Abstract:
It is an object of the present invention to provide an apparatus and method for evaluating an organic EL display, with which there is a simple drive circuit used for testing an organic EL display  10 , evaluation of high reliability can be achieved, and an evaluation of the organic EL display  10  itself is performed prior to the installation of finished product drive circuits to the organic EL display  10 , which makes it possible to suppress the decrease in yield caused by dealing with defective products due to the evaluation results. It was noticed that if the drive (testing) of a pixel  11  is performed during the discharge of the previous pixel  11  after the supply of drive current to that previous pixel  11 , and if the difference between the drive current and discharge current values of the organic EL pixel  14  of the pixel  11  is under a specific level, then it is possible to decide that the pixels constituted by the various organic EL elements are operating normally, and the present invention is characterized in that pixel defects are detected by detecting the difference between drive current and discharge current values for every pixel  11  constituted by an organic EL element  1.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to an apparatus and method for evaluating an organic electroluminescence display (hereinafter referred to as an “organic EL display”) that makes use of an organic substance for its light-emitting substance, and more particularly relates to an apparatus and method for evaluating an organic EL display used in any of various types of display devices, such as the display panels of cellular telephones, the display panels of car audio systems, display panels for still or moving pictures, and the image displays of digital cameras.  
           [0003]    2. Description of the Related Art  
           [0004]    Organic electroluminescence elements (hereinafter referred to as “organic EL elements”) have been the subject of considerable research and practical application in recent years.  
           [0005]    [0005]FIG. 7 is an enlarged cross section of the main components of a conventional type of organic EL element  1 . This organic EL element  1  comprises a glass substrate  2 , an anode  3 , a hole transport layer  4 , an electron transport light-emitting layer  5  and a cathode  6 . Direct current supplied by applying a specific voltage between the anode  3  and the cathode  6  from a DC power supply  7 .  
           [0006]    A transparent electrode made of ITO (Indium Tin Oxide) or the like is employed for the anode  3 , a diamine dielectric (TPAC) is employed for the hole transport layer  4 , an aluminum complex (Alq) is employed for the electron transport light-emitting layer  5 , and the carrier rebonding rate is raised by laminating materials with different carrier transport capabilities. Magnesium (Mg), aluminum (Al), or the like is employed for the cathode  6 .  
           [0007]    With an organic EL element  1  structured such as this, the carriers (hole and electron charges) injected from the anode  3  and the cathode  6  are confined in the organic layer of the electron transport light-emitting layer  5 , the carrier rebonding efficiency rises sharply, and a high level of brightness (over 1000 cd/m 2 ) can be obtained at a voltage of 10 volts or less.  
           [0008]    Such elements are therefore expected to find use in cellular telephones, car audio systems, household electronics, and so forth.  
           [0009]    [0009]FIG. 8 is a circuit diagram illustrating one pixel  11  in an active matrix type of organic EL display  10 . The organic EL display  10  comprises a plurality of selection lines VG (scanning lines) and signal lines (VD) arranged in a matrix, with the pixel  11  connected at the intersection of these lines.  
           [0010]    The pixel  11  comprises a switching circuit  12 , a constant current circuit  13 , and an organic EL pixel  14  constituted by the above-mentioned organic EL element  1 . The organic EL pixel  14  emits light when supplied with a constant current by the application of a fairly constant specific voltage from a voltage supply line VLC to the constant current circuit  13 .  
           [0011]    The pixel  11  has been disclosed in Japanese Laid-Open Patent Application H5-107561 and elsewhere; for example, as shown in FIG. 9, a first transistor  15  consisting of a thin film transistor (TFT) or the like is employed as the switching circuit  12 , and a second transistor  16 , similarly made of TFT or the like, and a capacitor  17  are employed as the constant current circuit  13 .  
           [0012]    The first transistor  15  is switched in order to supply a constant current to the organic EL pixel  14 .  
           [0013]    The second transistor  16  is switched by the first transistor and is connected to the organic EL pixel  14 .  
           [0014]    The capacitor  17  is charged with a specific electrical capacitance, and aids in the supply of a constant current to the organic EL pixel  14  according to the specific discharge time thereof.  
