Illumination device and method for avoiding flicker

Abstract

An illumination device comprising a plurality of light emitting diodes (leds) and a method for controlling the illumination device while avoiding flicker in the led output is provided herein. According to one embodiment, the method may include driving the plurality of leds substantially continuously with drive currents configured to produce illumination, periodically turning the plurality of leds off for short durations of time during a first period to take measurements or communicate optical data, and increasing the drive currents supplied to the plurality of leds by a small amount when the leds are on during the first period to compensate for lack of illumination when the leds are periodically turned off during the first period.

Description

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where ‘x’ is Vf (or temperature), ‘y’ is luminous flux, and ‘a,’ ‘b’ and ‘c’ are coefficients. If forward voltage and luminous flux values were previously obtained during the calibration phase at three different temperatures, the ‘a,’ ‘b’ and ‘c’ coefficient values may be precisely determined by inserting the stored calibration values into EQ. 1 and separately solving the equation for ‘a,’ ‘b’ and ‘c’. If, on the other hand, the LED was calibrated at only two different temperatures, the ‘a’ coefficient may be obtained from data sheets provided by the LED manufacturer, while the ‘b’ and ‘c’ coefficients are determined from the calibration values, as described above. While the latter method (sometimes referred to as a “poor man's quadratic interpolation”) may sacrifice a small amount of accuracy, it may in some cases represent an acceptable trade-off between accuracy and calibration costs.

In some embodiments, a relationship (solid black line in FIG. 6) between the luminous flux values calculated at the three X's may be determined through another interpolation technique, if a desired luminous flux (Lx) differs from one of the calculated values. However, since the relationship between luminous flux and drive current is non-linear for all LED colors (see, FIGS. 18-19), the relationship may be derived, in some embodiments, through a higher-order interpolation of the calculated luminous flux values. Alternatively, a piece-wise linear interpolation could be used to characterize the non-linear relationship between the calculated luminous flux values, or a typical curvature could be assumed from data sheets provided by the LED manufacturer.

In some embodiments, an appropriate interpolation technique may be selected based on trade-offs between memory and processing requirements, and/or based upon the particular color of LED being compensated. As noted above, some LED colors, such as blue and green, exhibit a comparatively more non-linear luminous flux vs. drive current relationship than other LED colors, such as red and red-orange (see, FIGS. 18-19). LED colors exhibiting substantially greater non-linear behaviors (such as blue and green) may be more accurately compensated by obtaining more luminous flux calibration values and using a piece-wise linear interpolation technique, or by obtaining fewer calibration values and using a higher-order interpolation technique or an assumed curvature to generate the non-linear relationship between luminous flux and drive current.

Once the relationship between luminous flux and drive current is derived for a given LED, the drive current (Ix) needed to produce a desired luminous flux (Lx) may be selected from the generated relationship, as shown in the example of FIG. 6. The selected drive current may then be used to drive the LED to produce illumination having the desired luminous flux (in step 38 of FIG. 5). This process is then repeated for each of the plurality of LEDs, until each is configured for producing a desired luminous flux at the present operating temperature. The drive currents supplied to the LEDs may be adjusted to meet the selected drive currents either by adjusting the drive current level (i.e., current dimming), or by changing the duty cycle of the drive current through Pulse Width Modulation (PWM) dimming.

As with the calibration method of FIG. 1, the compensation method shown in FIG. 5 provides many advantages over conventional compensation methods. For example, the present compensation method uses a relatively small drive current to obtain operating forward voltage measurements from each LED individually, while turning off all emission LEDs not currently under test. This improves the accuracy of the operating forward voltage values and enables each LED to be individually compensated for temperature and process. Unlike conventional methods, some of which rely on typical values or linear relationships between luminous flux and drive current, the compensation method described herein derives a non-linear relationship between luminous flux and drive current for each LED at the present operating temperature (or Vf) using the stored calibration values taken at the different temperatures during the calibration process. This enables the present compensation method to precisely characterize the luminous flux vs. drive current relationship for each LED, and provide accurate temperature compensation, regardless of process. As a consequence, the compensation method described herein is able to more precisely control the luminous flux (if the LEDs are all of the same color), or the luminous flux and color point (if the illumination device comprises two or more differently colored LEDs).

