Patent Publication Number: US-11663973-B1

Title: External compensation for displays using sensing and emission differences

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from and the benefit of U.S. Provisional Application Ser. No. 62/669,898, entitled “EXTERNAL COMPENSATION FOR DISPLAYS USING SENSING AND EMISSION DIFFERENCES,” filed May 10, 2018, which is hereby incorporated by reference in its entirety for all purposes. 
    
    
     SUMMARY 
     A summary of certain embodiments disclosed herein is set forth below. It should be understood that these aspects are presented merely to provide the reader with a brief summary of these certain embodiments and that these aspects are not intended to limit the scope of this disclosure. Indeed, this disclosure may encompass a variety of aspects that may not be set forth below. 
     Pixel-based display panels may generate images by the use of driving signals (e.g., a voltage or a current) provided to the individual pixels of the display. Due to inhomogeneities across pixels of a display, the brightness level of the pixel in response to a specific electrical signal may vary. Compensation circuitry that receives data from sensing circuitry may be used to correct the driving signals and prevent image artifacts from appearing. However, coupling the sensing circuitry to a pixel may change the electrical characteristics of the pixel circuitry. To prevent the changes caused by the presence of the sensing circuitry, such differences may be calibrated and correction factors may be programmed into the compensation circuitry of the display. Embodiments described herein include systems and methods that are capable of performing such calibrations and employing the correction factors during compensation using measurements from the sensing circuitry. The use of the embodiments described herein may improve the quality of the images provided by the display. 
     In one embodiment, an electronic device is described. The electronic device may include a pixel panel having multiple pixels, sensing circuitry that can be coupled or decoupled to the pixels, and compensation circuitry that may process image signals for the pixel. Processing of the image signals may use data including a received image signal from processing circuitry of the electronic device and received measurements from the sensing circuitry. The compensation circuitry may also employ a correction factor formula that may use the received image signal, the received measurements, and correction factor that is calculated to compensate an effect of the measurement circuitry on the pixels. Using the received data, as well as a correction factor, the compensation circuitry may generate a compensated signal, which may be provided to the pixel. 
     In another embodiment, a method for calibration is described. The method may include a determination of a current-voltage characteristic for pixels of the pixel panel in a condition in which the sensing circuitry is not coupled to the pixels or does not affect the pixel. The method may also include a determination of a current-voltage characteristics for pixels of the pixel panel in a condition in which the measurement circuitry is coupled to the pixels. Based on the two current-voltage characteristics calculated, a correction factor may be determined. The correction factor may be stored in a compensation circuitry of the pixel panel and may be used as part of a formula for compensation of signals. 
     In another embodiment, a method for compensating brightness in a pixel panel is described. The method may include a process for receiving a driving signal from processing circuitry, which is expected to generate a target current in the pixel, which may be associated with a target brightness for the pixel. The method may also include a process for receiving a measurement of an actual current generated in the pixel in response to the electric signal. The method may also include a process for generation of a compensated signal, which takes into account a difference between the target and the actual current in the pixel as well as a correction factor that may be stored in the compensation circuitry. 
     The correction factor is calculated to compensate for the impact of the measurement circuitry. The compensated signal may be provided to the pixel. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various aspects of this disclosure may be better understood upon reading the following detailed description and upon reference to the drawings in which: 
         FIG.  1    is a schematic block diagram of an electronic device that may implement the external compensation in pixel-based displays, in accordance with an embodiment; 
         FIG.  2    is a perspective view of a notebook computer representing an embodiment of the electronic device of  FIG.  1   , in accordance with an embodiment; 
         FIG.  3    is a front view of a hand-held device representing another embodiment of the electronic device of  FIG.  1   , in accordance with an embodiment; 