           [0015]    With a pixel  11  structured such as this, the selection of the pixel  11  is made by the first transistor  15 , the result of the selection is transmitted to the second transistor  16 , the voltage applied to the pixel  11  is controlled by the second transistor  16  and by the capacitor  17 , which is able to hold a specific electrical capacitance for a specific length of time, and a fairly constant specific voltage from the voltage supply line VLC is maintained, thereby reducing the difference in voltage between the various pixels  11 .  
           [0016]    In order to evaluate an organic EL display  10  structured such as this, in the past the organic EL display  10  was actually driven only after a drive circuit (not shown) was attached to the organic EL display  10  and everything put together in a form similar to that of an actual finished product, and the work of detecting line defects or dot defects was performed by separate image evaluation devices.  
           [0017]    Therefore, a problem was that discrepancies occurred between the various evaluation devices or in the evaluation standards, and this led to lower detection accuracy.  
           [0018]    Another method is for the drive or luminescence state of the organic EL display  10  to be visually evaluated by a human, but a problem was that there was variance in the evaluation results depending on the experience of the evaluator and how well he or she was performing on a given day.  
           [0019]    Furthermore, if an element is decided to be defective as a result of evaluation, the organic EL display  10  ends up being discarded along with the above-mentioned drive circuit parts attached to it, which is a problem in that it is wasteful. This also results in a waste of the time spent in evaluation.  
           [0020]    Organic EL elements have been disclosed in the above-mentioned Japanese Laid-Open Patent Application H5-107561, as well as in Japanese Laid-Open Patent Applications H9-260061 and H10-321367 and elsewhere.  
         SUMMARY OF THE INVENTION  
         [0021]    The present invention was conceived in light of the above problems, and it is an object thereof to provide an apparatus and method for evaluating an organic EL display, with which the drive circuit used for testing the organic EL display has a simple circuit configuration, and which yields evaluation results of high reliability.  
           [0022]    It is another object of the present invention to provide an apparatus and method for evaluating an organic EL display, with which the detection accuracy is high and it is possible to evaluate the organic EL display itself, before the finished product drive circuits have been incorporated into the organic EL display.  
           [0023]    It is another object of the present invention to provide an apparatus and method for evaluating an organic EL display, with which pixel defects of organic EL display elements can be detected by efficiently detecting, with a simple circuit configuration, the micro-current flowing to the organic EL elements.  
           [0024]    It is another object of the present invention to provide an apparatus and method for evaluating an organic EL display, with which the micro-current can be efficiently detected so that the drive current supplied to the organic EL elements for the purpose of detection does not become superposed between a number of organic EL elements.  
           [0025]    It is yet another object of the present invention to provide an apparatus and method for evaluating an organic EL display, with which it is possible to suppress the decrease in yield caused by dealing with defective products due to the evaluation results.  
           [0026]    Specifically, the present invention is the result of noticing that if signal lines and selection lines (plus voltage supply lines in the case of an active matrix system) capable of supplying drive current to the various pixels (organic EL elements) of an organic EL display are readied, if the drive (testing) of an element is performed during the discharge of the previous element after the supply of drive current to that previous element, if the difference is measured between the drive current and discharge current values of the organic EL elements, and if the difference between these current values is under a specific level, then it is possible to decide that the pixels constituted by the various organic EL elements are operating normally. The first invention is an apparatus for evaluating an organic EL display having organic EL elements as pixels, wherein the drive current and discharge current values are measured for each pixel constituted by an organic EL element, and pixel defects are detected by detecting a difference in the current values thereof.  
           [0027]    The above organic EL display can have a constant current circuit for driving the organic EL elements, and a switch for switching the voltage in order to make the constant current produced by this constant current circuit variable.  
           [0028]    The above organic EL display can have a constant current circuit such as TFT for driving the organic EL elements, and signal lines and selection lines for selecting the organic EL elements.  
           [0029]    There can be provided signal lines and selection lines for selecting the organic EL elements, and the drive current and discharge current values can be measured by switching either the signal line or the selection line for each pixel constituted by an organic EL element.  