A further advantage of the present compensation method is the ability to provide accurate temperature compensation while avoiding undesirable visible artifacts in the generated light. One undesirable artifact, called “brightness banding,” often occurs whenever LEDs are periodically turned on and off for any reason, even at imperceptibly high rates. This banding artifact is demonstrated in the photograph of FIG. 21 and described above as alternating bands of light and dark areas on a display screen backlit by an array of LEDs. The bright and dark bands that develop across the display screen may occur whenever light emitted by the LEDs is modulated or turned on/off for any reason, such as when obtaining forward voltage measurements for temperature compensation or when modulating light output to communicate optical data in visible light communication (VLC) systems.

Another visual artifact, called “flicker,” may occur during times when the LEDs are periodically turned off to measure forward voltage or communicate optical data. When the LEDs are turned off, the amount of light produced by the illumination device decreases, and when the LEDs are turned back on, the amount of light produced by the illumination device increases. This phenomenon may cause the illumination device to appear to flicker in either brightness and/or color. A solution for avoiding brightness banding and flicker during temperature compensation is illustrated in FIGS. 5 and 8. A solution for avoiding flicker in a VLC system is illustrated in FIG. 9.

In some embodiments, the compensation method described herein may achieve a desired luminous flux and/or color point from an illumination device, while avoiding undesirable visual artifacts, such as brightness banding and flicker. Such embodiments are illustrated in the optional method steps of FIG. 5 and the timing diagrams of FIG. 8.

As shown in the uppermost graph of FIG. 8, the ambient temperature (Ta) surrounding an illumination device increases steadily over time during operation of the device, until the temperature stabilizes (e.g., at Ta2). Since it is only necessary to perform the compensation method while the ambient temperature changes, alternative embodiments of the compensation method described herein may determine if there has been a significant change in ambient temperature (in optional step 31 of FIG. 5) before proceeding with steps 32-38. A “significant change” may be any incremental increase or decrease in ambient temperature. For example, a “significant change” may be a 1° C. increase or decrease in temperature. Other temperature increments may be used in other examples.

At specific increments of ambient temperature (e.g., 1° C.), the plurality of LEDs are turned off for short durations of time (in step 32 of FIG. 5), and the LED forward voltages (e.g., Vf1, Vf2, Vf3, and Vf4 of FIG. 8) are measured using a relatively small drive current (in step 34 of FIG. 5). New LED drive currents are calculated and applied to compensate for the change in temperature (in steps 36-38 of FIG. 5). However, once the ambient temperature stabilizes (“No” branch of step 31 of FIG. 5; Ta2 in the uppermost graph of FIG. 8), forward voltage measurements are no longer needed and the LEDs are driven to produce continuous illumination so that brightness banding does not occur.

In addition to brightness banding, FIG. 8 provides a solution for avoiding flicker during the times when the LEDs are periodically turned off to measure forward voltage. When forward voltage is measured from a given LED, all other emission LEDs are turned off to avoid inducing photocurrents within the LED under test. Because the amount of light produced by the illumination device decreases when the LEDs are turned off, and increases when the LEDs are turned back on, the illumination device may appear to flicker in brightness and/or color. The flicker phenomenon is avoided in the present compensation method by increasing the drive currents supplied to the LEDs by a small amount when the LEDs are turned on during the compensation period (in optional step 35 of FIG. 5). This is illustrated graphically in the two lowermost graphs of FIG. 8.

As shown in the two lowermost graphs of FIG. 8, the LEDs are driven with a first drive current level (denoted generically as I1) to produce continuous illumination. During the compensation period (shown most clearly in the bottom graph of FIG. 8), the LEDs are momentarily and periodically turned off to take forward voltage measurements (e.g., Vf1 , Vf2, Vf3, and Vf4) from each LED, one LED at a time. Whenever the plurality of LEDs are turned on during the compensation period, the drive currents supplied to the plurality of LEDs are increased or boosted to a second drive current level (denoted generically as I2). By increasing the drive currents supplied to the plurality of LEDs by a small amount when the LEDs are turned on during the compensation period, the compensation method described herein avoids flicker by compensating for the lack of illumination when the LEDs are periodically turned off during the compensation period.

