# Patent

## Title
Light emitting diode based lighting system with time division ambient light feedback response

## Abstract

A lighting system includes time division light output sensing and adjustment for ambient light. In at least one embodiment, time division light output sensing involves modulating power to a light emitting diode (LED) set, and the set of LEDs includes one or more LEDs. In at least one embodiment, each LED in the LED set is included in a single lamp, and, in at least one embodiment, the set of LEDs is contained in multiple lamps. In at least one embodiment, for each lamp, a controller modulates power to the LED set by selectively reducing power to the LED set using time division algorithm to allow a light sensor to sense the brightness of ambient light with a reduced contribution from the LED set. In at least one embodiment, a lighting system also includes time division light output sensing and adjustment for different spectra LEDs.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
This application claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional Application No. 61/122,198, filed Dec. 12, 2008, and entitled “Single Photo-Detector for Color Balance of Multiple LED Sources”. U.S. Provisional Application No. 61/122,198 includes exemplary systems and methods and is incorporated by reference in its entirety.


BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates in general to the field of lighting and signal processing, and more specifically to a system and method of time division light output sensing and adjustment for different spectra light emitting diodes.
2. Description of the Related Art
Light emitting diodes (LEDs) are becoming particularly attractive as main stream light sources in part because of energy savings through high efficiency light output and environmental incentives, such as the reduction of mercury. LEDs are a type of semiconductor devices and are driven by direct current. The brightness (i.e. luminous intensity) of the LED approximately varies in direct proportion to the current flowing through the LED. Thus, increasing current supplied to an LED increases the intensity of the LED and decreasing current supplied to the LED dims the LED. Current can be modified by either directly reducing the direct current level to the LEDs or by reducing the average current through duty cycle modulation.
FIG. 1 depicts lighting system 100, which includes lamp 102. Lamp 102 includes a light source 104 to generate artificial light 106. Light 106 reflects off the interior surface of the housing of lamp 102 and propagates through diffuser 108 to generate light 110. Lighting system 100 also includes an ambient light sensor 112 to facilitate light harvesting. Light harvesting involves supplementing artificial light 110 with natural light 114 and correlating adjustments in the artificial light with variations in the natural light. The physical location of ambient light sensor 112 is a matter of design choice. In at least one embodiment, ambient light sensor 112 is physically attached to the exterior of lamp 102. Location of ambient light sensor 112 on the exterior of the housing of lamp 102 assists in minimizing the contribution of artificial light 106 to the natural light 114 received by ambient light sensor 112. In at least one embodiment, ambient light sensor 112 is remotely located and communicates with controller 124 using any available communication technology.
Lamp 102 receives an alternating current (AC) voltage VAC—SUPPLY from supply voltage source 116 terminals 118 and 120. The voltage source 116 is, for example, a public utility, and the AC supply voltage VAC—SUPPLY is, for example, a 60 Hz/110 V line voltage in the United States of America or a 50 Hz/220 V line voltage in Europe.
Lighting system 100 includes a power control system 122 that includes controller 124 to control power provided to light source 104 and, thus, control the brightness of artificial light 110 generated by light source 104. Controller 124 generates control signal CS0 and provides control signal CS0 to lamp driver 126 to control power delivered by lamp driver 126 to light source 104. The particular configuration of lamp driver 126 is a matter of design choice and, in part, depends upon the configuration of light source 104. Light source 104 can be any type of light source, such as an incandescent, fluorescent, or LED based source. Lamp driver 126 provides power to light source 104 in accordance with control signal CS0.
Ambient light sensor 112 generates sense signal SEN1. Sense signal SEN1 indicates the brightness of ambient light. Controller 124 causes lamp driver 126 to increase or decrease the brightness of artificial light 110 if the ambient light is respectively too low or too high.
Light harvesting by lighting system 100 does not accurately account for the brightness of light 110 because the ambient light sensed by ambient light sensor 112 includes a contribution by artificial light 110.
SUMMARY OF THE INVENTION
In one embodiment of the present invention, an apparatus includes a controller configured to couple to a sensor and a first light emitting diode (LED) of a lamp. The controller is further configured to reduce power to a first light emitting diode (LED) coupled to the controller and receive a signal from a sensor indicating a brightness of light received by the sensor while the power to the first LED is reduced. The controller is also configured to adjust the brightness of the first LED in accordance with a brightness related target value.
In another embodiment of the present invention, an apparatus includes a lamp having at least a first light emitting diode (LED). The apparatus also includes a sensor to sense brightness of received light. The apparatus further includes a controller coupled to the lamp and the sensor. The controller is configured to reduce power to the first LED and receive a signal from the sensor indicating the brightness of light received by the sensor while the power to the first LED is reduced. The controller is further configured to adjust the brightness of the first LED in accordance with a brightness related target value.
In a further embodiment of the present invention, a method for light harvesting includes reducing power to a first light emitting diode (LED). The method further includes receiving a signal indicating a brightness of light while the power to the first LED is reduced and adjusting the brightness of the first LED in accordance with a brightness related target value.
In another embodiment of the present invention, an apparatus for light harvesting includes means for reduce power to a first light emitting diode (LED). The apparatus further includes means for receiving a signal indicating a brightness of light while the power to the first LED is reduced and means for adjusting the brightness of the first LED in accordance with a brightness related target value.



