Patent Publication Number: US-2005128193-A1

Title: Methods and apparatus for a display

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS  
      This application is a continuation of U.S. Ser. No. 10/816,742, filed Apr. 2, 2004, which claims the benefit of: 
          U.S. Provisional Patent Application No. 60/460,393, filed Apr. 7, 2003;     U.S. Provisional Patent Application No. 60/470,978, filed May 19, 2003;     U.S. Provisional Patent Application No. 60/479,955, filed Jun. 20, 2003; and        

      U.S. Provisional Patent Application No. 60/497,698, filed Aug. 26, 2003; 
          and incorporates the disclosure of each application by reference. To the extent that the present disclosure conflicts with any referenced application, however, the present disclosure is to be given priority.       

    
    
     FIELD OF THE INVENTION  
      The invention relates to methods and apparatus for implementing and/or controlling displays using organic light emitting diodes.  
     BACKGROUND OF THE INVENTION  
      Display systems find many uses in a variety of applications, such as computers, televisions, point-of-sale terminals, mobile phones, test equipment, and other electronic systems. For displays that use organic light emitting diodes, display brightness and uniformity deteriorate as the organic light emitting diodes that form the display age. Brightness may also vary significantly according to manufacturing variations, operating conditions, interference due to crosstalk, and other causes. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
      A more complete understanding of the present invention may be derived by referring to the detailed description when considered in connection with the following illustrative figures. In the following figures, like reference numbers refer to similar elements and steps.  
       FIG. 1  is a block diagram of a display system according to the various aspects of the present invention.  
       FIG. 2  is a schematic diagram of a display system with two row electrodes and three column electrodes per optical cell.  
      FIGS.  3 A-B are diagrams of photo resistors adapted for use as a feedback sensor.  
       FIGS. 4-6  are block diagrams of systems for storing pre-adjustment values for use with an incremental target value.  
       FIG. 7  is a block diagram of a display system having a signal generator, a comparing circuit, and a switch.  
       FIG. 8  is a block diagram of a control circuit having a voltage generator and a selector.  
       FIG. 9  is a block diagram of a display system with a common voltage generator for all columns.  
       FIG. 10  is a block diagram of a display system with a common increasing voltage generator and a pixel discharge circuit.  
       FIG. 11  is a block diagram of a pixel control circuit implemented with a voltage controlled current source.  
       FIG. 12  is a schematic diagram of a display system using a resistor feedback sensor that is common to all optical cells of a column.  
       FIG. 13  is a diagram of a display system using a resistor feedback sensor in each optical cell.  
       FIG. 14  is a diagram of a display with one row electrode and two column electrodes per optical cell and a feedback sensor that continuously reads the currents flowing in the optical cells of a column.  
       FIG. 15  is a diagram of an active maintenance circuit. 
    
    
      Elements and steps in the figures are illustrated for simplicity and clarity and have not necessarily been rendered according to any particular sequence. For example, steps that may be performed concurrently or in different order are illustrated in the figures to help to improve understanding of embodiments of the present invention.  
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
      The present invention is described partly in terms of functional components and various assembly and/or operating steps. Such functional components and steps may be realized by any number of components and steps configured to perform the specified functions and achieve the various results. For example, the present invention may employ various elements, materials, configurations, sensors, displays, circuit elements, integrated circuits, and the like, which may carry out a variety of functions. In addition, the present invention may be practiced in conjunction with any number of applications, environments, and display systems, and the systems and components described are merely exemplary applications for the invention. Further, the present invention may employ any number of conventional techniques for manufacturing, assembling, integration of elements, and the like.  
      Referring now to  FIG. 1 , a display system  100  configured for controlling pixel brightness according to various aspects of the present invention comprises a display panel  102 , a feedback sensor  104 , and a control circuit  106 . The display system  100  may be used for any suitable purpose or combination of purposes, such as displaying alpha-numeric information, graphical representations, information or graphics as a still image, a series of still images, or images displayed to form motion pictures. Information for display can come from any suitable source, such as a computer, a memory device, an electromechanical sensor, a television signal feed, a camera, a transducer, or the like. The display system  100  provides images on the display panel  102  according to the received information. The feedback sensor  104  generates a feedback signal  105 , which is provided to the control circuit  106  to adjust the brightness of the individual optical cells.  
      The display panel  102  is configured to display information in response to signals from a source. Any suitable technology or combination of technologies may be used to implement the display panel  102 , such as thin film components, semiconductors, and organic light emitting diodes (OLEDs). An exemplary display panel  102  of the present embodiment comprises an array of optical cells, for example OLEDs accessed via a set of row electrodes and a set of column electrodes. Each optical cell may include one or more OLEDs, the brightness of which may be individually controlled to emit light to form a composite image.  
      The optical cell configuration of the display panel  102  may be selected according to any appropriate criteria or design. Referring to  FIG. 2 , in an exemplary embodiment, the display panel  102  comprises a matrix of optical cells  212 A-D organized into multiple rows  220 A-B and columns  224 A-B. The number of rows  220  and columns  224  may be selected according to any appropriate criteria, such as the size and resolution of the display panel  102 .  
