Abstract:
A system and method for operating a display at a constant luminance even as some of the pixels in the display are degraded over time. Each pixel in the display is configured to emit light when a voltage is supplied to the pixel&#39;s driving circuit, which causes a current to flow through a light emitting element. Degraded pixels are compensated by supplying their respective driving circuits with greater voltages. The display data is scaled by a compression factor less than one to reserve some voltage levels for compensating degraded pixels. As pixels become more degraded, and require additional compensation, the compression factor is decreased to reserve additional voltage levels for use in compensation.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation-in-part of prior application Ser. No. 11/402,624, filed Apr. 12, 2006, which claims priority to Canadian Patent No. 2,504,571, filed Apr. 12, 2005, each of which is incorporated herein by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to display technologies, more specifically a method and system for compensating for non-uniformities of elements in light emitting device displays. 
       BACKGROUND 
       [0003]    Active-matrix organic light-emitting diode (AMOLED) displays are well known art. Amorphous silicon is, for example, a promising material for AMOLED displays, due to its low cost and vast installed infrastructure from thin film transistor liquid crystal display (TFTLCD) fabrication. 
         [0004]    All AMOLED displays, regardless of backplane technology used, exhibit differences in luminance on a pixel to pixel basis, primarily as a result of process or construction inequalities, or from aging caused by operational use over time. Luminance non-uniformities in a display may also arise from natural differences in chemistry and performance from the OLED materials themselves. These non-uniformities must be managed by the AMOLED display electronics in order for the display device to attain commercially acceptable levels of performance for mass-market use. 
         [0005]      FIG. 1  illustrates an operational flow of a conventional AMOLED display  10 . Referring to  FIG. 1 , a video source  12  contains luminance data for each pixel and sends the luminance data in the form of digital data  14  to a digital data processor  16 . The digital data processor  16  may perform some data manipulation functions, such as scaling the resolution or changing the color of the display. The digital data processor  16  sends digital data  18  to a data driver integrated circuit (IC)  20 . The data driver IC  20  converts that digital data  18  into an analog voltage or current  22 , which is sent to thin film transistors (TFTs)  26  in a pixel circuit  24 . The TFTs  26  convert that voltage or current  22  into another current  28  which flows through an organic light-emitting diode (OLED)  30 . The OLED  30  converts the current  28  into visible light  36 . The OLED  30  has an OLED voltage  32 , which is the voltage drop across the OLED. The OLED  30  also has an efficiency  34 , which is a ratio of the amount of light emitted to the current through the OLED. 
         [0006]    The digital data  14 , analog voltage/current  22 , current  28 , and visible light  36  all contain the exact same information (i.e. luminance data). They are simply different formats of the initial luminance data that came from the video source  12 . The desired operation of the system is for a given value of luminance data from the video source  12  to always result in the same value of the visible light  36 . 
         [0007]    However, there are several degradation factors which may cause errors on the visible light  36 . With continued usage, the TFTs will output lower current  28  for the same input from the data driver IC  20 . With continued usage, the OLED  30  will consume greater voltage  32  for the same input current. Because the TFT  26  is not a perfect current source, this will actually reduce the input current  28  slightly. With continued usage, the OLED  30  will lose efficiency  34 , and emit less visible light for the same current. 
         [0008]    Due to these degradation factors, the visible light output  36  will be less over time, even with the same luminance data being sent from the video source  12 . Depending on the usage of the display, different pixels may have different amounts of degradation. 
         [0009]    Therefore, there will be an ever-increasing error between the required brightness of some pixels as specified by the luminance data in the video source  12 , and the actual brightness of the pixels. The result is that the decreased image will not show properly on the display. 
         [0010]    One way to compensate for these problems is to use a feedback loop.  FIG. 2  illustrates an operational flow of a conventional AMOLED display  40  that includes the feedback loop. Referring to  FIG. 2 , a light detector  42  is employed to directly measure the visible light  36 . The visible light  36  is converted into a measured signal  44  by the light detector  42 . A signal converter  46  converts the measured visible light signal  44  into a feedback signal  48 . The signal converter  46  may be an analog-to-digital converter, a digital-to-analog converter, a microcontroller, a transistor, or another circuit or device. The feedback signal  48  is used to modify the luminance data at some point along its path, such as an existing component (e.g.  12 ,  16 ,  20 ,  26 ,  30 ), a signal line between components (e.g.  14 ,  18 ,  22 ,  28 ,  36 ), or combinations thereof. 
         [0011]    Some modifications to existing components, and/or additional circuits may be required to allow the luminance data to be modified based on the feedback signal  48  from the signal converter  46 . If the visible light  36  is lower than the desired luminance from video source  12 , the luminance signal may be increased to compensate for the degradation of the TFT  26  or the OLED  30 . This results in that the visible light  36  will be constant regardless of the degradation. This compensation scheme is often known as Optical Feedback (OFB). However, in the system of  FIG. 2 , the light detector  42  must be integrated onto a display, usually within each pixel and coupled to the pixel circuitry. Not considering the inevitable issues of yield when integrating a light detector into each pixel, it is desirable to have a light detector which does not degrade itself, however such light detectors are costly to implement, and not compatible with currently installed TFT-LCD fabrication infrastructure. 
         [0012]    Therefore, there is a need to provide a method and system which can compensate for non-uniformities in displays without measuring a light signal. 
         [0013]    AMOLED displays are conventionally operated according to digital data from a video source. The OLEDs within the display can be programmed to emit light with luminance according to a programming voltage or a programming current. The programming current or programming voltage are conventionally set by a display driver that takes digital data as input and has an analog output for sending the programming current or programming voltage to pixel circuits. The pixel circuits are configured to drive current through OLEDs based on the programming current or programming voltage. 
       SUMMARY 
       [0014]    It is an object of the invention to provide a method and system that obviates or mitigates at least one of the disadvantages of existing systems. 
         [0015]    In accordance with an aspect of the present invention there is provided a system for compensating non-uniformities in a light emitting device display which includes a plurality of pixels and a source for providing pixel data to each pixel circuit. The system includes: a module for modifying the pixel data applied to one or more than one pixel circuit, an estimating module for estimating a degradation of a first pixel circuit based on measurement data read from a part of the first pixel circuit, and a compensating module for correcting the pixel data applied to the first or a second pixel circuit based on the estimation of the degradation of the first pixel circuit. 
         [0016]    In accordance with a further aspect of the present invention there is provided a method of compensating non-uniformities in a light emitting device display having a plurality of pixels, including the steps of: estimating a degradation of the first pixel circuit based on measurement data read from a part of the first pixel circuit, and correcting pixel data applied to the first or a second pixel circuit based on the estimation of the degradation of the first pixel circuit. 
         [0017]    The present disclosure provides a method of maintaining uniform luminosity of an AMOLED display. The AMOLED display includes an array of pixels having light emitting devices. The light emitting devices are configured to emit light according to digital input from a video source. The video source includes digital data corresponding to a desired luminance of each pixel in the AMOLED display. Over time, aspects within the light emitting devices and their associated driving circuits degrade and require compensation to continue to emit light with the same luminance for a given digital input. 
         [0018]    Degradation of the pixels in the light emitting display are compensated by incrementing the digital inputs of the pixels according to a measured or estimated degradation of the pixels. To allow for compensation to occur, the digital input is compressed to a range of values less than an available range. Compressing the digital input is carried out according to a compression factor, which is a number less than one. In an implementation of the present disclosure, the digital inputs are multiplied by the compression factor, which compresses the digital input to a range less than the available range. The remaining portion of the digital range can be used to provide compensation to degraded pixels based on measured or estimated degradation of the pixels. The present disclosure provides methods for setting and adjusting the compression factor to statically or dynamically adjust the compression factor and provide compensation to the display by incrementing the digital signals before the signals are sent to the driving circuits. 
         [0019]    The foregoing and additional aspects and embodiments of the present invention will be apparent to those of ordinary skill in the art in view of the detailed description of various embodiments and/or aspects, which is made with reference to the drawings, a brief description of which is provided next. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    These and other features of the invention will become more apparent from the following description in which reference is made to the appended drawings. 
           [0021]      FIG. 1  illustrates a conventional AMOLED system. 
           [0022]      FIG. 2  illustrates a conventional AMOLED system that includes a light detector and a feedback scheme that uses the signal from the light detector. 
           [0023]      FIG. 3  illustrates a light emitting display system to which a compensation scheme in accordance with an embodiment of the present invention is applied. 
           [0024]      FIG. 4  illustrates an example of the light emitting display system of  FIG. 3 . 
           [0025]      FIG. 5  illustrates an example of a pixel circuit of  FIG. 5 . 
           [0026]      FIG. 6  illustrates a further example of the light emitting display system of  FIG. 3 . 
           [0027]      FIG. 7  illustrates an example of a pixel circuit of  FIG. 6 . 
           [0028]      FIG. 8  illustrates an example of modules for the compensation scheme applied to the system of  FIG. 4 . 
           [0029]      FIG. 9  illustrates an example of a lookup table and a compensation algorithm module of  FIG. 7 . 
           [0030]      FIG. 10  illustrates an example of inputs to a TFT-to-pixel circuit conversion algorithm module. 
           [0031]      FIG. 11A  illustrates an experimental result of a video source outputting equal luminance data for each pixel for a usage time of zero hours. 
           [0032]      FIG. 11B  illustrates an experimental result of a video source outputting maximum luminance data to some pixels and zero luminance data to other pixels for a usage of time of 1000 hours. 
           [0033]      FIG. 11C  illustrates an experimental result of a video source outputting equal luminance data for each pixel after some pixels received maximum luminance data and others pixels received zero luminance data for a usage time of 1000 hours when no compensation algorithm is applied. 
           [0034]      FIG. 11D  illustrates an experimental result of a video source outputting equal luminance data for each pixel after some pixels received maximum luminance data and others pixels received zero luminance data for a usage time of 1000 hours when a constant brightness compensation algorithm is applied. 
           [0035]      FIG. 11E  illustrates an experimental result of a video source outputting equal luminance data for each pixel after some pixels received maximum luminance data and others pixels received zero luminance data for a usage time of 1000 hours when a decreasing brightness compensation algorithm is applied. 
           [0036]      FIG. 12  illustrates an example of a grayscale compression algorithm. 
           [0037]      FIG. 13  is a data flow chart showing the compression and compensation of luminosity input data used to drive an AMOLED display. 
           [0038]      FIG. 14  is a flowchart illustrating a method for selecting the compression factor according to display requirements and the design of the pixel circuit. 
           [0039]      FIG. 15  is a flowchart illustrating a method for selecting the compression factor according to a pre-determined headroom adjustment profile. 
           [0040]      FIG. 16  is a flowchart illustrating a method for selecting the compression factor according to dynamic measurements of degradation data exceeding a threshold over a previous compensation. 
           [0041]      FIG. 17  is a flowchart illustrating a method for selecting the compression factor according to dynamic measurements of degradation data exceeding a previously measured maximum. 
       