When current dimming techniques are used to control the illumination device, as shown in FIG. 8, flicker may be avoided by increasing the drive current level by approximately 1-10% during the compensation period, such that I2 is substantially 1-10% greater than I1. When PWM dimming techniques are used to control the illumination device (not shown), the duty cycle of the drive current may also be increased by approximately 1-10% during the compensation period to avoid flicker.

Brightness banding and flicker may also occur whenever light emitted by the LEDs is modulated to communicate optical data in visible light communication (VLC) systems. One example of a VLC system is described U.S. Publication No. 2011/0069960, which is assigned to the present inventor and incorporated by reference herein. In this patent, LEDs are used for producing illumination, transmitting and receiving optical data, detecting ambient light and measuring output characteristics of other LEDs. Synchronized timing signals are supplied to the LEDs to produce time division multiplexed communication channels in which data is communicated optically by the same LEDs that produce illumination. In one embodiment, the timing signals are synchronized in frequency and phase to a common source, preferentially to the AC mains, so that the LEDs within the illumination devices can be periodically turned off in synchronization with the AC mains to produce a plurality of time slots in a first communication channel for communicating optical data. Additional communication channels may be generated when additional timing signals are synchronized to the same frequency, but different phase, used to produce the first communication channel. During these time slots, data may be communicated optically between illumination devices when one device produces light modulated with data, while the LEDs of other illumination devices are configured to detect and receive the optically communicated data. In addition to communicating optical data, the time slots may be used for other purposes. For example, one or more of the LEDs can be configured to measure ambient light or an output characteristic (e.g., forward voltage, luminous flux or chromacity chromaticity) from other LEDs in the illumination device during the time slots.

Brightness banding and flicker occur in VLC systems, such as the one described in U.S. Publication No. 2011/0069960, whenever the LEDs of an illumination device are periodically turned off to receive data, measure ambient light or measure output characteristics from other LEDs during the time slots. In some cases, brightness banding may be reduced by limiting VLC communications to short period(s) of time. For example, VLC may only occur periodically, only when initiated manually by a user or automatically by a controlling system, or only at designated times, such as during start-up when the illumination device is initially turned on. In other cases, brightness banding may be reduced by restricting the use of VLC to certain applications. For example, VLC may be limited to commissioning a set of illumination devices (e.g., establishing groupings of devices, setting addresses and output characteristics of the devices, etc.) included in a lighting system. A solution for avoiding the flicker phenomenon in a VLC system is illustrated in FIG. 9.

FIG. 9 is an exemplary timing diagram for communicating optical data between illumination devices without producing flicker. In particular, FIG. 9 illustrates a relationship between the AC mains timing (typically 50 or 60 Hz) and four different communication channels labeled Channel 0 through Channel 3. The communication channels are generated by driving a plurality of LEDs substantially continuously with drive currents configured to produce illumination, and periodically turning the plurality of LEDs off for short durations of time to produce gaps within the illumination, or time slots, within which data can be communicated optically or measurements can be taken. In this example, Channels 0 through 3 provide a plurality of communication gaps or time slots having different non-overlapping phases relative to the AC mains timing.

In this example, the four communication channels comprise alternating illumination and gap times. During illumination times, light from an illumination device may be on continually to produce a maximum brightness, or Pulse Width Modulated (PWM) to produce less brightness. During the periodic time slots, data can be sent from one device to any or all other devices, or measurements can be taken. In this example, the time slot duration is one quarter of the AC mains period, which enables four data bytes to be communicated at an instantaneous bit rate of 60 Hz×4×32, or 7.68K bits per second, with an average bit rate of 1.92K bits per second for each channel.

