An illumination device and method is provided herein for controlling an led illumination device, so that a desired luminous flux and a desired chromaticity of the device can be maintained over time as the LEDs age. According to one embodiment, the method determines an expected wavelength value and an expected intensity value for each emission led included within the illumination device at the drive current currently applied to the emission led and the present emitter forward voltage. In addition, the method determines a photodetector responsivity for each emission led at the expected wavelength value and the present photodetector forward voltage. The photodetector responsivity calculated for each emission led is used as a reference for adjusting the lumen output of the emission led to account for led aging affects.

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Patent
   RE49246
Priority
Aug 28 2014
Filed
Nov 29 2018
Issued
Oct 11 2022
Expiry
Aug 28 2034

TERM.DISCL.
Inventors
Knapp, Dav…
Assg.orig
Lutron Tec…
Assg.curr
Lutron Tec…
Entity
Large
Referenced by
0
References
364
Maint.:
currently ok
Exemplary Embodiment…
Exemplary Embodiment…
1. A method for controlling an illumination device comprising a plurality of emission light emitting diodes (LEDs) and a photodetector, 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 to produce periodic intervals;
measuring a forward voltage presently developed across each emission led, one led at a time, during a first portion of the periodic intervals; and
determining, for each emission led, an expected wavelength value and an expected intensity value corresponding to the forward voltage measured across the emission led and the drive current currently applied to the emission led by applying one or more interpolation techniques to a table of stored calibration values correlating wavelength and intensity to drive current at a plurality of different temperatures.
16. An illumination device, comprising:
a plurality of emission light emitting diodes (LEDs);
a storage medium configured for storing a table of calibration values correlating wavelength and intensity to drive current at a plurality of different 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 to produce periodic intervals, and applying a non-operative drive current to each emission led, one led at a time, during the a first portion of the periodic intervals to measure a forward voltage presently developed across each emission led; and
a control circuit configured for determining, for each emission led, an expected wavelength value and an expected intensity value corresponding to the forward voltage presently measured across the emission led and the drive current currently applied to the emission led by applying one or more interpolation techniques to the table of stored calibration values.
0. 32. A method for controlling an illumination device comprising a plurality of emission light emitting diodes (LEDs) and a photodetector, 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;
turning the plurality of emission LEDs off for durations of time to produce periodic intervals;
based on measurements taken for each emission led, one led at a time, during a first portion of the periodic intervals, determining, for each emission led, an expected wavelength value;
measuring a photocurrent induced on the photodetector in response to the illumination produced by each emission led, one emission led at a time, and received by the photodetector during a second portion of the periodic intervals;
measuring a forward voltage developed across the photodetector by applying a non-operative drive current to the photodetector during a third portion of the periodic intervals; and
calculating, for each emission led, a responsivity of the photodetector using the expected wavelength value determined for the emission led, the forward voltage measured across the photodetector, and a plurality of coefficient values that are stored within the illumination device to characterize a change in the photodetector responsivity over emitter wavelength and photodetector forward voltage.
2. The method as recited in claim 1, wherein for each emission led, the table of stored calibration values comprises:
a first plurality of stored wavelength 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 ambient temperature;
a second plurality of stored wavelength 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, which is different than the first ambient temperature;
a first plurality of stored forward voltages, which were previously measured across the emission led before or after each of the different drive currents was applied to the emission led during the calibration phase when the emission led was subjected to the first ambient temperature; and
a second plurality of stored forward voltages, which were previously measured across the emission led before or after each of the different drive currents was applied to the emission led during the calibration phase when the emission led was subjected to the second temperature.
3. The method as recited in claim 2, wherein the step of determining an expected wavelength value for each emission led comprises:
calculating a third plurality of wavelength values corresponding to the forward voltage presently measured across the emission led by interpolating between the first plurality of stored wavelength values and the second plurality of wavelength values corresponding to the emission led;
generating a relationship between the third plurality of wavelength values; and
selecting the expected wavelength value from the generated relationship that corresponds to the drive current currently applied to the emission led.
4. The method as recited in claim 3, wherein the step of calculating the third plurality of wavelength values comprises using a linear interpolation technique to interpolate between the first and second plurality of stored wavelength values corresponding to the emission led.
5. The method as recited in claim 3, wherein the step of generating the relationship comprises applying a linear interpolation or a non-linear interpolation to the third plurality of wavelength values to generate a linear relationship or a non-linear relationship between wavelength and drive current for the emission led, wherein application of the linear interpolation or the non-linear interpolation is based on a color of the emission led.
6. The method as recited in claim 3, wherein the step of generating the relationship comprises applying a piece-wise linear interpolation to the third plurality of wavelength values to approximate a non-linear relationship between wavelength and drive current for the emission led.
7. The method as recited in claim 1 2, wherein for each emission led, the table of stored calibration values further comprises:
a first plurality of stored intensity 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 ambient temperature; and
a second plurality of stored intensity 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 ambient temperature.
8. The method as recited in claim 7, wherein the step of determining an expected intensity value for each emission led comprises:
calculating a third plurality of intensity values corresponding to the forward voltage presently measured across the emission led by interpolating between the first plurality of stored intensity values and the second plurality of intensity values corresponding to the emission led;
generating a relationship between the third plurality of intensity values; and
selecting the expected intensity value from the generated relationship that corresponds to the drive current currently applied to the emission led.
9. The method as recited in claim 8, wherein the step of calculating the third plurality of intensity values comprises using a linear interpolation technique to interpolate between the first and second plurality of stored intensity values corresponding to the emission led.
10. The method as recited in claim 8, wherein the step of generating the relationship comprises applying a linear interpolation to the third plurality of intensity values to generate a linear relationship between intensity and drive current for the emission led.
11. The method as recited in claim 8, wherein the step of generating the relationship comprises applying a piece-wise linear interpolation to the third plurality of intensity values to approximate a non-linear relationship between intensity and drive current for the emission led.
12. The method as recited in claim 8, wherein the first, second and third plurality of intensity values comprise radiance values, and wherein the expected intensity value is an expected radiance value.
13. The method as recited in claim 8, wherein the first, second and third plurality of intensity values comprise luminance values, and wherein the expected intensity value is an expected luminance value.
14. The method as recited in claim 1, further comprising:
measuring a photocurrent induced on the photodetector in response to the illumination produced by each emission led, one emission led at a time, and received by the photodetector during a second portion of the periodic intervals;
measuring a forward voltage presently developed across the photodetector by applying a non-operative drive current to the photodetector during a third portion of the periodic intervals; and
calculating, for each emission led, a responsivity of the photodetector using the expected wavelength value determined for the emission led, the forward voltage presently measured across the photodetector, and a plurality of coefficient values that were generated during a calibration phase and stored within the illumination device to characterize a change in the photodetector responsivity over emitter wavelength and photodetector forward voltage.
15. The method as recited in claim 14, wherein for each emission led, the method further comprises:
calculating an intensity value for the emission led by dividing the induced photocurrent measured during the measuring step by the photodetector responsivity calculated during the calculating step;
calculating a scale factor by dividing the expected intensity value determined for the emission led by the intensity value calculated for the emission led;
applying the scale factor to a desired luminous flux value for the emission led to obtain an adjusted luminous flux value for the emission led; and
adjusting the drive current currently applied to the emission led to achieve the adjusted luminous flux value.
17. The illumination device as recited in claim 16, wherein for each emission led, the table of stored calibration values comprises:
a first plurality of stored wavelength 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 ambient temperature;
a second plurality of stored wavelength 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, which is different than the first ambient temperature;
a first plurality of stored forward voltages, which were previously measured across the emission led before or after each of the different drive currents was applied to the emission led during the calibration phase when the emission led was subjected to the first ambient temperature; and
a second plurality of stored forward voltages, which were previously measured across the emission led before or after each of the different drive currents was applied to the emission led during the calibration phase when the emission led was subjected the second temperature.
18. The illumination device as recited in claim 17, wherein for each emission led, the control circuit is configured for determining the expected wavelength value by:
calculating a third plurality of wavelength values corresponding to the forward voltage presently measured across the emission led by interpolating between the first plurality of stored wavelength values and the second plurality of stored wavelength values corresponding to the emission led;
generating a relationship between the third plurality of wavelength values; and
selecting the expected wavelength value from the generated relationship that corresponds to the drive current currently applied to the emission led.
19. The illumination device as recited in claim 18, wherein the control circuit is configured for calculating the third plurality of wavelength values by using a linear interpolation technique to interpolate between the first and second plurality of stored wavelength values corresponding to the emission led.
20. The illumination device as recited in claim 18, wherein the control circuit is configured for generating the relationship by applying a linear interpolation or a non-linear interpolation to the third plurality of wavelength values to respectively generate a linear relationship or a non-linear relationship between wavelength and drive current for the emission led, wherein application of the linear interpolation or the non-linear interpolation is based on a color of the emission led.
21. The illumination device as recited in claim 18, wherein the control circuit is configured for generating the relationship by applying a piece-wise linear interpolation to the third plurality of wavelength values to approximate a non-linear relationship between wavelength and drive current for the emission led.
22. The illumination device as recited in claim 1 17, wherein for each emission led, the table of stored calibration values further comprises:
a first plurality of stored intensity 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 ambient temperature; and
a second plurality of stored intensity 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 ambient temperature.
23. The illumination device as recited in claim 22, wherein for each emission led, the control circuit is configured for determining the expected intensity value by:
calculating a third plurality of intensity values corresponding to the forward voltage presently measured across the emission led by interpolating between the first plurality of stored intensity values and the second plurality of stored intensity values corresponding to the emission led;
generating a relationship between the third plurality of intensity values; and
selecting the expected intensity value from the generated relationship that corresponds to the drive current currently applied to the emission led.
24. The illumination device as recited in claim 23, wherein the control circuit is configured for calculating the third plurality of intensity values comprises by using a linear interpolation technique to interpolate between the first and second plurality of stored intensity values corresponding to the emission led.
25. The illumination device as recited in claim 23, wherein the control circuit is configured for generating the relationship by applying a linear interpolation to the third plurality of intensity values to generate a linear relationship between intensity and drive current for the emission led.
26. The illumination device as recited in claim 23, wherein the control circuit is configured for generating the relationship by applying a piece-wise linear interpolation to the third plurality of intensity values to approximate a non-linear relationship between intensity and drive current for the emission led.
27. The illumination device as recited in claim 23, wherein the first, second and third plurality of intensity values comprise radiance values, and wherein the expected intensity value is an expected radiance value.
28. The illumination device as recited in claim 23, wherein the first, second and third plurality of intensity values comprise luminance values, and wherein the expected intensity value is an expected luminance value.
29. The illumination device as recited in claim 16, wherein the led driver and receiver circuit is further configured for:
measuring a photocurrent induced on the a photodetector in response to the illumination produced by each emission led, one emission led at a time, and received by the photodetector during a second portion of the periodic intervals; and
measuring a forward voltage presently developed across the photodetector by applying a non-operative drive current to the photodetector during a third portion of the periodic intervals.
30. The illumination device as recited in claim 29, wherein the control circuit is further configured for:
calculating, for each emission led, a responsivity of the photodetector using the expected wavelength value determined for the emission led, the forward voltage presently measured across the photodetector, and a plurality of coefficient values that were generated during a calibration phase and stored within the illumination device to characterize a change in the photodetector responsivity over emitter wavelength and photodetector forward voltage.
31. The illumination device as recited in claim 30, wherein for each emission led, the control circuit is further configured for:
calculating an intensity value for the emission led as a ratio of the induced photocurrent measured by the led driver and receiver circuit over the photodetector responsivity calculated by the control circuit;
calculating a scale factor by dividing the expected intensity value determined for the emission led by the intensity value calculated for the emission led; and
applying the scale factor to a desired luminous flux value for the emission led to obtain an adjusted luminous flux value for the emission led; and
adjusting the drive current currently applied to the emission led to achieve the adjusted luminous flux value.
0. 33. The method as recited in claim 32, wherein for each emission led, the method further comprises:
based on the responsivity of the photodetector calculated for the emission led during the calculating step, adjusting the drive current currently applied to the emission led.

