Patent Publication Number: US-2006017669-A1

Title: Method and apparatus for uniformity and brightness correction in an OLED display

Description:
FIELD OF THE INVENTION  
      The present invention relates to OLED displays having a plurality of light-emitting elements and, more particularly, correcting for non-uniformities in the display.  
     BACKGROUND OF THE INVENTION  
      Organic Light Emitting Diodes (OLEDs) have been known for some years and have been recently used in commercial display devices. Such devices employ both active-matrix and passive-matrix control schemes and can employ a plurality of pixels (each comprising one or more light-emitting elements). The pixels are typically arranged in two-dimensional arrays with a row and a column address for each pixel and having a data value associated with the pixel value. However, such displays suffer from a variety of defects that limit the quality of the displays. In particular, OLED displays suffer from non-uniformities in the pixels. These non-uniformities can be attributed to both the light emitting materials in the display and, for active-matrix displays, to variability in the thin-film transistors used to drive the light emitting elements.  
      A variety of schemes have been proposed to correct for non-uniformities in displays by using a display controller. For example, WO2004023446 A1 entitled “Electroluminescent Display Devices” by Knapp et al published 20040318 describes an active matrix electroluminescent display device having a signal processor to control the signals sent to the electroluminescent display device to reduce the non-uniformity in the display. Typically such schemes utilize some sort of calibration step to measure the non-uniformity in a display and the information from the measurement is stored in the display controller and used to correct an input signal. The corrected input signal is then applied to the display. Referring to  FIG. 3 , a controller  22  controls a display  10  and includes a correction circuit  30 . An input signal  32  is corrected by the controller  30  to create a corrected input signal  34  that is provided to the display  10 .  
      Other examples of such correction schemes include U.S. Pat. No. 6,081,073 entitled “Matrix Display with Matched Solid-State Pixels” by Salam granted Jun. 7, 2000, U.S. Pat. No. 6,414,661 B1 entitled “Method and apparatus for calibrating display devices and automatically compensating for loss in their efficiency over time” by Shen et al issued 20020702, U.S. Pat. No. 6,473,065 B1 entitled “Methods of improving display uniformity of organic light emitting displays by calibrating individual pixel” by Fan issued 20021029, and US20020030647 entitled “Uniform Active Matrix OLED Displays” by Hack et al published 20020314. These designs, however, require that the controller  22  having the correction information supplied within the correction circuit  30  must be permanently associated with the corresponding display. If the display  10  is calibrated at the time of manufacture, the display  10  must be sold with the controller  22  containing the calibration and any associated correction information. This is problematic because a controller is typically manufactured as part of an appliance and is not associated with a display until final assembly. Alternatively, the display may be calibrated and a controller loaded with calibration and correction information after an appliance is assembled. This is even more problematic in that the calibration must now be done by the assembler or purchaser.  
      An alternative means for providing uniformity correction is the so-called “system-on-glass”. In this alternative, processing circuitry is provided on the same substrate as the display. See for example, US20030025127 A1 entitled “Thin-Film Transistor Device and Method of Manufacturing the Same” published 20030206. Similarly, U.S. Pat. No. 6,501,230 entitled “Display with Aging Correction Circuit” by Feldman issued 20021231 describes a circuit integrated on the glass substrate of a display. However, it is difficult to manufacture high-performance or complex processing circuitry on a glass substrate using thin-film circuitry. Such an approach reduces manufacturing yields and increases the cost of display panels.  
      There is a need, therefore, for an improved system and method of providing uniformity correction in an OLED display that overcomes these objections.  
     SUMMARY OF THE INVENTION  
      In accordance with one embodiment, the invention is directed towards a system for the correction of brightness and uniformity variations in OLED displays, comprising:  
      a) an OLED display including a plurality of light-emitting elements;  
      b) a non-volatile memory having uniformity correction information for the OLED display stored therein and permanently associated with and physically attached to the OLED display; and  
      c) a controller connected to the OLED display and to the non-volatile memory for reading the information from the non-volatile memory, receiving an input signal, correcting the input signal using the information to form a corrected input signal, and transmitting the corrected input signal to the OLED display.  
      In accordance with further embodiments, the invention is directed towards an OLED display device comprising an OLED display and a permanently associated non-volatile memory, and a method for the correction of brightness and uniformity variations in OLED displays.  