      Each optical cell  212  is configured to selectively provide light according to the signals from the control circuit  106 . The individual optical cells  212  may be addressed and driven according to any appropriate system or technique to control the image rendered on the display panel  102  and the brightness of the individual optical cells  212 . Each optical cell  212  of the present embodiment includes at least one control input, such that signals applied to the control input control the brightness of the optical cell  212 . In the present matrix configuration, each optical cell  212  in a row  220  is connected to at least one row selection electrode  232 , and each optical cell  212  in a column  224  is selectively connected to at least one column electrode  228 , for example via the control input. The light generated or transmitted by the optical cell  212  varies according to signals received from the control circuit  106  via the column electrode  228 . The row selection electrode  232  facilitates selective connection of the optical cells  212  in a row  220  to their respective column electrodes  228 .  
      The optical cells  212  may be controlled according to any desired technique, such as controlling all optical cells  212  simultaneously, all optical cells  212  in a row  220 , all optical cells  212  in a column  224 , or by individually controlling each optical cell  212  in any row  220  or column  224 . In the present exemplary embodiment using a conventional OLED display panel  102 , the optical cells  212  are accessed by simultaneously addressing optical cells  212  in a row or column via the row and column electrodes  232 ,  228 . At least one row selection electrode  232  is shared by an entire row  220 , and at least one column electrode  228  is common to an entire column  224 . By selectively driving the various row and column electrodes  232 ,  228 , each optical cell  212  may be accessed.  
      The optical cells  212  may comprise any suitable systems for selectively providing light using an OLED. For example, each optical cell  212  may comprise at least one OLED  208 , control transistor  206 , row transistor  200 ,  202 ,  204 , and storage capacitor  210 . The OLED  208  for each optical cell  212  may comprise any suitable OLED, such as a fluorescent or phosphorescent OLED, which generates light according to current through the OLED. In the present embodiment, the OLED operates as a single-color gray-scale element, such that adjusting the current through the OLED controls the brightness of the OLED. In a color display, multiple OLEDs having different colors, such as red, green, and blue OLEDs, may be independently adjusted to achieve desired colors and overall brightness.  
      The current through the OLED  208  may be controlled to adjust the brightness. The current may be controlled using any appropriate current control system or mechanism, such as a variable resistance, a modulator, an amplifier, or other suitable system. For example, the OLED  208  of the present embodiment is suitably connected in series with the control transistor  206 , such that the control transistor  206  controls the amount of current through the OLED  208 . In particular, the OLED  208  is suitably connected between the source of the control transistor  206  and ground, while the drain of the control transistor  206  is connected, directly or indirectly, to a power supply  248 . Adjusting the conductivity of the control transistor  206  controls the amount of current through the OLED  208  and thus its brightness.  
      The conductivity of the control transistor  206  corresponds to the voltage applied to its gate. The gate voltage is suitably controlled by signals applied via the column electrode  228 . Any appropriate technique may be used to apply and hold the desired signal to control the brightness of the optical cell  212 , such as a storage element for storing a value, like a memory or a capacitor. In the present embodiment, the storage capacitor  210  is connected to the gate of the control transistor  206  to maintain the voltage applied to the control transistor  206  gate after the desired charge is applied via the column electrode  228 . Thus, a desired charge may be stored on the storage capacitor  210  by selectively connecting the storage capacitor  210  and the gate of the control transistor  206  to the column electrode  228  and applying an appropriate signal to the column electrode  228 . The charge stored on the storage capacitor  210  sets the control transistor  206  gate-to-source voltage, thus setting the current through the control transistor  206  and OLED  208 . The optical cell  212  brightness thus substantially corresponds to the charge placed on the storage capacitor  210 .  
      A signal applied to the column electrode  228  charges or discharges the storage capacitor  210 . The signal may be provided to the individual storage capacitors  210  or other storage mechanisms in any appropriate manner, such as controlling the drive signal applied to the column electrode  228  or the connection between the column electrode  228  and the storage capacitor  210 . In the present embodiment, the storage capacitor  210  may be selectively connected to the column electrode  228  to vary or maintain the charge on the storage capacitor  210 . For example, a row selection transistor  200  selectively connects the control transistor  206  and the storage capacitor  210  to the column electrode  228 . Either the drain or the source of the row selection transistor  200  is connected to the column electrode  228 , and the other terminal of the row selection transistor  200  is connected to the control transistor  206  gate and the storage capacitor  210 . The gate of the row transistor  200  is connected to the row selection electrode  232 . To connect the control transistor  206  gates and storage capacitors  210  for each optical cell  212  in a row  220  to their respective column electrodes  228 , the row selection electrode  232  is activated. When the row selection electrode  232  is deactivated, the connection between the column electrodes  228  and the node between the storage capacitor  210  and control transistor  206  gate is terminated, leaving the desired charge stored on the storage capacitor  210 .  