    
    
       [0042]    While the invention is susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and will be described in detail herein. It should be understood, however, that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
       DETAILED DESCRIPTION 
       [0043]    Embodiments of the present invention are described using an AMOLED display which includes a pixel circuit having TFTs and an OLED. However, the transistors in the pixel circuit may be fabricated using amorphous silicon, nano/micro crystalline silicon, poly silicon, organic semiconductors technologies (e.g. organic TFT), NMOS technology, CMOS technology (e.g. MOSFET), or combinations thereof. The transistors may be a p-type transistor or n-type transistor. The pixel circuit may include a light emitting device other than OLED. In the description below, “pixel” and “pixel circuit” may be used interchangeably. 
         [0044]      FIG. 3  illustrates the operation of a light emitting display system  100  to which a compensation scheme in accordance with an embodiment of the present invention is applied. A video source  102  contains luminance data for each pixel and sends the luminance data in the form of digital data  104  to a digital data processor  106 . The digital data processor  106  may perform some data manipulation functions, such as scaling the resolution or changing the color of the display. The digital data processor  106  sends digital data  108  to a data driver IC  110 . The data driver IC  110  converts that digital data  108  into an analog voltage or current  112 . The analog voltage or current  112  is applied to a pixel circuit  114 . The pixel circuit  114  includes TFTs and an OLED. The pixel circuit  114  outputs a visible light  126  based on the analog voltage or current  112 . 
         [0045]    In  FIG. 3 , one pixel circuit is shown as an example. However, the light emitting display system  100  includes a plurality of pixel circuits. The video source  102  may be similar to the video source  12  of  FIGS. 1 and 2 . The data driver IC  110  may be similar to the data driver  110  may be similar to the data driver IC  20  of  FIGS. 1 and 2 . 
         [0046]    A compensation functions module  130  is provided to the display. The compensation functions module  130  includes a module  134  for implementing an algorithm (referred to as TFT-to-pixel circuit conversion algorithm) on measurement  132  from the pixel circuit  114  (referred to as degradation data, measured degradation data, measured TFT degradation data, or measured TFT and OLED degradation data), and outputs calculated pixel circuit degradation data  136 . It is noted that in the description below, “TFT-to-pixel circuit conversion algorithm module” and “TFT-to-pixel circuit conversion algorithm” may be used interchangeably. 
         [0047]    The degradation data  132  is electrical data which represents how much a part of the pixel circuit  114  has been degraded. The data measured from the pixel circuit  114  may represent, for example, one or more characteristics of a part of the pixel circuit  114 . 
         [0048]    The degradation data  132  is measured from, for example, one or more thin-film-transistors (TFTs), an organic light emitting diode (OLED) device, or a combination thereof. It is noted that the transistors of the pixel circuit  114  are not limited to TFTs, and the light emitting device of the pixel circuit  114  is not limited to an OLED. The measured degradation data  132  may be digital or analog data. The system  100  provides compensation data based on measurement from a part of the pixel circuit (e.g. TFT) to compensate for non-uniformities in the display. The non-uniformities may include brightness non-uniformity, color non-uniformity, or a combination thereof. Factors for causing such non-uniformities may include, but are not limited to, process or construction inequalities in the display, aging of pixels, etc. 
         [0049]    The degradation data  132  may be measured at a regular timing or a dynamically regulated timing. The calculated pixel circuit degradation data  136  may be compensation data to correct non-uniformities in the display. The calculated pixel circuit degradation data  136  may include any parameters to produce the compensation data. The compensation data may be used at a regular timing (e.g. each frame, regular interval, etc.) or dynamically regulated timing. The measured data, compensation data, or a combination thereof may be stored in a memory (e.g.  142  of  FIG. 8 ). 
         [0050]    The TFT-to-pixel circuit conversion algorithm module  134  or the combination of the TFT-to-pixel circuit conversion algorithm module  134  and the digital data processor  106  estimates the degradation of the entire pixel circuit based on the measured degradation data  132 . Based on this estimation, the entire degradation of the pixel circuit  114  is compensated by adjusting, at the digital data processor  106 , the luminance data (digital data  104 ) applied to a certain pixel circuit(s). 
         [0051]    The system  100  may modify or adjust luminance data  104  applied to a degraded pixel circuit or non-degraded pixel circuit. For example, if a constant value of visible light  126  is desired, the digital data processor  106  increases the luminance data for a pixel that is highly degraded, thereby compensating for the degradation. 
         [0052]    In  FIG. 3 , the TFT-to-pixel circuit conversion algorithm module  134  is provided separately from the digital data processor  106 . However, the TFT-to-pixel circuit conversion algorithm module  134  may be integrated into the digital data processor  106 . 
         [0053]      FIG. 4  illustrates an example of the system  100  of  FIG. 3 . The pixel circuit  114  of  FIG. 4  includes TFTs  116  and OLED  120 . The analog voltage or current  112  is provided to the TFTs  116 . The TFTs  116  convert that voltage or current  112  into another current  118  which flows through the OLED  120 . The OLED  120  converts the current  118  into the visible light  126 . The OLED  120  has an OLED voltage  122 , which is the voltage drop across the OLED. The OLED  120  also has an efficiency  134 , which is a ratio of the amount of light emitted to the current through the OLED  120 . 
         [0054]    The system  100  of  FIG. 4  measures the degradation of the TFTs only. The degradation of the TFTs  116  and the OLED  120  are usage-dependent, and the TFTs  116  and the OLED  120  are always linked in the pixel circuit  114 . Whenever the TFT  116  is stressed, the OLED  120  is also stressed. Therefore, there is a predictable relationship between the degradation of the TFTs  116 , and the degradation of the pixel circuit  114  as a whole. The TFT-to-pixel circuit conversion algorithm module  134  or the combination of the TFT-to-pixel circuit conversion algorithm module  134  and the digital data processor  106  estimates the degradation of the entire pixel circuit based on the TFT degradation only. An embodiment of the present invention may also be applied to systems that monitor both TFT and OLED degradation independently. 
         [0055]    The pixel circuit  114  has a component that can be measured. The measurement obtained from the pixel circuit  114  is in some way related to the pixel circuit&#39;s degradation. 
         [0056]      FIG. 5  illustrates an example of the pixel circuit  114  of  FIG. 4 . The pixel circuit  114  of  FIG. 5  is a 4-T pixel circuit. The pixel circuit  114 A includes a switching circuit having TFTs  150  and  152 , a reference TFT  154 , a dive TFT  156 , a capacitor  158 , and an OLED  160 . 
         [0057]    The gate of the switch TFT  150  and the gate of the feedback TFT  152  are connected to a select line Vsel. The first terminal of the switch TFT  154  and the first terminal of the feedback TFT  152  are connected to a data line Idata. The second terminal of the switch TFT  150  is connected to the gate of the reference TFT  154  and the gate of the drive TFT  156 . The second terminal of the feedback TFT  152  is connected to the first terminal of the reference TFT  154 . The capacitor  158  is connected between the gate of the drive TFT  156  and ground. The OLED  160  is connected between voltage supply Vdd and the drive TFT  156 . The OLED  160  may also be connected between drive TFT  156  and ground in other systems (i.e. drain-connected format). 
         [0058]    When programming the pixel circuit  114 A, Vsel is high and a voltage or current is applied to the data line Idata. The data Idata initially flows through the TFT  150  and charges the capacitor  158 . As the capacitor voltage rises, the TFT  154  begins to turn on and Idata starts to flow through the TFTs  152  and  154  to ground. The capacitor voltage stabilizes at the point when all of Idata flows through the TFTs  152  and  154 . The current flowing through the TFT  154  is mirrored in the drive TFT  156 . 
         [0059]    In the pixel circuit  114 A, by setting Vsel to high and putting a voltage on Idata, the current flowing into the Idata node can be measured. Alternately, by setting Vsel to high and putting a current on Idata, the voltage at the Idata node can be measured. As the TFTs degrade, the measured voltage (or current) will change, allowing a measure of the degradation to be recorded. In this pixel circuit, the analog voltage/current  112  shown in  FIG. 4  is connected to the Idata node. The measurement of the voltage or current can occur anywhere along the connection between the data diver IC  110  and the TFTs  116 . 