BRIEF DESCRIPTION OF THE DRAWINGS
The present invention may be better understood, and its numerous objects, features and advantages made apparent to those skilled in the art by referencing the accompanying drawings. The use of the same reference number throughout the several figures designates a like or similar element.
FIG. 1 (labeled prior art) depicts a lighting system for light harvesting.
FIG. 2 depicts a lighting system with time division light output sensing and adjustment for light harvesting.
FIG. 3 depicts a time division algorithm for light harvesting.
FIG. 4 depicts a light sensing/drive current modulation timing diagram.
FIGS. 5, 6, and 7 depict respective lighting systems with multiple lamps and synchronization schemes.
FIG. 8 depicts a lighting system with time division light output sensing and brightness adjustment for different spectrum light emitting diodes.
FIG. 9 depicts an embodiment of the lighting system of FIG. 8.
FIG. 10 depicts a time division and adjustment algorithm for sensing and adjusting the brightness of light in the lighting system of FIG. 9.
FIG. 11 depicts an LED drive current signal timing diagram which illustrates an interspacing time division for the algorithm of FIG. 10.
FIG. 12 depicts an LED drive current signal timing diagram which illustrates an interspersed time division for the algorithm of FIG. 10.
FIG. 13 depicts an LED drive current signal timing diagram which illustrates a unitary time division for the algorithm of FIG. 10.
FIG. 14 depicts another embodiment of a time division and adjustment algorithm for the lighting system of FIG. 9.
FIG. 15 depicts an embodiment of a controller of the lighting system of FIG. 8.