     ADVANTAGES  
      The present invention has the advantage of providing improved uniformity, reduced manufacturing costs, and increased flexibility of use in an OLED display. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a perspective view of an embodiment of the present invention;  
       FIG. 2  is a schematic diagram of an embodiment of the present invention;  
       FIG. 3  is a prior art illustration of a uniformity compensation design;  
       FIG. 4  is a flow graph illustrating the method of the present invention; and  
       FIGS. 5   a  and  5   b  are photographs of an OLED device with and without uniformity correction according to the present invention.  
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      The present invention is directed to a system for the correction of brightness and uniformity variations in OLED displays, comprising an OLED display having a plurality of light-emitting elements; a non-volatile memory having uniformity correction information stored therein and permanently associated with and physically attached to the OLED display; and a controller connected to the OLED display and to the non-volatile memory for reading the information from the non-volatile memory, receiving an input signal, correcting the input signal using the information to form a corrected input signal, and transmitting the corrected input signal to the OLED display. In accordance with one embodiment, the OLED display may comprise a substrate where the plurality of light-emitting elements are formed on the substrate and are electrically connected through electrodes located on the substrate, and the non-volatile memory may be formed on a separate substrate. Alternatively, the non-volatile memory may be formed on the same substrate. Forming the non-volatile memory on a separate substrate advantageously improves yields, reduces costs, and reduces the physical size of the display. Use of a common substrate, on the other hand, reduces the number of components. The common substrate design is further advantaged over prior “system-on-glass” designs including processing circuitry provided on the same substrate as the display, in that the non-volatile memory circuitry of the present invention is smaller and less complex.  
      Referring to  FIG. 1 , an OLED display  10  having a substrate  12  and a plurality of light-emitting elements  13  electrically connected through electrodes  14  located on the substrate  12 . A non-volatile memory  20  is formed on a separate substrate having uniformity correction information stored therein and permanently associated with and physically attached to the OLED display  10 ; and a controller  22  connected to the OLED display  10  and to the non-volatile memory  20  for reading the information from the non-volatile memory  20 , receiving an input signal, correcting the input signal using the information to form a corrected input signal, and transmitting the corrected input signal to the OLED display  10 . The electrodes  14  are connected to an integrated circuit comprising the non-volatile memory device  20  through a signal cable  16  permanently affixed (e.g., by soldering) to the substrate  12 . The cable  16  is further connected through an external printed circuit board  24  to a controller  22 . The cable may be a conventional flexible wiring cable carrying one or more electrical wires for conducting signals to and from the OLED display  10 , the non-volatile memory  20 , and the controller  22 . Means for affixing and connecting the non-volatile memory  20  to the flexible wiring cable  16  are well-known in the electronics manufacturing art (e.g., adhesives) as are means to connect the flexible wiring cable  16  to the printed circuit board  24  (e.g., socket connectors) and OLED display  10  (e.g., by soldering). The printed circuit board  24  may include additional electronic components  26  as may be useful in an application.  
      Referring to  FIG. 2 , the OLED display  10  and non-volatile memory  20  are associated and packaged as an OLED display device unit  11 . The non-volatile memory  20  may be physically affixed to a connecting cable, as shown in  FIG. 1 , or alternatively formed on a common substrate of the OLED display. Other means of permanently associating the non-volatile memory  20  to the OLED display may be employed, for example by affixing the memory  20  to the substrate  12  or a cover of the OLED display (not shown). The controller  22  is removably connected to the non-volatile memory  20  and includes signal and control circuitry for reading information from the non-volatile memory  20 . As understood in the electrical arts, a non-volatile memory is a memory whose stored information is not lost when power is removed from the memory. The non-volatile memory  20  may be a Read-Only Memory (ROM), such as a programmable read only memory (PROM), including one-time programmable electrically programmable read only memory (OTP EPROM), and an electrically erasable programmable read only memory (EEPROM), that can be used to both read and write non-volatile information. Signals and control for such memory devices are very well known in the electronics industry. The controller  22  also includes circuitry for accepting an input signal  32  and correcting the input signal  32  using a correction circuit  30  to form a corrected input signal  34  that is supplied to the OLED display  10 .  
      Referring to  FIG. 4 , an OLED display  10  and non-volatile memory  20  are first manufactured  108  using methods known in the OLED industry. Because of variability in the manufacturing process, the OLED display  10  is likely to include non-uniform light-emitting elements  13 . The display is tested  110  by measuring the light output and uniformity of the display  10  and the measurements are used to calculate  112  corrections to reduce the non-uniformity of the OLED display  10 . These corrections can be stored  114  in a look-up table in the non-volatile memory  20 , and the memory can be permanently associated with and affixed to the display. A controller  22  for the OLED display  10  may be independently manufactured  116 .  
