Method and apparatus for calibrating display devices and automatically compensating for loss in their efficiency over time

• US 6,414,661 B1
• Filed: 07/05/2000
• Issued: 07/02/2002
• Est. Priority Date: 02/22/2000
• Status: Expired due to Term

• Current Assignee: Mind Fusion, LLC (Ascend Innovation Management, LLC)
• Original Assignee: Sarnoff Corporation (SRI International, Inc.)
• Inventors: Atherton, James H., Shen, Zilan, Matthies, Dennis Lee, Stewart, Roger Green
• Primary Examiner(s): NGUYEN, CHANH DUY
• Application Number:
  US09/610,159
• Time in Patent Office: 727 Days
• Field of Search: 345/82, 345/39, 345/83, 345/76, 345/80, 345/204, 345/44, 345/46, 345/78, 315/169.1, 315/169.3, 313/503, 313/504, 313/507, 428/690, 428/691, 257/88, 257/98, 340/815.45
• US Class Current: 345/82
• CPC Class Codes: G09G 2300/026 Video wall, i.e. juxtaposition of a plurality of screens to create a display screen of bigger dimensions
• CPC Class Codes: G09G 2300/08 Active matrix structure, i.e. with use of active elements, inclusive of non-linear two terminal elements, in the pixels together with light emitting or modulating elements
• CPC Class Codes: G09G 2310/027 Details of drivers for data electrodes, the drivers handling digital grey scale data, e.g. use of D/A converters
• CPC Class Codes: G09G 2320/029 by monitoring one or more pixels in the display panel, e.g. by monitoring a fixed reference pixel
• CPC Class Codes: G09G 2320/0295 by monitoring each display pixel
• CPC Class Codes: G09G 2320/043 Preventing or counteracting the effects of ageing
• CPC Class Codes: G09G 2320/048 using evaluation of the usage time
• CPC Class Codes: G09G 2320/0693 Calibration of display systems
• CPC Class Codes: G09G 2360/145 the light originating from the display screen
• CPC Class Codes: G09G 3/3208 organic, e.g. using organic light-emitting diodes [OLED]

Abstract

A method and associated system that compensates for long-term variations in the light-emitting efficiency of individual organic light emitting diodes (OLEDs) in an OLED display device, calculates and predicts the decay in light output efficiency of each pixel based on the accumulated drive current applied to the pixel and derives a correction coefficient that is applied to the next drive current for each pixel. In one exemplary embodiment of the invention, the calculation is based the accumulated current that has been passed through the device. In another exemplary embodiment, the calculation is based on a difference in voltage across the pixel at two instants. The compensation system is best used after the display device has been calibrated to provide uniform light output. The present invention further provides a method for calibrating a display device comprising an array of individually adjustable discrete light emitting devices (pixels) using a camera having an array of radiation sensors or a single photodetector. According to this method, the camera captures respective images of substantially equal-sized first sub-areas and adjusts the driving current for each of said pixels within said first sub-areas to achieve a desired light output. Next, the camera captures images of a plurality of second sub-areas, each of the second sub-areas including multiple ones of the first sub-areas second-level sub-areas are adjusted such that each of the first-level sub areas provide substantially equal light output. In an alternative embodiment, each of the first level sub-areas overlaps and, after adjusting a first sub-area to a desired brightness level, the invention adjusts the pixels in each overlapping sub-area to have the same brightness as the overlapping pixels.

Claims

1. A method for correcting non uniformities in light emitted by an organic light emitting display device comprising a plurality of addressable discrete picture elements (pixels), each of said pixels being driven by a driving current and each having a light emitting efficiency, the method comprising:

a) predicting a decay in the light emitting efficiency for each of said plurality of pixels over a time period by accumulating for each of said pixels a total driving current for each of said pixels during said time period,

b) deriving a correction coefficient for each of said pixels based on said predicted decay in the light emitting efficiency, and

c) using said correction coefficients, altering said driving current for each of said pixels to compensate for said predicted decay in the light emitting efficiency of each of said plurality of pixels.

2. A method for correcting non uniformities in light emitted by an organic light emitting display device comprising a plurality of addressable discrete picture elements (pixels), each of said pixels being driven by a driving current and each having a light emitting efficiency, the method comprising:

a) predicting a decay in the light emitting efficiency for each of said plurality of pixels over a time period by measuring for each one of said pixels a differential voltage representing a difference between (1) a voltage across the one pixel at one instant which produces a desired current and (2) a voltage across the one pixel at an instant prior to the one instant that produces the desired current,

b) deriving a correction coefficient for each of said pixels based on said predicted decay in the light emitting efficiency, and

c) using said correction coefficients, altering said driving current for each of said pixels to compensate for said predicted decay in the light emitting efficiency of each of said plurality of pixels.