DETAILED DESCRIPTION
In at least one embodiment, a lighting system includes time division light output sensing and adjustment for ambient light. In at least one embodiment, time division light output sensing involves modulating power to a set of LEDs, and the set of LEDs includes one or more LEDs. In at least one embodiment, each LED in the LED set is included in a single lamp, and, in at least one embodiment, the set of LEDs are contained in multiple lamps. In at least one embodiment, for each lamp, a controller modulates power to the LED set by selectively reducing current to the LED set using time division algorithm to allow a light sensor to sense the brightness of ambient light with a reduced contribution from the LED set. In at least one embodiment, power can be reduced to zero by reducing the current to zero, thus, turning each LED in the lamp “off” or can be reduced to a value greater than zero. In at least one embodiment, the controller compares the determined brightness of the ambient light against a target value for brightness and adjusts the brightness of the light emitted by the LED set to meet the target values.
In at least one embodiment, power modulation to multiple lamps is synchronized so that light from multiple lamps using one or more LEDs as a light source (referred to herein as “LED lamps”) is reduced to allow one or more light sensors to sense ambient light with reduced (including eliminated) contribution by the LED lamps. The human eye generally takes 1-10 milliseconds (ms) to sense changes in light brightness. Light sensors can sense changes in light brightness in less than 1 ms. Thus, in at least one embodiment, current is reduced to the LEDs for 1 ms or less to allow the light sensor to sense ambient light without noticeable effect to the human eye.
In at least one embodiment, for lighting systems with multiple LEDs, in addition to sensing ambient light, a lighting system includes time division light output sensing and brightness adjustment for different spectra light emitting diodes (LEDs). In at least one embodiment, brightness of light emitted from multiple LEDs is adjusted by modifying power to subgroups of the multiple LEDs during different times and detecting the brightness of the LEDs during the reductions of power. In at least one embodiment, once the brightness of the LEDs are determined, a controller determines if the brightness meet target brightness values, and, if not, the controller adjusts each LED with the goal meet the target brightness values. In at least one embodiment, a process of modifying power to the subgroups of multiple LEDs over time and adjusting the brightness of the LEDs is referred as “time division and light output sensing and adjusting. Thus, in at least one embodiment, a lighting system includes time division light output sensing and adjustment for different spectrum light emitting diodes (LEDs).
In at least one embodiment, an LED set is a set of one or more LEDs whose brightness is collectively adjusted. For example, a first LED set could include four red LEDs, and a second LED set could include three blue LEDs. The brightness of each LED set can be collectively determined and adjusted. In at least one embodiment, time division light output sensing involves modulating power over time, e.g. changing current over time, to multiple LEDs to different subgroups of the LEDs. The number of LEDs in each subgroup is a matter of design choice and can be a single LED.
In at least one embodiment, a controller performs time division power modulation of the LEDs by modulating power to the LEDs by selectively reducing power for a limited duration of time to a subgroup of one or more LEDs having a spectrum of interest and repeating power reductions for each LED set having spectrums of interest using a time division algorithm. The time division power modulation allows the controller to determine a relative contribution to the brightness of the light received by one or more sensors for each LED set. In at least one embodiment, a controller correlates the different brightness of received light sensed during different times in accordance with the time division power modulation of the LEDs to determine the brightness of individual sets of LEDs. In at least one embodiment, a controller compares the determined brightness of individual sets of LEDs against target values and adjusts the brightness of the light emitted by the LEDs to meet the target values.
In at least one embodiment, the spectrum of light emitted by the LEDs is a matter of design choice. In at least one embodiment, the LEDs represent at least two different spectra. In at least one embodiment, the one or more sensors are photosensitive transistors and are calibrated to compensate for one or more variations in operating characteristics due to factors such as increasing operating temperatures.
FIG. 2 depicts a lighting system 200 that, for example, facilitates light harvesting by adjusting power provided to LED set 202 to complement the brightness of sensed ambient light 204. LED set 202 includes one or more LEDs. The particular type and light spectrum of each LED in LED set 202 is a matter of design choice. The “spectrum” of an LED refers to the wavelength or wavelengths of light emitted by the LED. Wavelengths of light determine the color of the light. Thus, the spectrum of an LED refers to the color of light emitted by the LED. For example, in one embodiment, a blue-green spectrum LED 106 emits blue-green light, and a red spectrum LED 108 emits red light. For example, the light spectrum of LED set 202 can be red, green, blue, amber, white, and so on including any combination of the foregoing colors. Lamp 206 of lighting system 200 includes a power control system 208. Power control system 208 includes controller 210 and LED driver 212 to control LED drive current iLED—1. The brightness of LED set 202 is directly proportional to the LED drive current iLED—1. Controller 210 includes a time division module 214 to facilitate time division power modulation to sense ambient light 204 with reduced contribution by LED set 202. In at least one embodiment, time division power modulation refers to changing power provided to one or more LEDs for a period of time. Lamp 206 receives power from power source 216. Power source 216 can be any power source, such as the AC voltage supply 110 (FIG. 1).
Light sensor 218 senses the brightness of light reaching light sensor 218 and generates a sense signal SENAL. Sense signal SENAL represents the brightness of light sensed by light sensor 218. Light sensor 218 provides the sense signal SENAL to controller 210. As subsequently described in more detail, in at least one embodiment, controller 210 utilizes sense signal SENAL to adjust the brightness of LED set 202. Light sensor 218 can be any type of light sensor. In at least one embodiment, light sensor 218 is a phototransistor or photodiode based light sensor that can sense the brightness of light received by the light sensor 218 in 1 ms or less. Lamp user interface 220 is an optional component of lighting system 200 that provides a target parameter to controller 210 to set the value of target data for adjusting the brightness of LED set 202. In at least one embodiment, lamp user interface 220 is an integrated part of lamp 206. The physical disposition of power control system 208, LED set 202, and light source 218 is a matter of design choice. In at least one embodiment, lamp 206 encloses power control system 208, LED set 202, and light sensor 218. In at least one embodiment, lamp 206 includes a diffuser at the base lamp 206 to soften and mix light provided by LED set 202. Light sensor 218 can be any type of light sensor that can produce sense signal SENAL. In at least one embodiment, light sensor 218 is a photo-diode or photo-transistor based light sensor.