      The OLED display  10 ;and permanently associated non-volatile memory  20  may then be sold as a display device unit  11 . The purchaser may also separately purchase a controller  22 . The display  10 , memory  20 , and controller  22  are integrated into a product. In operation, the controller  22  reads  118  information from the non-volatile memory  20 . The information from the memory  20  is used to provide correction values to a correction circuit  30 . An input signal  32  is input  120  to the controller  22 . The correction circuitry  30  corrects  122  the input signal  32  using the information supplied from the non-volatile memory  20  to form a corrected input signal  34  that is transmitted  124  to the OLED display  10  and displayed  126 . Referring to  FIGS. 5   a  and  5   b , e.g., an OLED display having non-uniform light-emitting elements is shown with a flat field before uniformity correction  40  ( FIG. 5   a ) and after luminance uniformity correction is applied  42  ( FIG. 5   b ).  
      The information stored in the non-volatile memory  20  may include a record of the light output from each, light-emitting element of each pixel of the OLED display. It may also include brightness information for the OLED display as a-whole and include an identifier for the OLED display  10  and associated memory  20  assembly. Additional information may be included in the non-volatile memory  20 , for example size, type, aging characteristics, resolution, color, pixel patterns, materials, control signal, and display type information. As is known in the art, OLED devices also tend to age and decrease their light output over time as the OLEDs are used. In a further embodiment, the non-volatile memory is a read/write memory (e.g., an EEPROM), and the controller  22  writes information back to the non-volatile memory  20 , for example a record of OLED display use. This record of use may also be used by the controller  22  to provide aging compensation in the correction circuitry  30 . In yet another embodiment, the non-volatile memory  20  is included in an integrated circuit that also performs signal processing on the corrected input signal  34  before it is transmitted to the OLED display  10 .  
      Certain embodiments of the present invention have an advantage in that the OLED display  10  and associated non-volatile memory  20  are manufactured separately and with relatively improved yields. Moreover, the memory  20  is very simple and low-cost, especially in comparison to the OLED display  10 . The integration of the non-volatile memory  20  and OLED display  10  is a straightforward and low-cost manufacturing task.  
      The controller  22  is a relatively intelligent controller and, as is common practice, may be formed in a separate integrated circuit. Such circuits are well known and the correction circuitry incorporated into the controller may rely upon conventional integrated circuit manufacturing technologies. The OLED display and non-volatile memory unit  11  may be replaced with a different unit  11  if the first unit is no longer adequate. Hence, the present invention allows devices to be upgraded over time without regard to the characteristics of the OLED display. An intelligent controller such as that described will simply read new information from the non-volatile memory  20  and adapt the correction circuit to the new information. This adaptation may include uniformity correction, aging compensation, image rendering for alternative display resolutions, graphic rendering techniques, and many other image correction operations, for example color correction.  
      A variety of techniques for measuring the uniformity of an OLED display are known in the art which may be employed to provide the uniformity correction information stored on the non-volatile memory in the system of the invention. U.S. Pat. No. 6,414,661 B1, e.g., describes measuring the display characteristics of all organic-light-emitting-elements of a display, and obtaining calibration parameters for each organic-light-emitting-element from the measured display characteristics of the corresponding Organic-light-emitting-element. The described technique acquires information about each pixel in turn using a photo-detector. An additional technique for measuring uniformity which may be employed to provide the uniformity correction information stored on the non-volatile memory in the system of the invention is described in copending, commonly assigned U.S. Ser. No. 10/858,260, filed Jun. 1, 2004, the disclosure of which is incorporated by reference herein. Correction circuitry  30  may be implemented in a variety of conventional ways known in the art. An additional correction circuitry technique which may be employed in the system of the present invention is described in copending, commonly assigned U.S. Ser. No. 10/869,009, filed Jun. 16, 2004, the disclosure of which is incorporated by reference herein.  
      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.  
     PARTS LIST  
     
         
           10  OLED display  
           11  OLED display device unit  
           12  substrate  
           13  light-emitting element  
           14  electrode  
           16  cable  
           18  connector  
           20  non-volatile memory  
           22  controller  
           24  printed circuit board  
           26  integrated circuits  
           30  correction circuitry  
           32  input signal  
           34  corrected input signal  
           40  uncorrected OLED display  
           42  corrected OLED display  
           108  manufacture display step  
           110  test display step  
           112  calculate correction step  
           114  store correction step  
           116  manufacture controller step  
           118  read corrections step  
           120  input signal step  
           122  correct signal step  
           124  output corrected signal step  
           126  display corrected signal step