to the relationships shown in FIGS. 11A-11C to characterize the change in the photodetector responsivity over emitter wavelength and photodetector forward voltage (in step 30). In this example, the coefficient ‘m’ corresponds to the slope of the lines shown in FIGS. 11A-11C, the coefficient ‘b’ corresponds to the offset or y-axis intercept value, and the coefficient ‘d’ corresponds to the shift due to temperature. In some cases, the slope of the lines may also vary over temperature. Thus, in accordance with another embodiment, the change in photodetector responsivity may be more accurately characterized by applying a first-order polynomial of:
Responsivity=(m+km)*λ+b+d*Vfd  EQ. 2
to the relationships shown in FIGS. 11A-11C, where the coefficient ‘km’ corresponds to a difference in the slope of the lines generated at T0 and T1. As shown in FIG. 12, the coefficient values in (and possibly kin), b and d may be stored within the calibration table in step 32 of the calibration method to characterize the photodetector responsivity over wavelength and temperature separately for each emission LED (e.g., LED1, LED2 and LED3).

The calibration table shown in FIG. 12 represents only one example of the calibration values that may be stored within an LED illumination device, in accordance with the calibration method described herein. In some embodiments, the calibration method shown in FIG. 8 may be used to store substantially different calibration values, or substantially different numbers of calibration values, within the calibration table of the LED illumination device. In some embodiments, the calibration table shown in FIG. 12 may also include additional columns for storing calibration values attributed to additional LEDs.

In one alternative embodiment of the invention, the calibration method shown in FIG. 8 may be used to obtain additional measurements, which may be later used to compensate for phosphor aging, and thereby, control the chromaticity of a phosphor converted white LED over time. For example, some embodiments of the invention may include a phosphor converted white emission LED within the emitter module. These LEDs may be formed by coating or covering, e.g., a blue LED having a peak emission wavelength of about 400-500 nm with a phosphor material (e.g., YAG) having a peak emission wavelength of about 500-650 nm to produce substantially white light with a CCT of about 3000K. Other combinations of LEDs and phosphors may be used to form a phosphor converted LED, which is capable of producing white or near-white light with a CCT in the range of about 2700K to about 10,000K.

In phosphor converted LEDs, the spectral content of the LED combines with the spectral content of the phosphor to produce white or near-white light. In general, the combined spectrum may include a first portion having a first peak emission wavelength (e.g., about 400-500), and a second portion having a second peak emission wavelength (e.g., about 500-650), which is substantially different from the first peak emission wavelength. In this example, the first portion of the spectrum is generated by the light emitted by the blue LED, and the second portion is generated by the light that passes through the phosphor (e.g., YAG).