FIG. 3 depicts a time division algorithm 300 for light harvesting that represents one embodiment of an algorithm to allow controller 210 (FIG. 2) to perform time division power modulation to sense ambient light. The particular implementation of controller 210 and time division algorithm 300 is a matter of design choice. Controller 210 can be implemented using digital, analog, or digital and analog technology. In at least one embodiment, controller 210 is fabricated as an integrated circuit. In at least one embodiment, controller 210 includes a processor and time division algorithm 300 is implemented in code and executed by the processor. The code can be stored in a memory (not shown) included in controller 210 or accessible to controller 210.
FIG. 4 depicts a light sensing/drive current modulation timing diagram 400 that represents one embodiment of a power modulation and ambient light sensing timing diagram utilized by time division module 214 of controller 210.
Referring to FIGS. 2, 3, and 4, in at least one embodiment, for more advanced light harvesting controller 210 utilizes time division algorithm 300 in accordance with light sensing/drive current modulation timing diagram 400 to adjust the brightness of LED set 202 to complement ambient light 204. In operation 301, at time t0, the lamp 206 is turned “on” by applying power from power source 216 to lamp 206. When the lamp is initially turned “on” at time t0, between times t0 and t3 controller 210 sets the LED drive current iLED—1 to a predetermined value. In at least one embodiment, the predetermined value is zero as shown in the LED drive current timeline 402. The predetermined value of the LED drive current iLED—1 can be any value that corresponds with a known brightness of LED set 202.
In operation 302, between times t1 and t2, light sensor 218 senses ambient light 204 received by light sensor 218 and generates sense signal SENAL as indicated in the ambient light sensing timeline 404. Light sensor 218 provides sense signal SENAL to controller 206. After receiving sense signal SENAL, in operation 304, controller 210 compares the value of ambient light represented by sense signal SENAL to target data. The target data is set so that a comparison of the target data with the ambient light value indicates whether controller 210 should adjust the LED drive current iLED—1 by increasing or decreasing the LED drive current iLED—1. In at least one embodiment, if LED drive current iLED—1 is zero between times t0 and t3, the target data represents a desired brightness of artificial light generated by LED set 202 plus natural light. If the LED drive current iLED—1 is not set to zero between times t0 and t3, LED set 202 contributes to the brightness of light received by light sensor 218 and, thus, the contribution is represented in sense signal SENAL. Thus, if the LED drive current iLED—1 is not set to zero, controller 210 compensates for the contribution of LED set 202 to the brightness of the ambient light 204 represented by sense signal SENAL in order to determine any adjustment for LED drive current iLED—1. In at least one embodiment, to compensate for the contribution of LED set 202, the target data compared to the sense signal SEN1 includes a known contribution of the brightness of LED set 202. Thus, comparing the target data to sense signal SEN1 negates the contribution of LED set 202 to the ambient light represented by sense signal SEN1.
The brightness level of natural light plus artificial light represented in the target data can be stored in a memory (not shown) accessible to controller 210 or can be communicated to controller 210. In at least one embodiment, lamp user interface 220 is a manual entry device that allows a user to set target data representing a desired brightness level of natural light plus artificial light. For example, in at least one embodiment, lamp user interface 220 is a digital device that allows the user to enter a setting corresponding to the desired brightness level. In at least one embodiment, lamp user interface 220 is a potentiometer having a resistance that indicates a desired brightness level. In at least one embodiment, the brightness level is received via a separate wired or wireless connection from a remote input device (not shown), such as a dimmer or a remotely located lamp user interface 220. In at least one embodiment, the connection is dedicated for communication with lamp 206. In at least one embodiment, a dimmer signal is received via terminals 222 and 224, and controller 210 interrupts a dimming level indicated by the dimmer signal as a brightness level for the target data. The dimmer signal can be any type of dimmer signal, such as a phase modified signal from a conventional triac based dimmer.
In operation 306, controller 210 adjusts LED current iLED—1 to adjust the brightness of LED set 202 to achieve a target brightness of artificial light plus natural light. In at least one embodiment, if sense signal SEN1 indicates that the ambient light brightness has increased, controller 210 decreases the value of LED current iLED—1 so that the brightness of LED set 202 is decreased by the same amount of the increase in ambient light brightness. Likewise, if sense signal SEN1 indicates that the ambient light brightness has decreased, controller 210 increases the value of LED current iLED—1 so that the brightness of LED set 202 is increased by the same amount of the decrease in ambient light brightness. Operation 306 can include an integration function so that adjustments to LED drive current iLED—1 are integrated over time to reduce perceptions of changes in brightness of LED set 202 by a human eye.
Operation 308 stops time division algorithm 300 until a time to perform operation 310 and repeat operations 302-308 is reached. The frequency of repeating time division algorithm 300 is a matter of design choice. In at least one embodiment, time division algorithm 300 is repeated every second. In at least one embodiment, time division algorithm 300 is repeated often enough to sense changes in the ambient light and changes in the brightness of LED set 202 so that the adjustments to the brightness of light emitted from LED set 202 is virtually imperceptible to a human eye. In at least one embodiment, time division algorithm 300 is repeated in multiples of 8.3 ms or 10 ms, which represent respective periods of rectified 60 Hz and 50 Hz public utility supply voltages.
In operation 310, at time t4 of FIG. 4, time division module 214 causes controller 210 to reduce LED drive current iLED—1 to zero (as shown in the LED drive current timeline 402 or to a value greater than zero as previously described). At time t5 and t6, operation 302 repeats so that light sensor 218 senses the brightness of light received by light sensor 218 between times t5 and t6. Between times t6 and t7, controller 210 repeats operations 304 and 306. LED drive current line 406 indicates an increase in LED drive current iLED—1 when the ambient light detected by light sensor 218 decreases between times t2 and t5. LED drive current line 408 indicates a decrease in LED drive current iLED—1 when the ambient light detected by light sensor 218 increases between times t2 and t5.
Controller 210 generates control signal CS1 in any of a variety of ways. U.S. patent application Ser. No. 11/864,366, entitled “Time-Based Control of a System having Integration Response,” inventor John L. Melanson, and filed on Sep. 28, 2007 describes an exemplary system and method for generating a control signal which can be used for driving current for an LED. U.S. patent application Ser. No. 11/864,366 is referred to herein as “Melanson II” and is incorporated by reference in its entirety. U.S. patent application Ser. No. 12/415,830, entitled “Primary-Side Based Control Of Secondary-Side Current For An Isolation Transformer,” inventor John L. Melanson, and filed on Mar. 31, 2009 also describes an exemplary system and method for generating a control signal which can be used for driving current for an LED. U.S. patent application Ser. No. 12/415,830 is referred to herein as “Melanson III” and is incorporated by reference in its entirety. In at least one embodiment, controller 210 is implemented and generates control signal CS1 in the same manner as the generation of a control signal described in Melanson II or Melanson III with the exception of the operation of time division module 214 as subsequently described. In at least one embodiment, controller 210 controls the LED drive current iLED1 using linear current control.