           [0030]    There can be provided signal lines and selection lines for selecting the organic EL elements, and the signal line or the selection line can be switched for each pixel constituted by an organic EL element so that drive current is supplied to the organic EL elements and the charge stored in the organic EL elements is discharged.  
           [0031]    There can be provided a capacitor for supplying a constant current to the organic EL elements, drive voltage can be supplied to each pixel constituted by an organic EL element, and the charge stored in the capacitor can be discharged.  
           [0032]    There can be provided a first transistor that performs switching for supplying a constant current to the organic EL elements, and a second transistor that is switched by the first transistor and is connected to the organic EL elements, and the first transistor can be switched for each pixel constituted by an organic EL element, whereby the drive current is supplied to the organic EL elements over a first specific time, and the second transistor is kept in a non-conducting state over a second specific time following this first specific time.  
           [0033]    There can be provided signal lines and selection lines for selecting the organic EL elements, and voltage supply lines for supplying voltage to the organic EL elements, the signal line or the selection line can be switched for each pixel constituted by an organic EL element, in a state in which this voltage supply line is ON, and the drive current and discharge current flowing to the organic EL elements can be measured.  
           [0034]    There can be provided signal lines and selection lines for selecting the organic EL elements, the signal line or the selection line can be switched for each pixel constituted by an organic EL element, and the current supplied to the organic EL elements can be controlled and the drive current and discharge current flowing to the organic EL elements measured.  
           [0035]    The second invention is an apparatus for evaluating an organic EL display, especially applicable to an active matrix type thereof, having organic EL elements as pixels, signal lines and selection lines for selecting the organic EL elements, and voltage supply lines for supplying voltage to the organic EL elements, said evaluation apparatus having a detection voltage generation circuit that generates a detection voltage to the signal lines, selection lines, and voltage supply lines, a control signal generation circuit that generates a control signal for sequentially applying this detection voltage at a specific period to the signal lines, selection lines, and voltage supply lines, a connection switching circuit for connecting this control signal to the organic EL elements via the signal lines, selection lines, and voltage supply lines, a current detection circuit for detecting the drive current and discharge current flowing to the organic EL elements, and a defect decision circuit that decides whether the organic EL elements are defective or non-defective based on the detected current values.  
           [0036]    The third invention is an apparatus for evaluating an organic EL display, especially applicable to a simple matrix type thereof wherein, different from an active matrix type, an organic EL element is not equipped with a switching circuit and a voltage supply line, having organic EL elements as pixels, and signal lines and selection lines for selecting the organic EL elements, said evaluation apparatus having a detection current generation circuit that generates a detection current to the signal lines and selection lines, a control signal generation circuit that generates a control signal for sequentially applying this detection current at a specific period to the signal lines and selection lines, a connection switching circuit for connecting this control signal to the organic EL elements via the signal lines and selection lines, a current detection circuit for detecting the drive current and discharge current flowing to the organic EL elements, and a defect decision circuit that decides whether the organic EL elements are defective or non-defective based on the detected current values.  
           [0037]    In particular for a simple matrix type of organic EL display, there can be provided a forward voltage drop detection circuit that measures the forward voltage drop of the organic EL elements, and the defect decision circuit can decide whether the organic EL elements are defective or non-defective based on the detected current values and on this forward voltage drop value.  
           [0038]    There can be provided a current amplification circuit for amplifying the current detected by the current detection circuit, and an A/D conversion circuit for converting this amplified current into a digital signal.  
           [0039]    In particular for an active matrix type of organic EL display, the setting of the detection voltage in the detection voltage generation circuit, the generation of the control signal in the control signal generation circuit, and the setting of the degree of current amplification in the current amplification circuit can be carried out as desired by means of bus data from a central control circuit.  
           [0040]    In particular for a simple matrix type of organic EL display, the setting of the detection current in the detection current generation circuit, the generation of the control signal in the control signal generation circuit, and the setting of the degree of current amplification in the current amplification circuit can be carried out as desired by means of bus data from a central control circuit.  
