Abstract:
A display includes an electroluminescent display panel containing a light emitting material; a video interface circuit for producing an analog video signal for driving the display; an age circuit for supplying a signal representing the age of the light emitting material; an aging correction circuit responsive to the age signal for forming an analog aging correction signal, the aging correction circuit including, controller means responsive to the age signal for producing a digital correction value, and a digital to analog converter for converting the digital correction value to an analog correction signal; and a summing amplifier for summing the analog aging correction signal with the video signal.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to color flat panel displays and, more particularly, to an electroluminescent flat panel display with aging correction. 
     BACKGROUND OF THE INVENTION 
     Electroluminescent displays are flat panel displays that emit light from pixel locations based on the level of signal applied to each pixel location. Organic electroluminescent displays employ organic thin films deposited at pixel locations to emit light. The intensity of the emitted light is proportional to current passing through the organic thin films. The color of the emitted light and the efficiency of the energy conversion from current to light are determined by the composition of the organic thin films. 
     FIG. 1 shows a cross section view of a typical prior art active matrix bottom emitting electroluminescent display such as an organic light emitting diode (OLED) flat panel display  10  of the type shown in U.S. Pat. No. 5,937,272, issued Aug. 10, 1999 to Tang. The OLED display  10  includes a transparent substrate  12  that provides mechanical support for the display device, a transistor switching matrix layer  14 , a light emission layer  18  containing materials forming organic light emitting diodes, and a cable  20  for connecting circuitry within the flat panel display to video interface circuit  24 , located on printed circuit board  26 . The transparent substrate  12  is typically glass, but other materials, such as plastic, may be used. The transistor switching matrix layer  14  contains a two-dimensional matrix of thin film transistors (TFTs)  16  that are used to select which pixel in the OLED display receives image data at a given time. The thin film transistors  16  are manufactured using conventional semiconductor manufacturing processes, and therefore extra thin film transistors  16  may be used to form circuitry for a variety of uses. As taught in U.S. Ser. No. 09/774,221 filed Jan. 30, 2001, by Feldman et al., the presence of TFTs within an active matrix flat panel display allows functions other than display to be implemented on the same substrate as the display function, producing a system-on-panel. The OLED display is responsive to digital control signals and analog video signals generated by video interface circuit  24 . 
     It is known to those skilled in the art that organic light emitting materials undergo an aging process, where changes in the materials cause the light output of a given material for a constant input current stimulus to change with age. This causes a given image signal to produce a different image as the materials age. However, users of organic electroluminescent displays expect a given image signal to produce the same image, regardless of the ages of the organic light emitting materials. Alternatively, users may expect a given image signal to produce a pleasing image, although not necessarily the same image, regardless of the ages of the organic light emitting materials. For example, a dimmer image, but with proper color balance, may be acceptable, rather than the same, brighter image. 
     P. Salam, in his paper “OLED and LED Displays with Autonomous Pixel Matching,” published in the SID 2001 Digest, pages 67-69, describes a closed-loop luminance control system that utilizes light sensors placed around the periphery of an OLED display for sensing pixel light output to feed back luminance information to a color correction circuit. A “monitoring mode” is used for the pixel light display when the display is not in use wherein a single or area of pixels, is addressed one color channel at a time, and the emitted light is detected to generate a control signal. 
     The signal from the light sensor undergoes analog-to-digital conversion, and a processor calculates the measured light value and stores it. During normal image display, the display controller utilizes this stored luminance information to correct for non-uniformities in the light outputs from the pixels, which may occur due to aging. This method is complex because it requires a “pixel luminance map” that must store information regarding each pixel, or each area of pixels, which may require a lot of memory. This can be expensive to implement, particularly for portable devices, which are often price sensitive. Additionally, the “pixel luminance map” memories must be updated periodically, and therefore must be either a volatile or a rewritable volatile memory. Volatile memories typically consume more power than non-volatile memories, in order to maintain their contents. Non-volatile memories such as FLASH consume less power when not being written, but have a limited number of update cycles prior to failure. Therefore, a “pixel luminance map” may be costly, power inefficient, and have limited life. 