FIG. 5 depicts a lighting system 500 having N+1 number of lamps 502.0-502.N, and “N” is an integer greater than or equal to 1. In at least one embodiment, each of lamps 502.0-502.N are identical to lamp 206 with an exception for lamp 502.0 as subsequently discussed. Lamps 502.0-502.N perform time division algorithm 300. To sense the brightness of ambient light without an unknown contribution from one or more of lamps 502.0-502.N, lamp 502.0 includes a synchronization module 504 to synchronize modulation of LED power using time division and sensing ambient light in respective operations 310 and 302. In at least one embodiment, lamp 502.0 sends a synchronization signal via signal path 506 to lamps 502.1-502.N so that lamps 502.0-502.N utilize output remapping-delta-sigma modulator module 400 with synchronized timing. By using synchronized timing, lamps 502.0-502.N perform time division algorithm 300 at the same time, and none of lamps 502.0-502.N contribute an unknown brightness of light to sensed ambient light.
FIG. 6 depicts a lighting system 600 having N+1 number of lamps 602.0-602.N, and “N” is an integer greater than or equal to 1. In at least one embodiment, each of lamps 602.0-602.N is identical to lamp 206 with an exception for lamp 602.0 as subsequently discussed. Lamps 602.0-602.N perform time division algorithm 300. To sense the brightness of ambient light without an unknown contribution from one or more of lamps 602.0-602.N, lamp 602.0 includes a synchronization module 604 to synchronize modulation of LED power using time division and sensing ambient light in respective operations 310 and 302. In at least one embodiment, lamp 602.0 sends a synchronization signal via a wireless signal to lamps 602.1-602.N so that lamps 602.0-602.N utilize output remapping-delta-sigma modulator module 400 with synchronized timing. In at least one embodiment, lamp 602.0 sends a synchronization signal by modulating the brightness of light emitted from lamp 602.0, and lamps 602.1-602.N are configured to sense the brightness modulation and utilize the brightness modulation as a synchronization signal so that lamps 602.0-602.N utilize output remapping-delta-sigma modulator module 400 with synchronized timing. By using synchronized timing, lamps 602.0-602.N perform time division algorithm 300 at the same time, and none of lamps 602.0-602.N contribute an unknown brightness of light to sensed ambient light.
FIG. 7 depicts a lighting system 700 having N+1 number of lamps 702.0-702.N, and “N” is an integer greater than or equal to 1. In at least one embodiment, each of lamps 702.0-702.N is identical to lamp 206. Lamps 702.0-702.N perform time division algorithm 300. To sense the brightness of ambient light without an unknown contribution from one or more of lamps 702.0-702.N, controller 704 sends a synchronization signal to lamps 702.0-702.N via signal path 706 to synchronize modulation of LED power using time division and sensing ambient light in respective operations 310 and 302. In at least one embodiment, lamps 702.0-702.N utilize output remapping-delta-sigma modulator module 400 with synchronized timing. By using synchronized timing, lamps 702.0-702.N perform time division algorithm 300 at the same time, and none of lamps 702.0-702.N contribute an unknown brightness of light to sensed ambient light. In at least one embodiment, signal path 706 is the supply voltage signal path, and controller 704 uses a protocol, such as the X10 protocol, to send the synchronization signal to lamps 702.0-702.N. In at least one embodiment, controller 704 can also send the synchronization signal via a wireless signal. Synchronization of multiple lamps, such as multiple copies of lamp 206 can be accomplished in many other ways as well. The particular synchronization process is a matter of design choice.
FIG. 8 depicts lighting system 800 which includes lamp 802. Lamp 802 includes N+1 LEDs 808.0-808.N driven by separate, respective LED drivers 810.0-810.N. “N” is an integer greater than or equal to one. Lamp 802 includes controller 806 with time division module 812 to generate control signals CS10-CS1N to control respective LED drivers 810.0-810.N and, thus, control respective LED drive currents iLED—0-iLED—N. In at least one embodiment, controller 806 and time division module 812 of power control system 804 are identical to respective controller 210 and time division module 214 except controller 806 generates a separate control signal for each of LED drivers 810.0-810.N. Each of LED drivers 810.0-810.N provides respective LED drive currents iLED—0-iLED—N to respective LED sets 808.0-808.N. LED sets 808.0-808.N can be any combination of LED color to generate a virtually limitless variety of spectra. Each of LED sets 808.0-808.N includes one or more LEDs. In at least one embodiment, controller 806 utilizes time division algorithm and output remapping-delta-sigma modulator module 400 for light harvesting.
In addition to light harvesting, in at least one embodiment, lighting system 800 includes time division light output sensing and adjustment for different spectra light emitting diodes. Lighting system 800 includes a power control system 802 that, in at least one embodiment, receives power from power source 216.
In at least one embodiment, each LED in an LED set 808 has approximately the same light spectrum. The particular spectrum is a matter of design choice and includes red, blue, amber, green, blue-green, and white.
Lighting system 800 includes a light sensor 814 to sense the brightness of light received by light sensor 814. In at least one embodiment, light sensor 814 is a single, broad spectrum light sensor that senses all the spectra of light emitted by LED sets 808.0-808.N. The physical location of light sensor 814 is a matter of design choice.
Controller 806 includes time division module 812 to, for example, selectively modulate power to LED sets 808.0-808.N to allow controller 806 to determine the brightness of at least two of the LED sets 808.0-808.N. In at least one embodiment, controller 806 decreases power to LED sets 808.0-808.N in accordance with a time division algorithm that allows controller 806 to determine the brightness of light 816 emitted from at least two of the LED sets 808.0-808.N. The controller 806 decreases power to different subgroups of the LED sets to allow the controller to determine the brightness of individual LED sets. Embodiments of the time division algorithm are discussed in more detail below.
The particular implementation of controller 806 is a matter of design choice. Controller 806 can be implemented using digital, analog, or digital and analog technology. In at least one embodiment, controller 806 is fabricated as an integrated circuit. In at least one embodiment, controller 806 includes a processor and algorithms performed by controller 806 are implemented in code and executed by the processor. The code can be stored in a memory (not shown) included in controller 806 or accessible to controller 806.
FIG. 9 depicts lighting system 900, which represents one embodiment of lighting system 300. Lamp 902 receives power from power source 304 via terminals 901 and 903. Lamp 902 includes LED 904, LED 906, and LED 908, which have different respective spectra. For purposes of description, LED 904, LED 906, and LED 908 will be discussed as respectively red, green, and blue LEDs, i.e. LED 904 emits red spectrum light, LED 906 emits green spectrum light, and LED 908 emits blue spectrum light. Lamp 902 also includes a power control system 910, which represents one embodiment of power control system 302. Power control system 910 includes controller 912 to control LED drivers 914, 916, and 918 and, thereby, control respective LED drive currents iLED—R, iLED—G, and iLED—B. In at least one embodiment, controller 912 generates control signals CSR, CSG, and CSB in the same manner that controller 306 generates control signals CS10-CS1N with N=2. Controller 912 represents one embodiment of controller 306.