         FIG.  4    is a front view of another hand-held device representing another embodiment of the electronic device of  FIG.  1   , in accordance with an embodiment; 
         FIG.  5    is a front view of a desktop computer representing another embodiment of the electronic device of  FIG.  1   , in accordance with an embodiment; 
         FIG.  6    is a front view and side view of a wearable electronic device representing another embodiment of the electronic device of  FIG.  1   , in accordance with an embodiment; 
         FIG.  7    is a circuit diagram of the pixel-based display of  FIG.  1   , in accordance with an embodiment; 
         FIG.  8    is a diagram of pixel-driving circuitry that may employ external sensing-based compensation, in accordance with an embodiment; 
         FIG.  9    is a chart illustrating, using an example, the impact of the sensing circuitry in the current-voltage (IV) diagram, in accordance with an embodiment; 
         FIG.  10 A  is a diagram of pixel circuitry during sensing, in accordance with an embodiment; 
         FIG.  10 B  is a diagram of the pixel circuitry of  FIG.  10 A  during normal operation, in accordance with an embodiment: 
         FIG.  11    is a diagram illustrating a process to implement external compensation, in accordance with an embodiment; 
         FIG.  12    is a diagram illustrating a process to identify a correction factor for external compensation, in accordance with an embodiment; and 
         FIG.  13    is a flow chart of a method to implement external compensation, in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS 
     One or more specific embodiments will be described below. In an effort to provide a concise description of these embodiments, not all features of an actual implementation are described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure. 
     When introducing elements of various embodiments of the present disclosure, the articles “a,” “an,” and “the” are intended to mean that there are one or more of the elements. The terms “comprising,” “including,” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. Additionally, it should be understood that references to “one embodiment” or “an embodiment” of the present disclosure are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features. 
     Many electronic devices may use display panels to display images or provide user interfaces. The displays may be line-based displays, such as cathode-ray tube (CRT) displays, or pixel-based displays, such as light-emitting diode (LED) displays, organic LED (OLED) displays, active-matrix OLED (AMOLED) displays, electronic-ink displays, electronic paper displays, among others. Pixel-based displays may operate by means of driving circuitry (or circuitries) that provides an electrical signal (e.g., a current or a voltage) to each pixel. In response to the electric signal, each the pixel circuitry may provide a specific level of brightness or color. For example, in LED displays, each pixel circuitry may receive a voltage corresponding to a target brightness and may drive a current through the LED. In this example, the brightness of the pixel may be associated to the current passing through the LED. 
     Many electronic devices, such as televisions, smartphones, computer panels, smartwatches, and automobile dashboards, among others, include electronic displays that can display content and provide user interfaces. The electronic displays may employ pixel panels, which may be operatively coupled to image generation circuitry in the electronic device. The electronic display may receive image data from image generation circuitry or processing circuitry, and generate driving signals to the individual pixels in the pixel panel. As an example, in panels using pixels formed from light-emitting diodes (LEDs) or organic light-emitting diodes (OLEDs), pixel-driving circuitry in the display may receive image data and may set a target pixel brightness for each pixel and form the image, by providing a voltage signal to the individual pixels. The current induced through the LED or OLED through in response to the voltage signal may cause the target brightness. 
     Due to variations that may occur by, for example, fabrication artifacts, component age, temperature, humidity variations, or material variations, different pixels may respond differently to the driving signals. For example, in an OLED-based pixel panel, different OLED pixels circuits may induce different currents in response to a given input voltage. To correct such errors, pixel circuitry may be coupled to sensing circuitry, and the data generated by the sensing circuitry may be used to adjust the input voltage. The use of compensation circuitry may improve the quality of images and prevent artifacts in the display panel due to the pixel inhomogeneities through the display. 