As the phosphor converted LED ages, the efficiency of the phosphor decreases, which causes the chromaticity of the phosphor converted LED to appear “cooler” over time. In order to accurately characterize a phosphor converted LED, it may be desirable in some embodiments of the calibration method shown in FIG. 8 to characterize the LED portion and the phosphor portion of the phosphor converted LED separately. Thus, some embodiments of the invention may use two different colors of photodetectors to measure photocurrents, which are separately induced by different portions of the phosphor converted LED spectrum. In particular, an emitter module of the illumination device may include a first photodetector whose detection range is configured for detecting only the first portion of the spectrum emitted by the phosphor converted LED, and a second photodetector whose detection range is configured for detecting only the second portion of the spectrum emitted by the phosphor converted LED.

In general, the detection range of the first and second photodetectors may be selected based on the spectrum of the phosphor converted LED being measured. In the exemplary embodiment described above, in which a phosphor converted white emission LED is included within the emitter module and implemented as described above, the detection range of the first photodetector may range between about 400 nm and about 500 nm for measuring the photocurrents induced by light emitted by the blue LED portion, and the detection range of the second photodetector may range between about 500 nm and about 650 nm for measuring the photocurrents induced by light that passes through the phosphor portion of the phosphor converted white LED. The first and second photodetectors may include dedicated photodetectors and/or emission LEDs, which are configured at certain times for detecting incident light.

As noted above, the emitter module of the illumination device preferably includes at least one dedicated photodetector. In one embodiment, the emitter module may include two different colors of dedicated photodetectors, such as one or more dedicated green photodetectors and one or more dedicated red photodetectors. In another embodiment, the emitter module may include only one dedicated photodetector, such as a single red, orange or yellow photodetector. In such an embodiment, one of the emission LEDs (e.g., a green emission LED) may be configured, at times, as a photodetector for measuring a portion of the phosphor converted LED spectrum.

In the calibration method described above and shown in FIG. 8, a first photodetector may be used in step 16 to measure the photocurrents, which are induced in the first photodetector by the illumination produced by each of the emission LEDs when the emission LEDs are successively driven to produce illumination at the plurality of different drive current levels and the plurality of different temperatures. In some embodiments, the first photodetector may be, e.g., a red LED, and may be used to measure the photocurrent induced by the light that passes through the phosphor. Sometime before or after each of the photocurrent measurements is obtained from the first photodetector, a forward voltage is measured across the first photodetector to provide an indication of the detector junction temperature at each of the calibrated drive current levels.

In some embodiments, a second dedicated photodetector (or one of the emission LEDs) may be used to measure the photocurrent, which is induced by the light emitted by the LED portion of the phosphor converted white LED. This photodetector may be, for example, a dedicated green photodetector or one of the green emission LEDs. Sometime before or after each of the photocurrent measurements is obtained from the second photodetector, a forward voltage is measured across the second photodetector to provide an indication of the detector junction temperature at each of the calibrated drive current levels.

In addition to measuring separate photocurrent and detector forward voltages for the phosphor converted white LED, the calibration method may also obtain separate wavelength and intensity measurements (and optionally, separate luminous flux and/or x and y chromaticity measurements) for the LED portion and the phosphor portion of the phosphor converted white LED spectrum at each of the calibrated drive currents and temperatures. This would enable the calibration method to characterize the LED portion and the phosphor portion of the phosphor converted white LED, separately, as if the phosphor converted white LED were two different LEDs. It would also enable the calibration method to characterize the responsivity of the first and second photodetectors separately for the phosphor converted white LED (in steps 28-30).

Sometime after the wavelength and intensity measurement values are obtained for the LED and phosphor portions of the phosphor converted white LED (in step 14), and the photodetector responsivity coefficients are determined (in steps 28 and 30), the measurement values and coefficients may be stored within the calibration table. In some embodiments, the calibration table shown in FIG. 12 may correspond to an LED illumination device comprising two different colors of LEDs (e.g., a phosphor converted white LED and a red LED) within each emitter module. In such embodiments, two of the columns in the calibration table (e.g., LED1 and LED2) may be used to store the calibration values for the different spectral portions of the white LED, as if the white LED were two different LEDs. In other embodiments, the calibration table of FIG. 12 may correspond to an LED illumination device comprising three different colors of LEDs (e.g., red, green and blue LEDs) within the emitter module. If a phosphor converted white LED is also included within the emitter module, two additional columns may be added to the calibration table shown in FIG. 12 to accommodate the calibration values for the two distinct spectral portions of the phosphor converted LED.

Exemplary methods for calibrating an illumination device comprising a plurality of emission LEDs and one or more photodetectors has now been described with reference to FIGS. 8-12. Although the method steps shown in FIG. 8 are described as occurring in a particular order, one or more of the steps of the illustrated method may be performed in a substantially different order.

The calibration method provided herein improves upon conventional calibration methods in a number of ways. First, the method described herein calibrates each emission LED (or chain of LEDs) individually, while turning off all other emission LEDs not currently under test. This not only improves the accuracy of the stored calibration values, but also enables the stored calibration values to account for process variations between individual LEDs, as well as differences in output characteristics that inherently occur between different colors of LEDs.

Accuracy is further improved herein by supplying a relatively small (i.e., non-operative) drive current to the emission LEDs and the photodetector(s) when obtaining forward voltage measurements, as opposed to the operative drive current levels typically used in conventional calibration methods. By using non-operative drive currents to obtain the forward voltage measurements, the present invention avoids inaccurate compensation by ensuring that the forward voltage measurements for a given temperature and fixed drive current do not change significantly over time (due to parasitic resistances in the junction when operative drive currents are used to obtain forward voltage measurements).

As another advantage, the calibration method described herein obtains a plurality of optical measurements from each emission LED and a plurality of electrical measurements from each emission LED and photodetector at a plurality of different drive current levels and a plurality of different temperatures. This further improves calibration accuracy by enabling non-linear relationships between wavelength and drive current and non-linear relationships between intensity and drive current to be precisely characterized for certain colors of LEDs. Furthermore, obtaining the calibration values at a number of different ambient temperatures improves compensation accuracy by enabling the compensation method (described below) to interpolate between the stored calibration values, so that accurate compensation values may be determined for current operating temperatures.

As yet another advantage, the calibration method described herein may use different colors of photodetectors to measure photocurrents, which are induced by different portions (e.g., an LED portion and a phosphor portion) of a phosphor converted LED spectrum. By storing these calibration values separately within the illumination device, the calibration values can be used to characterize the LED portion and the phosphor portion of the phosphor converted LED, separately, as if the phosphor converted LED were two different LEDs. It also enables the calibration method to characterize the responsivity of the two different photodetectors separately for the phosphor converted LED.

As described in more detail below, the calibration values stored within the calibration table can be used in the compensation method described herein to adjust the individual drive currents supplied to the emission LEDs, so as to obtain a desired luminous flux and a desired chromaticity over time, as the LEDs age. In some embodiments, the calibration and compensation methods described herein may be combined, or used along with, one or more of the calibration and compensation methods described in commonly assigned U.S. application Ser. Nos. 14/314,451; 14/314,482; 14/314,530; 14/314,556; and 14/314,580 to provide accurate control of the illumination device over changes in drive current and temperature, as well as time. While the most accurate results may be obtained by utilizing all such methods when operating an LED illumination device, one skilled in the art would understand how the calibration and compensation methods specifically described herein may be used to improve upon the compensation methods performed by prior art illumination devices.

Exemplary Embodiments of Improved Methods for Controlling an Illumination Device

FIGS. 13-16 illustrate an exemplary embodiment of an improved method for controlling an illumination device that generally includes a plurality of emission LEDs and at least one dedicated photodetector. More specifically, FIGS. 13-16 illustrate an exemplary embodiment of an improved compensation method that may be used to adjust the drive currents supplied to individual LEDs of an LED illumination device, so as to obtain a desired luminous flux and a desired chromaticity over time, as the LEDs age.