         [0060]    In  FIG. 4 , the TFT-to-pixel circuit conversion algorithm is applied to the measurement  132  from the TFTs  116 . However, current/voltage information read from various places other than TFTs  116  may be usable. For example, the OLED voltage  122  may be included with the measured TFT degradation data  132 . 
         [0061]      FIG. 6  illustrates a further example of the system  100  of  FIG. 3 . The system  100  of  FIG. 6  measures the OLED voltage  122 . Thus, the measured data  132  is related to the TFT  116  and OLED  120  degradation (“measured TFT and OLED voltage degradation data  132 A” in  FIG. 6 ). The compensation functions module  130  of  FIG. 6  implements the TFT-to-pixel circuit conversion algorithm  134  on the signal related to both the TFT degradation and OLED degradation. The TFT-to-pixel circuit conversion algorithm module  134  or the combination of the TFT-to-pixel circuit conversion algorithm module  134  and the digital data processor  106  estimates the degradation of the entire pixel circuit based on the TFT degradation and the OLED degradation. The TFT degradation and OLED degradation may be measured separately and independently. 
         [0062]      FIG. 7  illustrates an example of the pixel circuit  114  of  FIG. 6 . The pixel circuit  114 B of  FIG. 7  is a 4-T pixel circuit. The pixel circuit  114 B includes a switching circuit having TFTs  170  and  172 , a reference TFT  174 , a drive TFT  176 , a capacitor  178 , and an OLED  180 . 
         [0063]    The gate of the switch TFT  170  and the gate of the switch TFT  172  are connected to a select line Vsel. The first terminal of the switch TFT  172  is connected to a data line Idata while the first terminal of the switch TFT  170  is connected to the second terminal of the switch TFT  172  which is connected to the gate of the reference TFT  174  and the gate of the dive TFT  176 . The second terminal of the switch TFT  170  is connected to the first terminal of the reference TFT  174 . The capacitor  178  is connected between the gate of the dive TFT  176  and ground. The first terminal of the dive TFT  176  is connected to voltage supply Vdd. The second terminal of the reference TFT  174  and the second terminal of the drive TFT  176  are connected to the OLED  180 . 
         [0064]    When programming the pixel circuit  114 B, Vsel is high and a voltage or current is applied to the data line Idata. The data Idata initially flows through the TFT  172  and charges the capacitor  178 . As the capacitor voltage rises, the TFT  174  begins to turn on and Idata starts to flow through the TFTs  170  and  174  and OLED  180  to ground. The capacitor voltage stabilizes at the point when all of Idata flows through the TFTs  152  and  154 . The current flowing through the TFT  154  is mirrored in the drive TFT  156 . In the pixel circuit  114 A, by setting Vsel to high and putting a voltage on Idata, the current flowing into the Idata node can be measured. Alternately, by setting Vsel to high and putting a current on Idata, the voltage at the Idata node can be measured. As the TFTs degrade, the measured voltage (or current) will change, allowing a measure of the degradation to be recorded. It is noted that unlike the pixel circuit  114 A of  FIG. 5 , the current now flows through the OLED  180 . Therefore the measurement made at the Idata node is now partially related to the OLED voltage, which will degrade over time. In the pixel circuit  114 B, the analog voltage/current  112  shown in  FIG. 6  is connected to the Idata node. The measurement of the voltage or current can occur anywhere along the connection between the data driver IC  110  and the TFTs  116 . 
         [0065]    Referring to  FIGS. 3 ,  4 , and  6 , the pixel circuit  114  may allow the current out of the TFTs  116  to be measured, and to be used as the measured TFT degradation data  132 . The pixel circuit  114  may allow some part of the OLED efficiency to be measured, and to be used as the measured TFT degradation data  132 . The pixel circuit  114  may also allow a node to be charged, and the measurement may be the time it takes for this node to discharge. The pixel circuit  114  may allow any parts of it to be electrically measured. Also, the discharge/charge level during a given time can be used for aging detection. 
         [0066]    Referring to  FIG. 8 , an example of modules for the compensation scheme applied to the system of  FIG. 4  is described. The compensation functions module  130  of  FIG. 8  includes an analog/digital (A/D) converter  140 . The A/D converter  140  converts the measured TFT degradation data  132  into digital measured TFT voltage/current  112  shown in  FIG. 4  is connected to the Idata node. The measurement of the voltage or current can occur anywhere along the connection between the data driver IC  110  and the TFTs  116 . 
         [0067]    In  FIG. 4 , the TFT-to-pixel circuit conversion algorithm is applied to the measurement  132  from the TFTs  116 . However, current/voltage information read from various places other than TFTs  116  may be usable. For example, the OLED voltage  122  may be included with the measured TFT degradation data  132 . 
         [0068]      FIG. 6  illustrates a further example of the system  100  of  FIG. 3 . The system  100  of the  FIG. 6  measured the OLED voltage  122 . Thus, the measured data  132  is related to the TFT  116  and OLED  120  degradation (“measured TFT and OLED voltage degradation data  132 A” in  FIG. 6 ). The compensation functions module  130  of  FIG. 6  implements the TFT-to-pixel circuit conversion algorithm  134  on the signal related to both the TFT degradation and OLED degradation. The TFT-to-pixel circuit conversion algorithm module  134  or the combination of the TFT-to-pixel circuit conversion algorithm module  134  and the digital data processor  106  estimates the degradation of the entire pixel circuit based on the TFT degradation and the OLED degradation. The TFT degradation and OLED degradation may be measured separately and independently. 
         [0069]      FIG. 7  illustrates an example of the pixel circuit  114  of  FIG. 6 . The pixel circuit  114 B of  FIG. 7  is a 4-T pixel circuit. The pixel circuit  114 B includes a switching circuit having TFTs  170  and  172 , a reference TFT  174 , a drive TFT  176 , a capacitor  178 , and an OLED  180 . 
         [0070]    The gate of the switch TFT  170  and the gate of the switch TFT  172  are connected to a select line Vsel. The first terminal of the switch TFT  172  is connected to a data line Idata while the first terminal of the switch TFT  170  is connected to the second terminal of the switch TFT  172 , which is connected to the gate of the reference TFT  174  and the gate of the drive TFT  176 . The second terminal of the switch TFT  170  is connected to the first terminal of the reference TFT  174 . The capacitor  178  is connected between the gate of the drive TFT  176  and ground. The first terminal of the drive TFT  176  is connected to voltage supply Vdd. The second terminal of the reference TFT  174  and the second terminal of the drive TFT  176  are connected to the OLED  180 . 
         [0071]    When programming the pixel circuit  114 B, Vsel is high and a voltage or current is applied to the data line Idata. The data Idata initially flows through the TFT  172  and charges the capacitor  178 . As the capacitor voltage rises, the TFT  174  begins to turn on and Idata starts to flow through the TFTs  170  and  174  and OLED  180  to ground. The capacitor voltage stabilizes at the point when all of Idata flows through the TFTs  152  and  154 . The current flowing through the TFT  154  is mirrored in the drive TFT  156 . In the pixel circuit  114 A, by setting Vsel to high and putting a voltage on Idata, the current flowing into the Idata node can be measured. Alternately, by setting Vsel to high and putting a current on Idata, the voltage at the Idata node can be measured. As the TFTs degrade, the measured voltage (or current) will change, allowing a measure of the degradation to be recorded. It is noted that unlike the pixel circuit  114 A of  FIG. 5 , the current now flows through the OLED  180 . Therefore the measurement made at the Idata node is now partially related to the OLED voltage, which will degrade over time. In the pixel circuit  114 B, the analog voltage/current  112  shown in  FIG. 6  is connected to the Idata node. The measurement of the voltage or current can occur anywhere along the connection between the data driver IC  110  and the TFTs  116 . 
         [0072]    Referring to  FIGS. 3 ,  4 , and  6 , the pixel circuit  114  may allow the current out of the TFTs  116  to be measured, and to be used as the measured TFT degradation data  132 . The pixel circuit  114  may allow some part of the OLED efficiency to be measured, and to be used as the measured TFT degradation data  132 . The pixel circuit  114  may also allow a node to be charged, and the measurement may be the time it takes for this node to discharge. The pixel circuit  114  may allow any parts of it to be electrically measured. Also, the discharge/charge level during a given time can be used for aging detection. 
         [0073]    Referring to  FIG. 8 , an example of modules for the compensation scheme applied to the system of  FIG. 4  is described. The compensation functions module  130  of  FIG. 8  includes an analog/digital (A/D) converter  140 . The A/D converter  140  converts the measured TFT degradation data  132  into digital measured TFT degradation data  132 B. The digital measured TFT degradation data  132 B is converted into the calculated pixel circuit degradation data  136  at the TFT-to-pixel circuit conversion algorithm module  134 . The calculated pixel circuit degradation data  136  is stored in a lookup table  142 . Since measuring TFT degradation data from some pixel circuits may take a long time, the calculated pixel circuit degradation data  136  is stored in the lookup table  142  for use. 