     As an example of inhomogeneities in a display, consider an OLED-based panel in which each pixel is driven using a voltage signal received from the driver. A transistor associated with the pixel may receive the voltage signal and may drive a current through the OLED of the pixel. The brightness of the OLED pixel may be proportional to the source current (I S ), which may be determined, among other things, by the gate-source voltage (V GS ) of the transistor and the impedance displayed by the OLED. The relationship between the V GS  and the Is (i.e., the IV characteristic of the pixel circuitry) may be different across the display panel due to differences among the transistors or the OLEDs. 
     In order to prevent variations in the IV characteristic from causing visual artifacts in the display panel, compensation systems may be used. Compensation systems may include sensing circuitry that can measure the actual source current I S  obtained in response to the input electrical signal, and compensation circuitry may be use the measured Is to adjust the input electrical signal based on the measurements. However, the IV characteristic of the pixel circuitry during sensing may be different from the IV characteristic of the pixel circuitry under normal conditions (i.e., not sensing). This may be caused, for example, by impact of the coupling to the sensing circuitry, as detailed below. This effect may impact the quality of the compensation system in the display panel. The embodiments of the present application detailed below include methods and systems that take into account the impact of the sensing circuitry to perform a calibration and generate a correction factor for compensation systems. As detailed below are method and system embodiments that employ the calibrated correction factor to improve the image quality of the display. 
     With the foregoing in mind, a general description of suitable electronic devices with reduced bezel dimensions that may compensation circuitry for pixels, as discussed herein, are provided below. Turning first to  FIG.  1   , an electronic device  10  according to an embodiment of the present disclosure may include, among other things, one or more processor(s)  12 , memory  14 , nonvolatile storage  16 , a display  18 , input structures  22 , an input/output (I/O) interface  24 , a network interface  26 , and a power source  28 . The various functional blocks shown in  FIG.  1    may include hardware elements (including circuitry), software elements (including computer code stored on a computer-readable medium) or a combination of both hardware and software elements. It should be noted that  FIG.  1    is merely one example of an implementation and is intended to illustrate the types of components that may be present in electronic device  10 . 
     By way of example, the electronic device  10  may represent a block diagram of the notebook computer depicted in  FIG.  2   , the handheld device depicted in  FIG.  3   , the handheld device depicted in  FIG.  4   , the desktop computer depicted in  FIG.  5   , the wearable electronic device depicted in  FIG.  6   , or similar devices. It should be noted that the processor(s)  12  and other related items in  FIG.  1    may be generally referred to herein as “data processing circuitry.” Such data processing circuitry may be embodied wholly or in part as software, firmware, hardware, or any combination thereof. Furthermore, the data processing circuitry may be a single contained processing module or may be incorporated wholly or partially within any of the other elements within the electronic device  10 . 
     In the electronic device  10  of  FIG.  1   , the processor(s)  12  may be operably coupled with the memory  14  and the nonvolatile storage  16  to perform various algorithms. Such programs or instructions executed by the processor(s)  12  may be stored in any suitable article of manufacture that includes one or more tangible, computer-readable media at least collectively storing the instructions or routines, such as the memory  14  and the nonvolatile storage  16 . The memory  14  and the nonvolatile storage  16  may include any suitable articles of manufacture for storing data and executable instructions, such as random-access memory, read-only memory, rewritable flash memory, hard drives, and optical discs. In addition, programs (e.g., an operating system) encoded on such a computer program product may also include instructions that may be executed by the processor(s)  12  to enable the electronic device  10  to provide various functionalities. 
     In certain embodiments, the display  18  may be a liquid-crystal display (LCD), which may allow users to view images generated on the electronic device  10 . In some embodiments, the display  18  may include a touch screen, which may allow users to interact with a user interface of the electronic device  10 . Furthermore, it should be appreciated that, in some embodiments, the display  18  may include one or more organic light emitting diode (OLED) displays, or some combination of LCD panels and OLED panels. The display  18  may receive images, data, or instructions from processor(s)  12  or memory  14 , and provide an image in display  18  for interaction. 