      Referring again to  FIGS. 1 and 2 , the feedback sensor  104  is configured to generate a signal corresponding, directly or indirectly, to the brightness of one or more optical cells  212 . Brightness may be measured for any individual optical cell  212  and/or grouping of optical cells  212 , such as all optical cells, a row of optical cells, a column of optical cells, an area of optical cells, or individual optical cells. Any suitable technique for measuring brightness may be used, such as directly or indirectly measuring current through the OLED  208 , voltage across the OLED  208 , luminance from the OLED  208 , heat of the OLED  208 , or another signal substantially corresponding or relating to the brightness of the optical cell  212 . The feedback signal  105  may be communicated in any suitable manner. The feedback sensor  104  may measure the current flowing in an optical cell  212 , and convert the current to a voltage for use by the control circuit  106 , or the feedback sensor  104  may measure a voltage and convert it to a current.  
      The feedback sensor  104  may be implemented in any suitable manner, such as a light sensor, a voltage sensor, a current sensor, or other sensor. In one embodiment, the feedback sensor  104  measures electrical characteristics, such as a current through the OLED  208  or a voltage across the OLED  208 . For example, referring to  FIG. 2 , the feedback sensor  104  of the present embodiment suitably comprises a resistor, a diode, or diode-connected transistor connected in series with the OLED  208 , such that the current through the feedback sensor  104  substantially corresponds to the current through the OLED  208 . The feedback sensor  104  of the present embodiment suitably comprises a resistor having a relatively low resistance so that the connection and disconnection of the feedback sensor  104  to the various optical cells  212  does not significantly affect the signal provided to the optical cells  212 . Enabling the row selection electrode  232  activates a measurement selection transistor  202 , and disabling a row maintenance electrode  230  disables a maintenance transistor  204 , directing current to flow from supply  248  through the feedback sensor  104 , the measurement selection transistor  202 , the control transistor  206 , and the OLED  208 . The voltage drop across the feedback sensor  104  is substantially proportional to the current through the OLED  208 , which corresponds to the OLED  208  brightness.  
      The control transistor  206  may be configured to operate in the saturation region or the subsaturation region. Operation in the saturation region tends to simplify the operating requirements, for example by allowing a relatively large resistor to be used as the feedback sensor  104 . Operating the control transistor  206  in the subsaturation region, however, decreases power consumption. The display system  100  may be configured to facilitate operation of the control transistor  206  in the subsaturation region. For example, referring to  FIG. 12 , the feedback sensor  104  may comprise a low value resistor. The feedback signal  105 , which may be as low as a fraction of a millivolt, may be amplified by an amplifier  336  for the pixel control circuit  246 . Amplifier  336  may be implemented in any suitable manner, such as a single amplifier, multiple amplifiers in series, or amplifiers integrated into the pixel control circuit  246 .  
      In an alternate subsaturation operation embodiment, referring to  FIG. 13 , each optical cell  212  may be associated with a dedicated feedback sensor  104  comprising a low-value resistor, instead of having a single resistor or other feedback sensor  104  for an entire column. Disabling the row selection electrode  232  isolates the storage capacitor  210  from the column electrode  228  and disconnects the feedback sensor  104  from the pixel control circuit  246 . The resistor may be placed anywhere in the circuit branch where the current related to brightness of the optical cell  212  flows. For example, the resistor may be connected between the cathode of the OLED  208  and source of the control transistor  206 , which may facilitate fabrication of the resistors using thin film technology on top of the individual cathodes.  
      In another exemplary embodiment, the feedback sensor  104  measures optical characteristics of the optical cells  212 . For example, referring to  FIG. 3A , the feedback sensor  104  may comprise a photo resistor  300 . The photo resistor  300  is deposited over any suitable number of optical cells  212 , such as an individual optical cell  212  or a row  220  or a column  224  of optical cells  212 . The photo resistor  300  is deposited in any suitable manner to allow the photo resistor  300  to detect the luminance from each optical cell  212  with minimal interference with viewing the optical cell  212 . For example, a narrow photo resistor  300  may be deposited over a column of optical cells  302 ,  304 ,  306 ,  308 ,  310 ,  312 ,  314 . In an alternative embodiment, the feedback sensor  104  may comprise multiple photo resistors dedicated to one or a few pixels. For example, referring to  FIG. 3B , the feedback sensor  104  may comprise multiple photo resistors  300 , each of which is suitably dedicated to a single optical cell  212 . The photo resistors  300  may be positioned in any suitable position to measure the brightness of the optical cell  212 , such as over the OLED  208  or along the edge of the optical cell  212 . In the present embodiment, the photo resistors  300  are connected in series.  
