Illumination Device and Method for Controlling an Illumination Device over Changes in Drive Current and Temperature

• US 20150382424A1
• Filed: 06/25/2014
• Published: 12/31/2015
• Est. Priority Date: 06/25/2014
• Status: Active Grant

• Current Assignee: Lutron Technology Company LLC
• Original Assignee: Ketra, Inc. (Lutron Electronics)
• Inventors: Knapp, David J., Savage, Joseph A.
• Granted Patent: US 9,392,663 B2
• Time in Patent Office: Days
• US Class Current: 1/1
• CPC Class Codes: G01R 31/2635 Testing light-emitting diodes, laser diodes or photodiodes
• CPC Class Codes: G01R 31/44 Testing lamps
• CPC Class Codes: H05B 45/20 Controlling the colour of the light
• CPC Class Codes: H05B 45/24 using electrical feedback from LEDs or from LED modules
• CPC Class Codes: H05B 45/375 using buck topology
• CPC Class Codes: H05B 45/46 having LEDs disposed in parallel lines
• CPC Class Codes: H05B 47/19 via wireless transmission
• CPC Class Codes: Y10S 362/80 Light emitting diode

Abstract

An illumination device and method is provided herein for controlling individual light emitting diodes (LEDs) in an LED illumination device, so that a desired luminous flux and a desired chromaticity of the device can be maintained over changes in drive current and temperature. According to one embodiment, the illumination device comprises a plurality of emission LEDs, a storage medium, an LED driver and receiver circuit and a control circuit. The storage medium may store a table of calibration values correlating forward voltage and drive current to chromaticity and luminous flux at a plurality of temperatures for each of the plurality of emission LEDs. The LED driver and receiver circuit may apply respective drive currents to the emission LEDs to produce substantially continuous illumination, and may periodically turn the emission LEDs off to measure operating forward voltages that develop across each emission LED. The control circuit may determine whether a target luminance setting or a target chromaticity setting for the illumination device has changed, and if so, may determine new respective drive currents needed to achieve the target luminance setting and the target chromaticity setting using the operating forward voltages measured across each emission LED, the table of calibration values and one or more interpolation techniques.

Claims

1. A method for controlling an illumination device comprising a plurality of emission light emitting diodes (LEDs), wherein the method comprises:

applying respective drive currents to the plurality of emission LEDs to drive the plurality of emission LEDs substantially continuously to produce illumination;

periodically turning the plurality of emission LEDs off for short durations of time;

measuring a forward voltage presently developed across each emission LED by applying a non-operative drive current to each emission LED, one LED at a time, during the short durations of time the plurality of emission LEDs are periodically turned off; and

determining chromaticity values that are expected for each emission LED using the forward voltage measured across each emission LED, the respective drive current supplied to each emission LED, a table of stored calibration values correlating forward voltage and drive current to chromaticity at a plurality of different temperatures, and one or more interpolation techniques.

2. The method as recited in claim 1, wherein the non-operative drive current ranges between approximately 0.1 mA and approximately 10 mA.

3. The method as recited in claim 1, further comprising monitoring a target luminance setting and a target chromaticity setting of the illumination device, and performing the steps of measuring and determining only when a change is detected in at least one of the target luminance setting and the target chromaticity setting.

4. The method as recited in claim 1, wherein the table of stored calibration values comprises chromaticity calibration values corresponding to a CIE 1931 XYZ color space, a CIE 1931 RGB color space, or a CIE 1976 LUV color space.

5. The method as recited in claim 1, wherein for each emission LED, the table of stored calibration values comprises:

a first plurality of x chromaticity values and a first plurality of y chromaticity values, which were previously detected from the emission LED upon applying a plurality of different drive currents to the emission LED during a calibration phase when the emission LED was subjected to a first temperature;

a second plurality of x chromaticity values and a second plurality of y chromaticity values, which were previously detected from the emission LED upon applying the plurality of different drive currents to the emission LED during the calibration phase when the emission LED was subjected to a second temperature;

a first plurality of forward voltage values, which were previously measured across the emission LED upon applying a non-operative drive current to the emission LED, before, during or after each of the different drive currents was applied to the emission LED, when the emission LED was subjected to the first temperature; and

a second plurality of forward voltage values, which were previously measured across the emission LED upon applying a non-operative drive current to the emission LED, before, during or after each of the different drive currents was applied to the emission LED, when the emission LED was subjected to the second temperature.