     The input structures  22  of the electronic device  10  may enable a user to interact with the electronic device  10  (e.g., pressing a button to increase or decrease a volume level). The I/O interface  24  may enable electronic device  10  to interface with various other electronic devices, as may the network interface  26 . The network interface  26  may include, for example, one or more interfaces for a personal area network (PAN), such as a Bluetooth network, for a local area network (LAN) or wireless local area network (WLAN), such as an 802.11x Wi-Fi network, and/or for a wide area network (WAN), such as a 3rd generation (3G) cellular network, 4th generation (4G) cellular network, long term evolution (LTE) cellular network, or long term evolution license assisted access (LTE-LAA) cellular network. The network interface  26  may also include one or more interfaces for, for example, broadband fixed wireless access networks (WiMAX), mobile broadband Wireless networks (mobile WiMAX), asynchronous digital subscriber lines (e.g., ADSL, VDSL), digital video broadcasting-terrestrial (DVB-T) and its extension DVB Handheld (DVB-H), ultra-Wideband (UWB), alternating current (AC) power lines, and so forth. As further illustrated, the electronic device  10  may include a power source  28 . The power source  28  may include any suitable source of power, such as a rechargeable lithium polymer (Li-poly) battery and/or an alternating current (AC) power converter. 
     In certain embodiments, the electronic device  10  may take the form of a computer, a portable electronic device, a wearable electronic device, or other type of electronic device. Such computers may include computers that are generally portable (such as laptop, notebook, and tablet computers) as well as computers that are generally used in one place (such as conventional desktop computers, workstations, and/or servers). In certain embodiments, the electronic device  10  in the form of a computer may be a model of a MacBook®, MacBook® Pro, MacBook Air®, iMac®, Mac® mini, or Mac Pro® available from Apple Inc. 
     By way of example, the electronic device  10 , taking the form of a notebook computer  10 A, is illustrated in  FIG.  2    in accordance with one embodiment of the present disclosure. Notebook computer  10 A, or laptop computer, may be a MacBook®, MacBook® Pro, MacBook Air® by Apple, Inc. The depicted computer  10 A may include a housing or enclosure  36 , a display  18  framed by a bezel  38  of the enclosure  36 , input structures  22 , and ports of an I/O interface  24 . In one embodiment, the input structures  22  (such as a keyboard and/or touchpad) may be used to interact with the computer  10 A, such as to start, control, or operate a GUI or applications running on computer  10 A. For example, a keyboard and/or touchpad may allow a user to navigate a user interface or application interface displayed on display  18 . 
       FIG.  3    depicts a front view of a handheld device  10 B, which represents one embodiment of the electronic device  10 . The handheld device  10 B may represent, for example, a portable phone, a media player, a personal data organizer, a handheld game platform, or any combination of such devices. By way of example, the handheld device  10 B may be a model of an iPhone® available from Apple Inc. of Cupertino, Calif. The handheld device  10 B may include an enclosure  36  to protect interior components from physical damage and to shield them from electromagnetic interference. The enclosure  36  may include bezel  38 , which surrounds the display  18 . The I/O interfaces  24  may open through the enclosure  36  and may include, for example, an I/O port for a hard-wired connection for charging and/or content manipulation using a standard connector and protocol, such as the Lightning connector provided by Apple Inc., a universal service bus (USB), or other similar connector and protocol. 
     User input structures  22 , in combination with the display  18 , may allow a user to control the handheld device  10 B. For example, the input structures  22  may activate or deactivate the handheld device  10 B, navigate user interface to a home screen, a user-configurable application screen, and/or activate a voice-recognition feature of the handheld device  10 B. Other input structures  22  may provide volume control, or may toggle between vibrate and ring modes. The input structures  22  may also include a microphone may obtain a user&#39;s voice for various voice-related features, and a speaker may enable audio playback and/or certain phone capabilities. The input structures  22  may also include a headphone input may provide a connection to external speakers and/or headphones. 