3. A method for correcting non uniformities in light output by an organic light emitting display device, said device comprising a plurality of addressable discrete picture elements (pixels), each of said pixels driven by a driving current and each pixel having a light output which is a function of the driving current, the method comprising:

a) predicting a change in the light output for each of said plurality of pixels by accumulating, for each of said pixels, a driving current for each of said pixels during an elapsed time,

b) compensating for said change in said light output of each of said plurality of pixels by calculating a corresponding change in said driving current, based on the predicted change in light output, and applying said change in said driving current for each of said pixels, respectively.

4. The method according to claim 3 wherein the step of compensating for said change in light output of each of said plurality of pixels further comprises

a) measuring a first driving current for each of said pixels and a corresponding first light efficiency at a first time;

b) calculating a second light efficiency for each of said pixels at a second time as function of driving current applied to each of said pixels between said first and second times;

c) altering said first driving current for each of said pixels by a factor proportional to the ratio of the first and second light efficiencies.

5. The method according to claim 3 wherein the step of compensating for said change in light output of each of said plurality of pixels comprises:

a) identifying an initial driving current I_o and decay factor τ

_o for each of said pixels;

b) identifying a first driving current I<sub>N−

1</sub> for each of said pixels at a first time t<sub>n−

1</sub>

c) compensating for said change in light output for each of said plurality of pixels by applying a driving current I_N at a second time t_N such that

6. The method according to claim 3 wherein the step of predicting said change in light output further includes establishing an initial state of uniform device light output wherein each of said plurality of pixels is driven by an initial driving current such that each of said pixels provides a desired light output which is substantially the same for all of said plurality of pixels.

7. The method according to claim 6 wherein the step of establishing said initial state further includes the steps of:

a) driving said plurality of pixels each with a driving current corresponding to the desired light output;

b) subdividing said plurality of pixels into a first plurality of pixel arrays each of said first pixel arrays having fewer pixels than the plurality of pixels;

c) observing a light output of said driven pixels in each of said first plurality of pixel arrays with a photodetector device and adjusting the driving current for each of said pixels in each of said first pixel arrays to generate a substantially same photodetector output signal for each pixel in the first plurality of pixel arrays;

d) subdividing said plurality of pixels into a second plurality of arrays each of said second plurality of arrays including more than one of said first pixel arrays;

e) observing the light output of each of said second arrays with the photodetector and adjusting the driving current for each of said first pixel arrays such that each of the second pixel arrays generate a substantially same photodetector output signal for each of the first pixel arrays of said second plurality of arrays;

f) repeating steps (d) and (e) at least one more time increasing the number of pixels in each pixel array until said number of pixels in said pixel array equals the plurality of pixels.

8. The method according to claim 7 wherein said plurality of pixels defines a display area and wherein each of said pixel arrays comprise sub-arrays of pixels defining sub-areas of said display.

9. The method according to claim 8 wherein said sub-arrays of pixels define overlapping sub-areas.

10. The method according to claim 6 wherein the plurality of pixels form an array comprising rows and columns, and wherein the step of establishing said initial state further comprises the steps of:

a) driving said plurality of pixels each with a same driving current;

b) subdividing said plurality of pixels into a plurality of adjacent first sub-arrays of pixels along a row of said array of pixels said sub arrays comprising fewer pixels than a row of said array of pixels;

c) observing a light output of said driven pixels in each of said first plurality of pixel sub-arrays along each row of said array with a CCD detector device and adjusting the driving current for each of said pixels in each of said first plurality of pixel sub-arrays to generate a substantially same CCD output.