In some embodiments, the compensation methods shown in FIGS. 13-16 may be used to control an illumination device having LEDs all of the same color. However, the compensation method described herein is particularly well-suited for controlling an illumination device comprising two or more differently colored LEDs (i.e., a multi-colored LED illumination device), since output characteristics of differently colored LEDs vary differently over time.

Exemplary embodiments of an illumination device will be described below with reference to FIGS. 17-19, which show various components of an exemplary LED illumination device, where the illumination device is assumed to have one or more emitter modules. In general, each emitter module may include a plurality of emission LEDs arranged in an array, and one or more photodetectors spaced about a periphery of the array. In one exemplary embodiment, the array of emission LEDs may include red, green, blue and white (or yellow) LEDs, and the one or more photodetectors may include one or more red, orange, yellow and/or green LEDs. In other exemplary embodiments, one or more of the emission LEDs may be configured at certain times to detect light from at least some of the emission LEDs, and therefore, may be used in place of (or in addition to) the one or more of the dedicated photodetectors. The present invention is not limited to any particular color, number, combination or arrangement of emission LEDs and photodetectors. Furthermore, while the present invention is particularly well-suited to emitter modules, which do not control the temperature difference between the emission LEDs and the photodetector(s), a skilled artisan would understand how the method steps described herein may be applied to other LED illumination devices having substantially any emitter module design.

In general, the compensation method shown in FIG. 13 may be performed repeatedly throughout the lifetime of the illumination device to account for LED aging effects. The method shown in FIG. 13 may be performed at substantially any time, such as when the illumination device is first turned “on,” or at periodic or random intervals throughout the lifetime of the device. In some embodiments, the compensation method shown in FIG. 13 may be performed after a change in temperature, dimming level or color point setting is detected to fine tune the drive current values determined in one or more of the compensation methods disclosed in commonly assigned U.S. patent application Ser. Nos. 14/314,451; 14/314,482; 14/314,530; 14/314,556; and 14/314,580. This would provide accurate compensation for all LEDs used in the illumination device not only over time, but also over changes in drive current and temperature.

As shown in FIG. 13, the age compensation method may generally begin by driving the plurality of emission LEDs substantially continuously to produce illumination, e.g., by applying operative drive currents (Idrv) to each of the plurality of emission LEDs (in step 40). As noted above, the term “substantially continuously” means that an operative drive current is applied to the plurality of emission LEDs almost continuously, with the exception of periodic intervals during which the plurality of emission LEDs are momentarily turned off for short durations of time to produce periodic intervals (in step 42). In the method shown in FIG. 13, a first portion of the periodic intervals may be used for measuring a forward voltage (Vfe) presently developed across each emission LED, one LED at a time (in step 44). A second portion of the periodic intervals may be used for measuring a photocurrent, which is induced on the photodetector(s) in response to the illumination produced by each emission LED, one LED at a time, and received by the photodetector(s) (in step 48). A third portion of the periodic intervals may be used for measuring a forward voltage (Vfd) presently developed across the photodetector (in step 50). As in the calibration method, the Vfe and Vfd forward voltages are measured upon applying a relatively small (i.e., non-operative) drive current to the emission LEDs and the photodetector.

FIG. 14 is an exemplary timing diagram illustrating steps 40, 42, 44, 48 and 50 of the compensation method shown in FIG. 13, according to one embodiment of the invention. As shown in FIGS. 13 and 14, the plurality of emission LEDS are driven substantially continuously with operative drive current levels (denoted generically as I1 in FIG. 14) to produce illumination (in step 40 of FIG. 13). At periodic intervals, the plurality of emission LEDs are turned “off” for short durations of time (in step 42 of FIG. 13) by removing the drive currents, or at least reducing the drive currents to non-operative levels (denoted generically as I0 in FIG. 14). Between the periodic intervals, the illumination device produces continuous illumination with DC current supplied to the emission LEDs.

During a first portion of the periodic intervals, one emission LED is driven with a relatively small, non-operative drive current level (e.g., approximately 0.1-0.3 mA), while the remaining LEDs remain “off,” and the forward voltage (e.g., Vfe1) developed across that LED is measured. The forward voltages (e.g., Vfe1, Vfe 2, and Vfe 3) developed across each of the emission LEDs are measured, one LED at a time, as shown in FIG. 14 and step 44 of FIG. 13. These forward voltage measurements (also referred to herein as Vfe_present) provide an indication of the current junction temperature of the emission LEDs.

During a second portion of the periodic intervals, one emission LED is driven with an operative drive current level (II) to produce illumination, while the remaining LEDs remain “off,” and the photocurrent (e.g., Iph1) induced in the photodetector by the illumination from the driven LED is measured. The photocurrents (e.g., Iph1, Iph2, and Iph3) induced in the photodetector by the illumination produced by each of the emission LEDs are measured, one LED at a time, as shown in FIG. 14 and step 48 of FIG. 13. Sometime before or after the photocurrent (Iph) measurements are obtained, a forward voltage (Vfd) is measured across the photodetector by applying a relatively small, non-operative drive current (e.g., approximately 0.1-0.3 mA) to the photodetector (in step 50 of FIG. 13) during a third portion of the periodic intervals. This forward voltage measurement (also referred to herein as Vfd_present) provides an indication of the current junction temperature of the photodetector.

FIG. 14 provides an exemplary timing diagram for an illumination device comprising three emission LEDs, such as RGB. However, one skilled in the art would understand how the timing diagram could be easily modified to accommodate a fewer or greater number of emission LEDs. It is further noted that, although the timing diagram of FIG. 14 shows only one forward voltage (Vfd) measurement obtained from a single photodetector, the timing diagram can be easily modified to accommodate a greater number of photodetectors.

In one exemplary embodiment, the presently described compensation method may be utilized within an illumination device comprising a plurality of photodetectors implemented with differently colored LEDs. In particular, each emitter module of the illumination device may include one or more red LEDs and one or more green LEDs as photodetectors. In such an embodiment, a forward voltage measurement (Vfd) may be obtained from each photodetector by applying a small drive current thereto (in step 50). In some cases, the photocurrents associated with each emission LED (e.g., Iph1, Iph2, and Iph3) and the forward voltage(s) associated with each photodetector (Vfd) may be independently averaged over a period of time, filtered to eliminate erroneous data, and stored for example in a register of the illumination device.

In addition to the photocurrents, emitter forward voltages and detector forward voltage(s), the periodic intervals shown in FIG. 14 may be used to obtain other measurements not specifically illustrated herein. For example, some periodic intervals may be used by the photodetector to detect light originating from outside of the illumination device, such as ambient light or light from other illumination devices. In some cases, ambient light measurements may be used to turn the illumination device on when the ambient light level drops below a threshold (i.e., when it gets dark), and turn the illumination device off when the ambient light level exceeds another threshold (i.e., when it gets light). In other cases, the ambient light measurements may be used to adjust the lumen output of the illumination device over changes in ambient light level, for example, to maintain a consistent level of brightness in a room. If periodic intervals are used to detect light from other illumination devices, the detected light may be used to avoid interference from the other illumination devices when obtaining the photocurrent and detector forward voltage measurements in the compensation method of FIG. 13.