       FIG.  4    depicts a front view of another handheld device  10 C, which represents another embodiment of the electronic device  10 . The handheld device  10 C may represent, for example, a tablet computer, or one of various portable computing devices. By way of example, the handheld device  10 C may be a tablet-sized embodiment of the electronic device  10 , which may be, for example, a model of an iPad® available from Apple Inc. of Cupertino, Calif. The handheld device  10 C may include an enclosure  36  to protect interior components from physical damage and to shield them from electromagnetic interference. The enclosure  36  may include bezel  38 , which surrounds the display  18 . The I/O interfaces  24  may open through the enclosure  36  and may include, for example, an I/O port for a hard-wired connection for charging and/or content manipulation using a standard connector and protocol, such as the Lightning connector provided by Apple Inc., a universal service bus (USB), or other similar connector and protocol. 
     Turning to  FIG.  5   , a computer  10 D may represent another embodiment of the electronic device  10  of  FIG.  1   . The computer  10 D may be any computer, such as a desktop computer, a server, or a notebook computer, but may also be a standalone media player or video gaming machine. By way of example, the computer  10 D may be an iMac®, or other similar device by Apple Inc. It should be noted that the computer  10 D may also represent a personal computer (PC) by another manufacturer. A similar enclosure  36  may be provided to protect and enclose internal components of the computer  10 D such as the display  18 . The display  18  may be surrounded by a bezel  38  of the enclosure  36 . In certain embodiments, a user of the computer  10 D may interact with the computer  10 D using various peripheral input devices, such as the keyboard  22 A or mouse  22 B (e.g., input structures  22 ), which may connect to the computer  10 D. 
     Similarly,  FIG.  6    depicts a wearable electronic device  10 E representing another embodiment of the electronic device  10  of  FIG.  1    that may operate using the techniques described herein. By way of example, the wearable electronic device  10 E, which may include a wristband  43 , may be an Apple Watch® by Apple, Inc. However, in other embodiments, the wearable electronic device  10 E may include any wearable electronic device such as, for example, a wearable exercise monitoring device (e.g., pedometer, accelerometer, heart rate monitor), or other device by another manufacturer. The display  18 , framed by a bezel  38  of the enclosure  36  of the wearable electronic device  10 E may include a touch screen display  18  (e.g., LCD, OLED display, active-matrix organic light emitting diode (AMOLED) display, and so forth), as well as input structures  22 , which may allow users to interact with a user interface of the wearable electronic device  10 E. 
     As shown in  FIG.  7   , the display  18  may include a pixel array  80  having an array of one or more of pixels  82 . The display  18  may include any suitable circuitry to drive the pixels  82 . In the example of  FIG.  7   , the display  18  includes a controller  84 , a power driver  86 A, an image driver  86 B, and the array of the pixel  82 . The power driver  86 A and image driver  86 B may individually drive the pixels  82 . In some embodiments, the power driver  86 A and the image driver  86 B may include multiple channels for independently driving multiple of the pixel  82 . Each of the pixel  82  may include pixel circuitry, capable of receiving the electrical signals (e.g., driving signals from the power driver  86 A or image driver  86 B) and provide a current through a suitable light emitting element, such as a LED, one example of which is an OLED that causes light emission. 
     The scan lines S 0 , S 1 , . . . , and Sm and driving lines D 0 , D 1 , . . . , and Dm may connect the power driver  86 A to the pixel  82 . The pixel  82  may receive on or off instructions through the scan lines S 0 , S 1 , . . . , and Sm and may generate programming voltages corresponding to data voltages transmitted from the driving lines D 0 , D 1 , . . . , and Dm. The programming voltages may be transmitted to each of the pixel  82  and cause emission of light according to instructions from the image driver  86 B through driving lines M 0 , M 1 , . . . , and Mn. Both the power driver  86 A and the image driver  86 B may transmit voltage signals at programmed voltages through respective driving lines to operate each pixel  82  at a state determined by the controller  84  to emit light. Each driver may supply voltage signals at a duty cycle or amplitude sufficient to operate each pixel  82 . 
     The target brightness of each of the pixels  82  may be defined by the received image data. In this way, a first brightness of light may emit from a pixel  82  in response to a first value of the image data and the pixel  82  may emit a second brightness of light in response to a second value of the image data. Thus, image data may form images by generating driving signals to each individual pixel  82  that causes the individual pixels  82  to provide the target brightness. 