Lighting system 900 also includes a light sensor 920 to sense incoming light 922 from LEDs 904, 906, and 908 and ambient light 923 and generate a sense signal SEN1. Ambient light 923 represents light that is received by light sensor 920 but not generated by LEDs 904, 906, and 908. In at least one embodiment, ambient light 923 represents light from other artificial light sources or natural light such as sunlight. In at least one embodiment, light sensor 314 is a broad spectrum sensor that senses light 922 from LEDs 904, 906, and 908 and senses ambient light 923.
The human eye generally cannot perceive a reduction in brightness from a light source if the reduction has a duration of 1 millisecond (ms) or less. Thus, in at least one embodiment, power, and thus, brightness, is reduced to LEDs 904, 906, and 908 in accordance with a time division power modulation algorithm for 1 ms or less, and light sensor 920 senses light whose brightness is reduced for 1 ms or less and generates sense signal SEN1 to indicate the brightness of light 922 received by light sensor 920. In at least one embodiment, light sensor 920 is any commercially available photosensitive transistor-based or diode-based light sensor that can detect brightness of light and generate sense signal SEN1. The particular light sensor 920 is a matter of design choice. Controller 912 includes a time division module 924. As subsequently explained in more detail, time division module 924 in conjunction with LED drivers 914, 916, and 918 selectively modulates drive currents iLED—R, iLED—G, and iLED—B in accordance with a time division algorithm that allows controller 912 to determine the individual brightness of LEDs 904, 906, and 908. By determining the individual brightness of LEDs 904, 906, and 908, in at least one embodiment, controller 912 individually adjusts drive currents iLED—R, iLED—G, and iLED—B to obtain a target brightness of light emitted from respective LEDs 904, 906, and 908.
FIG. 10 depicts an exemplary time division sensing and LED adjustment algorithm 1000 (referred to herein as the “time division and adjustment algorithm 1000”) for sensing and adjusting the brightness of light emitted by LEDs 904, 906, and 908 of lighting system 900. In general, time division and adjustment algorithm 1000 obtains a brightness value for ambient light and reduces the brightness of subgroups of LEDs 904, 906, and 908 over time, determines the brightness of each of LEDs 904, 906, and 908.
FIG. 11 depicts interspacing time division 1100 for power modulation of LEDs 904, 906, and 908 (FIG. 9). In general, in interspacing time division 1100, ambient light brightness is determined by reducing power to all of LEDs 904, 906, and 908, then current, and, thus, brightness, is reduced to two of LEDs 904, 906, and 908 at a time until the brightness of light from each of LEDs 904, 906, and 908 plus ambient light is sensed. Since the ambient light brightness is known, controller 912 can determine the individual brightness of light from each of LEDs 904, 906, and 908, compare each brightness to target data, and adjust the brightness of light from each of LEDs 904, 906, and 908 in accordance with results of the comparison. In at least one embodiment, the brightness of light from each of LEDs 904, 906, and 908 is adjusted by increasing or decreasing current to the LEDs 904, 906, and 908. Increasing current increases brightness, and decreasing current decreases brightness. In interspacing time division 1100 power to the LEDs 904, 906, and 908 is reduced to zero. However, the particular amount of reduction is a matter of design choice.
Referring to FIGS. 9, 10, and 11, an exemplary operation of lighting system 900 involves time division and adjustment algorithm 1000 and interspacing time division 1100. In at least one embodiment, to sense the brightness of light emitted from each of LEDs 904, 906, and 908, in operation 1002, lighting system 900 senses ambient light 923. In at least one embodiment, ambient light is light received by light sensor 920 that is not emitted by LEDs 904, 906, or 908. To sense only the ambient light, between times t0 and t1, LED drive currents iLED—R, iLED—G, and iLED—B are reduced to zero, thereby turning “off” LEDs 904, 906, or 908. Light sensor 920 senses the ambient light between times t0 and t1 and generates signal SEN1, which is representative of the amount of ambient light 923 sensed by light sensor 920. In operation 1004, controller 912 stores a value of sensed ambient light indicated by signal SEN1. In operation 1006, the time division module 924 modulates power to LEDs 904 and 906 by causing LED drivers 914 and 916 to reduce drive currents iLED—R and iLED—G to zero between times t2 and t3. Light sensor 920 senses the ambient light 923 and light emitted by LED 908 and, in operation 1008, generates sense signal SEN1 to indicate a brightness value of the sensed light.
As previously discussed, the human eye generally cannot perceive a reduction in brightness from a light source if the reduction has a duration of 1 millisecond (ms) or less. Thus, in at least one embodiment, each time division of power to LEDs 904, 906, and 908 as indicated by the LED drive current reduction times t0-t1, t2-t3, t4-t5, and t6-t7 in time division and adjustment algorithm 1000 has a duration of 1 ms or less so that turning LEDs 904, 906, and 908 “off” and “on” during time division and adjustment algorithm 1000 is imperceptible to a human.
In operation 1010, controller 912 compares values of the sense signal to values of target data. The target data includes a target brightness value for sense signal SEN1 in which the target brightness value is representative of a target brightness for the combination of the ambient light and light emitted from the blue LED 908. In operation 1012, controller 912 adjusts the LED drive current iLED—B based on the comparison between the target brightness value and the brightness value indicated by sense signal SEN1. If the comparison indicates that the brightness of LED 908 is low, controller 912 increases the drive current iLED—B. If the comparison indicates that the brightness of LED 908 is high, controller 912 decreases the drive current iLED—B. Determining the amount and rate of change to drive current iLED—B is a matter of design choice. In at least one embodiment, the amount of drive current iLED—B change is determined based on the brightness-to-current relationship of LED 908 and the difference between the target brightness value and the brightness value of the sensed light indicated by sense signal SEN1. In at least one embodiment, the rate of change for drive current iLED—B is low enough, e.g. less than 1 ms, to prevent an instantaneously noticeable change by a human.
Controller 912 adjusts the drive current iLED—B by adjusting control signal CSB provided to lamp driver 918. In at least one embodiment, controller 912 generates control signal CSB in accordance with Melanson II or Melanson III so that lamp driver 918 provides a desired drive current iLED—B.
In operation 1014, controller 912 determines if operations 1006-1012 have been completed for all LEDs 904, 906, and 908. If not, the time division and adjustment algorithm 1000 returns to operation 1006 and repeats operations 1006-1012 for the next LED. In the currently described embodiment, in operation 1006, time division module 924 reduces drive currents iLED—R and iLED—B to zero between times t4 and t5. Operations 1008-1012 then repeat to adjust drive current iLED—G as indicated by operation 1012. Again, in operation 1014, controller 912 determines if operations 1006-1012 have been completed for all LEDs 904, 906, and 908. In the currently described embodiment, in operation 1006, time division module 924 reduces drive currents iLED—G and iLED—B to zero between times t6 and t7. Operations 1008-1012 then repeat to adjust drive current iLED—R as indicated by operation 1012. After performing operations 1008-1012 for LEDs 904, 906, and 908, time division and adjustment algorithm 1000 proceeds from operation 1014 to operation 1016. Operation 1016 causes time division and adjustment algorithm 1000 to stop until the next cycle. The next cycle repeats operations 1002-1016 as previously described to reevaluate the brightness of light from LEDs 904, 906, and 908.