      The resistance of the photo resistor  300  is substantially inversely proportional to the brightness of each optical cell  302 - 314 . Consequently, the electrical characteristics of the photo resistor  300  may be measured to determine the brightness of the pixels. For example, a constant current source  318  is suitably connected to the photo resistor  300 . The current source  318  drives a constant current through the photo resistor  300 , causing the voltage on a measurement electrode  316  to vary substantially proportionally to the resistance in the photo resistor  300 . As the brightness of each individual optical cell  302 - 314  is adjusted, the voltage on the measurement electrode  316  across the photo resistor  300  varies accordingly and thus generates the feedback signal  105 . The sensitivity of the sensor  104  can be enhanced by increasing the constant sensor current, which also increases the incremental voltage change corresponding to a brightness change. The sensor current may be increased without influencing the operation of other systems, as the sensor has no electronic connection to other circuits.  
      The control circuit  106  adjusts optical cell  212  brightness in conjunction with the feedback signal  105 . In addition, the control circuit  106  may receive the display information and provide drive signals to drive the display panel  102 . Any suitable control circuit  106  may be used to receive display information and control optical cell  212  brightness, such as conventional analog and/or digital circuits. In the present embodiment, the control circuit  106  suitably comprises an integrated circuit for driving the display panel  102  according to the received display information. The control circuit  106  suitably receives display information, determines the target brightness for each relevant optical cell  212  based on the display information, adjusts optical cell  212  brightness, and uses the feedback signal  105  to determine when the selected optical cell  212  reaches the target brightness.  
      Referring again to  FIG. 2 , a control circuit  106  according to various aspects of the present invention comprises a translation circuit  244  and a pixel control circuit  246 . The translation circuit  244  translates the display information into a target value corresponding to a desired brightness or corresponding value for an optical cell  212 . The pixel control circuit  246  activates the row electrodes  230 ,  232  for the row  220  of optical cells  212  to be adjusted, and drives the column electrode  228  to change the brightness of the OLEDs  208 . The feedback sensor  104  monitors optical cell  212  brightness and feeds the brightness information to the pixel control circuit  246 . The pixel control circuit  246  substantially maintains the state of the optical cell  212  when it reaches the desired brightness.  
      More particularly, the translation circuit  244  converts display information into a target value representing desired optical cell  212  brightness. The display information may be received in any suitable manner, such as via a port  242 . The display information may be any suitable type of signal. For example, the display information may include digital signals, analog signals, row and column timing signals, voltages, currents, electromagnetic levels, and/or optical signals. Further, the display information may be of any suitable format, such as broadcast format signals, computer display signals, and the like. In addition, the display information may represent absolute optical cell  212  brightness, the incremental difference between the present and the desired optical cell brightness, or total brightness for a row or a column of optical cells  212  with a variant factor from the total brightness for each optical cell  212 . Likewise, the target signal may be of any suitable type, for example a digital or analog signals corresponding to voltage, current, luminance, or heat. In the present embodiment, the target value is expressed as an analog voltage value to be compared to the voltage signal generated by the feedback sensor  104 .  
      The translation circuit  244  may generate target values corresponding directly to the display information, such that if the display information calls for a first pixel brightness that is twice as bright as a second pixel brightness, the target value for the first pixel brightness is about twice the target value for the second pixel brightness. Alternatively, the translation circuit  244  may be configured to generate target values for incremental changes. The translation circuit  244  may use any suitable technique to translate the display information into the target value. For example, the translation circuit  244  may comprise a lookup table, a signal processor, a microprocessor, digital logic gates or arrays (synchronous or otherwise), and/or an analog signal processor. The translation circuit  244  may also include storage capacity to store display information or target signal values. Any suitable type and size of storage may be used, and the information stored may be for immediate or later use.  
      The translation circuit  244  may also perform calculations to determine the target value based on the display information. For example, the translation circuit  244  may convert the display information from a current to a voltage, or a voltage to a luminance. In an alternative embodiment, the translation circuit  244  may store the current flowing through each OLED  208  and may store the display data before converting the information.  
      Referring to  FIG. 2 , in the present control circuit  106 , the translation circuit  244  includes a look-up table to translate display information into a target value. The look-up table generates predetermined target values, such as target voltages, to drive the display panel  102  and achieve the desired image effects from the received display information. The target values are provided to the pixel control circuit  246  for control of the optical cells  212  in conjunction with the feedback signal  105 .  
      The pixel control circuit  246  adjusts optical cell brightness. The pixel control circuit  246  may adjust the optical cell brightness according to any appropriate mechanism and/or process. For example, the pixel control circuit  246  may adjust the voltage across, the current through, or the temperature of the OLEDs  208 . The pixel control circuit  246  may simultaneously adjust any suitable number of optical cells  212 , such as a single optical cell  212 , one optical cell  212  in each column  224 , all optical cells  212  of a row  220 , all optical cells  212  in a column  224 , or all optical cells  212 .  