     The controller  84  may retrieve image data stored in the storage device(s)  14  indicative of the target brightness for the colored light outputs of individual pixels  82 . In some embodiments, the processing circuit(s)  12  may provide image data directly to the controller  84 . The controller  84  may coordinate the signals provided to each pixel  82  from the power driver  86 A or image driver  86 B. The pixel  82  may include pixel circuitry, which may include a controllable element, such as a transistor, one example of which is an MOSFET. The pixel circuitry may process the signals received from the power driver  86 A or image driver  86 B, and may generate the target brightness. However, any other suitable type of controllable elements, including thin film transistors (TFTs), p-type and or n-type MOSFETs, and other transistor types, may also be used. 
     The diagram in  FIG.  8    illustrates a compensation system  100 . The compensation system  100  may correct the driving signals provided to a pixel panel  102  from the driving circuitry  104 . Driving circuitry  104 , such as the power driver  86 A or the image driver  68 B described above, may generate signals directed to individual pixels. Signal from the driving circuitry  104  may go through compensation circuitry  106 . The compensation circuitry  106  may process the driving signals from the driving circuitry  104  using measurement data  110  received from a sensing circuitry  108 . The measurement data  110  may include a current or a voltage in one or more pixels of the pixel panel  102 . Using the measurement data  110 , the compensation circuitry  106  may adjust the driving signal received from the driving circuitry, and may generate a compensated signal to pixels of the pixel panel  102 . 
     To perform the measurement and obtain the measurement data  110 , sensing circuitry  108  may be coupled to the pixels of the pixel panel  102  through an electrical coupling  112 . The electrical coupling  112  may be configurable (e.g., switchable), such that the sensing circuitry  108  is coupled to the pixel circuitry during sensing, and uncoupled from the pixel circuitry during normal operations. However, as discussed above and detailed below, the sensing circuitry  108  may, through the electrical coupling  112 , impact the IV characteristic of the pixel circuitry in the pixel panel  102 . To compensate for differences in the IV characteristics between sensing and normal conditions, the compensation circuitry  106  may employ a correction factor in the compensation of the driving signal, which is detailed below. 
     The chart  130  in  FIG.  9    illustrates the impact of the sensing circuitry  108  on the IV curves for pixel circuitry of a pixel which may be driven using a transistor. Chart  130  shows the source current I S    132  through the transistor as a function of the gate-source voltage V GS    134  for a pixel during normal conditions  136  (i.e., when it is not being measured and, thus, uncoupled from the sensing circuitry), and during sensing conditions  138  (i.e., when it is measured and, thus, coupled to the sensing circuitry). The source current I S    132  may be the current driven through the light-emitting element (e.g., LED). 
     As illustrated in chart  130 , during sensing conditions  138 , the IV characteristic may be shifted up relative to the normal conditions  136 . For example, at a voltage of approximately 1.5V (voltage  140 ), the pixel current may be approximately 22 nA in normal conditions  136  and may be 31 nA in sensing conditions  138 , resulting in a shift  142  of approximately 9 nA. As a result of the shift  142 , a system employing data obtained during sensing may underestimate the required V GS    134  that produces a particular I S    132 . Moreover, chart  130  illustrates that the difference is not uniform. As illustrated, at a voltage of approximately 2.5V (voltage  144 ), the pixel current may be approximately 80 nA in normal conditions  136 , but may be 140 nA in sensing conditions  138 , leading to a shift  146  at voltage  144  that is substantially different from the shift  142  at voltage  140 . Therefore, the compensation strategy may benefit from employing a content-dependent (e.g., current-dependent, voltage dependent, brightness-dependent) correction factor. 
     One cause for the impact of the sensing circuitry in the measurements is illustrated in  FIGS.  10 A and  10 B . The diagram in  FIG.  10 A  illustrates the configuration of a pixel  150  during sensing conditions (e.g., sensing condition  138  of  FIG.  9   ). In the arrangement, capacitors C P    152  and C GS    154  are configured to provide a charge from a reference signal to the transistor  156 . The diagram also illustrates the V GS  voltage between gate  160  and source  162  of the transistor  156 . The circuit in diagram may be under sensing conditions, in which sensing circuitry is coupled to the pixel  150 . As a result, the gap  164  between an input voltage V dataS    166 , and an anode voltage V sense    168  may lead to the V GS    158 . In fact, for pixel  150  in  FIG.  10 A , the V GS    158  may be described as:
 