In order to avoid the flicker phenomenon, the drive currents supplied to the plurality of LEDs may be increased by a small amount (e.g., about 1-10%) when the LEDs are turned on for producing illumination, thereby compensating for the lack of illumination when the LEDs are periodically turned off to receive optical data or take measurements. This is illustrated in FIG. 9 by increasing the current level from I1 to I2 over the entire illumination period between each of the gap times. If PWM dimming is used (not shown) instead of current dimming, the duty cycle of the drive current supplied to each of the LEDs during the illumination periods may be increased by about 1-10% to avoid flicker.

Although the timing diagram of FIG. 9 appears to indicate that the same drive currents (I1 and I2) are supplied to the plurality of LEDs to produce illumination, one skilled in the art would understand that the plurality of LEDs may each be driven with a respective drive current deemed appropriate for that particular LED. For example, each LED may be driven with a drive current needed to produce a desired luminous flux from that LED, regardless of process and temperature variations. The individual drive currents needed to drive each LED may be determined and applied according to the method steps shown in FIG. 5, and may be increased by approximately 1-10% during the illumination periods shown in FIG. 9 to avoid flicker during times of VLC communications.

FIG. 10 is one example of a block diagram of an illumination device 40, which is configured to accurately maintain a desired luminous flux and/or a desired color point over variations in temperature and process. The illumination device illustrated in FIG. 10 provides one example of the hardware and/or software that may be used to implement the calibration and compensation methods shown respectively in FIGS. 1 and 5.

In the illustrated embodiment, illumination device 40 is connected to AC mains 42 and comprises an AC/DC converter 44, a DC/DC converter 46, a phase locked loop (PLL) 48, a wireless interface 50, a control circuit 52, a driver circuit 54 and a plurality of LEDs 56. The LEDs 56, in this example, 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 receive the same drive current. In the illustrated embodiment, the LEDs 56 may include a chain of red LEDs, a chain of green LEDs, a chain of blue LEDs, and a chain of yellow LEDs. The present invention is noted 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. However, the present invention may be particularly well suited when one or more different colors of LEDs are included within the illumination device 40.

In the illustrated embodiment, the AC/DC converter 44 converts AC mains power (e.g., 120V or 240V) to a DC voltage (labeled V_DC in FIG. 10), which is supplied to the driver circuit 54 for producing the respective drive currents for LEDs 56. The DC/DC converter 46 converts the DC voltage V_DC (e.g., 15V) to a lower voltage V_L (e.g., 3.3V), which may be used to power the low voltage circuitry included within the illumination device, such as PLL 48, wireless interface 50, and control circuit 52.

In the illustrated embodiment, the PLL 48 locks to the AC mains frequency (50 or 60 HZ) and produces a high speed clock (CLK) signal and a synchronization signal (SYNC). The CLK signal provides the timing for the control circuit 52 and the driver circuit 54. In one example, the CLK signal is in the tens of mHz 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 52 to create the timing used to obtain the forward voltage measurements. 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, the wireless interface 50 may be used to calibrate the illumination device 40 during manufacturing. For example, an external production calibration tool (not shown) could communicate luminous flux measurements and other information to a device under test via the wireless interface 50. The calibration values may then be stored within a storage medium of the control circuit 52, for example. However, the wireless interface 50 is not limited to receiving only calibration data, and may be used for communicating information and commands for many other purposes. For example, the wireless interface 50 could be used during normal operation to communicate commands used to control the illumination device 40 or to obtain information about the illumination device. For example, commands may be communicated to the illumination device 40 via the wireless interface 50 to turn the illumination device on/off, to control the dimming and/or color of the illumination device, to initiate forward voltage measurements, or to store measurement results in memory. In other examples, the wireless interface 50 may be used to obtain status information or fault condition codes associated with the illumination device 40.

