Systems and methods for calibrating solid state lighting panels

• US 8,278,846 B2
• Filed: 11/17/2006
• Issued: 10/02/2012
• Est. Priority Date: 11/18/2005
• Status: Active Grant

• Current Assignee: Brightplus Ventures LLC (IPValuation Partners, LLC)
• Original Assignee: CREE Incorporated (Wolfspeed, Inc.)
• Inventors: Roberts, John K., Vadas, Keith J., You, Chenhua
• Primary Examiner(s): PHILOGENE, HAISSA
• Application Number:
  US11/601,410
• Publication Number:
  US 20070115228A1
• Time in Patent Office: 2,146 Days
• Field of Search: 315/291, 315/307, 315/224, 315/312, 315/149, 315/159, 315/308, 345/77, 345/82, 345/84, 345/87, 345/83, 345/102, 345/589, 345/592, 345/593, 345/597, 345/39, 362/219, 362/227, 362/230, 362/231, 362/236, 362/239, 362/251, 362/800, 358/504, 358/509, 358/512, 358/513, 358/520
• US Class Current: 315/308
• CPC Class Codes: G09G 2320/0233 Improving the luminance or brightness uniformity across the screen
• CPC Class Codes: G09G 2320/041 Temperature compensation
• CPC Class Codes: G09G 2320/064 by time modulation of the brightness of the illumination source
• CPC Class Codes: G09G 2320/0653 Controlling or limiting the speed of brightness adjustment of the illumination source
• CPC Class Codes: G09G 2320/0666 for control of colour parameters, e.g. colour temperature
• CPC Class Codes: G09G 2320/0693 Calibration of display systems
• CPC Class Codes: G09G 2360/144 the light being ambient light
• CPC Class Codes: G09G 2360/145 the light originating from the display screen
• CPC Class Codes: G09G 3/3413 Details of control of colour illumination sources
• CPC Class Codes: G09G 3/342 using several illumination sources separately controlled corresponding to different display panel areas, e.g. along one dimension such as lines
• CPC Class Codes: H05B 45/20 Controlling the colour of the light
• CPC Class Codes: H05B 45/22 using optical feedback
• CPC Class Codes: H05B 45/28 using temperature feedback
• CPC Class Codes: H05B 45/325 Pulse-width modulation [PWM]
• CPC Class Codes: H05B 45/33 Pulse-amplitude modulation [PAM]
• CPC Class Codes: H05B 45/395 Linear regulators

Abstract

Methods of calibrating a lighting panel including a plurality of segments, each of said segments configured to emit a first color light and a second color light in response to pulse width modulation control signals having respective duty cycles include determining an average segment luminance for the lighting panel, determining a luminance variation of each segment to the average segment luminance, comparing the luminance variation of each segment to a threshold, and adjusting the duty cycle of at least one color of at least one segment to reduce the luminance variation in response to the luminance variation of a segment exceeding the threshold. Calibration systems are also disclosed.

Claims

1. A method of calibrating a lighting panel comprising a plurality of segments, each of said segments configured to emit a first color light and a second color light in response to pulse width modulation control signals having respective duty cycles, the method comprising:

determining an average segment luminance for the lighting panel;

determining a luminance variation of each segment to the average segment luminance;

comparing the luminance variation of each segment to a threshold; and

in response to the luminance variation of a segment exceeding the threshold, adjusting the duty cycle of at least one color of at least one segment to reduce the luminance variation.

2. The method of claim 1, wherein determining the average segment luminance comprises:

sequentially illuminating a plurality of segments;

measuring a display luminance from the illuminated segments at a measurement location; and

averaging the display luminance measurements.

3. The method of claim 2, wherein the measurement location comprises a location at about the center of the lighting panel.

4. The method of claim 2, wherein sequentially illuminating a plurality of segments comprises applying a pulse width modulation control signal having an adjusted duty cycle to at least one of the plurality of segments.

5. The method of claim 1, wherein the luminance variation to the average segment luminance is calculated according to the equation:

Δ

Y_mn=[Y_mn−

max(Y_mn)]/Y_center

where Y_mn represents the luminance of an mth segment measured at an nth measurement location, and Y_center represents the average segment luminance.

6. The method of claim 1, wherein adjusting the duty cycle of at least one color of at least one segment comprises:

determining a maximum duty cycle for all colors/segments; and

dividing the duty cycle of the at least one color of the at least one segment by the maximum duty cycle.