      The pixel control circuit  246  is suitably configured to adjust the optical cell  212  brightness according to the target values received from the translation circuit  244  and the feedback signal  105 , and may be configured in any suitable manner to control the optical cell  212  brightness. For example, referring to  FIG. 7 , the pixel control circuit  246  of the present embodiment comprises a variable signal generator  320 , a comparison circuit  322 , a switch  324 , and a row addressing circuit  326 . The row addressing circuit  326  controls the row electrodes  230 ,  232 . The variable signal generator  320  generates a variable signal that may be applied to the column electrodes  228  to adjust the brightness of the optical cells  212 . The comparison circuit  322  compares the feedback signal  105  to the target value. When the target value is reached, the comparison circuit  322  opens the switch  324  to maintain the desired charge on the column electrode  228 .  
      More particularly, the variable signal generator  320  of the present embodiment provides a variable signal through the switch  324  to the column electrode  228  to change the optical cell  212  brightness. The variable signal generator  320  may comprise any suitable system for providing a variable signal. Referring to  FIG. 8 , the variable signal generator of the present embodiment comprises a selector  330  and a voltage generator  332 . The selector  330  selects either an increasing or decreasing signal for application to the column electrode  228 . Selecting the increasing signal causes the voltage applied to the column electrode  228  to increase, and vice versa with selecting the decreasing signal. The selector  330  may select the signal to be applied according to any appropriate criteria, such as the relative values of the current target value and the preceding target value or the feedback signal  105  for the relevant optical cell  212 .  
      The voltage generator  332  generates the variable signal that varies in the direction designated by the selector  330 . The voltage generator  332  may generate any appropriate waveform or set of waveforms for selection, such as a step voltage, a ramp, a modulated step voltage, a piece-wise linear voltage, a current, or a DC power supply. In one embodiment, the voltage generator  332  produces an increasing step voltage and a decreasing step voltage. In an alternate embodiment, the voltage generator  332  comprises a power sink, such as a low voltage supply, and a power source, such as a high voltage supply, that may be selectively connected to the column electrode by the selector  330  to discharge or charge, respectively, the column electrode  228 . In an alternate embodiment, the voltage generator  332  produces increasing and decreasing ramp voltages. Ramp voltages tend to reduce cross talk between the column electrode  228  and the optical cells  212  not selected for adjustment, which tends to increase visual quality.  
      The variable signal generator  320  may be configured in any suitable manner to drive the various electrodes. Referring again to  FIG. 7 , each column electrode  228  may be associated with a dedicated voltage generator  332 . Alternatively, referring to  FIG. 9 , the voltage generator  332  may be associated with multiple column electrodes  228 . A selector  330  unique to each column electrode  228  selects the appropriate signal for adjusting optical cell  212  brightness in the column  224 .  
      In another embodiment, referring to  FIG. 10 , the voltage generator  332  produces a signal that only increases or only decreases. Before the variable signal is applied to the column electrode  228 , the row addressing circuit  326  initializes the optical cells  212 , for example by activating a pixel discharge circuit  334  to drain the charge from the column electrode  228  and storage capacitor  210  before applying a drive voltage to the column electrode. Conversely, the voltage generator  332  may produce only a decreasing signal, such that the column electrode  228  and storage capacitor  210  are pre-charged to the highest level required for maximum pixel brightness before driving the column electrode  228  with the variable signal.  
      In another embodiment, the variable signal generator  320  may be replaced with a constant current source. Referring to  FIG. 11 , the current source, such as a voltage-controlled current source  328 , supplies a current to the column electrode  228 . The current is suitably regulated by a comparison between the feedback signal  105  and the target value, such as using a conventional comparator circuit as the current source  328 . In an additional alternative embodiment, the variable signal generator  320  includes a constant current source and sink. The selector  330  selects the current source or sink required to properly adjust the optical cell brightness.  
      The pixel control circuit  246  is configured to determine whether the feedback signal  105  has approached the target value, and may include any suitable configuration or element to compare the target value with the feedback signal  105  and control the switch  324 . For example, the present pixel control circuit  246  includes the comparison circuit  322  to compare the feedback signal  105  to the target value. The comparison circuit  322  may comprise any suitable system or component for comparing signals, such as a conventional operational amplifier configured as a comparator. In the present embodiment, the comparison circuit  322  compares the target value received from the translation circuit  244  with the feedback signal  105 . When the feedback signal  105  is substantially equal to the target value, the comparison circuit  322  disables the switch  324  and isolates the column electrode  228  from the variable signal generator  320 . Disabling the switch  324  substantially maintains the voltage on the column electrode  228  at the desired target voltage.  
      The comparison circuit  322  may also be configured to maintain the brightness for the optical cell. For example, after the initial charging of the storage capacitor  210 , the comparison circuit  322  opens the switch  324  to maintain the charge on the storage capacitor  210 . In some systems, however, the charge on the storage capacitor  210  may be redistributed from the storage capacitor  210  onto column parasitic capacitances or other unwanted capacitances. Accordingly, the charge on the storage capacitor  210  may drop, causing an undesired loss of brightness.  