         [0074]    In  FIG. 8 , the TFT-to-pixel circuit conversion algorithm  134  is a digital algorithm. The digital TFT-to-pixel circuit conversion algorithm  134  may be implemented, for example, on a microprocessor, an FPGA, a DSP, or another device, but not limited to these examples. The lookup table  142  may be implemented using memory, such as SRAM or DRAM. This memory may be in another device, such as a microprocessor or FPGA, or may be an independent device. 
         [0075]    The calculated pixel circuit degradation data  136  stored in the lookup table  142  is always available for the digital data processor  106 . Thus, the TFT degradation data  132  for each pixel does not have to be measured every time the digital data processor  106  needs to use the data. The degradation data  132  may be measured infrequently (for example, once every 20 hours, or less). Using a dynamic time allocation for the degradation measurement is another case, more frequent extraction at the beginning and less frequent extraction after the aging gets saturated. 
         [0076]    The digital data processor  106  may include a compensation module  144  for taking input luminance data for the pixel circuit  114  from the video source  102 , and modifying it based on degradation data for that pixel circuit or other pixel circuit. In  FIG. 8 , the module  144  modifies luminance data using information from the lookup table  142 . 
         [0077]    It is noted that the configuration of  FIG. 8  is applicable to the system of  FIGS. 3 and 6 . It is noted that the lookup table  142  is provided separately from the compensating functions module  130 , however, it may be in the compensating functions module  130 . It is noted that the lookup table  142  is provided separately from the digital data processor  106 , however, it may be in the digital data processor  106 . 
         [0078]    One example of the lookup table  142  and the module  144  of the digital data processor  106  is illustrated in  FIG. 9 . Referring to  FIG. 9 , the output of the TFT-to-pixel circuit conversion algorithm module  134  is an integer value. This integer is stored in a lookup table  142 A (corresponding to  142  of  FIG. 8 ). Its location in the lookup table  142 A is related to the pixel&#39;s location on the AMOLED display. Its value is a number, and is added to the digital luminance data  104  to compensate for the degradation. 
         [0079]    For example, digital luminance data may be represented to use 8-bits (256 values) for the brightness of a pixel. A value of 246 may represent maximum luminance for the pixel. A value of 128 may represent approximately 50% luminance. The value in the lookup table  142 A may be the number that is added to the luminance data  104  to compensate for the degradation. Therefore, the compensation module ( 144  of  FIG. 7 ) in the digital data processor  106  may be implemented by a digital adder  144 A. It is noted that digital luminance data may be represented by any number of bits, depending on the driver IC used (for example, 6-bit, 8-bit, 10-bit, 14-bit, etc.). 
         [0080]    In  FIGS. 3 ,  4 ,  6 ,  8 , and  9 , the TFT-to-pixel circuit conversion algorithm module  134  has the measured TFT degradation data  132  or  132 A as an input, and the calculated pixel circuit degradation data  136  as an output. However, there may be other inputs to the system to calculate compensation data as well, as shown in  FIG. 10 .  FIG. 10  illustrates an example of inputs to the TFT-to-pixel circuit conversion algorithm module  134 . In  FIG. 10 , the TFT-to-pixel circuit conversion algorithm module  134  processes the measured data ( 132  of  FIGS. 3 ,  4 ,  8 , and  9 ;  132 A of  FIG. 5 ;  132 B of  FIGS. 8 and 9 ) based on additional inputs  190  (e.g. temperature, other voltages, etc.), empirical constants  192 , or combinations thereof. 
         [0081]    The additional inputs  190  may include measured parameters such as a voltage reading from current-programming pixels and a current reading from voltage-programming pixels. These pixels may be different from a pixel circuit from which the measured signal is obtained. For example, a measurement is taken from a “pixel under test” and is used in combination with another measurement from a “reference pixel.” As described below, in order to determine how to modify luminance data to a pixel, data from other pixels in the display may be used. The additional inputs  190  may include light measurements, such as measurement of an ambient light in a room. A discrete device or some kind of test structure around the periphery of the panel may be used to measure the ambient light. The additional inputs may include humidity measurements, temperature readings, mechanical stress readings, other environmental stress readings, and feedback from test structures on the panel 
         [0082]    It may also include empirical parameters  192 , such as the brightness loss in the OLED due to decreasing efficiency (ΔL), the shift in OLED voltage over time (ΔVoled), dynamic effects of Vt shift, parameters related to TFT performance such as Vt, ΔVt, mobility (μ), inter-pixel non-uniformity, DC bias voltages in the pixel circuit, changing gain of current-mirror based pixel circuits, short-term and long-term based shifts in pixel circuit performance, pixel-circuit operating voltage variation due to IR-drop and ground bounce. 
         [0083]    Referring to  FIGS. 8 and 9 , the TFT-to-pixel-circuit conversion algorithm in the module  134  and the compensation algorithm  144  in the digital data processor  106  work together to convert the measured TFT degradation data  132  into a luminance correction factor. The luminance correction factor has information about how the luminance data for a given pixel is to be modified, to compensate for the degradation in the pixel. 
         [0084]    In  FIG. 9 , the majority of this conversion is done by the TFT-to-pixel-circuit conversion algorithm module  134 . It calculates the luminance correction values entirely, and the digital adder  144 A in the digital data processor  106  simply adds the luminance correction values to the digital luminance data  104 . However, the system  100  may be implemented such that the TFT-to-pixel circuit conversion algorithm module  134  calculates only the degradation values, and the digital data processor  106  calculates the luminance correction factor from that data. The TFT-to-pixel circuit conversion algorithm  134  may employ fuzzy logic, neural networks, or other algorithm structures to convert the degradation data into the luminance correction factor. 
         [0085]    The value of the luminance correction factor may allow the visible light to remain constant, regardless of the degradation in the pixel circuit. The value of the luminance correction factor may allow the luminance of degraded pixels not to be altered at all; instead, the luminance of the non-degraded pixels to be decreased. In this case, the entire display may gradually lose luminance over time, however the uniformity may be high. 
         [0086]    The calculation of a luminance correction factor may be implemented in accordance with a compensation of non-uniformity algorithm, such as a constant brightness algorithm, a decreasing brightness algorithm, or combinations thereof. The constant brightness algorithm and the decreasing brightness algorithm may be implemented on the TFT-to-pixel circuit conversion algorithm module (e.g.  134  of  FIG. 3 ) or the digital data processor (e.g.  106  of  FIG. 3 ). The constant brightness algorithm is provided for increasing brightness of degraded pixels so as to match nondegraded pixels. The decreasing brightness algorithm is provided for decreasing brightness of non-degraded pixels  244  so as to match degraded pixels. These algorithm may be implemented by the TFT-to-pixel circuit conversion algorithm module, the digital data processor (such as  144  of  FIG. 8 ), or combinations thereof. It is noted that these algorithms are examples only, and the compensation of non-uniformity algorithm is not limited to these algorithms. 
         [0087]    Referring to  FIGS. 11A-11E , the experimental results of the compensation of non-uniformity algorithms are described in detail. Under the experiment, an AMOLED display includes a plurality of pixel circuits, and is driven by a system as shown in  FIGS. 3 ,  4 ,  6 ,  8  and  9 . It is noted that the circuitry to drive the AMOLED display is not shown in  FIGS. 11A-11E . 
         [0088]      FIG. 11A  schematically illustrates an AMOLED display  240  which starts operating (operation period t=0 hour). The video source ( 102  of  FIGS. 3 ,  4 ,  7 ,  8  and  9 ) initially outputs maximum luminance data to each pixel. No pixels are degraded since the display  240  is new. The result is that all pixels output equal luminance and thus all pixels show uniform luminance. 
         [0089]    Next, the video source outputs maximum luminance data to some pixels in the middle of the display as shown in  FIG. 11B .  FIG. 11B  schematically illustrates the AMOLED display  240  which has operated for a certain period where maximum luminance data is applied to pixels in the middle of the display. The video source outputs maximum luminance data to pixels  242 , while it outputs minimum luminance data (e.g. zero luminance data) to pixels  244  around the outside of the pixels  242 . It maintains this for a long period of time, for example 1000 hours. The result is that the pixels  242  at maximum luminance will have degraded, and the pixels  244  at zero luminance will have no degradation. 