7. The method of claim 6, further comprising:

determining a uniformity coefficient for the at least one segment; and

adjusting the duty cycles of each color of the at least one segment using the uniformity coefficient before determining the maximum duty cycle.

8. The method of claim 7, wherein the uniformity coefficient is determined according to the equation

C_m=[1−

min(Δ

Y_m1, . . . , Δ

Y_mn)]/1.1

where Δ

Y_mn represents the luminance variation of the mth segment at the nth location to the average segment luminance and Cm represents the uniformity coefficient for the at least one segment.

9. The method of claim 1, wherein determining the luminance variation of each segment to the average segment luminance comprises determining the luminance variation of each segment to the average segment luminance for each color.

10. The method of claim 9, wherein adjusting the duty cycle of at least one color of at least one segment comprises:

for each color, determining a maximum duty cycle for all segments; and

dividing the duty cycle of the at least one color of the at least one segment by the maximum duty cycle for the at least one color.

11. The method of claim 1, wherein each segment comprises a group of tiles.

12. The method of claim 1, wherein each segment comprises a bar of tiles.

13. The method of claim 1, further comprising adjusting the duty cycles of a segment to reduce the maximum color variation of the segment.

14. The method of claim 13, wherein adjusting the duty cycles of a segment to reduce the maximum color variation for the segment comprises:

for each color, measuring a luminance of each segment at a first duty cycle;

for each color, determining a nominal luminance ratio comprising a ratio of a total luminance of each color divided by a total luminance of the lighting panel;

for the segment, determining a luminance ratio for each color comprising a ratio of a total luminance of a color of the segment to a total luminance of the segment;

determining a variation of luminance ratios for each color of the segment from the nominal luminance ratio; and

in response to at least one variation of luminance ratios from the nominal luminance ratio exceeding a second threshold, adjusting a duty cycle of at least one color of the segment to reduce the at least one variation of luminance ratios from the nominal luminance ratio.

15. The method of claim 14, wherein the first duty cycle comprises a maximum duty cycle.

16. The method of claim 14, wherein determining a variation of luminance ratios from the nominal luminance ratio for each color comprises determining a maximum variation of luminance ratios from the nominal luminance ratio for each color.

17. The method of claim 14, wherein determining a luminance ratio for each color comprises determining a total luminance for each segment for each color.

18. The method of claim 14, wherein adjusting a duty cycle of at least one color of the segment comprises selecting a color with a lowest relative luminance, and multiplying a duty cycle by a coefficient generated based on the luminance of the selected color.

19. A calibration system for calibrating a lighting panel comprising a plurality of segments, each of said segments configured to emit a first color light and a second color light in response to pulse width modulation control signals having respective duty cycles, the calibration system comprising:

a calibration controller configured to be coupled to the lighting panel; and

a calibration unit coupled to the calibration controller and comprising a calorimeter;

wherein the calibration controller is configured to determine an average segment luminance for the lighting panel, to determine a luminance variation of each segment to the average segment luminance, to compare the luminance variation of each segment to the average segment luminance to a threshold, and, in response to the luminance variation of a segment exceeding the threshold, to adjust the duty cycle of at least one color of at least one segment to reduce the luminance variation.

20. The calibration system of claim 19, wherein the calibration controller is further configured to cause the lighting panel to individually illuminate a plurality of segments, to measure a display- luminance from the illuminated segments at a measurement location, and to average the display luminance measurements.

21. The calibration system of claim 20, wherein the measurement location comprises a location at about the center of the lighting panel.

22. The calibration system of claim 19, wherein the calibration controller is configured to calculate the luminance variation to the average segment luminance according to the equation:

Δ

Y_mn=[Y_mn−

max(Y_mn)]/Y_center

where Y_mn represents the luminance of an mth segment measured at an nth measurement location, and Y_center represents the average segment luminance.

23. The calibration system of claim 19, wherein the calibration controller is further configured to adjust the duty cycle of at least one color of at least one segment by determining a maximum duty cycle for all colors/segments and dividing the duty cycle of the at least one color of the at least one segment by the maximum duty cycle.

24. The calibration system of claim 23, wherein the calibration controller is further configured to determine a uniformity coefficient for the at least one segment, and to adjust the duty cycles of each color of the at least one segment using the uniformity coefficient before determining the maximum duty cycle.

25. The calibration system of claim 24, wherein the calibration controller is configured to determine the uniformity coefficient according to the equation

C_m=[1−

min(Δ

Y_m1, . . . , Δ

Y_mn)]/1.1

where Δ

Y_mn represents the luminance variation of the mth segment at the nth location to the average segment luminance and Cm represents the uniformity coefficient for the at least one segment.