     U.S. Pat. No. 6,081,073, issued Jun. 27, 2000 to Salam, describes a circuit and method for minimizing luminance and color variation in a light emitting diode matrix display, where light output is measured and stored, and a microprocessor or controller controls the measurement and correction process. Again, this method of performing luminance and color correction utilizes a memory map storing information about each display pixel, and therefore may be unnecessarily complex. 
     International application WO99/41732, published Aug. 19, 1999, by Matthies et al. describes several methods for correcting brightness due to OLED materials aging and pixel non-uniformity. These methods include measuring a physical aspect regarding light emitting pixels, performing calculations, and changing the current supplied to these light emitting pixels based on these measurements, in relation to stored accumulated current-values. This method directly modulates OLED current, and must therefore operate at the pixel level. However, display systems typically supply image information to displays using analog voltages and electronics within the display, or within the drivers that directly supply pixel current to pixels, to convert the voltage information into current. It is often desirable to modify these analog voltages, and not the currents to which the input analog voltages are converted. 
     There is a need therefore for an improved means of modifying analog video signals in a display device for the purpose of compensating for the aging of the corresponding organic light emitting materials that the signals drive, utilizing a simplified circuit. 
     SUMMARY OF THE INVENTION 
     The need is met according to the present invention by providing a display, that includes an electroluminescent display panel containing a light emitting material; a video interface circuit for producing an analog video signal for driving the display; an age circuit for supplying a signal representing the age of the light emitting material; an aging correction circuit responsive to the age signal for forming an analog aging correction signal, the aging correction circuit including, controller means responsive to the age signal for producing a digital correction value, and a digital to analog converter for converting the digital correction value to an analog correction signal; and a summing amplifier for summing the analog aging correction signal with the video signal. 
     Advantages 
     The display according to the present invention is advantageous in that it exhibits a near constant luminance and/or color balance for given analog video voltages as the light emitting materials of an electroluminescent display age. The circuit is optimized for compensating analog video channels, and therefore is simpler, more cost efficient, and benefits from higher manufacturing yields than previous methods. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram showing the structure of a prior art organic electroluminescent display; 
     FIG. 2 is a schematic diagram showing an aging correction circuit for an organic electroluminescent display according to the present invention, where analog aging correction signals are combined with analog video signals; 
     FIG. 3 is a schematic diagram showing one embodiment of the aging correction circuit containing a controller, and one memory, one latch, and one digital-to-analog converter per analog video channel; 
     FIG. 4 is a schematic diagram showing an alternative embodiment of the aging correction circuit containing a controller, one memory, one latch, and one digital-to-analog converter controlling all analog video channels; 
     FIG. 5 is a schematic diagram showing a further alternative embodiment of the aging correction circuit containing a controller connected to one memory, and one latch and digital-to-analog converter per analog video channel; 
     FIG. 6 is a schematic diagram showing one implementation of the aging correction circuit containing a controller connected to one latch and digital-to-analog converter per analog video channel; 
     FIG. 7 is a schematic diagram showing a conventional implementation of the aging correction circuit where all components of the aging correction circuit are implemented on a printed circuit board; 
     FIG. 8 is a diagram of an organic electroluminescent display showing its active area and an area where additional circuitry may be implemented; 
     FIG. 9 is a schematic diagram showing an implementation of the present invention where amplifiers are implemented within the organic electroluminescent display, and all other components are implemented on a printed circuit board; 
     FIG. 10 is a schematic diagram showing an implementation of the present invention where latches, digital-to-analog converters, and amplifiers are implemented within the organic electroluminescent display, and all other components are implemented on a printed circuit board; 
     FIG. 11 is a schematic diagram showing an implementation of the present invention where memories, latches, digital-to-analog converters, and amplifiers are implemented within the organic electroluminescent display, and all other components are implemented on a printed circuit board; 
     FIG. 12 is a schematic diagram showing an implementation of the present invention where a controller, memories, latches, digital-to-analog converters, and amplifiers are implemented within the organic electroluminescent display, and all other components are implemented on a printed circuit board; and 
     FIG. 13 is a schematic diagram showing an implementation of the present invention where an age circuit is implemented within the organic electroluminescent display; 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 2, an electroluminescent display system generally designated  100  according to the present invention includes an electroluminescent display panel  102  driven by one or more aging corrected analog video signals  104 . A video interface circuit  24  produces analog video signals  108  and control signals  110 . An aging correction circuit  112  is responsive to an electroluminescent display age input from an age circuit  118 , to produce analog correction signals  114 . The analog correction signals are combined with the analog video signals  108  within summing amplifiers  116 . The electroluminescent display panel  102  contains light emitting materials. For multicolor displays, the light emitting materials that emit the different colors of light may age at different rates. 