      The comparison circuit  322  may be configured to counteract the redistribution of the charge or other unwanted loss of brightness. For example, the comparison circuit  322  may be configured to exhibit hysteresis, such that the storage capacitor  210  may be charged to an upper threshold beyond the target value, such as a percentage or preselected amount more than the target value, before opening the switch  324 . In addition, after the comparison circuit  322  opens the switch  324 , the comparison circuit  322  may continue to monitor the feedback signal  105  for the remainder of the row address cycle. If the feedback signal  105  drops below a lower threshold, such as a percentage or preselected difference from the target voltage, the comparison circuit  322  may close the switch  324  to recharge the capacitor  210  and increase the brightness until the upper threshold is again reached. The hysteresis may be symmetrical or asymmetrical around the target value. Further, the comparison circuit  322  with hysteresis may be used in conjunction with pre-charging or presetting the charge on the storage capacitor  210  to any value before adjusting the optical cell  212  to the desired brightness.  
      Any suitable system and/or technique may be used to maintain charge on the feedback capacitor  210  between the time that the feedback signal  105  reaches the target value and completion of the row address cycle, at which time the row electrode  232  is disabled. For example, the variable signal generator  320  and the current source  328  may be configured to selectively operate sequentially or simultaneously in parallel. The signal generator  320  may perform fast charging of the capacitor  210 , while the current source  328  suitably adjusts for small deviations from the target voltage.  
      In one embodiment, referring to  FIG. 15 , the variable signal generator  320  drives the column electrode  228  through the switch  324  until the feedback signal  105  is substantially equal to the target value. As soon as the feedback signal  105  is equal to the target value, the switch  324  is opened and a second switch  354  is closed. Closing the second switch  354  enables the current source  328  to drive the column electrode  228  whenever the feedback signal  105  is not equal to the target value. The current source  328  maintains the column electrode  228  and storage capacitor  210  at the target value until the end of the row address cycle for the row and the row electrode  232  is deactivated. The current source  328  compares the target value with the feedback signal  105  and charges and discharges the column electrode  228  to maintain the desired charge on the storage capacitor  210 . The current source  328  may be implemented using any suitable circuit; for example, it may have multiple stages to provide greater gain or drive strength.  
      In an additional alternative embodiment, the pixel control circuit  246  is configured to quickly charge or discharge the column electrode  228  by using both the variable signal generator  320  and the difference amplifier  356  to drive the column electrode  228  simultaneously until the feedback signal  105  is equal to the target value. Once the feedback signal  105  reaches the target value, the first switch  324  is opened to disconnect the variable signal generator  320  from the column electrode  228 , while the second switch  354  remains closed (or is omitted altogether, for example if the signal from the variable signal generator  320  does not adversely affect the current source  328 ), so the current source  328  maintains the charge on the storage capacitor  210  until the row addressing cycle for the row is complete.  
      The switch  324  selectively connects the column electrode  228  to the variable signal generator  320  or other power source. The switch  324  may comprise any suitable system for selectively enabling and disabling the connection, such as a relay, a switch, a transistor, and the like. In the present embodiment, the switch  324  comprises a transistor where the source or drain is connected to the variable signal generator  320 , the opposite terminal is connected to the column electrode  228 , and the gate is driven by the comparison circuit  322  output. When the switch  324  is opened, the variable signal generator  320  is disconnected from the column electrode  228 . When the switch  324  is closed, the signal from the variable signal generator  320  drives the column electrode  228 .  
      The row addressing circuit  326  selects a row  220  of optical cells  212  for brightness adjustment. Any suitable technique may be used for controlling the order and the method of row  220  selection, such as selecting an entire row, a fraction of a row, multiple rows, or multiple fractions of different rows. For example, referring again to  FIG. 7 , each row may be addressed using the row selection electrode  232  and the row maintenance electrode  230 . The row selection electrode  232  controls connection of the optical cell  212  to the column electrode  228  and the feedback sensor  104 . The row maintenance electrode  230  controls connection of the optical cell  212  to the power supply while bypassing the feedback sensor  104 . When the row selection electrode  232  is active, the optical cells  212  in the corresponding row  220  may be adjusted. When the row selection electrode  232  is inactive, the row maintenance electrode  230  is suitably activated to provide power to each optical cell  212  in the row  220 . All optical cells  212  of a row  220  are suitably adjusted simultaneously.  
      By driving the row and column electrodes  228 ,  230 ,  232 , the control circuit  106  can control the display panel  102 , measure the brightness of the individual optical cells  212 , and adjust the brightness of the individual optical cells  212 . Disabling the row maintenance electrode  230  disables the maintenance transistors  204  for each optical cell  212  in the row. Activating the row selection electrode  232  activates the row selection transistors  200 ,  202 , which connect the control transistor  206  to the column electrode  228  and the power supply via the feedback sensor  104 , respectively. Thus, the brightness of each optical cell  212  may be adjusted by driving the column electrode  228  with the appropriate signal and monitoring the optical cell  212  brightness via the feedback sensor  104 . The comparison circuit  322  detects when the current through the control transistor  206  and OLED  208  reaches the target value and opens the switch  324  to maintain the charge on the storage capacitor  210 . When the row addressing cycle is complete, the row selection electrode  232  is disabled to inhibit further modification of the current through the optical cell  212 . Further, row maintenance electrode  230  is enabled to connect the control transistor  206  and OLED  208  to the supply  248  through the maintenance transistor  204 .  