6. The method as recited in claim 5, wherein for each emission LED, the step of determining chromaticity values comprises:

calculating a third plurality of x chromaticity values corresponding to the forward voltage measured across the emission LED by interpolating between the first plurality of x chromaticity values and the second plurality of x chromaticity values;

generating a relationship between the third plurality of x chromaticity values, if the respective drive current supplied to the emission LED differs from one of the plurality of different drive currents applied to the emission LED during the calibration phase; and

determining, from the generated relationship, an expected x chromaticity value for the emission LED corresponding to the respective drive current applied to the emission LED.

7. The method as recited in claim 6, wherein for each emission LED, the step of determining chromaticity values further comprises:

calculating a third plurality of y chromaticity values corresponding to the forward voltage measured across the emission LED by interpolating between the first plurality of y chromaticity values and the second plurality of y chromaticity values;

generating a relationship between the third plurality of y chromaticity values, if the respective drive current supplied to the emission LED differs from one of the plurality of different drive currents applied to the emission LED during the calibration phase; and

determining, from the generated relationship, an expected y chromaticity value for the emission LED corresponding to the respective drive current applied to the emission LED.

8. The method as recited in claim 7, wherein the steps of calculating a third plurality of x chromaticity values and a third plurality of y chromaticity values each comprise using a linear interpolation technique to interpolate between the first and second plurality of x and y chromaticity values.

9. The method as recited in claim 7, wherein the step of generating a relationship comprises applying a higher-order interpolation to the third plurality of x and y chromaticity values to generate a non-linear relationship between x and y chromaticity and drive current at the forward voltage measured across the emission LED.

10. The method as recited in claim 7, wherein the step of generating a relationship comprises applying a piece-wise linear interpolation to the third plurality of x and y chromaticity values to approximate a non-linear relationship between x and y chromaticity and drive current at the forward voltage measured across the emission LED.

11. The method as recited in claim 7, wherein the step of generating a relationship comprises assuming a typical curvature from data sheets provided by an LED manufacturer.

12. The method as recited in claim 7, further comprising calculating a relative luminous flux needed from each emission LED to achieve a target luminance setting and a target chromaticity setting of the illumination device, wherein the relative luminous flux for each emission LED is calculated using the target luminance setting, the target chromaticity setting, the expected x chromaticity value and the expected y chromaticity value determined from the generated relationships.

13. The method as recited in claim 12, further comprising:

determining a drive current needed to achieve the relative luminous flux calculated for each emission LED using the forward voltage measured across each emission LED, the table of stored calibration values, which further correlates forward voltage and drive current to luminous flux at the plurality of different temperatures, and one or more interpolation techniques; and

driving each emission LED with the determined drive current to produce illumination having the calculated relative luminous flux.

14. The method as recited in claim 13, wherein for each emission LED, the table of stored calibration values further comprises:

a first plurality of luminous flux values, which were previously detected from the emission LED upon applying the plurality of different drive currents to the emission LED during the calibration phase when the emission LED was subjected to the first temperature; and

a second plurality of luminous flux values, which were previously detected from the emission LED upon applying the plurality of different drive currents to the emission LED during the calibration phase when the emission LED was subjected to the second temperature.

15. The method as recited in claim 14, wherein for each emission LED, the step of determining a drive current comprises:

calculating a third plurality of luminous flux values corresponding to the forward voltage measured across the emission LED by interpolating between the first plurality of luminous flux values and the second plurality of luminous flux values;

generating a relationship between the third plurality of luminous flux values, if the relative luminous flux calculated for the emission LED differs from one of the third plurality of luminous flux values; and

determining the drive current needed to achieve the calculated relative luminous flux by selecting, from the generated relationship, a drive current corresponding to the calculated relative luminous flux.

