Method and apparatus for sensing the color point of an RGB LED white luminary using photodiodes

• US 6,630,801 B2
• Filed: 10/22/2001
• Issued: 10/07/2003
• Est. Priority Date: 10/22/2001
• Status: Expired due to Term

• Current Assignee: Koninklijke Philips N.V.
• Original Assignee: Lumileds LLC (Apollo Global Management, Inc.)
• Inventors: Schuurmans, Frank Jeroen Pieter
• Primary Examiner(s): Wong, Don
• Assistant Examiner(s): TRAN, THUY V
• Application Number:
  US10/083,329
• Publication Number:
  US 20030076056A1
• Time in Patent Office: 715 Days
• Field of Search: 315/291, 315/299, 315/300, 315/301, 315/302, 315/307, 315/308, 315/363, 362/230, 362/231, 362/800, 340/815.45, 345/39, 345/82, 345/83, 347/232, 347/237, 347/238
• US Class Current: 315/307
• CPC Class Codes: G09G 3/3406 Control of illumination source illumination devices structurally associated with liquid crystal cells G02F1/1336
• CPC Class Codes: H05B 45/22 using optical feedback
• CPC Class Codes: Y10S 362/80 Light emitting diode

Abstract

Method and apparatus for controlling an RBG based LED luminary which measures the output signals of filtered photodiodes and unfiltered photodiodes and correlates these values to chromaticity coordinates for each of the red, green and blue LEDs of the luminary. Forward currents driving the LED luminary are adjusted in accordance with differences between the chromaticity coordinates of each of the red, green and blue LEDs and chromaticity coordinates of a desired mixed color light.

Claims

1. A method of controlling an LED luminary including red, green and blue (RGB) light emitting diodes (LEDs) driven by forward currents to produce a mixed color light comprising the steps of:

measuring an output signal of a filtered photodiode for each of the red, green and blue LEDs of the LED luminary;

measuring an output signal of an unfiltered photodiode for each of the red, green and blue LEDs of the LED luminary;

calculating a photodiode output signal ratio by dividing the output signal for the filtered photodiode with the output signal of the unfiltered photodiode for each of the red, green and blue LEDs;

utilizing the photodiode output signal ratio to determine chromaticity coordinates for each of the red, green and blue LEDs; and

adjusting the forward currents for each of the red, green and blue LEDs to produce a desired color light.

2. The method of controlling an LED luminary according to claim 1, wherein the utilizing step comprises a step of:

accessing a look-up table (LUT) which includes a correlation between the photodiode output signal ratio and chromaticity coordinates for each of the red, green and blue LEDs.

3. The method of controlling an LED luminary according to claim 2 further comprising a step of generating a LUT for each of the red, green and blue LEDs by:

calculating a photodiode output signal ratio for each of a plurality of red, green and blue LEDs;

measuring chromaticity coordinates for each of the plurality of red, green and blue LEDs; and

determining a relationship between the calculated photodiode output signal ratio and the chromaticity coordinates for each of the red, green and blue LEDs.

4. The method of controlling an LED luminary according to claim 1 wherein the chromaticity coordinates correspond to a CIE 1931 chromaticity coordinate system.

5. The method of controlling an LED luminary according to claim 1 wherein the chromaticity coordinates correspond to a new RGB colorimetric system.

6. The method of controlling an LED luminary according to claim 1 further comprising the step of:

selecting said desired color light.

7. The method of controlling an LED luminary according to claim 6 wherein said step of selecting said desired color light includes selecting a desired color point of the LED luminary.

8. The method of controlling an LED luminary according to claim 7 further comprising the steps of:

pre-storing a plurality of desired color points of the LED luminary in a memory; and

selecting one of the plurality of desired color points as the desired color light.

9. The method of controlling an LED luminary according to claim 1 wherein said adjusting step comprises the steps of:

generating control voltages for each of the red, green and blue LEDs based on said chromaticity coordinates for each of the red, green and blue LEDs and chromaticity coordinates for the desired color light; and

applying said control voltages to LED drivers for each of the red, green and blue LEDs so as to adjust forward currents for each of the red, green and blue LEDs to produce the desired color light.