In other embodiments, periodic intervals may be used to measure different portions of a particular LED's spectrum using two or more different colors of photodetectors. For example, the spectrum of a phosphor converted white LED may be divided into two portions, and each portion may be measured separately during two different periodic intervals using two different photodetectors. Specifically, a first periodic interval may be used to detect the photocurrent, which is induced on a first photodetector (e.g., a green photodetector) by a first spectral portion (e.g., about 400 nm to about 500 nm) of the phosphor converted white LED. A second periodic interval may then be used to detect the photocurrent, which is induced on a second photodetector (e.g., a red photodetector) by a second spectral portion (e.g., about 500 nm to about 650 nm) of the phosphor converted white LED.

Sometime after the emitter forward voltage(s) are measured (in step 44), the compensation method shown in FIG. 13 may determine expected wavelength values (λ_exp) and expected intensity values (Rad_exp) for each emission LED (in step 46) using the forward voltage (Vfe_present) presently measured across the emission LED, the drive current (Idrv) presently applied to the emission LED, the table of stored calibration values generated during the calibration method of FIG. 8, and one or more interpolation techniques. FIGS. 15 and 16 illustrate how one or more interpolation techniques may be used to determine the expected wavelength values (λ_exp) and the expected intensity values (Rad_exp) for a given LED at the present operating temperature (Vfe_present) and the present drive current (Idrv) from the table of stored calibration values.

In FIG. 15, the solid dots (•) represent the wavelength calibration values, which were obtained during the calibration method of FIG. 8 at a plurality of different drive currents (e.g., 50 mA, 100 mA, 150 mA, 200 mA, 250 mA, 300 mA, 350 mA and 400 mA) and two different ambient temperatures (e.g., T0 and T1). The wavelength calibration values (•) were previously stored within a table of calibration values (see, e.g., FIG. 12) for each emission LED included within the illumination device. To determine the expected wavelength value (λ_exp) for a given LED, the compensation method of FIG. 13 interpolates between the stored calibration values (•) to calculate the wavelength values (Δ), which should be produced at the present operating temperature (Vfe_present) when using the same drive currents (e.g., 50 mA, 100 mA, 150 mA, 200 mA, 250 mA, 300 mA, 350 mA and 400 mA) that were used during calibration. In most cases, a linear interpolation technique can be used to calculate the wavelength values (Δ's) at the present operating temperature for all colors of LEDs. While this is illustrated for only a red LED, the same method may be used to calculate the wavelength values (Δ) that are expected to be produced at the present operating temperature and each of the calibrated drive currents for all colors of LEDs.

If the drive current (Idrv) presently supplied to the emission LED differs from one of the calibrated drive current levels, the compensation method of FIG. 13 may apply another interpolation technique to the calculated wavelength values (Δ) to generate a relationship there between (denoted by a dashed line in FIG. 15). In some cases, a linear interpolation or a non-linear interpolation of the calculated wavelength values (Δ) may be used to generate a linear relationship or a non-linear relationship between wavelength and drive current. As noted above and shown in FIGS. 9A-9C, the relationship between wavelength and drive current tends to be relatively linear for red LEDs, but significantly more non-linear for green and blue LEDs. In some cases, a linear interpolation may be selected to generate the relationship between the calculated wavelength values for red LEDs, while a non-linear interpolation is used for green and blue LEDs. In other cases, a piece-wise linear interpolation could be used to characterize the relationship between the calculated wavelength values for one or more of the LED colors. From each generated relationship, the expected wavelength value (λ_exp) may be determined for the drive current (Idrv) currently applied to the emission LED.

The expected intensity (e.g., Rad_exp) may be determined in substantially the same manner. For example, the solid dots (•) shown in FIG. 16 represent the intensity calibration values, which were obtained during the calibration method of FIG. 8 at a plurality of different drive currents (e.g., 50 mA, 100 mA, 150 mA, 200 mA, 250 mA, 300 mA, 350 mA and 400 mA) and two different ambient temperatures (e.g., T0 and T1). The wavelength calibration values (•) were previously stored within a table of calibration values (see, e.g., FIG. 12) for each emission LED included within the illumination device. Although FIG. 16 illustrates the use of radiance calibration values, some embodiments of the invention may instead utilize luminance.

To determine the expected intensity value (e.g., Rad_exp) for a given LED, the compensation method of FIG. 13 interpolates between the stored calibration values (•) to calculate the intensity values (Δ), which should be produced at the present operating temperature (Vfe_present) when using the same drive currents (e.g., 50 mA, 100 mA, 150 mA, 200 mA, 250 mA, 300 mA, 350 mA and 400 mA) that were used during calibration. In most cases, a linear interpolation technique can be used to calculate the intensity values (Δ) at the present operating temperature for all colors of LEDs. While this is illustrated for only a red LED, the same method may be used to calculate the intensity values (Δ) that are expected to be produced at the present operating temperature and each of the calibrated drive currents for all colors of LEDs.

If the drive current (Idrv) presently supplied to the emission LED differs from one of the calibrated drive current levels, the compensation method of FIG. 13 may apply another interpolation technique to the calculated intensity values (Δ) to generate a relationship there between (denoted by a dashed line in FIG. 16). In some cases, a linear interpolation or a non-linear interpolation of the calculated intensity values (Δ) may be used to generate a linear relationship or a non-linear relationship between intensity and drive current. As noted above and shown in FIGS. 10A-10C, the relationship between intensity and drive current tends to be relatively linear for red LEDs, but significantly more non-linear for green and blue LEDs. In some cases, a linear interpolation may be selected to generate the relationship between the calculated wavelength values for red LEDs, while a non-linear interpolation is used for green and blue LEDs. In other cases, a piece-wise linear interpolation could be used to characterize the relationship between the calculated intensity values for one or more of the LED colors. From each generated relationship, the expected intensity value (e.g., Rad_exp) may be determined for the drive current (Idrv) currently applied to the emission LED.

Sometime after the expected wavelength (λ_exp) value is determined for each emission LED (in step 46), the compensation method shown in FIG. 13 calculates a photodetector responsivity for each emission LED (in step 52) using the forward voltage (Vfd) measured across the photodetector in step 50, the expected wavelength value (λ_exp) determined for the emission LED in step 46 and a plurality of coefficient values, which were generated during the calibration method of FIG. 8 and stored within the illumination device to characterize a change in the photodetector responsivity over emitter wavelength and photodetector forward voltage.

As noted above, the photodetector responsivity may be expressed as a first-order polynomial in the form of:
Responsivity=m*λ+b+d*Vfd, or  EQ. 1
Responsivity=(m+km)*λ+b+d*Vfd  EQ. 2
where the coefficient ‘m’ corresponds to the slope of the lines shown in FIGS. 11A-11C, the coefficient ‘km’ corresponds to a difference in the slope of the lines generated at T0 and T1, the coefficient ‘b’ corresponds to the offset or y-axis intercept value, and the coefficient ‘d’ corresponds to the shift due to temperature. These coefficient values were calculated and stored within the calibration table during the calibration phase to characterize the change in the photodetector responsivity over emitter wavelength and photodetector forward voltage for each emission LED. In step 52 of the compensation method shown in FIG. 13, the photodetector responsivity is again calculated for each emission LED at the present operating temperature by inserting the forward voltage (Vfd) presently measured across the photodetector in step 50, the expected wavelength value (λ_exp) determined for the emission LED in step 46 and the stored coefficient values (e.g., m, km, b, and d) within EQ. 1 or EQ. 2.