      In an alternative embodiment, the row selection and maintenance system may be configured to facilitate the use of a single row electrode  232  for each row  220 . Instead of connecting the feedback sensor  104  to each individual optical cell  212  in the column  228 , the feedback sensor  104  may remain connected to multiple optical cells  212  in the column  224 . To compare the feedback signal  105  to the target value, the pixel control circuit  246  suitably compares the target value to the change in the feedback signal  105  attributable to the relevant optical cell  212 . Various aspects previously discussed may be applied to the present embodiment as well, such as multiple or single signal generators, operation of the control transistor in subsaturation region, different placements of the sensor  105 , and the like.  
      For example, referring to  FIG. 14 , a single row electrode  232  controls connection of the control transistors  206  and storage capacitors  210  in the row to their respective column electrodes  228 . The feedback sensor  104  comprises a column sensor that is connected to each control transistor  206  in the column, for example via a power electrode  1402 . The brightness of each optical cell  212  may be adjusted by activating the row electrode  232  and driving a signal on the column electrode  228 . The feedback signal  105  comprises a brightness sum signal comprising the currents for all of the optical cells  212  in the column, and the brightness sum signal varies as the conductivity of the control transistor  206  in the relevant row changes. As each optical cell  212  is adjusted for each row, the feedback signal  105  changes, such that the feedback signal  105  comprises a sequential signal corresponding to varying brightnesses of multiple optical cells in the column. By monitoring the amount of change in the feedback signal  105  for each optical cell  212 , the control circuit  106  may control the brightness of each optical cell  212  in the column.  
      Current from multiple optical cells  212  continuously flows through the feedback sensor  104  and a brightness change of the optical cell  212  results in an incremental change in the feedback signal  105 . When the target value is reached, the pixel control circuit  246  terminates the connection to the variable signal generator  320 . The row electrode  232  is disabled at the end of the row address cycle, deactivating the row transistor  200  and isolating the storage capacitor  210 .  
      The change in the feedback signal  105  and/or target value may be measured and/or calculated according to any suitable technique or mechanism. For example, the target value may be calculated in the translation circuit  244  by summing or otherwise summarizing the target values for all of the other optical cells  212  in the column at the beginning of the row address cycle, and adjusting the summary number according to the current target value for the current optical cell  212  to determine an overall target value for the entire column.  
      For example, the preceding target value for a first row address cycle may be stored, and then retrieved and subtracted from the current target value for a second row address cycle. The resulting value corresponds to the desired change in the feedback signal  105  for the relevant optical cell  212 . Alternatively, the control circuit  106  may store the an initial feedback signal  105  for a first row address cycle, and then subtract the initial feedback signal  105  value from the current target value from a second row address cycle to arrive at a desired change in the feedback signal  105 . The pixel control circuit  246  may then monitor the feedback signal  105  for the desired change and control the control transistor  206  and storage capacitor  210  accordingly.  
      Referring to  FIG. 4 , in an exemplary embodiment, the pre-adjustment feedback signal  105 , meaning the feedback signal  105  before responding to the current display information, of the selected optical cell  212  may be stored on a storage capacitor  328  as a reference voltage by activating a storage transistor  326 . The translation circuit  244  converts the current display information into a target voltage change attributable to the optical cell  212  in the row being addressed. The storage transistor  326  is deactivated and a sum transistor  324  is activated, such that the charge on the capacitor  328  is provided to a summing circuit  322 . The summing circuit  322  adds the current target value change to the previous feedback signal  105  value stored on the capacitor  328 . The resulting sum is the target value used by the pixel control circuit  246  to adjust the optical cell  212  brightness.  
      In another alternative embodiment, referring to  FIG. 5 , the pre-adjustment voltage is stored on the capacitor  328 . The translation circuit  244  provides the pixel control circuit  246  with its calculated incremental change value as the target value. The pre-adjustment value stored on the capacitor  328  is subtracted from the feedback signal  105  to produce an incremental change from the feedback sensor  104 . When the feedback signal  105  changes by the target incremental amount, the optical cell  212  under adjustment has reached the desired brightness.  
      In yet another alternative embodiment, referring to  FIG. 6 , the translation circuit  244  uses stored information to calculate the pre-adjustment value, and calculates a current target value based on the display information. The translation circuit  244  subtracts the pre-adjustment value from the target value to find the incremental change value. The translation circuit  244  provides the incremental change value to the pixel control circuit  246  and the pre-adjustment value to a difference circuit  602  so that the signal  604  provided to the pixel control circuit  246  is also an incremental change value. When the feedback signal  105  changes by the same amount as the target incremental change value, the optical cell  212  has been adjusted properly.  