           [0041]    The fourth invention is an apparatus for evaluating an organic EL display having signal lines and selection lines arranged in a matrix, and organic EL elements as pixels connected to said signal lines and selection lines at the intersections between these signal lines and selection lines, wherein the signal line or the selection line is switched for each pixel constituted by an organic EL element, that organic EL element is driven, and the drive current and discharge current values are measured for said organic EL elements, and pixel defects are detected by detecting a difference in the current values thereof.  
           [0042]    The fifth invention is an apparatus for evaluating an organic EL display having signal lines and selection lines arranged in a matrix, and organic EL elements as pixels connected to said signal lines and selection lines at the intersections between these signal lines and selection lines, wherein the signal line or the selection line is switched for each pixel constituted by an organic EL element, and that organic EL element is driven, a first sampling is performed for the drive current values within the drive time of the organic EL elements, and a second sampling is performed for the discharge current values at the end of the discharge time following this drive time, whereby the drive current and discharge current values are measured, and pixel defects are detected by detecting a difference in the current values thereof.  
           [0043]    The sixth invention is a method for evaluating an organic EL display having organic EL elements as pixels, wherein the drive current and discharge current values are measured for each pixel constituted by an organic EL element, and pixel defects are detected by detecting a difference in the current values thereof.  
           [0044]    With the apparatus and method of the present invention for evaluating an organic EL display, the drive (i.e., the testing) of an element is performed during the discharge of the previous element after the supply of drive current to the various pixels (organic EL elements) of the organic EL display; that is, the difference is measured between the drive current and discharge current values of the organic EL elements, so the supply and discharge of drive current can be performed for each pixel (organic EL element), and the organic EL elements can be tested one at a time.  
           [0045]    If the difference between the current values is under the specified level, it can be concluded that the organic EL element that constitutes a pixel is operating normally.  
           [0046]    With the first invention in particular, any difference between the drive current and discharge current values is detected for each pixel constituted by an organic EL element, so the next pixel (organic EL element) is always tested in the same way upon completion of the discharge of the previous pixel, the drive current value resulting from the previous detection does not remain in the next pixel, and successive evaluations can be carried out for all of the pixels in a reliable manner.  
           [0047]    With the second invention in particular, there are provided a detection voltage generation circuit, a control signal generation circuit, a connection switching circuit for connecting to the organic EL display, a current detection circuit, and a defect decision circuit, so unlike with a conventional evaluation apparatus, in which a drive circuit was attached to the organic EL display and everything put together in a form similar to that of an actual finished product, the evaluation work can be carried out for just the organic EL display.  
           [0048]    With the third invention in particular, for a simple matrix type of organic EL display that is not equipped with a constant current circuit for each pixel constituted by an organic EL element, a detection current generation circuit that generates a detection current to the signal lines and selection lines is provided instead of the detection voltage generation circuit used in an apparatus for evaluating an active matrix type of organic EL display, but the same evaluation work as in the second invention can be conducted.  
           [0049]    With the fourth invention in particular, the signal line or the selection line is switched for each pixel constituted by an organic EL element connected at the various intersection between these signal lines and selection lines arranged in a matrix, and the difference between the drive current and discharge current values is detected, so each pixel can be evaluated quickly by selecting a signal line or selection line.  
           [0050]    With the fifth invention in particular, a first sampling is performed for the drive current values within the drive time of the organic EL elements, and a second sampling is performed for the discharge current values at the end of the discharge time following this drive time, so it is possible to measure a current value that is suitable for the evaluation of each organic EL element.  