 V   gs   =V   ref   −V   dataS   +k ( V   dataS   −V   sense ).  (1)
 
     In the above equation, as well as in the following descriptions, k is determined by the voltage divider expression: 
     
       
         
           
             
               
                 
                   k 
                   = 
                   
                     
                       
                         C 
                         p 
                       
                       
                         
                           C 
                           gs 
                         
                         + 
                         
                           C 
                           p 
                         
                       
                     
                     . 
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     The diagram in  FIG.  10 B  illustrates the configuration of the pixel  150  during normal conditions (e.g., normal conditions  136  of  FIG.  9   ). In such system, the pixel  150  is arranged in the same circuit as the one illustrated in  FIG.  10 A , but is under normal conditions, in which sensing circuitry is decoupled. As a result, the gap  174  between an input voltage V dataD    176 , and the normal anode voltage V anode    178  may lead to a V GS    158 . 
     As a result, the V GS    158  in pixel  150  in the  FIG.  10 B  may be described as:
 
 V   gs   =V   ref   −V   dataD   +k ( V   dataD   −V   anode ).  (3)
 
     Note that the V anode    178  may be different from the V sense    168 . As a result, if V dataS    166  and V dataD    176  are equal, the current I S , and thus, the brightness may be different. In order to prevent the difference in brightness, and due to the fact that the V GS    158  may determine the brightness of the pixel, the V GS  expressions (1) and (3) may be equated, to identify a calibration curve, or compensation curve. To that end, an expression for the input voltage under normal conditions, V dataD    176 , as a function of V dataS    166  may be identified as: 
     
       
         
           
             
               
                 
                   
                     V 
                     dataD 
                   
                   = 
                   
                     
                       V 
                       dataS 
                     
                     + 
                     
                       
                         k 
                         
                           k 
                           + 
                           1 
                         
                       
                       ⁢ 
                       
                         
                           ( 
                           
                             
                               
                                 V 
                                 OLED 
                               
                               ( 
                               I 
                               ) 
                             
                             + 
                             
                               
                                 V 
                                 SSEL 
                               
                               ( 
                               DBV 
                               ) 
                             
                             - 
                             
                               V 
                               sense 
                             
                           
                           ) 
                         
                         . 
                       
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     In the above expression, V OLED (I) corresponds to the correction applied in view of the current going through the OLED, and V SSEL (DBV) corresponds to a baseline or bias voltage that may be associated with the global display brightness level. The above expression allows calculation of the V dataD    176  that should be used under normal conditions to obtain a target brightness when the V dataS    166  is the voltage that provided that brightness during sensing. 
     Diagram  200  in  FIG.  11    illustrates a process for determining differences in IV between the normal and sensing conditions. In order to obtain the IV curves, a reference current  202  may be used. The reference current  202  may be adjusted using a pixel-level non-uniformity compensation  204 , to prevent interference from high-frequency noise or other pixel-to-pixel variations. The compensated electrical signal may go to pixels of the pixel panel  102 . The emission current from pixels in the pixel panel  102  may be measured by a process  208  to produce an emission current reading  210 . That measurement may take place during calibration in a factory, and may include the use of highly sensitive low impedance current measuring instruments or through the measurement of the brightness of the pixel. The electrical signal may also go through a spatial averaging  212  of the driving voltages, to provide an averaged voltage reading  214 . Based on the emission current reading  210  and the averaged voltage reading  214 , an emission IV curve  216  may be generated. The sensing circuit  218  of the display panel may be used to obtain the sensing IV curve  220 . As discussed above, based on a difference between the emission IV curve  216  and the sensing IV curve  220 , a correction term  222  may be obtained. 
     The characterization described in diagram  200  may be performed in the production of the electronic device  10  (e.g., during manufacturing, testing, or quality control), and the identified correction term  222  may be stored in the compensation circuitry or in a memory of the electronic device  10 . The calibration and generation of the correction term  222  may be generated automatically by a calibration electronic device. Such calibration electronic device may include, or be coupled to, low-impedance current sensors, brightness sensors, or any other instrument capable of measuring currents or brightness without affecting any biasing voltage in the pixel circuitry. In some electronic devices  10 , the calibration device may be included, and may be configured to perform the characterization process described in diagram  200  periodically (e.g., after a time period established by a wall clock, after a number of initializations, after a number of hours of uptime of the device), to recalculate the correction term  222  and incorporate variations resulting from regular usage of the display after the initial programming of the compensation circuitry. 
     The process illustrated described by diagram  200  employs a spatial averaging  212 . As a result, the correction term  222  described may be specific to a region of the display panel. For example, a display panel having 1920×1080 pixels may be divided into 200 regions in a 20×10 grid with 96×108 pixels in each region, and the compensation circuitry may store one correction term  222  for each region. In some embodiments, the process illustrated by diagram  200  may bypass the spatial averaging  212 , and the compensation circuitry may perform compensation on an individual pixel basis. 
       FIG.  12    further details the calculation of the correction term  222 . As shown in expression (4) above, the correction factor may be determined by an application of a discrete differentiation. That is, by applying differentiation or a discrete differentiation of the expression (4), a compensation ratio may be obtained as: 
     
       
         
           
             
               
                 
                   
                     C 
                     ratio 
                   
                   = 
                   
                     
                       
                         Δ 
                         ⁢ 
                         
                           V 
                           data 
                         
                       
                       