16. The method as recited in claim 15, wherein the step of calculating a third plurality of luminous flux values comprises using a linear interpolation technique or a non-linear interpolation technique to interpolate between the first and second plurality of luminous flux values, and wherein selection between the linear interpolation technique and the non-linear interpolation technique is independently made for each emission LED based on a color of the emission LED.

17. The method as recited in claim 15, wherein the step of generating a relationship comprises applying a higher-order interpolation to the third plurality of luminous flux values to generate a non-linear relationship between luminous flux and drive current.

18. The method as recited in claim 15, wherein the step of generating a relationship comprises applying a piece-wise linear interpolation to the third plurality of luminous flux values to approximate a non-linear relationship between luminous flux and drive current.

19. The method as recited in claim 15, wherein the step of generating a relationship comprises assuming a typical curvature from data sheets provided by an LED manufacturer.

20. An illumination device, comprising:

a plurality of emission light emitting diodes (LEDs);

a storage medium configured for storing a table of calibration values correlating forward voltage and drive current to chromaticity and luminous flux at a plurality of temperatures for each of the plurality of emission LEDs;

an LED driver and receiver circuit configured for applying respective drive currents to the plurality of emission LEDs to drive the plurality of emission LEDs substantially continuously to produce illumination, periodically turning the plurality of emission LEDs off for short durations of time, and applying a non-operative drive current to each emission LED, one LED at a time, during the short durations of time to measure an operating forward voltage developed across each emission LED; and

a control circuit configured for determining whether a target luminance setting or a target chromaticity setting for the illumination device has changed, and if so, for determining new respective drive currents needed to achieve the target luminance setting and the target chromaticity setting using the operating forward voltages measured across each emission LED, the table of calibration values and one or more interpolation techniques.

21. The illumination device as recited in claim 20, wherein each emission LED is configured for producing illumination at a different peak wavelength.

22. The illumination device as recited in claim 20, wherein the non-operative drive current ranges between approximately 0.1 mA and approximately 10 mA.

23. The illumination device as recited in claim 20, wherein the table of calibration values stored within the storage medium comprises chromaticity calibration values corresponding to a CIE 1931 XYZ color space, a CIE 1931 RGB color space, or a CIE 1976 LUV color space.

24. The illumination device as recited in claim 20, wherein for each emission LED, the table of calibration values comprises:

a first plurality of x chromaticity values and a first plurality of y chromaticity values, which were previously detected from the emission LED upon applying a plurality of different drive currents to the emission LED during a calibration phase when the emission LED was subjected to a first temperature;

a second plurality of x chromaticity values and a second plurality of y chromaticity values, which were previously detected from the emission LED upon applying the plurality of different drive currents to the emission LED during the calibration phase when the emission LED was subjected to a second temperature;

a first plurality of forward voltage values, which were previously measured across the emission LED upon applying a non-operative drive current to the emission LED, after each of the different drive currents was applied to the emission LED, when the emission LED was subjected to the first temperature; and

a second plurality of forward voltage values, which were previously measured across the emission LED upon applying a non-operative drive current to the emission LED, after each of the different drive currents was applied to the emission LED, when the emission LED was subjected to the second temperature;

a first plurality of luminous flux values, which were previously detected from the emission LED upon applying the plurality of different drive currents to the emission LED during the calibration phase when the emission LED was subjected to the first temperature; and

a second plurality of luminous flux values, which were previously detected from the emission LED upon applying the plurality of different drive currents to the emission LED during the calibration phase when the emission LED was subjected to the second temperature.

25. The illumination device as recited in claim 24, wherein for each emission LED, the control circuit is further configured for:

calculating a third plurality of x chromaticity values corresponding to the operating forward voltage measured across the emission LED by interpolating between the first plurality of x chromaticity values and the second plurality of x chromaticity values;

generating a relationship between the third plurality of x chromaticity values, if the respective drive current applied to the emission LED differs from one of the plurality of different drive currents applied to the emission LED during the calibration phase; and

determining, from the generated relationship, an expected x chromaticity value for the emission LED corresponding to the respective drive current applied to the emission LED.

