included in the display device. Each unit pixel is formed of EL elements  18 - 10 ,  18 - 21 ,  18 - 22 ,  18 - 31 ,  18 - 32 ,  18 - 33  and  18 - 34 , and is a 3 bits-8 gradation display. In FIGS. 2A-D, EL element  18 - 10  is an EL element for 0-bit display. The on/off states of EL elements  18 - 21  and  18 - 22  are simultaneously controlled by the same driving transistor, and EL element  18 - 21  is a first EL element for 1-bit display and EL element  18 - 22  is a second EL element for 1-bit display. Similarly, the on/off states of EL elements  18 - 31 ,  18 - 32 ,  18 - 33  and  18 - 34  are simultaneously controlled by the same driving transistor. EL element  18 - 31  is a first EL element for 2-bit display, EL element  18 - 32  is a second EL element for 2-bit display, EL element  18 - 33  is a third EL element for 2-bit display, and EL element  18 - 34  is a fourth EL element for 2-bit display. Each EL element is driven and controlled by three scanning lines (for 0 to 2 bit display) which are not shown. 
     Furthermore, although FIGS. 2A-D only show the unit pixel element  11 , in reality the unit pixel elements  11  are arranged in a matrix over the entire screen of the display device. 
     FIG. 2A shows that none of the EL elements emit light (gradation “0”); FIG.  2 B—emission only by 0-bit display EL element  18 - 10  (gradation “1”); FIG.  2 C—emission by only 1-bit display EL elements  18 - 21  and  18 - 22  (gradation “2”); and FIG. 2D, emission by 0-bit and 1-bit display EL elements  18 - 10 ,  18 - 21  and  18 - 22  (gradation “3”). Furthermore, FIG. 3A shows emission of only 2-bit display EL elements  18 - 31 ,  18 - 32 ,  18 - 33  and  18 - 34  (gradation “4”); FIG.  3 B—emission of only 0-bit and 2-bit display EL elements  18 - 10 ,  18 - 31 ,  18 - 32 ,  18 - 33  and  18 - 34  (gradation “5”); FIG.  3 C—emission of only 1-bit and 2-bit display EL elements  18 - 21 ,  18 - 22 ,  18 - 31 ,  18 - 32 ,  18 - 33  and  18 - 34  (gradation “6”); and FIG. 3D emission of all 0-bit, 1-bit and 2-bit display EL elements  18 - 10 ,  18 - 21 ,  18 - 22 ,  18 - 31 ,  18 - 32 ,  18 - 33  and  18 - 34  (gradation “7”). 
     As shown in FIGS. 2A-D and  3 A-D, the luminous center  40  for each gradation is located at the same position as the center point of the luminescent portion (EL element  18 - 10 ), and structured so as to avoid shifting for each gradation. In other words, the luminescent portion corresponding to gradation “2” is located point-symmetrically with respect to the luminescent portion corresponding to gradation “1”. The luminescent portion corresponding to gradation “3” is located point-symmetrically with respect to the luminescent portion corresponding to gradation “1”, and the luminescent portion corresponding to gradation “7” is located point-symmetrically with respect to the luminescent portion corresponding to gradation “1”. By arranging luminescent portions point-symmetrically around a prescribed point provided at the center, easily obtained is a configuration which prevents shifting of the luminous center  40 . Accordingly, even when the brightness of a displayed image is changed, unfavorable shifting of the display position does not take place. Therefore, the present invention solves disadvantages related to the picture quality, such as flickering of images, or impression of unnatural display or fatigue caused to the viewer. 
     Furthermore, although respective EL elements are shaped in quadrilaterals (squares) in FIGS. 2A-D, they may be configured as circles or ovals. Moreover, by making the respective areas of EL elements  18 - 10 ,  18 - 21 , and  18 - 22 , for example, uniform, light emission intensity for respective gradations may be increased or decreased linearly. 
     Furthermore, although the present embodiment is explained with eight gradations, different gradations may be obtained by adjusting the number of EL elements. The display device according to the present invention may be used for video cameras, digital cameras, car stereos, video CD players, portable terminals, laptop personal computers, etc.