           [0051]    With the sixth invention in particular, just as with the first invention, detection is carried out for a given pixel (organic EL element) after completion of discharge of the previous pixel, so the drive current value resulting from the previous detection does not remain in the next pixel, and successive evaluations can be carried out for all of the pixels in a reliable manner. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0052]    [0052]FIG. 1 is a schematic circuit diagram of an organic EL display evaluation device  20  pertaining to a first embodiment of the present invention;  
         [0053]    [0053]FIG. 2 is a timing chart for driving the organic EL display evaluation device  20  and evaluating the organic EL display  10 ;  
         [0054]    [0054]FIG. 3 is a graph of the pixel current values for the various organic EL pixels  14 ;  
         [0055]    [0055]FIG. 4 is a graph as in FIG. 3, illustrating a testing procedure in which the next organic EL pixel  14  is tested without waiting for a fall time t 2  and a discharge time t 3  after the supply of drive current (after drive time t 1 );  
         [0056]    [0056]FIG. 5 is a schematic diagram of a conventional, simple matrix type of organic EL display  40 ;  
         [0057]    [0057]FIG. 6 is a schematic circuit diagram of an evaluation device  50  for the organic EL display  40  pertaining to a second embodiment of the present invention;  
         [0058]    [0058]FIG. 7 is an enlarged cross section of the main components of a conventional type of organic EL element  1 ;  
         [0059]    [0059]FIG. 8 is a circuit diagram of one pixel  11  in an active matrix type of organic EL display  10 ; and  
         [0060]    [0060]FIG. 9 is a circuit diagram of one pixel  11  in an active matrix type of organic EL display  10 , shown in more detail than in FIG. 8. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0061]    Next, the organic EL display evaluation device  20  pertaining to the first embodiment of the present invention will be described along with an evaluation method through reference to FIGS.  1  to  4 . Those components that are the same as in FIGS.  7  to  9  are numbered the same, and will not be described again in detail.  
         [0062]    [0062]FIG. 1 is a schematic circuit diagram of the evaluation device  20  for the organic EL display  10 . The organic EL display evaluation device  20  evaluates the various pixels  11  of the organic EL display  10 , the wiring thereof, and so forth to find whether these components are defective or non-defective, and comprises a central control circuit  21  (CPU), a control bus  22 , a test voltage generation circuit  23 , a current detection circuit  24 , a current amplification circuit  25 , an A/D conversion circuit  26 , a defect decision circuit  27 , a control signal generation circuit  28 , a signal line connection switching circuit  29  (connection switching circuit), a selection line connection switching circuit  30  (connection switching circuit), and a voltage supply line connection switching circuit  31  (connection switching circuit).  
         [0063]    The central control circuit  21  controls the overall system through the control bus  22 .  
         [0064]    The test voltage generation circuit  23  generates a test voltage for testing the organic EL display  10 , and is connected to the signal line connection switching circuit  29 , the selection line connection switching circuit  30 , and the current detection circuit  24 .  
         [0065]    The test voltage should be low enough that it will not damage the organic EL display  10 , and is preferably lower than the light-emitting voltage. A favorable voltage will allow the current required for evaluation of the organic EL display  10  to flow. The light-emitting voltage of the organic EL pixels  14  of the organic EL display  10  will vary with the organic materials and electrode materials being used, but is usually about 2 to 4 volts. The test voltage should therefore be generated at up to about 4 volts.  
         [0066]    The test voltage generation circuit  23  should be capable of generating the desired voltage. For instance, the test voltage can be easily generated by using a constant voltage circuit, a regulator circuit, or the like.  
         [0067]    The signal line connection switching circuit  29  serves to sequentially switch and connect signal lines VD in the organic EL display  10  to the various pixels  11 , and the control signals for this switching are supplied from the control signal generation circuit  28 .  
         [0068]    The selection line connection switching circuit  30  serves to sequentially switch and connect selection lines VG in the organic EL display  10  to the various pixels  11 , and the control signals for this switching are supplied from the control signal generation circuit  28 .  
         [0069]    The voltage supply line connection switching circuit  31  serves to sequentially switch and connect voltage supply lines VLC in the organic EL display  10  to the various pixels  11 , and the control signals for this switching are supplied from the control signal generation circuit  28 .  
         [0070]    Therefore, the test voltage generation circuit  23  is able to supply its test voltage to the signal lines VD, the selection lines VG, and the voltage supply lines VLC through the signal line connection switching circuit  29  and selection line connection switching circuit  30 , and through the current detection circuit  24  and voltage supply line connection switching circuit  31 .  