In step 54, an intensity value (e.g., Rad_calc) is calculated for each emission LED by dividing the photocurrent, which was induced in the photodetector from the illumination produced by the emission LED at the present drive current and measured in step 48, by the photodetector responsivity calculated in step 52 for that LED. Next, a scale factor is calculated for each emission LED (in step 56) by dividing the expected intensity value (e.g., Rad_exp) determined for the emission LED in step 46 by the intensity value (e.g., Rad_calc) calculated for the emission LED in step 54. Once the scale factor is calculated, the compensation method applies each scale factor to a desired luminous flux value for each emission LED to obtain an adjusted luminous flux value for each emission LED (in step 58). In some embodiments, the desired luminous flux values may be relative lumen values (Y1, Y2, Y3 or Y4), which are calculated during one of the compensation methods disclosed in the prior applications to account for changes in the target luminance (Ym) and/or target chromaticity (xm, ym) settings stored within the illumination device. Finally, the drive currents currently applied to the emission LEDs are adjusted (in step 60) to achieve the adjusted luminous flux values if a difference exists between the expected and calculated intensity values for any of the emission LEDs.

The compensation method described above and illustrated in FIG. 13 provides an accurate method for adjusting the individual drive currents applied to the emission LEDs, so as to compensate for the degradation in lumen output that occurs over time as the LEDs age. By accurately controlling the luminous flux produced by each emission LED, the compensation method accurately controls the color of an LED illumination device comprising a plurality of multi-colored emission LEDs.

The compensation method shown in FIG. 13 and described above provides many advantages over conventional compensation methods. For example, the compensation method improves the accuracy with which emitter and detector forward voltage(s) are measured by applying a relatively small drive current (e.g., about 0.1 mA to about 0.3 mA) to the emission LEDs and photodetector(s). In addition, the compensation method interpolates between a plurality of stored wavelength and intensity values taken at different drive currents and different temperatures to derive relationships between wavelength, intensity and drive current for each emission LED at the present operating temperature (Vfe_present). By accurately and individually characterizing the wavelength vs. drive current relationship and the intensity vs. drive current relationship for each individual LED, the present compensation method is able to determine the wavelength and intensity, which would be expected from the emission LED at the present drive current and temperature, with a high degree of precision.

Furthermore, the compensation method described herein characterizes photodetector responsivity as a function of emitter wavelength and photodetector forward voltage separately for each emission LED. In preferred embodiments, a photodetector configured to operate at a relatively low current is used, so that aging of the photodetector is negligible over the lifetime of the illumination device. This allows the photodetector responsivity values calculated in step 52 to be used as a reference for the emission LEDs when the intensity values are calculated in step 54. The scale factors calculated in step 56 will account for any differences between the expected intensity (e.g., Rad_exp) and the calculated intensity (e.g., Rad_calc) at the drive current presently applied to an emission LED. If a difference exists, a scale factor>1 will be applied to the desired luminous flux value to increase the drive current applied to the emission LED, thereby increasing the lumen output.

Exemplary Embodiments of Improved Illumination Devices

The improved methods described herein for calibrating and controlling an illumination device may be used within substantially any LED illumination device having a plurality of emission LEDs and one or more photodetectors. As described in more detail below, the improved methods described herein may be implemented within an LED illumination device in the form of hardware, software or a combination of both.

Illumination devices, which benefit from the improved methods described herein, may have substantially any form factor including, but not limited to, parabolic lamps (e.g., PAR 20, 30 or 38), linear lamps, flood lights and mini-reflectors. In some cases, the illumination devices may be installed in a ceiling or wall of a building, and may be connected to an AC mains or some other AC power source. However, a skilled artisan would understand how the improved methods described herein may be used within other types of illumination devices powered by other power sources (e.g., batteries or solar energy).

Exemplary embodiments of an improved illumination device will now be described with reference to FIGS. 17-19, which show various components of an LED illumination device, where the illumination device is assumed to have one or more emitter modules. Each emitter module included within the LED illumination device may generally include a plurality of emission LEDs and at least one dedicated photodetector, all of which are mounted onto a common substrate and encapsulated within a primary optics structure. Although examples are provided herein, the inventive concepts described herein are not limited to any particular type of LED illumination device, any particular number of emitter modules that may be included within an LED illumination device, or any particular number, color or arrangement of emission LEDs and photodetectors that may be included within an emitter module. Instead, the present invention may only require an LED illumination device to include at least one emitter module comprising a plurality of emission LEDs and at least one dedicated photodetector. In some embodiments, a dedicated photodetector may not be required, if one or more of the emission LEDs is configured, at times, to provide such functionality. While the present invention is particularly well-suited to emitter modules, which do not control the temperature difference between the emission LEDs and the photodetector(s), a skilled artisan would understand how the method steps described herein may be applied to other types of LED illumination devices having substantially different emitter module designs.

One embodiment of an exemplary emitter module 70 that may be included within an LED illumination device is shown in FIG. 17. In the illustrated embodiment, emitter module 70 includes four emission LEDs 72, which are mounted onto a substrate 76 and encapsulated within a primary optics structure 78. The primary optics structure 78 may be formed from a variety of different materials and may have substantially any shape and/or dimensions necessary to shape the light emitted by the emission LEDs in a desirable manner. Although the primary optics structure is described below as a dome, one skilled in the art would understand how the primary optics structure may have substantially any other shape or configuration, which encapsulates the emission LEDs and the at least one photodetector. In some embodiments, a heat sink 79 may be coupled to a bottom surface of the substrate 76 for drawing heat away from the heat generating components of the emitter module. In other embodiments, the heat sink 79 may be omitted.

In some embodiments, the emission LEDs 72 may be arranged in a square array and placed as close as possible together in the center of the dome 78, so as to approximate a centrally located point source. In some embodiments, the emission LEDs 72 may each be configured for producing illumination at a different peak emission wavelength. For example, the emission LEDs 72 may include RGBW LEDs or RGBY LEDs. In some embodiments, the array of emission LEDs 72 may include a chain of four red LEDs, a chain of four green LEDs, a chain of four blue LEDs, and a chain of four white or yellow LEDs. Each chain of LEDs may be coupled in series and driven with the same drive current. In some embodiments, the individual LEDs in each chain may be scattered about the array, and arranged so that no color appears twice in any row, column or diagonal, to improve color mixing within the emitter module 70.

In addition to the emission LEDs 72, one or more dedicated photodetectors 74 may be mounted onto the substrate 76 and arranged within the dome 78 somewhere around the periphery of the array. The dedicated photodetector(s) 74 may be any device (such as a silicon photodiode or an LED) that produces current indicative of incident light. In one embodiment, at least one of the dedicated photodetectors 74 is an LED with a peak emission wavelength in the range of approximately 550 nm to 700 nm. A photodetector with such a peak emission wavelength will not produce photocurrent in response to infrared light, which reduces interference from ambient light sources. The at least one photodetector 74 is preferably implemented with a small red, orange or yellow LED. Such a photodetector may be configured to operate at a relatively low current, so that aging of the at least one photodetector is negligible over the lifetime of the illumination device. In some embodiments, the at least one photodetector 74 may be arranged to capture a maximum amount light, which is reflected from a surface of the dome 78 from the emission LEDs having the shortest wavelengths (e.g., the blue and green emission LEDs).

In some embodiments, four dedicated photodetectors 74 may be included within the dome 78 and arranged around the periphery of the array. In some embodiments, the four dedicated photodetectors 74 may be placed close to, and in the middle of, each edge of the array and may be connected in parallel to a receiver of the illumination device. By connecting the four dedicated photodetectors 74 in parallel with the receiver, the photocurrents induced on each photodetector may be summed to minimize the spatial variation between the similarly colored LEDs, which may be scattered about the array.