11. A method for calibrating a display device comprising an array of individually adjustable discrete light emitting devices (pixels) using a photodetector, the method comprising:

a) observing with said photodetector a first area of said display device array forming a first level sub-array having a first number of pixels and adjusting each of said pixels within said first sub-array to a desired light output;

b) observing with said photodetector a second area forming a first level second sub-array and adjusting each of said pixels within said second sub-array to the desired light output;

c) repeating steps (a) and (b) until all of the display pixels have been adjusted to the desired light output;

d) observing with said photodetector another first area of the device array containing a plurality of said first level sub-arrays to form a second level sub-array;

e) adjusting as a unit each of said first level sub-arrays in said second level sub-array, to have a common light output;

f) observing with said photodetector another second level sub-array containing a plurality of said first level sub-arrays to form another second level sub-array;

g) adjusting as a unit each of said first level sub-arrays in said another second level sub-array, to have a common light output;

h) repeating steps (e) through (g) until all of the display first level sub-arrays have been adjusted to have common outputs;

i) repeating steps (d) through (h) with respectively larger sub-arrays until the sub-array has a size that spans the display array.

12. A method for calibrating a display device comprising an array of individually adjustable discrete light emitting devices (pixels) using an array of photodetectors, the method comprising:

a) observing with said array of photodetector a first area of said display device array forming a first level sub-array having a first number of pixels and adjusting each of said pixels within said first sub-array to a desired light output;

b) observing with said photodetector a second area forming a first level second sub-array, said second sub-array overlapping said first sub-array in at least one pixel position and adjusting each of said pixels within said second sub-array to have a brightness output substantially equal to the light output of the at least one overlapping pixel;

c) repeating step (b) until all of the display pixels have been adjusted to the desired light output.

14. The system according to claim 12 wherein the means for calculating said corrected current include means for receiving an input comprising a first current value I_N−

1, a value representing

I<sub>N−

1</sub>Δ

t<sub>N−

1</sub>/I_oτ

_o and for generating an output current value

16. The system according to claim 12 wherein the means for accumulating, calculating and applying said corrected current are digital means and their respective functions are implemented in software.

13. A system for correcting non uniformities in light output by an organic light emitting display device, said device comprising a plurality of addressable discrete picture elements (pixels), each of said pixels driven by a driving current and each pixel having a light output which is a function of the driving current, the system comprising:

a) accumulating means for integrating for each of said pixels the driving current for each of said pixels during elapsed time;

b) means associated with said accumulating means for calculating a corrected driving current,

b) means for applying said corrected current to each of said plurality of pixels.

15. The system according to claim 13 wherein the means for calculating said corrected current operates in the analog signal domain and comprise a bipolar transistor circuit.

17. Apparatus for calibrating a display device comprising an array of individually adjustable discrete light emitting devices (pixels), the apparatus comprising;

a light detector which receives light emitted by the display device;

light measuring means, coupled to the light detector, for measuring light output of a portion said display device array;

adjusting means, responsive to the light measuring means for controlling the portion of the display device array to change the light output of the portion of the display device array;

an xyz translation stage, which moves the light detector parallel to the display device to capture light emitted by respectively different portions of the display device and which moves the light detector perpendicular to the display device to cause the light measuring means to receive light from different-sized portions of the display device array;

a controller coupled to the light detector, the light measuring means, the adjusting means and the translation stage for moving the light detector to measure the light output of respective first portions of the display device array, each of the first portions being substantially equal in size, for adjusting the respective portions of the display device to provide a desired light output;

for moving the light detector to measure light output of respective second portions of the display device array, each of said second portions being substantially equal in size and including a plurality of the first portions; and

for adjusting each of the second portions to provide substantially equal light output.

18. Apparatus according to claim 17, wherein the light detector is a CCD camera having a plurality of light detecting elements arranged in an array.

19. Apparatus according to claim 17, wherein the light detector is a single photo detector and the controller controls the translation stage such that the first portions of the display device array correspond to respectively different pixels of the display device array.

20. Apparatus for calibrating a display device comprising an array of individually adjustable discrete light emitting devices (pixels), the apparatus comprising:

an array of light detecting elements which receives light emitted by the display device;

light measuring means, coupled to the array of light detecting elements, for measuring light output of a portion said display device array;

adjusting means, responsive to the light measuring means for controlling the portion of the display device array to change the light output of the portion of the display device array;

an xy translation stage, which moves the light detector parallel to the display device to capture light emitted by respective portions of the display device;

a controller coupled to the array of light detecting elements, the light measuring means, the adjusting means and the translation stage, the controller including

means for moving the light detector to measure the light output of respective overlapping portions of the display device array, each of the overlapping portions being substantially equal in size and overlapping a previously measured portion in at least one pixel position;

means for adjusting a first one of the portions of the display device to provide a desired light output; and

means for adjusting each one of the overlapping portions to have a light output substantially equal to the light output of the at least one overlapping pixel position.