         [0090]    At 1000 hours, the video source outputs maximum luminance data to all pixels. The results are different depending on the compensation algorithm used, as shown in  FIGS. 11C-11E . 
         [0091]      FIGS. 11C  schematically illustrates the AMOLED display  240  to which no compensation algorithm is applied. As shown in  FIG. 11C , if there was no compensation algorithm, the degraded pixels  242  would have a lower brightness than the nondegraded pixels  244 . 
         [0092]      FIG. 11D  schematically illustrates the AMOLED display  240  to which the constant brightness algorithm is applied. The constant brightness algorithm is implemented for increasing luminance data to degraded pixels, such that the luminance data of the degraded pixels matches that of non-degraded pixels. For example, the increasing brightness algorithm provides increasing currents to the stressed pixels  242 , and constant current to the unstressed pixels  244 . Both degraded and nondegraded pixels have the same brightness. Thus, the display  240  is uniform. Differential aging is compensated, and brightness is maintained, however more current is required. Since the current to some pixels is being increased, this will cause the display to consume more current over time, and therefore more power over time because power consumption is related to the current consumption. 
         [0093]      FIG. 11E  schematically illustrates the AMOLED display  240  to which the decreasing brightness algorithm is applied. The decreasing brightness algorithm decreases luminance data to nondegraded pixels, such that the luminance data of the nondegraded pixels match that of degraded pixels. For example, the decreasing brightness algorithm provides constant OLED current to the stressed pixels  242 , while decreasing current to the unstressed pixels  244 . Both degraded and non-degraded pixels have the same brightness. Thus, the display  240  is uniform. Differential aging is compensated, and it requires a lower Vsupply, however brightness decrease over time. Because this algorithm does not increase the current to any of the pixels, it will not result in increased power consumption. 
         [0094]    Referring to  FIG. 3 , components, such as the video source  102  and the data driver IC  110 , may use only 8-bits, or 256 discrete luminance values. Therefore if the video source  102  outputs maximum brightness (a luminance value of 255), there is no way to add any additional luminance, since the pixel is already at the maximum brightness supported by the components in the system. Likewise, if the video source  102  outputs minimum brightness (a luminance value of 0), there is no way to subtract any luminance. The digital data processor  106  may implement a grayscale compression algorithm to reserve some grayscales.  FIG. 12  illustrates an implementation of the digital data processor  106  which includes a grayscale compression algorithm module  250 . The grayscale compression algorithm  250  takes the video signal represented by 256 luminance values, and transforms it to use less luminance values. For example, instead of minimum brightness represented by grayscale 0, minimum brightness may be represented by grayscale 50. Likewise, maximum brightness may be represented by grayscale 200. In this way, there are some grayscales reserved for future increase and decrease. It is noted that the shift in grayscales does not reflect the actual expected shift in grayscales. 
         [0095]    According to the embodiments of the present invention, the scheme of estimating (predicting) the degradation of the entire pixel circuit and generating a luminance correction factor ensures uniformities in the display. According to embodiments of the present invention, the aging of some components or entire circuit can be compensated, thereby ensuring uniformity of the display. 
         [0096]    According to the embodiments of the present invention, the TFT-to-pixel circuit conversion algorithm allows for improved display parameters, for example, including constant brightness uniformity and color uniformity across the panel over time. Since the TFT-to-pixel circuit conversion algorithm takes in additional parameters, for example, temperature and ambient light, any changes in the display due to these additional parameters may be compensated for. 
         [0097]    The TFT-to-Pixel circuit conversion algorithm module ( 134  of  FIGS. 3 ,  4 ,  6 ,  8  and  9 ), the compensation module ( 144  of  FIG. 8 ,  144 A of  FIG. 9 , the compensation of non-uniformity algorithm, the constant brightness algorithm, the decreasing brightness algorithm and the grayscale compression algorithm may be implemented by any hardware, software or a combination of hardware and software having the above described functions. The software code, instructions and/or statements, either in its entirety or a part thereof, may be stored in a computer readable memory. Further, a computer data signal representing the software code, instructions and/or statements, which may be embedded in a carrier wave may be transmitted via a communication network. Such a computer readable memory and a computer data signal and/or its carrier are also within the scope of the present invention, as well as the hardware, software and the combination thereof. 
         [0098]    Referring again to  FIG. 3 , which illustrates the operation of the light emitting display system  100  by applying a compensation algorithm to digital data  104 . In particular,  FIG. 3  illustrates the operation of a pixel in an active matrix organic light emitting diode (AMOLED) display. The display system  100  includes an array of pixels. The video source  102  includes luminance input data for the pixels. The luminance data is sent in the form of digital input data  104  to the digital data processor  106 . The digital input data  104  can be eight-bit data represented as integer values existing between 0 and 255, with greater integer values corresponding to higher luminance levels. The digital data processor  106  can optionally manipulate the digital input data  104  by, for example, scaling the resolution of the video source  102  to a native screen resolution, adjusting the color balance, or applying a gamma correction to the video source  102 . The digital data processor  106  can also apply degradation corrections to the digital input data  104  based on degradation data  136 . Following the manipulations, the digital data processor  106  sends the resulting digital data  108  to the data driver integrated circuit (IC)  110 . The data driver IC  110  converts the digital data  108  into the analog voltage or current output  112 . The data driver IC  110  can be implemented, for example, as a module including a digital to analog converter. The analog voltage or current  112  is provided to the pixel circuit  114 . The pixel circuit  114  can include an organic light emitting diode (OLED) and thin film transistors (TFTs). One of the TFTs in the pixel circuit  114  can be a drive TFT that applies a drive current to the OLED. The OLED emits visible light  126  responsive to the drive current flowing to the OLED. The visible light  126  is emitted with a luminance related to the amount of current flowing to the OLED through the drive TFT. 
         [0099]    In a configuration where the analog voltage or current  112  is a programming voltage, the drive TFT within the pixel circuit  114  can supply the OLED according to the analog voltage or current  112  by, for example, biasing the gate of the drive TFT with the programming voltage. The pixel circuit  114  can also operate where the analog voltage or current  112  is a programming current applied to each pixel rather than a programming voltage. A display system  100  utilizing programming currents can use current minors in each pixel circuit  114  to apply a drive current to the OLED through the drive TFT according to the programming current applied to each pixel. 
         [0100]    The luminance of the emitted visible light  126  is affected by aspects within the pixel circuit  114  including the gradual degradation of hardware within the pixel circuit  114 . The drive TFT has a threshold voltage, and the threshold voltage can change over time due to aging and stressing of the drive TFT. The luminance of the emitted visible light  126  can be influenced by the threshold voltage of the drive TFT, the voltage drop across the OLED, and the efficiency of the OLED. The efficiency of the OLED is a ratio of the luminance of the emitted visible light  126  to the drive current flowing through the OLED. Furthermore, the degradation can generally be non-uniform across the display system  100  due to, for example, manufacturing tolerances of the drive TFTs and OLEDs and differential aging of pixels in the display system  100 . Non-uniformities in the display  100  are generally referred to as display mura or defects. In a display  100  with an array of OLEDs having uniform light emitting efficiency and threshold voltages driven by TFTs having uniform gate threshold voltages, the luminance of the display will be uniform when all the pixels in the display are programmed with the same analog voltage or current  112 . However, as the OLEDs and TFTs in each pixel age and the degradation characteristics change, the luminance of the display ceases to be uniform when programmed the same. 