10. The method of controlling an LED luminary according to claim 1 wherein said steps are implemented by analog circuitry.

11. The method of controlling an LED luminary according to claim 1 wherein said steps are implemented by digital circuitry.

12. The method of controlling an LED luminary according to claim 1 wherein the steps of measuring the output signal of a filtered photodiode and measuring the output signal of an unfiltered photodiode are carried out in predetermined time intervals.

13. A control system for an LED luminary including red, green and blue (RGB) light emitting diodes (LEDs) driven by forward currents to produce a mixed color light, comprising:

a feedback unit which generates feedback values representative of the mixed color light produced by said LED luminary, said feedback values corresponding to output signals of a photodiode; and

a controller operatively coupled to said feedback unit which determines a difference between said feedback values and reference values representative of a desired mixed color light, said controller adjusting at least one of control voltages and forward currents in accordance with said difference.

14. The control system for an LED luminary according to claim 13 wherein said feedback unit comprises a filtered photodiode and an unfiltered photodiode.

15. The control system for an LED luminary according to claim 14 wherein said filtered photodiode corresponds to an edge-filtered photodiode and wherein the edge filtered photodiode comprises an optical colored glass filter which transmits long wavelengths of light and absorbs short wavelengths of light and having a small wavelength transition region centered around a cut-off wavelength.

16. The control system for an LED luminary according to claim 15 wherein cut-off wavelengths for each of the filtered photodiodes for said red, green and blue LEDs are 610 nm, 530 nm and 470 nm respectively.

17. The control system for an LED luminary according to claim 13 wherein said feedback unit further comprises an amplifier and signal conversion circuitry for converting output photocurrents of said photodiode to voltage signals.

18. The control system for an LED luminary according to claim 13 wherein said feedback unit further comprises means for providing said feedback values to said controller.

19. The control system for an LED luminary according to claim 13 further comprising a user interface operatively coupled to said controller for a user to select said desired mixed color light.

20. The control system for an LED luminary according to claim 19 further comprising a memory operatively coupled to said controller for storing and providing to said controller reference values representative of said desired mixed color light.

21. The control system for an LED luminary according to claim 13 wherein said reference values correspond to chromaticity coordinates of the CIE 1931 chromaticity coordinate system.

22. The control system for an LED luminary according to claim 13 wherein said reference values correspond to chromaticity coordinates in a new RGB colorimetric system.

23. The control system for an LED luminary according to claim 13 further comprising a voltage generator operatively coupled to said controller which generates a control voltage in accordance to said difference between said feedback values and said reference values, said controller applying said control voltage to LED drivers for each of the red, green and blue LEDs of said LED luminary so as to adjust forward currents for each of the red, green, blue LEDs to produce the desired color light.

24. The control system for an LED luminary according to claim 13 wherein said controller comprises analog circuitry.

25. The control system for an LED luminary according to claim 13 wherein said controller comprises digital circuitry.

26. The control system for an LED luminary according to claim 13 further comprising a memory operatively coupled to the controller for pre-storing a plurality of desired mixed color lights that a user may select.

27. The control system for an LED luminary according to claim 13 wherein said feedback unit comprises further comprising first, second and third filtered photodiodes respectively corresponding to said red, green and blue LEDs, and an unfiltered photodiode.

28. The control system for an LED luminary according to claim 27 wherein said feedback unit measures an output signal of said first, second and third filtered photodiodes and an output of said unfiltered photodiode for each of said red, green and blue LEDs;

and

said controller calculates a photodiode output signal ratio by dividing filtered photodiode output signals for said red, green and blue LEDs with unfiltered photodiode output signals for said red, green and blue LEDs respectively, utilizes the photodiode output signal ratio to determine chromaticity coordinates for each of the red, green and blue LEDs, and adjusts forward currents for each of the red, green and blue LEDs to produce the desired mixed color light.

29. The control system for an LED luminary according to claim 28 wherein said controller determines the chromaticity coordinates for each of the red, green and blue LEDs by accessing a look-up table which includes a relationship between the photodiode output signal ratio and the chromaticity coordinates for each of the red, green and blue LEDs.