      Alternatively, the translation unit  244  may store the display information and determine the pre-adjustment value of the voltage across the feedback sensor  104 . Transistor  326  is closed to store the pre-adjustment voltage value of the feedback sensor  104  on capacitor  328 . The translation unit  244  calculates a new target voltage across the feedback sensor  104  according to the display information. The translation unit  244  subtracts the pre-adjustment value from the target voltage to produce an incremental change value. Transistor  326  is opened, transistor  324  is closed, and the incremental change value is summed with pre-adjustment value stored on capacitor  328  to produce the target value used by the pixel control circuit  246 .  
      In another alternative embodiment, the translation circuit  244  stores the current flowing in each optical cell  212 . Before adjusting a particular optical cell  212 , the translation circuit  244  stores an updated target current value for the optical cell  212  for the current row address cycle. The translation circuit  244  then sums all the stored current values and calculates the target voltage across the feedback sensor  104 . The calculated voltage value becomes the target value and is used by the pixel control circuit  246  to adjust the optical cell  212  to the correct brightness.  
      A feedback sensor  104  that measures the current through multiple pixels may have a large absolute voltage difference between its all-off and all-on values. The pixel control circuit  246  must respond to the voltage levels provided by both the translation circuit  244  and the feedback sensor  104 . Using an incremental change in voltage instead of the absolute voltage directly from the feedback sensor  104  tends to allow the pixel control circuit  246  to work at a lower voltage and consume less power.  
      Several implementations derive the current target value from the pre-adjustment value, including the preceding value in the optical cell  212  for which the brightness is updated. The values may accumulate errors during the frame time, such as errors from electromagnetic interference, capacitive coupling, and parasitic discharge of the capacitor  210 . To reduce such errors, the control voltage stored on the capacitor  210  may be initialized to a known value, such as discharged to zero, by activating the pixel discharge circuit  334  to close the switch  1010  ( FIG. 10 ). Initializing the charge on the storage capacitor  210  facilitates charging the capacitor  210  to the desired target value independent of the previous, possibly erroneous, value, which tends to suppress propagation of errors from frame to frame. The pixel discharge may reduce errors when using both electrical sensors (such as resistors) and optical sensors (such as photo resistors).  
      The display system  100  according to various aspects of the present invention provides for monitoring the brightness of individual pixels and dynamically, automatically adjusting the brightness to achieve the desired brightness, regardless of the age or operating characteristics of the OLED  208 . Any suitable method may be used to monitor and adjust optical cell brightness. In the present embodiments, the display information is received from a source, for example data for a frame. The display information suitably contains brightness information for each OLED  208  in the display system  100 .  
      The translation circuit  244  translates the display information into target values corresponding to the desired brightness or change in brightness of each optical cell  212 , such as a target current through an OLED required to achieve the desired brightness or a feedback sensor voltage from a photosensor corresponding to the desired brightness. The target values are provided to the pixel control circuit  246  to drive the display panel  102 , for example as a sequence of lines corresponding to data for each row in the frame.  
      The pixel control circuit  246  drives each pixel in the row of the display panel  102  and monitors the feedback signal  105 . For each optical cell  212  in the row, the selector  330  compares the target value with a preceding value, such as the preceding target value or the feedback signal  105  for the relevant optical cell from the preceding frame. The selector then provides either an increasing or a decreasing signal to the optical cell  212  via the column electrode  228 . Alternatively, the pixel control circuit  246  may initialize the optical cell to a known state, such as fully charged or fully discharged, and then provide a decreasing signal or an increasing signal to the optical cell  212 . The optical cells  212  are addressed by activating the appropriate row selection electrode  232  to connect the column electrode  228  to the optical cells  212 . If necessary, the feedback sensor  104  may also be connected to the optical cell, for example by deactivating row maintenance electrode  230 .  
      As the variable signal is applied to the optical cell  212 , the brightness changes, causing a corresponding change in the feedback signal  105 . The pixel control circuit  246  compares the feedback signal  105  to the target value and adjusts the signal provided to the optical cell  212  so that the brightness is maintained at the desired level. The pixel control circuit  246  may continue to monitor the brightness via the feedback signal  105  to maintain the desired brightness for the remaining portion of the row address cycle. At the end of the row address cycle, the row electrode is deactivated, leaving the optical cell  212  at the desired brightness. If appropriate, the feedback sensor  104  may be disconnected from the optical cell  212  as well and power may be provided via an alternate route, such as by activating the row maintenance electrode.  
      The particular implementations shown and described are illustrative of the invention and its best mode and are not intended to otherwise limit the scope of the present invention in any way. Indeed, for the sake of brevity, conventional manufacturing, connection, preparation, and other functional aspects of the system may not be described in detail. Furthermore, the connecting lines shown in the various figures are intended to represent exemplary functional relationships and/or physical couplings between the various elements. Many alternative or additional functional relationships or physical connections may be present in a practical system.  
      The present invention has been described above with reference to a preferred embodiment. However, changes and modifications may be made to the preferred embodiment without departing from the scope of the present invention. These and other changes or modifications are intended to be included within the scope of the present invention.