26. The calibration system of claim 19, wherein the calibration controller is configured to determine the luminance variation of each segment to the average segment luminance by determining the luminance variation of each segment to the average segment luminance for each color.

27. The calibration system of claim 26, wherein the calibration controller is configured to adjust the duty cycle of at least one color of at least one segment comprises by, for each color, determining a maximum duty cycle for all segments, and by dividing the duty cycle of the at least one color of the at least one segment by the maximum duty cycle for the at least one color.

28. The calibration system of claim 19, wherein the calibration controller is further configured to adjust the duty cycles of a segment to reduce the maximum color variation of the segment.

29. The calibration system of claim 28, wherein the calibration controller is further configured to measure a luminance of each segment at a first duty cycle for each color;

to determine a nominal luminance ratio for each color comprising a ratio of a total luminance of each color divided by a total luminance of the lighting panel;

to determine a luminance ratio for each color of the segment comprising a ratio of a total luminance of a color of the segment to a total luminance of the segment;

to determine a variation of luminance ratios for each color of the segment from the nominal luminance ratios, and in response to at least one variation of a luminance ratio from the nominal luminance ratio exceeding a second threshold, to adjust a duty cycle of at least one color of the segment to reduce the at least one variation of luminance ratios from the nominal luminance ratio.

30. The calibration system of claim 29, wherein the first duty cycle comprises a maximum duty cycle.

31. The calibration system of claim 29, wherein the calibration controller is configured to determine the variation of luminance ratios from the nominal luminance ratio for each color by determining a maximum variation of luminance ratios from the nominal luminance ratio for each color.

32. The calibration system of claim 29, wherein the calibration controller is configured to calculate the luminance ratio for each color by determining a total luminance for each segment for each color.

33. The calibration system of claim 29, wherein the calibration controller is configured to adjust the duty cycle of at least one color of at least one segment by selecting a color with a lowest relative luminance, and by multiplying a duty cycle by a coefficient generated based on the luminance of the selected color.

34. The calibration system of claim 19, wherein the calibration unit further comprises an XZ positioner connected to the calorimeter and configured to move the calorimeter in two dimensions.

35. The calibration system of claim 19, wherein the calibration unit further comprises:

an enclosure having an entrance;

a conveyor extending from outside the enclosure to inside the enclosure through the entrance; and

a pallet on the conveyor and configured to hold the lighting panel during calibration;

wherein the conveyor and the pallet are configured to bring the lighting panel into enclosure, and wherein the calorimeter is positioned within the enclosure so as to detect light emitted by the lighting panel.

36. A method of calibrating a lighting panel comprising a plurality of strings of solid state light emitting devices, each of said strings configured to emit light in response to a respective pulse width modulation control signal having a duty cycle and an on-state current level, the method comprising:

selectively energizing one of the plurality of strings;

measuring a dominant wavelength of the light emitted by the energized string;

comparing the dominant wavelength of the light emitted by the energized string to a desired dominant wavelength; and

adjusting the on-state current level of the pulse width modulation control signal for the energized string to reduce a difference of the dominant wavelength emitted by the energized string to the desired dominant wavelength.

37. The method of claim 36, wherein adjusting the on-state current level of the pulse width modulation control signal comprises increasing the on-state current level of the pulse width modulation control signal if the dominant wavelength of the light emitted by the energized string is greater than the desired dominant wavelength.

38. The method of claim 36, wherein adjusting the on-state current level of the pulse width modulation control signal comprises reducing the on-state current level of the pulse width modulation control signal if the dominant wavelength of the light emitted by the energized string is less than the desired dominant wavelength.

39. The method of claim 36, wherein the lighting panel comprises a plurality of strings configured to emit light of a first color, the method further comprising:

measuring the dominant wavelength of each of the strings configured to emit light of the first color; and

determining an average of the dominant wavelengths of each of the strings configured to emit light of the first color;

wherein comparing the dominant wavelength of the light emitted by the energized string to a desired dominant wavelength comprises comparing the dominant wavelength of the light emitted by the energized string to the average dominant wavelength.

40. The method of claim 39, further comprising:

determining a variance of the dominant wavelengths of each of the strings configured to emit light of the first color; and

adjusting the on-state current level of the pulse width modulation control signal for at least one string to reduce the variance of the dominant wavelengths emitted by the strings.