     Typically, an age for each color of light emitting materials is provided, since the light emitting materials for different colors age at different rates. However, a single overall age signal may be presented to a multiplicity of video channels when the light emitting materials of these video channels age similarly. 
     The age circuit  118  may supply one average age of the light emitting materials, an average age of each light emitting material in a multicolor electroluminescent display, an average age of the light emitting material for subset areas of the active area, an average age of each light emitting material for subset areas of the active area, an average age of the light emitting materials for each pixel within the active area, or the age of each light emitting material at each pixel of the active area. The granularity of age measurement depends on a tradeoff between design complexity, cost, the profiles of the aging of the light emitting materials, and power consumption. 
     Various means of measuring the age of the OLED may be used. For example, age may be measured by counting the time the OLED has been driven. Alternatively, age may be indirectly measured by using a light detector to measure the actual light output of the electroluminescent display, and comparing it to the expected light output for the given drive conditions, as described in U.S. Pat. No. 6,081,073, referenced above. This light output may be taken from the display of normal images, or from test patterns displayed during inactive periods, as described by Salam, in his paper “OLED and LED Displays with Autonomous Pixel Matching,” referenced above. Age may also be indirectly measured by using a reference pixel. In one method, the light output of a reference pixel formed within the electroluminescent display panel  102  emits light, and the light output is sensed, as described in copending U.S. patent application Ser. No. 09/707,223, filed Nov. 6, 2000 by Cok et al., and allowed Jul. 3, 2001. 
     In a second method, the electrical characteristics of a reference pixel are measured. The measured electrical characteristic must change proportionally to the OLED material&#39;s age. This method is described in copending U.S. patent application Ser. No. 09/577,241, filed May 24, 2000 by Cok et al. When indirect measurements are used for measuring the age of light emitting materials, the age circuit  118  may produce a measured value that is related to age, rather than an actual chronological age. The aging correction circuit  112  then converts this measured value into a chronological actual age via a functional relationship. 
     A number of implementations exist for the aging correction circuit  112 . One embodiment is shown in FIG.  3 . Here, the aging correction circuit  112  contains a controller  120 , one or more memories  122 , one or more latches  124 , and one or more digital-to-analog converters  126 . The controller  120  may be implemented as custom digital logic, a programmed microprocessor, a microcontroller, or digital signal processor, and is responsive to age input from age circuit  118 . The memories  122  hold age correction information, where each memory address corresponds to a predetermined age, and the contents of each memory location contain a digital voltage difference based on the predetermined age. Thus, the memories  122  are used as correction value look-up tables. Because these memories  122  do not contain a “pixel luminance map,” the size of the memories used in the current embodiment are typically much smaller than those used to store a pixel luminance map. The memories may be volatile or non-volatile. Volatile memories are useful for use with different electroluminescent display panels  102  having different aging characteristics, which occurs relatively often. However, volatile memories require initialization prior to use in aging correction. Alternatively, non-volatile memories are useful when the aging correction circuit  112  is normally operated with a single electroluminescent display, and therefore does not need initialization prior to every use. 