         [0101]    The degradation can be compensated for by increasing the amount of drive current sent through the OLED in the pixel circuit  114 . According to an implementation of the present disclosure, compensation for the degradation of the display  100  can be carried out by adjusting the digital data  108  output from the digital data processor  106 . The digital data processor  106  receives the degradation data  136  from the compensation module  130 . The compensation module  130  receives degradation data  132  based on measurements of parameters within the pixel circuit  114 . Alternatively, the degradation data  132  sent to the compensation module  130  can be based on estimates of expected performance of the hardware aspects within the pixel circuit  114 . The compensation module  130  includes the module  134  for implementing the algorithm  134 , such as the TFT-to-pixel circuit conversion algorithm. The degradation data  132  can be electrical data that represents how much a hardware aspect of the pixel circuit  114  has been degraded. The degradation data  132  measured or estimated from the pixel circuit  114  can represent one or more characteristics of the pixel circuit  114 . 
         [0102]    In a configuration where the analog voltage or current  112  is a programming voltage, the programming voltage is generally determined by the digital input data  104 , which is converted to a voltage in the data driver IC  110 . The present disclosure provides a method of compensating for non-uniform characteristics in each pixel circuit  114  that affect the luminance of the emitted visible light  126  from each pixel. Compensation is performed by adjusting the digital input data  104  in the digital data processor  106  before the digital data  108  is passed to the data driver IC  110 . 
         [0103]      FIG. 13  is a data flow chart showing the compression and compensation of luminosity input data  304  used to drive an AMOLED display. The data flow chart shown in  FIG. 13  includes a digital data processor block  306  that can be considered an implementation of the digital data processor  106  shown in  FIG. 3 . Referring again to  FIG. 13 , a video source provides the luminosity input data  304 . The input data  304  is a set of eight-bit integer values. The input data  304  includes integer values that exist between 0 and 255, with the values representing 256 possible programmable luminosity values of the pixels in the AMOLED display. For example, 255 can correspond to a pixel programmed with maximum luminance, and  127  can correspond to a pixel programmed with roughly half the maximum luminance. The input data  304  is similar to the digital input data  104  shown in  FIG. 3 . Referring again to  FIG. 13 , the input data  304  is sent to the digital data processor block  304 . In the digital data processor block  304 , the input data  304  is multiplied by four ( 310 ) in order to translate the eight-bit input data  304  to ten-bit resulting data  312 . Following the multiplication by four ( 310 ), the resulting data  312  is a set of ten-bit integers existing between 0 and 1020. 
         [0104]    By translating the eight-bit input data  304  to the ten-bit resulting data  312 , the resulting data  312  can be manipulated for compensation of luminance degradation with finer steps than can be applied to the eight-bit input data  304 . The ten-bit resulting data  312  can also be more accurately translated to programming voltages according to a gamma correction. The gamma correction is a non-linear, power law correction as is appreciated in the art of display technology. Applying the gamma correction to the input data can be advantageous, for example, to account for the logarithmic nature of the perception of luminosity in the human eye. According to an aspect of the present disclosure, multiplying the input data  304  by four ( 310 ) translates the input data  304  into a higher quantized domain. While the present disclosure includes multiplying by four ( 310 ), in an implementation the input data  304  can be multiplied by any number to translate the input data  310  into a higher quantized domain. The translation can advantageously utilize multiplication by a power of two, such as four, but the present disclosure is not so limited. Additionally, the present disclosure can be implemented without translating the input data  304  to a higher quantized domain. 
         [0105]    The resulting data  312  is multiplied by a compression factor, K ( 314 ). The compression factor, K, is a number with a value less than one. Multiplying the resulting data  312  by K ( 314 ) allows for scaling the ten-bit resulting data  312  into compressed data  316 . The compressed data  316  is a set of ten-bit integers having values ranging from 0 to the product of K and 1020. Next, the compressed data  316  is compensated for degradations in the display hardware ( 318 ). The compressed data  316  is compensated by adding additional data increments to the integers corresponding to the luminance of each pixel ( 318 ). The compensation for degradation is performed according to degradation data  336  that is sent to the digital data processor block  306 . The degradation data  336  is digital data representing an amount of compensation to be applied to the compressed data  316  within the digital data processor block  306  according to degradations in the display hardware corresponding to each pixel. Following the compensation for degradations ( 318 ), compensated data  308  is output. The compensated data  208  is a set of ten-bit integer values with possible values between 0 and 1023. The compensated data  308  is similar in some respects to the digital data  108  output from the digital data processor  106  in  FIG. 3 . Referring again to  FIG. 13 , the compensated data  308  is supplied to a display driver, such as a display driver incorporating a digital to analog converter, to create programming voltages for pixels in the AMOLED display. 
         [0106]    The degradations in the display hardware can be from mura defects (non-uniformities), from the OLED voltage drop, from the voltage threshold of the drive TFT, and from changes in the OLED light emitting efficiency. The degradations in the display hardware each generally correspond to an additional increment of voltage that is applied to the pixel circuit in order to compensate for the degradations. For a particular pixel, the increments of additional voltage necessary to compensate for the hardware degradations can be referred to as: V mura , V Th , V OLED , and V efficiency . Each of the hardware degradations can be mapped to corresponding increments in data steps according to a function of V mura , V Th , V OLED , V efficiency , D(V mura , V Th , V OLED , V efficiency ). For example, the relationship can be given by Expression 1: D(V mura , V Th , V OLED , V efficiency )=int[(2 nBits −1) (V mura +V Th +V OLED +V efficiency )/V Max ], where nBits is the number of bits in the data set being compensated and V Max  is the maximum programming voltage. In Expression 1, int[ ] is a function that evaluates the contents of the brackets and returns the nearest integer. The degradation data  336  sent to the digital data processor block  306  can be digital data created according to the relationship for D(V mura , V Th , V OLED , V efficiency ) provided in Expression 1. In an implementation of the present disclosure, the degradation data  336  can be an array of digital data corresponding to an amount of compensation to be applied to the compressed data of each pixel in an AMOLED display. The array of digital data is a set of offset increments that can be applied to the compressed data by adding the offset increments to the compressed data of each pixel or by subtracting the offset increments from the compressed data of each pixel. The set of offset increments can generally be a set of digital data with entries corresponding to an amount of compensation needed to be applied to each pixel in the AMOLED display. The amount of compensation can be the amount of increments in data steps needed to compensate for a degradation according to Expression 1. In a configuration, locations in the array of the degradation data  336  can correspond to locations of pixels in the AMOLED display. 
         [0107]    For example, Table 1 below provides a numerical example of the compression of input data according to  FIG. 13 . Table 1 provides example values for a set of input data  304  following the multiplication by four ( 310 ) and the multiplication by K ( 314 ). In the example provided in Table 1, K has a value of 0.75. In Table 1, the first column provides example values of integer numbers in the set of input data  304 . The second column provides example values of integer numbers in the set of resulting data  312  created by multiplying the corresponding input data values by four ( 310 ). The third column provides example values of numbers in the set of compressed data  316  created by multiplying the corresponding values of the resulting data  312  by K, where K has an example value of 0.75. The final column is the output voltage corresponding to the example compressed data  316  shown in the third column when no compensation is applied. The final column is created for an example display system having a maximum programming voltage of 18 V. In the numerical example illustrated in Table 1, the programming output voltage corresponding to the input data with the maximum input of two-hundred fifty-five is more than 4.5 V below the maximum voltage. The 4.5 V can be considered the compensation budget of the display system, and can be referred to as the voltage headroom, V headroom . According to an aspect of the present disclosure, the 4.5 V is used to provide compensation for degradation of pixels in the AMOLED display. 
         [0000]    
       
