Adaptive adjustment of light output of solid state lighting panels

• US 20070115662A1
• Filed: 03/06/2006
• Published: 05/24/2007
• 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, You, Chenhua, Vadas, Keith
• Granted Patent: US 7,926,300 B2
• Time in Patent Office: Days
• US Class Current: 362/249.160
• 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 2320/0626 for control of overall brightness
• CPC Class Codes: G09G 2320/0633 by amplitude modulation of the brightness of the illumination source
• CPC Class Codes: G09G 2320/064 by time modulation of the brightness 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

Abstract

A lighting panel system includes a lighting panel including a first string of solid state lighting devices configured to emit light at a first wavelength and a second string of solid state lighting devices configured to emit light at a second wavelength, different from the first wavelength, and a current supply circuit configured to supply a drive current to the first string in response to a control signal. A photosensor is arranged to receive light emitted by the panel, and a control system is configured to sample an output signal of the photosensor and adjust the control signal responsive thereto to thereby adjust an average current supplied to the first string by the current supply circuit. Methods of operating a lighting panel are also provided.

Claims

1. A lighting panel system, comprising:

a lighting panel including at least a first string of solid state lighting devices configured to emit light at a first dominant wavelength and a second string of solid state lighting devices configured to emit light at a second dominant wavelength, different from the first dominant wavelength;

a current supply circuit configured to supply an on-state drive current to the first string upon receipt of a control signal;

a photosensor arranged to receive light from at least one solid state lighting device in the first string; and

a control system configured to receive an output signal from the photosensor and to adjust the control signal responsive to the output signal of the photosensor to thereby adjust an average current supplied to the first string by the current supply circuit to thereby change a color point of the combined light emitted by the first string and the second string, wherein the photosensor, the control system and the current supply circuit form a feedback loop for the lighting panel.

2. The lighting panel system of claim 1, wherein the current supply circuit comprises a closed loop variable voltage boost converter current source.

3. The lighting panel system of claim 1, wherein the control system is configured to sample the output of the photosensor when current is not being supplied to the first string of solid state lighting devices or the second string of solid state lighting devices to obtain an ambient light value.

4. The lighting panel system of claim 3, wherein the control system is configured to increase average current to the first string as the ambient light value increases.

5. The lighting panel system of claim 1, wherein the control system is configured to sample the photosensor during an interval in which current is being supplied to the first string and/or the second string in order to obtain a display brightness value.

6. The lighting panel system of claim 5, wherein the control system is configured to decrease the average current to the first string as the display brightness value increases.

7. The lighting panel system of claim 5, wherein the control system is further configured to sample the output of the photosensor when current is not being supplied to the first string of solid state lighting devices or the second string of solid state lighting devices to obtain an ambient light value.

8. The lighting panel system of claim 7, wherein the control system is configured to adjust the average current supplied to the first LED string based on the ambient light value and the display brightness value.

9. The lighting panel system of claim 8, wherein the control system is configured to adjust the average current supplied to the first LED string based on a difference between the ambient light value and the display brightness value.

10. The lighting panel system of claim 8, wherein the control system is configured to adjust the average current supplied to the first LED string based on a ratio of the ambient light value and the display brightness value.

11. The lighting panel system of claim 8, wherein the control system is configured to maintain an average luminosity of the first string independent of an ambient/background illumination.

12. The lighting panel system of claim 8, wherein the control system is configured to maintain a relationship between an ambient/background illumination and an average luminosity of the first string by providing a positive feedback signal with respect to the ambient light value and a negative feedback signal with respect to the display brightness value.

13. The lighting panel system of claim 1, wherein the control system is configured to employ digital incremental logic in the feedback loop.

14. The lighting panel system of claim 13, wherein the digital incremental logic references indices in a lookup table.

15. The lighting panel system of claim 1, wherein the control system is configured to employ proportional control in the feedback loop.

16. The lighting panel system of claim 1, wherein the control signal comprises a pulse width modulation (PWM) signal, and wherein the control system is configured to control an average current supplied to the first string by varying a duty cycle of the PWM signal.

17. The lighting panel system of claim 16, wherein the duty cycle of the PWM signal corresponds to a value of a register in the control system.

18. The lighting panel system of claim 1, wherein the control system is configured to control an average current supplied to the first string by varying a pulse frequency of the control signal.

19. The lighting panel system of claim 1, wherein the current supply circuit is configured to maintain the on-state current supplied to the first string at a substantially constant value even as the average current supplied to the first string is varied.

20. The lighting panel system of claim 1, wherein the dominant wavelength of the first string remains more constant at various average current levels than if the average current was manipulated using a variable current.

21. The lighting panel system of claim 1, wherein the luminous flux per unit power dissipated by the first string remains more constant at various average current levels than if the average current was manipulated using a variable current.