In some embodiments, the wireless interface 50 could operate according to ZigBee, WiFi, Bluetooth, or any other proprietary or standard wireless data communication protocol. In other embodiments, the wireless interface 50 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 50 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 the PLL 48, the control circuit 52 calculates and produces values indicating the desired drive current to be used for each LED chain 56. This information may be communicated from the control circuit 52 to the driver circuit 54 over a serial bus conforming to a standard, such as SPI or I2C, for example. In addition, the control circuit 52 may provide a latching signal that instructs the driver circuit 54 to simultaneously change the drive currents supplied to each of the LED 56 to prevent brightness and color artifacts.

In one embodiment, the control circuit 52 may include a storage medium (e.g., non-volatile memory) for storing a table of calibration values correlating forward voltage and drive current to luminous flux at a plurality of different temperatures for each of the LEDs 56. The control circuit 52 may be configured for determining respective drive currents needed to achieve a desired luminous flux from each LED in accordance with the compensation method shown in FIG. 5 and described above. In some embodiments, the control circuit 52 may determine the respective drive currents by executing program instructions stored within the storage medium. Alternatively, the control circuit 52 may include combinatorial logic for determining the desired drive currents.

In general, the LED driver circuit 54 may include a number of driver blocks equal to the number of LED chains 56 included within the illumination device. In the exemplary embodiment discussed herein, LED driver circuit 54 comprises four driver blocks, each configured to produce illumination from a different one of the LEDs chains 56. The LED driver circuit 54 also comprises the circuitry needed to measure ambient temperature (optional) and forward voltage, and to adjust LED drive currents accordingly. Each driver block receives data indicating a desired drive current from the control circuit 52, along with a latching signal indicating when the driver block should change the drive current.

FIG. 11 is an exemplary block diagram of an LED driver circuit 54, according to one embodiment of the invention. As shown in FIG. 11, the driver circuit 54 includes four driver blocks, each block 58 including a buck converter 60, a current source 62, a difference amplifier 64, and an LC filter 66 for producing illumination and taking forward voltage measurements from a connected LED chain 56a. In addition, the LED driver circuit 54 includes an analog to digital converter (ADC) 68 for digitizing the output of the difference amplifiers 64 included within each driver block 58, and controller 70 for the control circuit 52 to use to adjust the current produced by the current source 62.

In some embodiments, the LED driver circuit 54 may include an optional temperature sensor 72 for taking ambient temperature (Ta) measurements, and a multiplexor (mux) 74 for multiplexing the ambient temperature (Ta) and forward voltage (Vf) measurements sent to the ADC 68. In some embodiments, the temperature sensor 72 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 a heat sink coupled to the LEDs. In other embodiments, the temperature sensor 72 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 chains 56.

In some embodiments, the buck converter 60 may produce a pulse width modulated (PWM) voltage output (Vdr) when the controller 70 drives the “Out_En” signal high. This voltage signal (Vdr) is filtered by the LC filter 66 to produce a forward voltage on the anode of the connected LED chain 56a. The cathode of the LED chain is connected to the current source 62, which forces a fixed drive current equal to the value provided by the “Current” signal through the LED chain 56a when the “Led_On” signal is high. The Vc signal from the current source 62 provides feedback to the buck converter 60 to output the proper duty cycle and minimize the voltage drop across the current source 62. The difference amplifier 64 produces a signal relative to ground that is equal to the forward voltage (Vf) drop across the LED chain 56a during forward voltage measurements. The ADC 68 digitizes the forward voltage (Vf) output from the difference amplifier 64 and provides the result to the controller 70. The controller 70 determines when to take forward voltage measurements and produces the Out_En, Current, and Led_On signals.

In some embodiments, such as those shown in FIGS. 8 and 9 and the optional method steps of FIG. 5, the forward voltage (Vf) output from the difference amplifier 64 may be multiplexed with the ambient temperature (Ta) output from the temperature sensor 72 and connected to the ADC 68. In these embodiments, the ADC 68 digitizes the temperature sensor and difference amplifier outputs and provides the results to the controller 70. The controller 70 determines when to take the temperature and forward voltage measurements and produces the Out_En, Current, and Led_On signals.

