Method and apparatus for uniformity and brightness correction in an electroluminescent display

1. A method for reducing brightness uniformity variations in an active-matrix electroluminescent (EL) display employing amorphous silicon thin-film transistors, comprising the steps of:

a) providing an active-matrix EL display having amorphous silicon thin-film transistors that drive a plurality of light-emitting elements responsive to an input signal that causes the light-emitting elements to emit light;

b) deriving a first correction value from a measured or estimated current used or from a measured light output emitted by the light-emitting element in response to known image signals at a first time;

c) identifying a plurality of predetermined groups of light-emitting elements, the plurality of predetermined light-emitting groups including all of the light-emitting elements in the EL display, wherein each predetermined group of light-emitting elements includes more than one light-emitting element;

d) selecting one or more representative light-emitting elements for each predetermined group of light-emitting elements, wherein, in each predetermined group, the number of the representative light-emitting elements is fewer than the number of the light-emitting elements;

e) measuring total representative current used by the representative light-emitting elements for each predetermined group of light-emitting element in response to known image signals at a second time;

f) deriving an estimated second correction value from i) the first correction value, or the measured or estimated current used or the measured light output emitted by the light-emitting element in response to known image signals at the first time, and ii) the measured total representative currents for each individual light-emitting elements; and

g) employing the estimated second correction value to correct image signals for the changes in the output of the light-emitting elements in each predetermined group and produce compensated image signals,

wherein the method employs only a single measurement of the total representative current per each predetermined group, and

the frequency of the measurements is tens of hours of use.

2. The method of claim 1 , wherein steps e) through g) are repeatable.

3. The method of claim 1 , wherein the estimates for each light-emitting element are calculated by interpolating from the total representative current measurements for each predetermined group.

4. The method of claim 1 , wherein a correction value for at least one light-emitting element is estimated by interpolating between correction values for other light-emitting elements.

5. The method of claim 1 , wherein a single representative light-emitting element is selected.

6. The method of claim 1 , wherein the representative light-emitting elements comprise more than one but fewer than all of the light-emitting elements in a group.

7. The method of claim 6 , wherein the representative light-emitting elements comprise a regular array of samples within a group.

8. The method of claim 1 , wherein the current used by the light-emitting element in the step of deriving the first correction value is measured at a plurality of luminance levels.

9. The method of claim 1 , wherein the correction values for one or more of the light-emitting elements is calculated by interpolating the measured total representative current values.

10. The method of claim 1 , wherein the EL display luminance is held substantially constant.

11. The method of claim 1 , further comprising the steps of re-determining the groups after the first correction value is derived and measuring the total representative current for each of the re-determined groups.

12. The method of claim 1 , wherein the EL display is a color display comprising light-emitting elements of multiple colors and wherein the measurements are done separately for each color of light-emitting element.

13. The method of claim 1 , wherein the total representative current for each group is measured for a plurality of different input signal values and a plurality of correction values are estimated for each light-emitting element.

14. The method of claim 1 , wherein different sets of representative light-emitting elements are specified for each group and different total representative currents are measured for each group and then combined to form a total representative current measurement.

15. An active-matrix EL display, comprising:

a) an active-matrix EL display having amorphous silicon thin-film transistors that drive a plurality of light-emitting elements responsive to an input signal that causes the light-emitting elements to emit light; the light-emitting elements divided into a plurality of predetermined groups, each group comprising more than one light-emitting element and one or more representative light-emitting elements selected for each group of light-emitting elements, wherein, in each predetermined group, the number of the representative light-emitting elements is fewer than the number of the light-emitting elements; and

b) a controller coupled to the active-matrix EL display that obtains a first correction value from a measured or estimated current used or from a measured light output emitted by the light-emitting elements in response to known image signals at a first time; and also that measures total representative current used by the representative light-emitting elements for each of the predetermined groups in response to known image signals at a second time, wherein the controller employs only a single measurement of the total representative current per each predetermined group, and the frequency of the measurements is tens of hours of use,

wherein the controller further comprises:

means for forming an estimated second value of the current used by individual light-emitting elements based on the measured total representative currents;

means for calculating correction values for individual light-emitting elements based on the difference between the first and second measurements; and

means for employing the correction values to compensate image signals for the changes in the output of the light-emitting elements in each predetermined group and produce compensated image signals.

16. The method of any of claims 1 and 15 , wherein the EL display is an organic light-emitting diode (OLED) display.

17. The active matrix EL display of claim 15 , wherein the EL display is an OLED display.

18. The method of claim 7 , wherein the predetermined group of the light-emitting elements comprises light-emitting elements arranged in two-dimensional arrays.

19. The active-matrix EL display of claim 15 , wherein the each predetermined group comprises more than one representative light-emitting elements selected for each predetermined group of the light-emitting elements.

20. The active-matrix EL display of claim 15 , wherein the predetermined group of the light-emitting elements comprises light-emitting elements arranged in two-dimensional arrays.

21. The method of claim 1 , wherein the frequency of the measurements is days of use.

22. The method of claim 1 , wherein the frequency of the measurements is one week of use.

23. The active-matrix EL display of claim 15 , wherein the frequency of the measurements is weeks of use.

24. The active-matrix EL display of claim 15 , wherein the frequency of the measurements is months of use.