22. The lighting panel system of claim 1, wherein the control system comprises:

a color management unit coupled to the photosensor and configured to sample and process the output signal of the photosensor and to provide the processed output signal to the control system.

23. The lighting panel system of claim 1, further comprising:

a temperature sensor configured to sense a temperature associated with the lighting panel, wherein the control system is configured to adjust an average current supplied to the first string in response to a change in the sensed temperature.

24. The lighting panel system of claim 1, further comprising:

a current supply circuit configured to supply an on-state drive current to the second string upon receipt of a second control signal;

wherein the control system is further configured to adjust the second control signal responsive to the output signal of the photosensor.

25. A lighting panel system, comprising:

a lighting panel including at least a first string of solid state lighting devices configured to emit light at a first dominant wavelength and a second string of solid state lighting devices configured to emit light at a second dominant wavelength, different from the first dominant wavelength;

a first current supply circuit configured to supply an on-state drive current to the first string in response to a first control signal;

a second current supply circuit configured to supply an on-state drive current to the second string in response to a second control signal;

a photosensor arranged to receive light from at least one solid state lighting device in the first string and at least one solid state lighting device in the second string; and

a control system configured to receive an output signal from the photosensor and to adjust the first control signal and/or the second control signal responsive to the output signal of the photosensor to thereby adjust an average current supplied to the first string by the first current supply circuit and/or to adjust an average current supplied to the second string by the second current supply circuit, such that the photosensor, the control system and the first and second current supply circuits thereby form a feedback loop for the lighting panel;

wherein the first and second control signals comprise pulse width modulation (PWM) signals, and wherein the control system is configured to control an average current supplied to the first and/or second string by varying a duty cycle of the first and/or second control signal.

26. The lighting panel system of claim 25, wherein a leading edge of a pulse of the first control signal occurs at a different time from a leading edge of a pulse of the second control signal.

27. The lighting panel system of claim 26, wherein the leading edge of the pulse of the first control signal is delayed from the leading edge of the pulse of the second control signal by a fixed delay.

28. The lighting panel system of claim 26, wherein the leading edge of the pulse of the first control signal is delayed from the leading edge of the pulse of the second control signal by a variable delay.

29. The lighting panel system of claim 28, wherein the variable delay changes within a range of delay intervals that is random, chaotic or determined by a sweep function, table or other technique, and/or is dependent on the pulse width of the first control signal and/or the second control signal.

30. The lighting panel system of claim 29, wherein the delay interval is dependent on the pulse width of the first control signal and/or the second control signal.

31. The lighting panel system of claim 26, wherein an external power factor of the lighting panel is more balanced than if the leading edges of the pulses of the first and second control signals were to occur at substantially the same moment.

32. The lighting panel system of claim 25, wherein a combined EMI/RFI emission of the first and second strings has an amplitude at one or more frequencies that is less than if the leading edges of the pulses of the first and second control signals were to occur at substantially the same moment.

33. A lighting panel system, comprising:

a lighting panel comprising first and second pulsed LED light sources configured to emit narrow band optical radiation having a first dominant wavelength when energized and third and fourth pulsed LED light sources configured to emit narrow band optical radiation having a second dominant-wavelength when energized, the second dominant wavelength being different from the first dominant wavelength;

a photosensor configured to be responsive to the first and second dominant wavelengths and configured to provide substantially independent outputs related to the sensed illumination levels in the first and second dominant wavelengths

first, second, third and fourth current sources configured to supply current to the first, second, third and fourth pulsed LED light sources, respectively, in response to control signals; and

a control system coupled to the lighting panel and to the photosensor and configured to provide a feedback loop from the photosensor to the lighting panel by sampling the photosensor output, and, responsive to the photosensor output samples, providing the control signals to the first and second current sources to adjust an average current supplied to at least the first and second pulsed LED light sources.

34. The lighting panel system of claim 33, wherein the control system is further configured to maintain the first and second pulsed LED light sources at different average current levels from one another and to maintain the third and fourth pulsed LED light sources at different average current levels than one another;

wherein the current sources are configured to provide a first on-state current level to the first and second pulsed LED light sources and to provide a second on-state current level, different from the first on-state current level, to the third and fourth pulsed LED light sources; and

wherein the control system is further configured to maintain a ratio of average current levels between the first and second pulsed LED light sources relatively constant while varying the average current level to the first and second pulsed LED light sources and without appreciably changing the on-state current of the first and second pulsed LED light sources.

35. The lighting panel system of claim 34, wherein the control system is further configured to maintain a ratio of average current levels between the third and fourth pulsed LED light sources relatively constant while varying the average current level to the third and fourth pulsed LED light sources and without appreciably changing the on-state current of the third and fourth pulsed LED light sources.