FIG. 12 is an exemplary block diagram of the current source 62 shown in FIG. 11, according to one embodiment of the invention. As shown in FIG. 12, the current source 62 includes a current digital to analog converter (DAC) 72 that is connected to ground through an N-channel Field Effect Transistor (NFET) 74, an error amplifier 76, and stack of two NFETs 78 and 80, which are connected to the cathode of the LED chain 56a. The DAC is coupled for receiving the “Current” signal from the controller 70 and for producing a reference current (Iref). The NFET 74 coupled to the DAC operates as a resistor to generate a reference voltage (Vref) for the error amplifier 76. The gate of the top NFET 78 provides the Vc signal, which is input to the buck converter 60 to adjust the voltage on the LED chain anode to a minimum needed by the current source 62.

In the current source 62 of FIG. 12, the error amplifier 76 adjusts the gate of the top NFET 78 until the drain voltage of the bottom NFET 80 is the same as the reference voltage (Vref). The impedance of the bottom NFET 80 is precisely 1/1000th that of NFET 74 when the “Led_On” signal is high. This forces the drive current (Idr) through the LED chain 56a to be precisely 1000 times the value of the reference current (Iref) generated by the current DAC 72. The value of the drive current is adjusted through the “Current” signal provided by the controller 70. In some embodiments, the reference current (Iref) generated by the DAC 72 may range from about 0.1 uA to about 1 mA, so that the LED drive current (Idr) can range between about 1 mA to about 1 A. As indicated above, a 0.1 mA-10 mA drive current setting is preferably used during forward voltage measurements, while substantially greater drive current settings (e.g., about 20 mA to about 500 mA) are used to produce illumination from the LED chain 56a.

FIG. 13 is an exemplary block diagram of a buck converter 60, according to one embodiment of the invention. As shown in FIG. 13, the buck converter 60 may include a reference voltage 82, a comparator 84, an up/down counter 86, a pulse width modulator (PWM) 88, and a driver 90. The comparator 84 compares the Vc signal from the current source 62 to the reference voltage 82 and produces an output, which causes the up/down counter 86 to increment or decrement whenever the Vc signal is lower or higher, respectively, than the fixed reference voltage 82. The up/down counter 86 operates as an integrator in the loop that adjusts the filtered buck converter output (LED chain anode) to a level that causes the Vc signal voltage to equal the reference voltage 82. The pulse width modulator 88 produces a PWM clock signal that has a duty cycle equal to the value in the up/down counter 86. When the “Out_En” signal is supplied to the driver 90 (FIG. 13) and the “Led_On” signal supplied to the current source 62 (FIG. 12) are both high, the driver 90 applies the PWM clock signal to the LC filter 66, which converts the PWM clock signal to a relatively constant voltage proportional to the duty cycle of the clock.

When the “Out_En” signal is low, the driver 90 is tri-stated. If the “Led_On” signal supplied to the current source 62 (see, FIG. 12) is high while “Out_En” is low, the LED drive current will cause the capacitor within the LC filter 66 to discharge. If the “Led_On” signal is low while “Out_En” signal is high, the buck converter 60 will charge the capacitor within the LC filter 66.

During forward voltage measurement times, the buck converters 60 and the current sources 62 connected to all LED chains that are not being measured, should be turned off at the same time by simultaneously applying low “Led_On” and “Out_En” signals to these LEDs. Since no current will be flowing through these LEDs, the LC capacitor voltage should not change during the forward voltage measurement times. Since no time is needed for the buck converter to settle, there should be no LED current transients to produce visible artifacts.

During forward voltage measurement times, the buck converter 60 connected to the LED chain under test should also be turned off to prevent the switching noise of the buck converter from interfering with the forward voltage measurement. While the current source 62 connected to this LED chain should remain on, the drive current (Idr) should be switched from the operating current level (e.g., about 20 mA to about 500 mA) to the relatively small drive current level used to take forward voltage measurements (e.g., about 0.1 mA-10 mA). Because this small drive current level will naturally cause the voltage on the LC capacitor to droop, the buck converter 60 should remain on for one or more PWM cycles after the LED current is switched to the relatively small drive current, but before the forward voltage measurements are taken. This enables the LC capacitor voltage to charge by a small amount to compensate for the voltage droop during the forward voltage measurement.