         
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Numerical Example of Input Data Compression 
               
             
          
           
               
                   
                   
                   
                 Output Voltage 
               
               
                   
                 Resulting Data 
                 Compressed Data 
                 (without degradation 
               
               
                 Input Data 
                 (×4) 
                 (×0.75) 
                 compensation) 
               
               
                   
               
             
          
           
               
                 255 
                 1020 
                 765 
                 13.46 V 
               
               
                 254 
                 1016 
                 762 
                 13.40 V 
               
               
                 253 
                 1012 
                 759 
                 13.35 V 
               
               
                 . . . 
                 . . . 
                 . . . 
                 . . . 
               
               
                 2 
                 8 
                 6 
                  0.10 V 
               
               
                 1 
                 4 
                 3 
                  0.05 V 
               
               
                 0 
                 0 
                 0 
                  0.00 V 
               
               
                   
               
             
          
         
       
     
         [0108]    According to an implementation of the present disclosure, the amount of voltage available for providing compensation degradation is V headroom . An amount of V headroom  can be advantageously reserved to compensate for a degradation of a pixel in an AMOLED display with the most severe luminance degradation. By reserving an amount of V headroom  to compensate for the most severely degraded pixel, the relative luminosity of the display can be advantageously maintained. The required amount of V headroom  to compensate for the pixel in an AMOLED display with a maximum amount of degradation is given by Expression 2: V headroom =max[V mura +V Th +V OLED +V efficiency ]. In Expression 2, V mura , V Th , V OLED , and V efficiency  can be an array of values corresponding to the amount of additional voltage necessary to compensate the pixels in the display, and the entries in the arrays of values can correspond to individual pixels in the display. That is, V mura  can be an array of voltages required to compensate display mura or non-uniform defects; V Th  can be an array of voltage thresholds of drive TFTs of pixels in the display; V OLED  can be an array of OLED voltages of the pixels in the display; and V efficiency  can be an array of voltages required to compensate for OLED efficiency degradations of pixels in the display. In Expression 2, max[ ] is a function evaluating an array of values in the brackets and returning the maximum value in the array. 
         [0109]    As can be appreciated with reference to  FIG. 13  and Table 1, the choice of K affects the amount of V headroom  available to compensate for degradations in the display. Choosing a lower value of K leads to a greater amount of V headroom . In a configuration of the present disclosure where the need for compensation increases over time due to aging of the display, the value of K can be advantageously decreased over time according to the degradation of the display over time. Decreasing K enables uniformity compensation across the display such that pixels receiving the same digital input data actually emit light with the same luminance, but the uniformity compensation comes at the cost of overall luminance reduction for the entire display.  FIGS. 14 through 17  provide methods for selecting and adjusting K. 
         [0110]      FIG. 14  is a flowchart illustrating a method for selecting the compression factor according to display requirements and the design of the pixel circuit. In operation of the method illustrated by the flowchart in  FIG. 14 , the display requirements and pixel circuit design of a display are analyzed to estimate maximum values of V mura , V Th , V OLED , and V efficiency  for the pixels in the display ( 405 ). The estimation ( 405 ) can be carried out based on, for example, empirical data from experimental results related to the aging of displays incorporating pixel circuits similar to the pixel circuit in the display  100 . Alternatively, the estimation ( 405 ) can be carried out based on numerical models or software-based simulation models of anticipated performances of the pixel circuit in the display  100 . The estimation ( 405 ) can also account for an additional safety margin of headroom voltage to account for statistically predictable variations amongst the pixel circuits in the display  100 . Responsive to the estimation ( 405 ), the required voltage headroom is calculated ( 410 ). The required voltage headroom, V headroom , is calculated according to Expression 2. Once V headroom  is calculated, the compression factor, K, is calculated ( 415 ) according to Expression 3: K= 1−V   headroom /V Max , where V Max  is a maximum programming voltage for the display  100 . The compression factor, K, is then set ( 420 ) for use in the compression and compensation algorithm, such as the compression algorithm illustrated in the data flow chart in  FIG. 13 . 
         [0111]      FIG. 15  is a flowchart illustrating a method for selecting the compression factor according to a pre-determined headroom adjustment profile. A headroom adjustment profile is selected ( 505 ). The first block  505  in the flowchart in  FIG. 15  graphically illustrates three possible headroom adjustment profiles as profile  1 , profile  2 , and profile  3 . The profiles illustrated are graphs of K versus time. The time axis can be, for example, a number of hours of usage of the display  100 . In all three profiles K decreases over time. By decreasing K over time, an additional amount of voltage (V headroom ) is available for compensation. The example profiles in the first block  505  include profile  1 , which maintains K at a constant level until a time threshold is reached and K decreases linearly with usage time thereafter. Profile  2  is a stair step profile, which maintains K at a constant level for a time, and then decreases K to a lower value, when it is maintained until another time, at which point it is decreased again. Profile  3  is a linear decrease profile, which provides for K to gradually decrease linearly with usage time. The profile can be selected by a user profile setting according to a user&#39;s preferences for the compensation techniques employed over the life of the display. For example, a user may want to maintain an overall maximum luminance for the display for a specific amount of usage hours before dropping the luminance. Another user may be fine with gradually dropping the luminance from the beginning of the display&#39;s lifetime. 
         [0112]    Once an headroom adjustment profile is selected ( 505 ), the display usage time is monitored ( 510 ). At a given usage time, the value of the compression factor, K, is determined according to the usage time and selected profile ( 515 ). The compression factor, K, is then set ( 520 ), and the display usage time continues to be monitored ( 510 ). After K is set ( 520 ), K can be used in the compression and compensation algorithm, such as the compression algorithm illustrated in the data flow chart in  FIG. 13 . According to an aspect of the present disclosure, the method of setting and adjusting K shown in  FIG. 15  is a dynamic method of setting and adjusting K, because the value of K is updated over time according to the usage time of the display  100 . 