     Non-volatile memories may be read-only memory (ROM), electrically programmable read-only memory (EPROM), electrically erasable programmable read-only memory (EEPROM), or FLASH memory. The latter two types of memory are reprogrammable, and therefore allow for updates to the lookup table for better accuracy in the future, or for re-use of the circuitry with different devices or organic light emitting materials. However, the frequency of update is relatively low compared to the frequency of update of a “pixel luminance map,” and therefore the number of write cycles for a FLASH device will usually not be a concern. 
     Latches  124  are used to synchronize the digital aging correction value to the clock controlling the digital-to-analog converters  126 . The latches  124  may be omitted if this synchronization occurs inherently as a part of circuit operation, or if the memories  122  contain a latching function. 
     Utilizing the input age, the controller  120  produces a corresponding memory address for the memories  122 . The address is sent to the memories  122 , and corresponding digital aging correction values are read from these memories  122 . When present, latches  124  then latch these digital aging correction values. The digital-to-analog converters  126  then convert these digital aging correction values to analog form. 
     FIG. 4 shows an embodiment of the present invention where the aging correction circuit  112  contains only one memory  122 , one latch  124 , and one digital-to-analog converter  126 . The output of the aging correction circuit  112  is combined with more than one video channel using summing amplifiers  116 . This embodiment is useful where two different light emitting materials exhibit the same, or relatively close, aging profiles, or where reduced complexity of the aging correction circuit  112  is desired. This embodiment has the advantages of being simple, thereby reducing cost, using fewer materials, and improving circuit yields. 
     FIG. 5 shows an embodiment of the present invention where the aging correction circuit  112  contains a single memory  122 , a controller  120 , one or more latches  124 , and one or more digital-to-analog converters  126 . Controller  120  is responsive to organic electroluminescent display age input  118  and reads appropriate aging-to-voltage correction data from memory  122 . When multiple color channels are present, the controller  120  computes the appropriate aging-to-voltage correction value for one color channel at a time, and stores this value in the appropriate latch  124 . Once a new value is stored in latch  124 , the corresponding digital-to-analog converter  126  converts this digital value to analog. 
     Controller  120  synchronizes the look-up operation, latching, and the digital-to-analog conversion. Because the light emitting materials aging process is relatively slow (measured in hours) as compared to the look-up, computation, latching, and digital-to-analog conversion process using conventional clock periods (measured in hundreds of microseconds), this embodiment may be utilized to reduce the parts count in the circuit. Therefore, this embodiment reduces materials and cost, and improves yields over more complex embodiments. 
     FIG. 6 shows an embodiment of the present invention where the aging correction circuit  112  contains controller  120 , one or more latches  124 , and one or more digital-to-analog converters  126 . This embodiment does not use a memory look-up table, but instead relies on a mathematical relationship to directly calculate an aging-to-voltage correction value via computations performed by controller  120 . Different mathematical relationships may be used for different light emitting materials within the organic electroluminescent display  102 . The digital age-to-voltage correction values are stored in latches  124 , and converted to analog voltages by digital-to-analog converters  126 . This embodiment is useful where the mathematical relationship between the age of the light emitting materials and light output is relatively simple and therefore does not require a very powerful controller for computation. Likewise, this embodiment is useful when the aging process of the materials is slow enough so that complex calculations may be performed over a relatively long period of time. This embodiment has the advantage of eliminating the memories associated with lookup tables, and therefore requires fewer materials, reduces cost, improves yields, and reduces manufacturing steps, since manufacturing processes associated with memories are often different from those associated with digital logic. 