         [0071]    The current detection circuit  24  serves to detect the drive current and discharge current flowing to the pixels  11  (the organic EL pixels  14  or the organic EL elements  1 ), and is connected to the voltage supply line connection switching circuit  31  so that the detected current values will be outputted to the current amplification circuit  25 .  
         [0072]    The current amplification circuit  25  amplifies the detected current.  
         [0073]    The A/D conversion circuit  26  converts the amplified current into a digital signal.  
         [0074]    The defect decision circuit  27  decides whether the pixels  11  or organic EL pixels  14  (organic EL elements  1 ) in the organic EL display  10  are defective or non-defective on the basis of the detected current values (this will be described in detail through reference to FIG. 4).  
         [0075]    The control signal generation circuit  28  supplies control signals to the A/D conversion circuit  26 , signal line connection switching circuit  29 , selection line connection switching circuit  30 , and voltage supply line connection switching circuit  31 .  
         [0076]    [0076]FIG. 2 is a timing chart for driving the organic EL display evaluation device  20  and evaluating the organic EL display  10 , and is drawn for first and second pixels.  
         [0077]    In the testing of the organic EL pixel  14  in the pixels  11  of the organic EL display  10 , upon completion of the testing of each pixel  11 , the organic EL pixels  14  selected in this testing must be sufficiently discharged. Specifically, in the testing of the organic EL pixels  14 , the timing is important as the signal lines VD, the selection lines VG, and the voltage supply lines VLC are switched on and off by the control signal generation circuit  28 .  
         [0078]    To describe this in specific terms through reference to FIGS. 1 and 2, at the start of testing, voltage is supplied to the selection line VG of the organic EL pixel  14  of a specific pixel  11  (first pixel) from the test voltage generation circuit  23  via the selection line connection switching circuit  30 , which switches ON the first transistor  15  (FIG. 9), and voltage (VD 1 ) is supplied to the signal line VD via the signal line connection switching circuit  29 , which results in voltage V 1  being supplied for driving the second transistor  16 , switching ON the second transistor  16 . The capacitor  17  is charged as this voltage V 1  rises.  
         [0079]    The voltage supply line VLC is switched ON in this state, and the voltage V 2  to the organic EL pixel  14  rises. Specifically, the test voltage is supplied to the organic EL pixel  14 , and the drive current of the organic EL pixel  14  is measured by performing a first sampling S 1  within this drive time t 1  (first specific time).  
         [0080]    After the drive current has been measured, the signal line VD is switched OFF, and after the fall time t 2  has elapsed, the charge of the capacitor  17  and the organic EL pixel  14  is completely released (discharge), this discharge state is stabilized and the second transistor  16  is switched completely OFF, and the discharge current is measured by performing a second sampling S 2  within the discharge time t 3  (second specific time).  
         [0081]    Any difference between the above-mentioned drive current (drive current data) and this discharge current (discharge current data) is determined, and defect detection is performed for the organic EL pixel  14  (first pixel) on the basis of this difference in current values (operating current differential), that is, current data (digital signal) for the pixel current value.  
         [0082]    In specific terms, FIG. 3 is a graph of the pixel current values for the various organic EL pixels  14 , and the evaluation standard value for the pixel current value is set to within a specific range.  
         [0083]    For this specific range or threshold value, the average operating current differential of a properly operating organic EL pixel  14  is determined ahead of time, and any organic EL pixels  14  with a detected current value outside this range is deemed a defective pixel. For instance, if the upper limit of this range is exceeded, there may be a defect in the first transistor  15 , the second transistor  16 , or the wiring portion, and in the illustrated example, the n+3 rd  organic EL pixel  14  emits light too intensely, and is therefore deemed a white defect.  
         [0084]    If the pixel current value drops under the lower limit of the range, the current itself may not flow well, and there may be a line defect or dot defect. For instance, the n+6 th  organic EL pixel  14  emits light too weakly, and is therefore deemed a black defect.  
         [0085]    Thus, in the testing of the second pixel (organic EL pixel  14 ), it is possible to achieve a state in which no effect whatsoever remains of the first pixel drive state or test state, allowing proper and accurate pixel testing to be continued successively.  