         [0113]      FIG. 16  is a flowchart illustrating a method for selecting the compression factor according to dynamic measurements of degradation data exceeding a threshold over a previous compensation. Measurements are taken from aspects of the pixel circuits of the pixels in the display  100  to measure V mura , V Th , V OLED , and V efficiency  ( 605 ) and compute V headroom  according to Expression 2. The difference between the value of V headroom  presently computed at time t 2  is then compared to the value of V headroom  computed at an earlier time t 1  by computing the difference ( 610 ). The difference is ΔV headroom , and is calculated according to Expression 5: ΔV headroom =(V headroom ) t2 −(V headroom ) t1 . In Expression 5, t 1  is the last time used to adjust the compensation factor, K, and t 2  is a present time. The subscripts in the right hand side of Expression 5 indicate a time of evaluation of the quantity in parentheses. 
         [0114]    The calculated value of ΔV headroom  is then compared to a compensation threshold, V thresh  ( 615 ). If ΔV headroom  exceeds V thresh , K is modified ( 620 ). If ΔV headroom  is less than or equal to V thresh , K is not modified. The value of K can be modified according to Expression 6: K new =K old /A−B, where K new  is the new value of K, K old  is the old value of K, and A and B are values set for applications and different technologies. For example, A and B can be set based on empirical results from experiments examining the characteristic degradation due to aging of pixel circuits similar to those used in the display  100  to drive OLEDs in each pixel. Similar measurements or user inputs can be used to set V thresh  as well. The compression factor, K, is then set ( 625 ) for use in the compression and compensation algorithm, such as the compression algorithm illustrated in the data flow chart in  FIG. 13 . Degradation measurements continue to be measured ( 605 ), ΔV headroom  continues to be calculated ( 610 ), and K is updated according to Expression 6 whenever ΔV headroom  exceeds V thresh  ( 515 ). According to an aspect of the present disclosure, the method of adjusting K shown in  FIG. 5  is a dynamic method of adjusting K, because the value of K is updated over time according to degradation measurements gathered from the pixel circuits within the display  100 . 
         [0115]    Alternatively, the compression factor can be modified ( 620 ) according to Expression 3 based on the measured V headroom . According to an aspect of the method provided in the flowchart shown in  FIG. 16 , the value of K is maintained until a threshold event occurs ( 615 ), when K is modified ( 620 ). Implementing the method provided in  FIG. 16  for adjusting the compression factor, K, can result in K being decreased over time according to a stair step profile. 
         [0116]      FIG. 17  is a flowchart illustrating a method for selecting the compression factor according to dynamic measurements of degradation data exceeding a previously measured maximum. Measurements are taken from aspects of the pixel circuits of the pixels in the display  100  to measure V mura , V Th , V OLED , and V efficiency  ( 605 ). The measurements of V mura , V Th , V OLED , and V efficiency  are referred to as degradation measurements. The maximum values of the degradation measurements are selected ( 710 ). The maximum values of the degradation can be selected according to Expression 2. The combination of measuring the degradation measurements ( 605 ) and selecting the maximum values ( 710 ) provides for ascertaining the maximum compensation applied to pixels within the display. The maximum values are compared to previously measured maximum values of previously measured degradation measurements ( 715 ). If the presently measured maximum values exceed the previously measured maximum values, V headroom  is calculated according to Expression 2 ( 410 ) based on the present degradation measurements. Next, the compression factor, K, is determined according to Expression 3 ( 720 ). The compression factor is set ( 725 ) and the maximum values are updated for comparison with new maximum values ( 715 ). The compression factor is set ( 725 ) for use in the compression and compensation algorithm, such as the compression algorithm illustrated in the data flow chart in  FIG. 13 . Similar to the method provided in  FIG. 16 , the method shown illustrated by the flowchart in  FIG. 17  is a dynamic method of adjusting K based on degradation measurements continually gathered from the pixel circuits within the display  100 . 
         [0117]    The present disclosure can be implemented by combining the above disclosed methods for setting and adjusting the compression factor, K, in order to create an adequate amount of voltage headroom that allows for compensation to be applied to the digital data before it is passed to the data driver IC. For example, a method of setting and adjusting K according to  FIG. 16  or  FIG. 17  can also incorporate a user selected profile as in  FIG. 15 . 
         [0118]    In an implementation of the present disclosure, the methods of selecting and adjusting the compression factor, K, provided in  FIGS. 14 through 17  can be used in conjunction with the digital data manipulations illustrated in  FIG. 13  to operate a display while maintaining the uniform luminosity of the display. In a configuration, the above described methods allow for maintaining the relative luminosity of a display by compensating for degradations to pixels within the display. In a configuration, the above described methods allow for maintaining the luminosity of a pixel in a display array for a given digital input by compensating for degradations within the pixel&#39;s pixel circuit. 
         [0119]    The present disclosure describes maintaining uniform luminosity of an AMOLED display, but the techniques presented are not so limited. The disclosure is applicable to a range of systems incorporating arrays of devices having a characteristic stimulated responsive to a data input, and where the characteristic is sought to be maintained uniformly. For example, the present disclosure applies to sensor arrays, memory cells, and solid state light emitting diode displays. The present disclosure provides for modifying the data input that stimulates the characteristic of interest in order to maintain uniformity. While the present disclosure for compressing and compensating digital luminosity data to maintain a luminosity of an AMOLED display is described as utilizing TFTs and OLEDs, the present disclosure applies to a similar apparatus having a display including an array of light emitting devices. 
         [0120]    While particular embodiments and applications of the present invention have been illustrated and described, it is to be understood that the invention is not limited to the precise construction and compositions disclosed herein and that various modifications, changes, and variations can be apparent from the foregoing descriptions without departing from the spirit and scope of the invention as defined in the appended claims.