36. The lighting panel system of claim 34, wherein the control system is configured to alter average current levels of the first and second pulsed LED light sources in order to maintain a white point of the lighting panel.

37. An LCD backlight for an LCD display having a visible area with a diagonal size greater than 17″

, the LCD backlight comprising;

a plurality of strings of red, green and blue LEDs arranged in a two-dimensional surface that is substantially parallel to a display surface of the LCD display, wherein the two-dimensional surface has an area greater than about 30% of the visible area of the LCD display;

wherein an average power dissipated by the LEDs, is less than about 0.3 Watts per square inch over the two-dimensional surface; and

wherein an average luminance of the LCD backlight at maximum brightness adjustment is greater than 200 Nit at 22 degrees C. ambient temperature when set to a white point with a correlated color temperature of between 4000k and 8000k.

38. The LCD backlight of claim 37, wherein an average luminance of the LCD backlight at maximum brightness adjustment is greater than 250 Nit at 22 degrees C. ambient temperature when set to a white point with a correlated color temperature of between 4000k and 8000k.

39. The LCD backlight of claim 37, wherein, for a given color, an average luminous flux generated per unit of power dissipated at a given junction temperature by LEDs nearest an edge of the LCD display is greater than an average luminous flux per unit of power dissipated at a given junction temperature by all LEDs of that color within the LCD backlight.

40. The LCD backlight of claim 37, wherein, for a given color, an average power dissipated by LEDs nearest the edge of the display is greater than the average power dissipated by all LEDs of that color in the LCD backlight.

41. The LCD backlight of claim 37, wherein, for a given color, an average power dissipated by LEDs nearest a bottom of the display is greater than an average power dissipated by all LEDs of that color within the LCD backlight.

42. The LCD backlight of claim 37, wherein, for a given color, an average luminous flux generated per unit of power dissipated at a given junction temperature for LEDs nearest a bottom of the display is less than an average luminous flux generated per unit of power dissipated at a given junction temperature for all LEDs of that color within the LCD backlight.

43. An LCD backlight system, comprising:

a lighting panel comprising a plurality of tiles, each of the plurality of tiles having thereon a plurality red, green and blue LED chips arranged in RGB clusters on a substrate, wherein the LED chips in the lighting panel are electrically connected into a plurality of red, green and blue LED strings;

a plurality of constant current sources, each configured to energize a different LED string in response to a corresponding control signal;

wherein an average luminance of the lighting panel at maximum brightness adjustment is greater than 220 Nit at 22 deg C. ambient temperature when set to a white point with a correlated color temperature of between 4000k and 8000k.

44. The LCD backlight system of claim 43, wherein the constant current sources are configured, in response to second control signals, to energize different LED strings of a same color at different on-state current levels.

45. A method of operating a lighting panel including first and second strings of solid state lighting devices configured to emit light having first and second dominant wavelengths, respectively, the method comprising:

supplying a first pulsed drive current the first string, the first drive current having a first pulse width at a pulse repetition rate;

supplying a second pulsed drive current the second string, the second drive current having a second pulse width at the pulse repetition rate;

sensing a light output from the lighting panel; and

adjusting the first pulse width in response to the sensed light output.

46-64. -64. (canceled)

65. A lighting panel system, comprising:

a lighting panel including a plurality of bar assemblies;

at least a first string of solid state lighting devices configured to emit light at a first dominant wavelength and a second string of solid state lighting devices configured to emit light at a second dominant wavelength, different from the first dominant wavelength, in each of the plurality of bar assemblies;

a plurality of current supply circuits configured to supply an on-state drive current to a corresponding string in response to a respective one of a plurality of control signals;

a plurality of photosensors arranged to receive light from the first and second strings of a corresponding bar assembly;

a control system configured to receive an output signal from the photosensors and to adjust the control signals responsive to the output signals of the photosensors to thereby adjust an average current supplied to the strings by the current supply circuits.

66-67. -67. (canceled)

68. A lighting panel system, comprising:

a lighting panel including a plurality of bar assemblies;

at least a first string of solid state lighting devices configured to emit light at a first dominant wavelength and a second string of solid state lighting devices configured to emit light at a second dominant wavelength, different from the first dominant wavelength, in each of the plurality of bar assemblies;

a plurality of current supply circuits configured to supply an on-state drive current to a corresponding string upon receipt of a respective one of a plurality of control signals;

a photosensor arranged to receive light from each of the bar assemblies; and

a control system configured to receive an output signal from the photosensor and to adjust the control signals responsive to the output signal of the photosensors to thereby adjust an average current supplied to the strings by the current supply circuits.

69-71. -71. (canceled)

72. 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 modulated control signals applied thereto, the method comprising:

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

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

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

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

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

73-79. -79. (canceled)