         [0086]    A drive circuit and other accessory parts are only added to an organic EL display  10  that has thus been evaluated to be normal, and this improves the yield in the manufacturing and evaluation steps.  
         [0087]    [0087]FIG. 4 is a graph as in FIG. 3, illustrating a testing procedure in which the next organic EL pixel  14  is tested without waiting for the fall time t 2  and the discharge time t 3  after the supply of drive current (after the drive time t 1 ), as discussed above. Because the superposition of the drive current occurs successively for each of the pixels, the pixel current value grows steadily larger, resulting in a white defect, and minute changes in a pixel current that has taken on a large absolute value mean that a defect decision must be made for each and every pixel  11 , making the evaluation work either very difficult or impossible for all practical purposes.  
         [0088]    The apparatus and method for evaluating an organic EL display pertaining to the present invention are not limited to an active matrix type of organic EL display  10 , and can also be applied to a simple matrix type of organic EL display  40  in which each organic EL pixel  14  is not equipped with the switching circuit  12  and the voltage supply line VLC (FIGS. 8 and 9).  
         [0089]    Specifically, FIG. 5 is a schematic diagram of a simple matrix type of organic EL display  40 , and this organic EL display  40  is such that current can be supplied to the organic EL pixels  14  (organic EL elements  1 ) from a current source  41  capable of supplying a constant current, and light is emitted when drive current is selectively supplied to the organic EL pixels  14  by means of a signal line selection circuit  42  (data line selection circuit) and a selection line selection circuit  43  (scanning line selection circuit).  
         [0090]    [0090]FIG. 6 is a simplified circuit diagram of an evaluation device  50  for the organic EL display  40  (simple matrix type) pertaining to a second embodiment of the present invention. The evaluation device  50  of the organic EL display  40  does not have the voltage supply line connection switching circuit  31  seen in the evaluation device  20  of the organic EL display  10  (FIG. 1), a test current generation circuit  51  is provided instead of the test voltage generation circuit  23  (FIG. 1), and a forward voltage drop detection circuit  52  connects this test current generation circuit  51  to the current amplification circuit  25 . This affords scanning of the organic EL pixels  14  (organic EL elements  1 ) in the organic EL display  40 .  
         [0091]    The test current generation circuit  51  serves to successively generate test current at the signal lines VD and the selection lines VG. Specifically, the organic EL pixels  14  (organic EL elements  1 ) in the simple matrix type of organic EL display  40  (FIG. 5) are not equipped with a constant current circuit  13  as was the active matrix type of organic EL display  10  (FIGS. 8 and 9), so the test current must be supplied directly from the test current generation circuit  51  to the organic EL pixels  14  in order to evaluate the organic EL pixels  14 .  
         [0092]    As indicated by the imaginary line in FIG. 5, the forward voltage drop detection circuit  52  detects forward drop voltage when the organic EL pixels  14  (organic EL elements  1 ) are operating properly. Specifically, current flows the same to the organic EL pixels  14  in the simple matrix type of organic EL display  40  whether these organic EL pixels  14  are operating normally or are short-circuited, but a forward drop voltage is generated when the operation is normal, whereas no forward drop voltage is generated when there is a short-circuit, so pixel defects are detected by detecting forward drop voltage along with the test current.  
         [0093]    The evaluation procedure with an evaluation device  50  structured such as this is the same as with the organic EL display evaluation device  20  described above through reference to FIGS.  1  to  4 , and therefore will not be described in detail, but the signal line VD is switched by the signal line selection circuit  42 , or the selection line VG by the selection line selection circuit  43 , so that the difference between the drive current and discharge current values is measured for every organic EL pixel  14 , and the defect decision circuit  27  decides whether an organic EL element  1  is defective or non-defective from the current value detected by the current detection circuit  24  and from the forward drop voltage value detected by the forward voltage drop detection circuit  52 .  
         [0094]    Thus, with the present invention, the difference between the drive current and discharge current is detected for the organic EL pixels or organic EL elements in an organic EL display, and a decision as to whether the organic EL display is defective or non-defective is made on the basis of this difference, allowing organic EL pixels to be properly evaluated one another the other.