     FIG. 7 shows the conventional placement of the various elements of the electroluminescent display system  100 . Conventionally, OLED age circuit  118 , the aging correction circuit  112 , the video interface circuit  24 , and the summing amplifiers  116  would be components mounted on a circuit board  30 . The aging corrected analog video signals  104  are then supplied to the organic electroluminescent display panel  102  via a cable  20 . The cable  20  connects to the printed circuit board  26  at connector  28 . 
     As disclosed in U.S. Ser. No. 09/774,221, referenced above, circuitry may be integrated on the same substrate as an active matrix electroluminescent display. Therefore, all, or a part of the circuitry described herein with respect to aging correction may be implemented on the electroluminescent display&#39;s substrate. FIG. 8 shows the basic structure of such an electroluminescent display system  100 . Electroluminescent display panel  102  includes an active area  130  in which the light emitting materials and pixels of the electroluminescent display panel  102  are located. Additionally, since thin film transistors are located within the active area  130 , it is relatively simple to form additional circuitry  132  including additional thin film transistors around the periphery of the active area  130 . For example, this circuitry  132  may be placed close to cable  20 , if it is responsive to signals carried over the cable  20 , and the output of the circuitry  132  is used to control circuitry within the active area  130 . Alternatively, existing circuitry within active area  130  may be modified to include all of, or portion of, additional circuitry  132 . 
     FIG. 9 shows an embodiment of the present invention where the summing amplifiers  116  are physically located on the electroluminescent display panel  102  within circuitry  132 . Preferably, the summing amplifiers  116  are placed around the periphery of the active area  130 . One or more analog correction signals  114 , along with analog video signals  108 , are transmitted over cable  20  to the electroluminescent display  102 . The summing amplifiers  116  easily integrate into the manufacturing process of the electroluminescent display panel  102 , since such analog circuitry is already formed within the electroluminescent display itself to accommodate the analog video channels. This embodiment has the advantage of reducing parts count on the printed circuit board  26 , utilizing high density integration technology on the electroluminescent display panel  102 , reducing overall system cost. 
     FIG. 10 shows a further embodiment of the present invention where the latches  124 , the digital-to-analog converters  126  and the summing amplifiers  116  are physically located on the electroluminescent display  102  within circuitry  132 . Typically, the latches  124 , the digital-to-analog converters  126 , and the summing amplifiers  116  are placed around the periphery of the active area  130 . The digital aging-to-color voltage correction output by the memories  122  on the printed circuit board  26 , along with analog video signals  108 , are transmitted over cable  20  to the electroluminescent display panel  102 . A digital transmission circuit  136  performs a conversion of the digital aging-to-color voltage correction value to the transmission format. A digital receiver circuit  140 , located within circuitry added to the electroluminescent display panel  102 , receives these transmitted correction values, and stores them in the appropriate latch  124  on the electroluminescent display panel  102 . The digital aging-to-color voltage correction values are transmitted digitally over cable  20 . This digital transmission can utilize serial or parallel transmission format. Serial transmission is preferred, since fewer conductors are required, minimizing materials and therefore cost. Serial transmission is often slower than parallel transmission. However, since the aging rate of the light emitting materials is very slow in comparison to conventional serial transmission rates, it is normally acceptable to transmit at this lower rate. This embodiment has the advantage of reducing parts count on the printed circuit board  26 , utilizing high density integration technology on the electroluminescent display panel  102 , reducing overall system cost. 
     FIG. 11 shows a further embodiment of the present invention where the memories  122 , latches  124 , the digital-to-analog converters  126  and the summing amplifiers  116  are physically located on the electroluminescent display panel  102  within circuitry  132 . Typically, the memories  122 , the latches  124 , the digital-to-analog converters  126  and the summing amplifiers  116  are placed around the periphery of the active area  130 . Controller  120  on the printed circuit board  26  computes the digital memory address values. These digital memory address values, along with analog video signals  108 , are transmitted over cable  20  to the electroluminescent display panel  102 . A digital transmission circuit  136  performs a conversion of the digital memory address values to the transmission format. A digital receiver circuit  140 , located within circuitry added to the electroluminescent display panel  102 , receives these transmitted correction values, and routes them to the appropriate memories  122  on the electroluminescent display panel  102 . This embodiment has the advantage of further reducing parts count on the printed circuit board  26 , utilizing high density integration technology on the electroluminescent display panel  102 , reducing overall system cost. 
