Effective modeling of CIE xy coordinates for a plurality of LEDs for white LED light control

• US 6,411,046 B1
• Filed: 12/27/2000
• Issued: 06/25/2002
• Est. Priority Date: 12/27/2000
• Status: Expired due to Term

• Current Assignee: Philips Lighting BV (Koninklijke Philips N.V.)
• Original Assignee: Koninklijke Philips N.V.
• Inventors: Muthu, Subramanian
• Primary Examiner(s): Wong, Don
• Assistant Examiner(s): TRAN, THUY V
• Application Number:
  US09/749,170
• Time in Patent Office: 545 Days
• Field of Search: 315/309, 362/227, 362/230, 362/231, 362/800, 250/553
• US Class Current: 315/309
• CPC Class Codes: H05B 45/20 Controlling the colour of the light
• CPC Class Codes: H05B 45/22 using optical feedback
• CPC Class Codes: Y02B 20/30 Semiconductor lamps, e.g. solid state lamps [SSL] light emitting diodes [LED] or organic LED [OLED]
• CPC Class Codes: Y10S 362/80 Light emitting diode

Abstract

The present invention describes a method and luminaire for driving an array of LEDs with at least one LED in each of a plurality of colors in a luminaire. This method controls the light output and color of the LEDs by measuring color coordinates for each LED light source for different temperatures, storing the expressions of the color coordinates as a function of the temperatures, deriving equations for the color coordinates as a function of temperature, calculating the color coordinates and lumen output fractions on-line, and controlling the light output and color of said LEDs based upon the calculated color coordinates and lumen output fractions.

Claims

1. A method for driving an array of LEDs comprising at least one LED in each of a plurality of colors in a luminaire comprising the steps of:

supplying electrical current to said LEDs in each said color, such that said LEDs have a light output with a nominal continuous value during ordinary operation;

measuring CIE xy coordinates for each LED light source for different temperatures;

storing the expressions of the CIE xy coordinates as a function of the temperatures;

deriving equations for the CIE x and CIE y coordinates as a function of temperature;

calculating the CIE xy coordinates and lumen output fractions on-line; and

controlling the light output and color of said LEDs based upon the calculated CIE xy coordinates and lumen output fractions.

2. The method of claim 1, wherein the deriving step further comprises deriving the equations for the CIE x and CIE y coordinates as a function of temperature using polynomials.

3. The method of claim 1, wherein the deriving step further comprises deriving the equations for the CIE x and CIE y coordinates as a function of temperature using higher order polynomials.

4. The method of claim 1, wherein reference lumens are calculated off-line for a fixed color of the white light.

5. The method of claim 1, wherein the temperatures are measured from a heat-sink.

6. A method of linearly varying the color temperature of white light produced by an array of LEDs comprising at least one LED in each of a plurality of colors in a luminaire to a desired color temperature comprising the steps of:

measuring a first lumen output of the white light;

measuring a temperature of the white light;

expressing CIE xy chromaticity coordinates of the white light as a function of color temperature using polynomials;

calculating a required reference second lumen output for LED light sources from CIE xy chromaticity coordinates of the white light for the desired color temperature and the CIE xy coordinates of the LED light sources depending on the temperature;

supplying the first and second lumen outputs to a lumen output control system and regulating the lumen output of the LED light source.

7. The method of claim 6, wherein the CIE xy chromaticity coordinates vary with forward current for the array of LEDs.

8. The method of claim 7, wherein the forward current is supplied by amplitude modulation.

9. The method of claim 7, wherein the CIE xy chromaticity coordinates are measured for the average current over an operation.

10. The method of claim 9, wherein the CIE xy chromaticity coordinates are measured at peak current.

11. The method of claim 6, wherein the forward current is supplied by pulse-width-modulation.

12. The method of claim 6, wherein the temperature is a temperature of a heat-sink.

13. A luminaire comprising:

an array of LEDs comprising at least one LED in each of a plurality of colors;

means for supplying electrical current to said LEDs in each said color, said LEDs in each said color having a light output, such that said light output has a nominal continuous value during ordinary operation;

at least one photodiode arranged to measure the light outputs of at least one of the LEDs in the array;

a means for measuring CIE xy coordinates for each LED light source for different heat-sink temperatures;

a storage means storing expressions of CIE xy coordinates as a function of the temperatures;

a calculating means for deriving equations for the CIE x and CIE y coordinates as a function of temperature and calculating the CIE xy coordinates and lumen output fractions on-line; and

a control means for controlling the light output and color temperature of said LEDs based upon the calculated CIE xy coordinates and lumen output fractions.

14. The luminaire as recited in claim 13, wherein the control means further comprises a potentiometer.

15. The luminaire as recited in claim 13, wherein the temperature is measured from a heat-sink encasing the LED array.