26. The illumination device as recited in claim 25, wherein for each emission LED, the control circuit is further configured for:

calculating a third plurality of y chromaticity values corresponding to the operating forward voltage measured across the emission LED by interpolating between the first plurality of y chromaticity values and the second plurality of y chromaticity values;

generating a relationship between the third plurality of y chromaticity values, if the respective drive current applied to the emission LED differs from one of the plurality of different drive currents applied to the emission LED during the calibration phase; and

determining, from the generated relationship, an expected y chromaticity value for the emission LED corresponding to the respective drive current applied to the emission LED.

27. The illumination device as recited in claim 26, wherein the control circuit is configured for calculating the third plurality of x and y chromaticity values for each emission LED by using a linear interpolation technique to interpolate between the first and second plurality of x and y chromaticity values corresponding to the emission LED.

28. The illumination device as recited in claim 26, wherein the control circuit is configured for applying a higher-order interpolation to the third plurality of x and y chromaticity values to generate non-linear relationships between x and y chromaticity and drive current for each emission LED.

29. The illumination device as recited in claim 26, wherein the control circuit is configured for applying a piece-wise linear interpolation to the third plurality of x and y chromaticity values to approximate non-linear relationships between x and y chromaticity and drive current for each emission LED.

30. The illumination device as recited in claim 26, wherein the control circuit is further configured for calculating a relative luminous flux needed from each emission LED to achieve the target luminance setting and the target chromaticity setting, and wherein the control circuit calculates the relative luminous flux for each emission LED using the target luminance setting and the target chromaticity setting, and the expected x chromaticity value and the expected y chromaticity value determined from the generated relationships.

31. The illumination device as recited in claim 30, wherein for each emission LED, the control circuit is further configured for determining a drive current needed to achieve the relative luminous flux calculated for the emission LED using the operating forward voltage measured across the emission LED, the table of stored calibration values, and one or more interpolation techniques.

32. The illumination device as recited in claim 31, wherein the driver circuit is further configured for driving each emission LED with the determined drive current to produce illumination having the calculated relative luminous flux.

33. The illumination device as recited in claim 31, wherein for each emission LED, the control circuit is further configured for:

calculating a third plurality of luminous flux values corresponding to the operating forward voltage measured across the emission LED by interpolating between the first plurality of luminous flux values and the second plurality of luminous flux values;

generating a relationship between the third plurality of luminous flux values, if the relative luminous flux calculated for the emission LED differs from one of the third plurality of luminous flux values; and

determining the drive current needed to achieve the calculated relative luminous flux by selecting, from the generated relationship, a drive current corresponding to the calculated relative luminous flux.

34. The illumination device as recited in claim 33, wherein the control circuit is configured for calculating the third plurality of luminous flux values for each emission LED by using a linear interpolation technique or a non-linear interpolation technique to interpolate between the first and second plurality of luminous flux values corresponding to the emission LED, and wherein selection between the linear interpolation technique and the non-linear interpolation technique is independently made for each emission LED based on a color of the emission LED.

35. The illumination device as recited in claim 33, wherein the control circuit is configured to generate the relationship by applying a higher-order interpolation to the third plurality of luminous flux values to generate a non-linear relationship between luminous flux and drive current for each emission LED.

36. The illumination device as recited in claim 33, wherein the control circuit is configured to generate the relationship by applying a piece-wise linear interpolation to the third plurality of luminous flux values to approximate a non-linear relationship between luminous flux and drive current for each emission LED.

37. The illumination device as recited in claim 33, wherein the control circuit is configured for generating the relationship by assuming a typical curvature from data sheets provided by a manufacturer of the emission LED.

38. The illumination device as recited in claim 20, further comprising a phase locked loop (PLL) coupled to an AC mains and configured for producing a timing signal in synchronization with a frequency of the AC mains, wherein the timing signal is supplied to the LED driver and receiver circuit for periodically turning the plurality of emission LEDs off for the short durations of time.

39. The illumination device as recited in claim 20, further comprising a temperature sensor configured for detecting an ambient temperature surrounding the plurality of emission LEDs, and wherein if the target luminance setting and the target chromaticity setting for the illumination device has not changed, the control circuit is further configured for determining the respective drive currents needed to achieve the target luminance setting and the target chromaticity setting only when the ambient temperature changes by a specified amount.