One implementation of an improved illumination device 40 has now been described in reference to FIGS. 10-13. 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/or a desired color point over variations in temperature and process. 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.

Claims

1. An illumination device, comprising:

a plurality of light emitting diode (led) chains;

a driver circuit configured for driving the plurality of led chains with drive currents substantially continuously to produce illumination, periodically turning the plurality of led chains off during a first period to take measurements or communicate optical data, and supplying a non-operative drive current to each led chain, one chain at a time, during short durations of time to measure an operating forward voltage developed across each led chain; and

a control circuit configured for determining respective drive currents needed to achieve a desired luminous flux from each led chain using the operating forward voltages measured across each led chain, a table of calibration values and one or more interpolation techniques, wherein during the first period, the control circuit instructs the driver circuit to increase the respective drive currents supplied to the plurality of led chains by a small amount when the plurality of led chains are on to compensate for a decrease in or lack of illumination when the led chains are periodically turned off during the first period.

2. The illumination device as recited in claim 1, wherein each led chain is configured for producing illumination at a different peak wavelength.

3. The illumination device as recited in claim 1, wherein the small amount comprises approximately 1% to approximately 10% of the drive currents supplied to the plurality of led chains to produce illumination substantially continuously.

4. The illumination device as recited in claim 1, 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 driver circuit for periodically turning the plurality of led chains off for the first period.

5. The illumination device as recited in claim 1, further comprising a storage medium configured for storing the table of calibration values, and wherein the table of calibration values correlate forward voltage and drive current to luminous flux at a plurality of temperatures for each of the plurality of led chains.

6. The illumination device as recited in claim 5, wherein for each led chain, the table of calibration values comprises:

a first forward voltage value measured across the led chain using a non-operative drive current when the led chain was previously subjected to a first temperature;

a second forward voltage value measured across the led chain using the non-operative drive current when the led chain was previously subjected to a second temperature;

a first plurality of luminous flux values detected from the led chain using a plurality of different drive currents when the led chain was previously subjected to the first temperature; and

a second plurality of luminous flux values detected from the led chain using the plurality of different drive currents when the led chain was previously subjected to the second temperature.

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

8. The illumination device as recited in claim 6, wherein the control circuit is further configured to:

calculate a third plurality of luminous flux values corresponding to an operating forward voltage measured across a given led chain by interpolating between the first plurality of luminous flux values and the second plurality of luminous flux values associated with the given led chain;

generate a relationship between the third plurality of luminous flux values, if the desired luminous flux differs from one of the third plurality of luminous flux values; and

determine a drive current needed to achieve a desired luminous flux from the given led chain by selecting, from the generated relationship, a drive current corresponding to the desired luminous flux.

9. The illumination device as recited in claim 8, wherein the control circuit is configured to calculate the third plurality of luminous flux values by 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 made based on a color of the given led chain.

10. The illumination device as recited in claim 8, 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 the given led chain.

11. The illumination device as recited in claim 8, 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 the given led chain.

12. The illumination device as recited in claim 8, wherein the control circuit is configured to generate the relationship by assuming a typical curvature from data sheets provided by a manufacturer of the given led chain.

13. The illumination device as recited in claim 1, further comprising a phase locked loop (PLL), which is coupled to an AC mains and configured to lock onto a frequency and phase of the AC mains.

14. The illumination device as recited in claim 13, wherein the control circuit is coupled to the PLL and configured to produce a timing signal in synchronization with the AC mains, wherein the timing signal is supplied to the driver circuit for periodically turning the led chains off in synchronization with the AC mains to produce a plurality of time slots in a first communication channel for communicating optical data.