                         Δ 
                         ⁢ 
                         
                           V 
                           sense 
                         
                       
                     
                     . 
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     In the above expression, the discrete differences ΔV data  (i.e., differential data voltage) and ΔV sense  (i.e., differential sense voltage) are calculated with respect to a baseline data voltage V data  and baseline sense voltage V sense  that provides a matching current I S  and in which ΔV data  and ΔV sense  lead to a similar change in current. The diagram on  FIG.  12    illustrates a 3-step process  250  for characterizing the correction factor based on this principle. The process  250  may have a first stage  252  in which a baseline data voltage is determined, a second stage  254  in which discrete differences are determined, and a third stage  256  in which the panel may be programmed. 
     In the first stage  252 , the pixel circuitry may be set to a baseline iteratively. The iterations loop between steps  262  and  264 . A V sense  voltage may be set in step  264 . With the set V sense  voltage set, a search for V data  voltage that reaches a target current, in step  262 , may be applied. The search for V data  may proceed by testing voltages over a range of values. In the second stage  254 , the pixel circuitry may iterate between steps  272  and  274 . In step  274 , a V sense +ΔV sense  voltage may be set in step  274 . In steps  272 , a search for a ΔV data  that causes V data +ΔV data  the pixel to provide the target current, may be performed. The correction factor may be then calculated by the expression (5), shown above. The search for the ΔV data  may be proceed by testing voltages over a range of values. Stages  252  and  254  may be repeated for multiple baselines of V sense  and V data . 
     Stages  252  and  254  may be performed with every pixel of the display panel or may implemented over a sparse subset of the pixels. The third stage  256  illustrates a sparse implementation. The sampling  282  illustrates a division that may be used for sparse calibration. For example, in a panel having 1920×1080 pixels, the display may be divided into 200 regions in a 20×10 grid with 96×108 pixels in each region, and stages  252  and  254  may be performed in one or few pixels for each region. The correction factor for the tested pixels in each region may be then averaged (process  284 ) to produce a grid  286  of correction factors. The correction factor for region of the grid  286  may be applied to all pixels of the region. The data for each region of the grid may be stored in the compensation circuitry, as discussed above. 
       FIG.  13    illustrates a method  300  for compensation for displays using the sensing and emission differences discussed above. Process  302  includes the measurement of the current-voltage characteristic of the pixels while the pixel is being sensed. Process  304  includes the measurement of the current-voltage characteristics of the pixels while the pixel is not being sensed. Process  302  and  304  may take place simultaneously or sequentially by any of the methods described above. Moreover, the measurements in processes  302  and  304  may be carried on every pixel of the display, or in a subset of pixels of the display, which may be determined by sampling. For example, in situations where the low-spatial artifacts is of concern may cause artifacts, the measurements may be performed on a sparse sample of the pixels of the display. 
     In process  306 , a correction factor may be determined based on the data obtained in processes  302  and  304 . It should be noted that the correction factor determination in process  306  may be integral to the processes  302  and  304 . For example, the calibration process may simultaneously perform processes  302  and  304  and may determine a correction factor using process  306  simultaneously, without long-term storage of the intermediate values. As discussed above, the correction factor calculated in process  306  may be used to provide improved images in process  308 . Process  308  may include programming the compensation circuitry using the correction factor calculated. As discussed above, the compensation circuitry may employ the correction factor for each individual pixel or for all pixels in a region. The distribution of pixels may be based on a spatial location, as discussed above. 
     The specific embodiments described above have been shown by way of example, and it should be understood that these embodiments may be susceptible to various modifications and alternative forms. It should be further understood that the claims are not intended to be limited to the particular forms disclosed, but rather to cover all modifications, equivalents, and alternatives falling within the spirit and scope of this disclosure. 
     The techniques presented and claimed herein are referenced and applied to material objects and concrete examples of a practical nature that demonstrably improve the present technical field and, as such, are not abstract, intangible or purely theoretical. Further, if any claims appended to the end of this specification contain one or more elements designated as “means for [perform]ing [a function] . . . ” or “step for [perform]ing [a function] . . . ” it is intended that such elements are to be interpreted under 35 U.S.C. § 112(f). However, for any claims containing elements designated in any other manner, it is intended that such elements are not to be interpreted under 35 U.S.C. § 112(f).