The emitter module shown in FIG. 17 is provided merely as an example of an emitter module that may be included in an LED illumination device. Further description of the emitter module may be found in commonly assigned U.S. application Ser. No. 14/097,339 and commonly assigned U.S. Application No. 61/886,471, which incorporated herein by reference in their entirety.

One problem with emitter modules, such as the one shown in FIG. 17, is that the temperature difference between the emission LEDs 72 and the photodetector(s) 74 is typically not well controlled. In particular, the junction temperature of the emission LEDs 72 tends to be about 10-20° C. higher than the junction temperature of the smaller, less frequently used photodetectors 74. Furthermore, because LED junction temperatures fluctuate with drive current, the temperature difference (ΔT) between the emission LEDs and the photodetectors tends to change with operating conditions.

The presently described calibration method address this problem by precisely characterizing how the wavelength and intensity of the emission LEDs changes over drive current and temperature, and precisely characterizing how the responsivity of the photodetector changes over emitter wavelength and detector forward voltage for each emission LED. During operation of the illumination device, the compensation method described herein calculates the responsivity, which is to be expected from the photodetector for the drive currently presently applied to the emission LED and the current junction temperature of the photodetector. Although the photodetector responsivity necessarily changes with emitter wavelength and detector junction temperature, it will not change significantly over time if a relatively small photodetector is used and driven with a relatively low current, This allows the compensation method described herein to use the photodetector responsivity as a reference when determining the difference between the intensity expected from the emission LED and the current intensity output by the emission LED. If a difference exists, a scale factor is generated to increase the lumen output from the emission LED to counteract LED aging affects.

FIG. 18 is one example of a block diagram of an illumination device 80, which is configured to accurately maintain a desired luminous flux and a desired chromaticity over variations in drive current, temperature and time. The illumination device illustrated in FIG. 18 provides one example of the hardware and/or software that may be used to implement the calibration method shown in FIG. 8 and the compensation method shown in FIG. 13.

In the illustrated embodiment, illumination device 80 comprises a plurality of emission LEDs 96 and one or more dedicated photodetectors 98. In this example, the emission LEDs 96 comprise four chains of any number of LEDs. In typical embodiments, each chain may have 2 to 4 LEDs of the same color, which are coupled in series and configured to receive the same drive current. In one example, the emission LEDs 96 may include a chain of red LEDs, a chain of green LEDs, a chain of blue LEDs, and a chain of white or yellow LEDs. However, the present invention is not limited to any particular number of LED chains, any particular number of LEDs within the chains, or any particular color or combination of LED colors.

Although the one or more dedicated photodetectors 98 are also illustrated in FIG. 18 as including a chain of LEDs, the present invention is not limited to any particular type, number, color, combination or arrangement of photodetectors. In one embodiment, the one or more dedicated photodetectors 98 may include a small red, orange or yellow LED. In another embodiment, the one or more dedicated photodetectors 98 may include one or more small red LEDs and one or more small green LEDs. In some embodiments, one or more of the dedicated photodetector(s) 98 shown in FIG. 18 may be omitted if one or more of the emission LEDs 96 are configured, at times, to function as a photodetector. The plurality of emission LEDs 96 and the (optional) dedicated photodetectors 98 may be included within an emitter module, as discussed above. In some embodiments, an illumination device may include more than one emitter module, as discussed above.

In addition to including one or more emitter modules, illumination device 80 includes various hardware and software components, which are configured for powering the illumination device and controlling the light output from the emitter module(s). In one embodiment, the illumination device is connected to AC mains 82, and includes AC/DC converter 84 for converting AC mains power (e.g., 120V or 240V) to a DC voltage (VDC). As shown in FIG. 18, this DC voltage (e.g., 15V) is supplied to the LED driver and receiver circuit 94 for producing the operative drive currents, which are applied to the emission LEDs 96 for producing illumination. In addition to the AC/DC converter, a DC/DC converter 86 is included for converting the DC voltage VDC (e.g., 15V) to a lower voltage VL (e.g., 3.3V), which may be used to power the low voltage circuitry included within the illumination device, such as PLL 88, wireless interface 90, and control circuit 92.

In the illustrated embodiment, PLL 88 locks to the AC mains frequency (e.g., 50 or 60 HZ) and produces a high speed clock (CLK) signal and a synchronization signal (SYNC). The CLK signal provides the timing for control circuit 92 and LED driver and receiver circuit 94. In one example, the CLK signal frequency is in the tens of megahertz range (e.g., 23 MHz), and is precisely synchronized to the AC Mains frequency and phase. The SNYC signal is used by the control circuit 92 to create the timing used to obtain the various optical and electrical measurements described above. In one example, the SNYC signal frequency is equal to the AC Mains frequency (e.g., 50 or 60 HZ) and also has a precise phase alignment with the AC Mains.

In some embodiments, a wireless interface 90 may be included and used to calibrate the illumination device 80 during manufacturing. As noted above, for example, an external calibration tool (not shown in FIG. 18) may communicate wavelength and intensity (and optionally, luminous flux and chromaticity) calibration values to an illumination device under test via the wireless interface 90. The calibration values received via the wireless interface 90 may be stored in the table of calibration values within a storage medium 93 of the control circuit 92, for example.

Wireless interface 90 is not limited to receiving only calibration data, and may be used for communicating information and commands for many other purposes. For example, wireless interface 90 could be used during normal operation to communicate commands, which may be used to control the illumination device 80, or to obtain information about the illumination device 80. For instance, commands may be communicated to the illumination device 80 via the wireless interface 90 to turn the illumination device on/off, to control the dimming level and/or color set point of the illumination device, to initiate the calibration procedure, or to store calibration results in memory. In other examples, wireless interface 90 may be used to obtain status information or fault condition codes associated with illumination device 80.

In some embodiments, wireless interface 90 could operate according to ZigBee, WiFi, Bluetooth, or any other proprietary or standard wireless data communication protocol. In other embodiments, wireless interface 90 could communicate using radio frequency (RF), infrared (IR) light or visible light. In alternative embodiments, a wired interface could be used, in place of the wireless interface 90 shown, to communicate information, data and/or commands over the AC mains or a dedicated conductor or set of conductors.

Using the timing signals received from PLL 88, the control circuit 92 calculates and produces values indicating the desired drive current to be used for each LED chain 96. This information may be communicated from the control circuit 92 to the LED driver and receiver circuit 94 over a serial bus conforming to a standard, such as SPI or I2C, for example. In addition, the control circuit 92 may provide a latching signal that instructs the LED driver and receiver circuit 94 to simultaneously change the drive currents supplied to each of the LEDs 96 to prevent brightness and color artifacts.

During calibration, the control circuit 92 may be configured for generating a plurality of photodetector responsivity coefficients (e.g., m, km, b, and d) for each of the emission LEDs, which may then be stored within the storage medium 93. In some embodiments, the control circuit 92 may determine the photodetector responsivity coefficients by executing program instructions stored within the storage medium 93. During operation of the illumination device, the control circuit 92 may be further configured for determining the respective drive currents needed to achieve a desired luminous flux and/or a desired chromaticity for the illumination device in accordance with the compensation method shown in FIG. 8 13. In some embodiments, the control circuit 92 may determine the respective drive currents by executing additional program instructions stored within the storage medium 93. In one embodiment, the storage medium 93 may be a non-volatile memory, and may be configured for storing the program instructions used by the control circuit during the calibration and compensation methods along with a table of calibration values, such as the table described above with respect to FIG. 12.