     FIG. 12 shows a further embodiment of the present invention where the entire aging correction circuit  112 , the electroluminescent display age input  118 , and the summing amplifiers  116  are physically located on the electroluminescent display panel  102  within circuitry  132 . Typically, aging correction circuit  112 , the organic electroluminescent display age input  118 , and the summing amplifiers  116  are placed around the periphery of the active area  130 . The video interface circuit  24  remains on the printed circuit board  26 . This embodiment places the entire aging correction functionality on the electroluminescent display panel  102  itself, making the aging correction operation and manufacturing independent of the system designer. 
     Additionally, the integration of the aging correction circuit  112 , the electroluminescent display age input  118 , and the summing amplifiers  116  on the electroluminescent display panel  102  requires fewer components to be placed on the printed circuit board  26 , reducing circuit board materials and cost. Since no additional signals must be transmitted over connector  28  and cable  20 , the same pinout may be used for an electroluminescent display with and without aging correction. This increases the flexibility of the electroluminescent display system  100 , and allows electroluminescent displays with and without aging correction to be used in the electroluminescent display system interchangeably. 
     FIG. 13 shows a further embodiment of the present invention where circuitry  132  on the electroluminescent display panel  102  includes the age circuit  118 . The age of the light emitting materials of the electroluminescent display panel  102  is supplied to the age correction circuit  112  located on printed circuit board  26  via one or more conductors in cable  20 . The placement of the age circuit  118  on the electroluminescent display panel  102  allows the measurement of materials age to be coupled to the electroluminescent display panel  102 , and not physically a part of circuitry external to the electroluminescent display panel  102 . Thus, a different electroluminescent display panel  102  can be plugged in to connector  28 , and the age correction circuit  112  would operate correctly for this new display, within the need for reprogramming. This increases the usability of the electroluminescent display system  100 , decreasing integration costs, and allowing complex aging correction circuitry  112  to be implemented off of the display substrate and in integrated circuitry, where manufacturing yields are currently higher. 
     The above embodiments described in relation to the integration onto the OLED substrate are in relation to the embodiment of FIG.  3 . Similar embodiments in relation to FIGS. 4-6, can be readily implemented by a person of ordinary skill in the art, since the basic methods and reasoning are similar. 
     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. For example, these aging correction techniques may be used for both passive and active matrix organic electroluminescent displays, although the integration of transistor circuitry on passive matrix displays is not likely. Additionally, organic electroluminescent displays with color changing materials or color filters typically use a single color light emitting material. The embodiments described herein may be used for such devices, and therefore fall within the scope of the present invention. Although the embodiments herein were described in relation to signal transmission between a printed circuit board and an electroluminescent display via a cable, other interface means, such as optical and electromagnetic transmission of signals, fall within the scope of the present invention of an aging correction means by altering analog video signals. 
     PARTS LIST 
       10  organic light emitting diode flat panel display 
       12  transparent substrate 
       14  transistor switch matrix layer 
       16  thin film transistor 
       18  light emission layer 
       20  cable 
       24  video interface circuit 
       26  printed circuit board 
       28  connector 
       100  electroluminescent display system 
       102  electroluminescent display panel 
       104  aging corrected analog video signal 
       108  analog video signal 
       110  control signals 
       112  aging correction circuit 
       114  analog correction signal 
       116  summing amplifier 
       118  age circuit 
       120  controller 
       122  memory 
       124  latch 
       124  digital-to-analog converter 
       126  active area 
       130  circuitry 
       132  digital transmission circuit 
       136  digital receiver circuit