15. A method for controlling an illumination device comprising a plurality of light emitting diodes (leds), the method comprising:

driving the plurality of leds substantially continuously with drive currents configured to produce illumination;

periodically turning the plurality of leds off during a first period to take measurements or communicate optical data;

supplying a non-operative drive current to each led, one led at a time, during the first period, in which the leds are periodically turned off, to measure an operating forward voltage developed across each led;

determining respective drive currents needed to achieve a desired luminous flux from each led using the operating forward voltages measured across each led, a table of calibration values and one or more interpolation techniques; and

increasing the respective drive currents supplied to the plurality of leds by a small amount when the leds are on during the first period to compensate for a decrease in or lack of illumination when the leds are periodically turned off during the first period.

16. The method as recited in claim 15, wherein the small amount comprises approximately 1% to approximately 10% of the drive currents configured to produce illumination.

17. The method as recited in claim 15, wherein the step of periodically turning the plurality of leds off for the short durations of time comprises periodically turning the plurality of leds off in synchronization with an AC mains frequency to generate a plurality of time slots in the first period.

18. The method as recited in claim 17, further comprising measuring an output characteristic of each led, one led at a time, during the time slots.

19. The method as recited in claim 18, wherein the output characteristic is selected from a group consisting of forward voltage, luminous flux or chromacity chromaticity.

20. The method as recited in claim 17, further comprising configuring at least one of the plurality of leds for communicating optical data during the time slots.

21. The method as recited in claim 17, further comprising configuring at least one of the plurality of leds for measuring ambient light during the time slots.

22. A device for controlling a plurality of light emitting diodes (leds) chains, the device comprising:

a driver circuit configured to drive the plurality of led chains with respective drive currents to produce illumination; and

a control circuit configured to:

periodically turn each of the plurality of led chains off during a compensation period;

measure a respective operating forward voltage across each led chain by supplying a non-operative drive current to each led chain, one chain at a time, for short durations of time during the compensation period; and

determine a respective drive current needed to achieve a desired luminous flux from each led chain using the operating forward voltage measured across each led chain, a table of calibration values, and one or more interpolation techniques.

23. The device as recited in claim 22, wherein each led chain is configured for producing illumination at a different peak wavelength.

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

25. The device as recited in claim 22, further comprising:

a storage medium configured to store the table of calibration values;

wherein the table of calibration values correlate forward voltage and drive current to luminous flux at a plurality of temperatures for each of the plurality of led chains.

26. The device as recited in claim 25, wherein for each led chain, the table of calibration values comprises:

a first forward voltage value measured across the led chain using a non-operative drive current when the led chain was previously subjected to a first temperature;

a second forward voltage value measured across the led chain using the non-operative drive current when the led chain was previously subjected to a second temperature;

a first plurality of luminous flux values detected from the led chain using a plurality of different drive currents when the led chain was previously subjected to the first temperature; and

a second plurality of luminous flux values detected from the led chain using the plurality of different drive currents when the led chain was previously subjected to the second temperature.

27. The device as recited in claim 26, wherein the control circuit is further configured to:

calculate a third plurality of luminous flux values corresponding to an operating forward voltage measured across a given led chain by interpolating between the first plurality of luminous flux values and the second plurality of luminous flux values associated with the given led chain;

generate a relationship between the third plurality of luminous flux values; and

determine a drive current needed to achieve a desired luminous flux from the given led chain by selecting, from the generated relationship, a drive current corresponding to the desired luminous flux for the given led chain.

28. The device as recited in claim 27, wherein the control circuit is configured to calculate the third plurality of luminous flux values by using a linear interpolation technique or a non-linear interpolation technique to interpolate between the first and second plurality of luminous flux values, the control circuit configured to select between the linear interpolation technique and the non-linear interpolation technique based on a color of the given led chain.

29. The device as recited in claim 26, wherein for each led chain, the table of calibration values further comprises:

a third forward voltage value measured across the led chain using the non-operative drive current when the led chain was previously subjected to a third temperature; and

a third plurality of luminous flux values detected from the led chain using the plurality of different drive currents when the led chain was previously subjected to the third temperature; and

wherein to determine the respective drive current needed to achieve the desired luminous flux from each led chain comprises using the table of calibration values measured at the first, second, and third temperatures.