In general, the LED driver and receiver circuit 94 may include a number (N) of driver blocks equal to the number of emission LED chains 96 included within the illumination device. In the exemplary embodiment discussed herein, LED driver and receiver circuit 94 comprises four driver blocks 100, each configured to produce illumination from a different one of the emission LED chains 96. The LED driver and receiver circuit 94 also comprises the circuitry needed to measure ambient temperature (optional), the detector and/or emitter forward voltages, and the detector photocurrents, and to adjust the LED drive currents accordingly. Each driver block receives data indicating a desired drive current from the control circuit 92, along with a latching signal indicating when the driver block should change the drive current.

FIG. 19 is an exemplary block diagram of an LED driver and receiver circuit 94, according to one embodiment of the invention. As shown in FIG. 19, the LED driver and receiver circuit 94 includes four driver blocks 100, each block including a buck converter 102, a current source 104, and an LC filter 108 for generating the drive currents that are supplied to a connected chain of emission LED 96a to produce illumination and obtain forward voltage (Vfe) measurements. In some embodiments, buck converter 102 may produce a pulse width modulated (PWM) voltage output (Vdr) when the controller 124 drives the “Out_En” signal high. This voltage signal (Vdr) is filtered by the LC filter 108 to produce a forward voltage on the anode of the connected LED chain 96a. The cathode of the LED chain is connected to the current source 104, which forces a fixed drive current equal to the value provided by the “Emitter Current” signal through the LED chain 96a when the “Led_On” signal is high. The “Vc” signal from the current source 104 provides feedback to the buck converter 102 to output the proper duty cycle and minimize the voltage drop across the current source 104.

As shown in FIG. 19, each driver block 100 includes a difference amplifier 106 for measuring the forward voltage drop (Vfe) across the chain of emission LEDs 96a. When measuring Vfe, the buck converter 102 is turned off and the current source 104 is configured for drawing a relatively small drive current (e.g., about 1 mA) through the connected chain of emission LEDs 96a. The voltage drop (Vfe) produced across the LED chain 96a by that current is measured by the difference amplifier 106. The difference amplifier 106 produces a signal that is equal to the forward voltage (Vfe) drop across the emission LED chain 96a during forward voltage measurements.

In addition to including a plurality of driver blocks 100, the LED driver and receiver circuit 94 may include one or more receiver blocks 110 for measuring the forward voltages (Vfd) and photocurrents (Iph) induced across the one or more dedicated photodetectors 98. Although only one receiver block 110 is shown in FIG. 19, the LED driver and receiver circuit 94 may generally include a number of receiver blocks 110 equal to the number of dedicated photodetectors included within the emitter module.

In the illustrated embodiment, receiver block 110 comprises a voltage source 112, which is coupled for supplying a DC voltage (Vdr) to the anode of the dedicated photodetector 98 coupled to the receiver block, while the cathode of the photodetector 98 is connected to current source 114. When photodetector 98 is configured for obtaining a forward voltage (Vfd) measurement, the controller 124 supplies a “Detector_On” signal to the current source 114, which forces a fixed drive current (Idrv) equal to the value provided by the “Detector Current” signal through photodetector 98.

When obtaining detector forward voltage (Vfd) measurements, current source 114 is configured for drawing a relatively small amount of drive current (Idrv) through photodetector 98. The voltage drop (Vfd) produced across photodetector 98 by that current is measured by difference amplifier 118, which produces a signal equal to the forward voltage (Vfd) drop across photodetector 98. As noted above, the drive current (Idrv) forced through photodetector 98 by the current source 114 is generally a relatively small, non-operative drive current. In the embodiment in which four dedicated photodetectors 98 are coupled in parallel, the non-operative drive current may be roughly 1 mA. However, smaller/larger drive currents may be used in embodiments that include fewer/greater numbers of photodetectors, or embodiments that do not connect the photodetectors in parallel.

In addition to measuring forward voltage, receiver block 110 also includes circuitry for measuring the photocurrents (Iph) induced on photodetector 98 by light emitted by the emission LEDs. As shown in FIG. 19, the positive terminal of transimpedance amplifier 115 is coupled to the Vdr output of voltage source 112, while the negative terminal is connected to the cathode of photodetector 98. When connected in this manner, the transimpedance amplifier 115 produces an output voltage relative to Vdr (e.g., about 0-1V), which is supplied to the positive terminal of difference amplifier 116. Difference amplifier 116 compares the output voltage to Vdr and generates a difference signal, which corresponds to the photocurrent (Iph) induced across photodetector 98. Transimpedance amplifier 115 is enabled when the “Detector_On” signal is low. When the “Detector_On” signal is high, the output of transimpedance amplifier 115 is tri-stated.

As noted above, some embodiments of the invention may scatter the individual LEDs within each chain of LEDs 96 about the array of LEDs, so that no two LEDs of the same color exist in any row, column or diagonal. By connecting a plurality of dedicated photodetectors 98 in parallel with the receiver block 110, the photocurrents (Iph) induced on each photodetector 98 by the LEDs of a given color may be summed to minimize the spatial variation between the similarly colored LEDs, which are scattered about the array.

As shown in FIG. 19, the LED driver and receiver circuit 94 may also include a multiplexor (Mux) 120, an analog to digital converter (ADC) 122, a controller 124, and an optional temperature sensor 126. In some embodiments, multiplexor 120 may be coupled for receiving the emitter forward voltage (Vfe) from the driver blocks 100, and the detector forward voltage (Vfd) and detector photocurrent (Iph) measurements from the receiver block 110. The ADC 122 digitizes the Vfe, Vfd and Iph measurements and provides the results to the controller 124. The controller 124 determines when to take forward voltage and photocurrent measurements and produces the “Out_En,” “Emitter Current” and “Led_On” signals, which are supplied to the driver blocks 100, and the “Detector Current” and “Detector_On” signals, which are supplied to the receiver block 110 as shown in FIG. 19.

In some embodiments, the LED driver and receiver circuit 94 may include an optional temperature sensor 126 for taking ambient temperature (Ta) measurements. In such embodiments, multiplexor 120 may also be coupled for multiplexing the ambient temperature (Ta) with the forward voltage and photocurrent measurements sent to the ADC 122. In some embodiments, the temperature sensor 126 may be a thermistor, and may be included on the driver circuit chip for measuring the ambient temperature surrounding the LEDs, or a temperature from the heat sink of the emitter module. In other embodiments, the temperature sensor 126 may be an LED, which is used as both a temperature sensor and an optical sensor to measure ambient light conditions or output characteristics of the LED emission chains 96.

One implementation of an improved illumination device 80 has now been described in reference to FIGS. 17-19. Further description of such an illumination device may be found in commonly assigned U.S. application Ser. Nos. 13/970,944; 13/970,964; and 13/970,990 and commonly assigned U.S. application Ser. Nos. 14/314,451; 14/314,482; 14/314,530; 14/314,556; and 14/314,580. A skilled artisan would understand how the illumination device could be alternatively implemented within the scope of the present invention.

It will be appreciated to those skilled in the art having the benefit of this disclosure that this invention is believed to provide an improved illumination device and improved methods for calibrating and compensating individual LEDs in the illumination device, so as to maintain a desired luminous flux and a desired chromaticity over time. Further modifications and alternative embodiments of various aspects of the invention will be apparent to those skilled in the art in view of this description. It is intended, therefore, that the following claims be interpreted to embrace all such modifications and changes and, accordingly, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense.

INVENTORS:

Knapp, David J., Ho, Horace C., Lewis, Jason E., Dias, Alcides Jose

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