The frequency of repeating time division and adjustment algorithm 1000 is a matter of design choice and can be, for example, on the order of one or more seconds, one or more minutes, one or more hours, or one or more days. In at least one embodiment, time division and adjustment algorithm 1000 is repeated every second. In at least one embodiment, time division and adjustment algorithm 1000 is repeated often enough to sense changes in the ambient light and changes in the brightness of LEDs 904, 906, and 908 so that the brightness of light 926 exiting diffuser 928 is a constant or at least approximately constant value. Additionally, the timing between each period of power modulation, e.g. between times t1 and t2, t3 and t4, and so on is a matter of design choice. The particular choice is, for example, long enough to perform operations 1006-1014 for an LED before repeating operations 1006-1014 for the next LED.
In at least one embodiment, the brightness of only a subset of LEDs 904, 906, and 908 are considered during operations 1006-1012. For example, if the red LED 904 is assumed to maintain a relatively constant brightness over time, then the modulation of power of LEDs 906 and 908 between times t6 and t7 in operation 1006 and subsequent processing in operations 1008-1012 for LED 904 is not performed. Additionally, the amount of power reduction to LEDs 904, 906, and 908 in time division and adjustment algorithm 1000 is a matter of design choice. Interspacing time division 1100 depicts drive currents iLED—R, iLED—G, and iLED—B reducing to zero during time division power modulation times. The reduction amount is a matter of design choice. In at least one embodiment, the drive currents iLED—R, iLED—G, and/or iLED—B are reduced a specific percentage between approximately 10% and 90%. By reducing the drive currents iLED—R, iLED—G, and/or iLED—B to a value less than a nominal value, controller 912 accounts for the brightness contribution of all LEDs 904, 906, and 908 to the brightness indicated by sense signal SEN1 when determining the adjustment to be made in operation 1012.
In at least one embodiment, LEDs 904, 906, and/or 908 each represent a single LED. In at least one embodiment, one, two, or all of LEDs 904, 906, and 908 represent a set of LEDs that includes multiple LEDs having the same spectrum. For example, in at least one embodiment, LED 904 represents multiple red LEDs, LED 906 represents multiple green LEDs, and LED 908 represents multiple blue LEDs. The time division and adjustment algorithm 1000 applies regardless of the number of LEDs in LEDs 904, 906, and 908.
The time division and adjustment algorithm 1000 also includes optional operation 1018 to calibrate the target data. In at least one embodiment, light sensor 920 is sensitive to temperature changes, which affects accuracy of the value provided for sense signal SEN1. For example, in at least one embodiment, as the temperature of light sensor 920 increases, the value of sense signal SEN1 changes for the same brightness level of light 922 received by light sensor 920. However, in at least one embodiment, the relationship between temperature changes of light sensor 920 and sense signal SEN1 is known. In at least one embodiment, light sensor 920 provides temperature information to controller 912, or controller 912 senses the temperature in or near light sensor 920. Using this relationship, controller 912 accordingly calibrates the target data to compensate for effects of temperature on the accuracy of the values for sense signal SEN1. In at least one embodiment, the light sensor 920 is self-compensating for temperature changes, thus, eliminating a need for optional operation 1018. In at least one embodiment, temperature effects on the accuracy of values for sense signal SEN1 are either negligible or not considered in time division and adjustment algorithm 1000. The target data can also be adjusted to compensate for operating characteristics associated with light sensor 920. For example, in at least one embodiment, the reception by broad spectrum light sensor 920 is not uniform across the spectrum. The target data can be adjusted to account for the non-uniformity. In at least one embodiment, the adjustment is made during a calibration test by a manufacturer or distributor of lamp 902.
The time division and adjustment algorithm 1000 represents one embodiment of a time division and adjustment algorithm that can be used to sense and, if appropriate, adjust the brightness of one or more LEDs in lighting system 900. The number of time division and adjustment algorithms that can be used by lighting system 900 is virtually limitless. For example, operations 1006 and 1008 can be executed for each of LEDs 904, 906, and 908, the sense signal SEN1 stored for each of LEDs 904, 906, and 908, and operations 1010 and 1012 repeated for each of LEDs 904, 906, and 908. Additionally, the time intervals for reduction of power, such as between t2 and t1, t4 and t3, and so on of time division power modulation in interspacing time division 1100 is a matter of design choice, and the range of power reductions is a matter of design choice. In at least one embodiment, the time intervals for reduction of power are less than an amount of time for a human to perceive a reduction in power by perceiving a change in brightness of the lighting system 900.
FIG. 12 depicts an LED current drive timing diagram 1200. Timing diagram 1200 illustrates interspersed time division, which represents another embodiment of a timing division power modulation scheme. Timing diagram 1200 is similar to interspacing time division 1100 except that the timing between reductions of power for different LEDs is clearly shown as interspersed over time. Time division and adjustment algorithm 1000 works identically with interspersed time division 1200 as time division and adjustment algorithm 1000 works with interspacing time division 1100. Using interspersed time division 1200 spreads out the times between reductions in drive currents iLED—R, iLED—G, and iLED—B, thereby reducing the perceptibility of altering the brightness of light 926 during execution of time division and adjustment algorithm 1000.
FIG. 13 depicts an LED current drive timing diagram 1300. Timing diagram 1300 illustrates unitary time division, which represents yet another embodiment of a timing division power modulation scheme. Unitary time division in timing diagram 1300 reduces current to LEDs 904, 906, and 908 one at a time during respective periods t2-t3, t6-t7, and t4-t5. FIG. 14 depicts a time division and adjustment algorithm 1400 for implementing unitary time division. In at least one embodiment, in order to utilize unitary time division, time division and adjustment algorithm 1000 is modified to, for example, include operations 1402-1406. In operation 1006, time division module 924 modulates power to LEDs 904, 906, and 908 in accordance with LED current drive timing diagram 1300. Operation 1402 stores each value of sense signal SEN1 for each reduction in power to LEDs 904, 906, and 908 in a memory (not shown) within, or accessible to, controller 912. Sense signal SEN1 is generated in operation 1008 for a brightness levels sensed during time t2-t3. Operation 1404 causes operations 1006, 1008, and 1402 to repeat until a sense signal SEN1 is generated in operation 1008 for brightness levels sensed during times t6-t7 and t4-t5.
Once a brightness level has been determined during each of power modulation periods t2-t3, t6-t7, and t4-t5, controller 912 determines in operation 1406 the brightness of each of LEDs 904, 906, and 908. Each stored value of sense signal SEN1 represents the brightness of the ambient light and the contribution of two of the LEDs 904, 906, and 908 as set forth in Equation [1]:

SEN1=BAL+BLEDx+BLEDy  [1],

where BAL=the brightness of the ambient light, and BLEDx and BLEDy equal the respective brightness contributions of the two LEDs of LEDs 904, 906, and 908 whose power is not reduced in operation 1006. Since the brightness of the ambient light, BAL, is known from operations 1002 and 1004, in at least one embodiment, controller 912 uses a multi-variable, linear equation solution process to solve for the three values of sense signal SEN1 stored in operation 1402 using three instances of Equation [1]. The particular linear equation solution process is a matter of design choice. For example, at time t3:

SEN1=BAL+BLED906+BLED908  [2],

at time t6:

SEN1=BAL+BLED904+BLED906  [3],

at time t7:

SEN1=BAL+BLED904+BLED908  [4].

Since the value of BAL and SEN1 is known, Equation [2] can be solved for BLED906 in terms of BLED908 and substituted into Equation [3]. After the substitution, Equation [3] can be solved in terms of BLED908 and substituted into Equation [4]. After substitution, Equation [4] can be solved for the value of BLED908. From the value of BLED908, BLED906 and BLED904 can then be solved from Equation [2] then Equation [3].
FIG. 15 depicts controller 1500, which represents one embodiment of controller 912. Controller 1500 includes control signal generators 1502.0-1502.N and pulse width modulators 1504.0-1504.N for generation of respective control signals CS10 and CS1N. In at least one embodiment, each of control signal generators 1502.0-1502.N and pulse width modulators 1504.0-1504.N operate in accordance with time division and adjustment algorithm 1000 or time division and adjustment algorithm 1400 to determine the brightness of light of at least two LEDs having different spectra and adjust the brightness in accordance with a comparison to values of target data 1506 representing a target brightness of the LEDs. Generally adjusting current to LEDs using pulse width modulated control signals control signals CS10 and CS1N is illustratively described in Melanson II. In at least one embodiment, control signal generators 1502.0-1502.N cause control signals CS10 and CS1N to have no pulse during sensing of ambient light in operation 1002 (FIGS. 10 and 14).
Thus, a lighting system includes time division light output sensing and adjustment for different spectra light emitting diodes (LEDs). In at least one embodiment, the time division light output sensing and adjustment allows the lighting system to individually adjust the brightness of LEDs to account for ambient light and changes in brightness of the LEDs.
Although the present invention has been described in detail, it should be understood that various changes, substitutions and alterations can be made hereto without departing from the spirit and scope of the invention as defined by the appended claims.

## Claims

1. An apparatus comprising:
a controller configured to couple to a sensor and a first light emitting diode (LED) of a lamp, wherein the controller is further configured to:
i. reduce power to a first light emitting diode (LED) coupled to the controller to a value greater than zero;
ii. receive a signal from the sensor indicating a brightness of light received by the sensor while the power to the first LED is reduced; and
iii. adjust the brightness of the first LED in accordance with a brightness related target value and the brightness indicated by the signal from the sensor.

2. The apparatus of claim 1 wherein the controller is configured to receive a sense signal from a light sensor indicating a brightness of ambient light and adjust the brightness of the LED to complement the ambient light.

3. The apparatus of claim 1 wherein reducing power to the first LED comprises reducing current to the first LED.

4. The apparatus of claim 1 wherein the controller is further configured to synchronize multiple controllers, wherein each of the multiple controllers is configured to:
i. reduce power to an LED coupled to the controller during a same period of time as a reduction in power to LEDs coupled to each of the other controllers;
ii. receive a signal from a sensor indicating a brightness of light received by the sensor while the power to the LED coupled to the controller is reduced; and
iii. adjust the brightness of the LED coupled to the controller in accordance with a brightness related target value.

5. The apparatus of claim 1 wherein the controller is further configured to:
i. receive a first signal indicating a brightness of received light at a first time;
ii. receive a second signal indicating a brightness of the received light at a second time, wherein a relative contribution to the brightness from the first LED and a second LED is different for the first and second times and wherein the light emitted from the first LED has a different spectrum than the light emitted from the second LED;
iii. determine the brightness of light emitted from the first LED and the brightness of light emitted from the second LED using information from the signals; and
iv. adjust the brightness of the light emitted from the first LED and the brightness of the light emitted from the second LED in accordance with one or more brightness related target values.

6. The apparatus of claim 5 wherein the first and second LEDs are members of groups consisting of: red and green, red and yellow, amber and blue, green and blue, and red and blue.

7. The apparatus of claim 5 wherein the first LED is a member of a first set of multiple LEDs having approximately identical spectra and the second LED is a member of a second set of multiple LEDs having approximately identical spectra.

8. The apparatus of claim 5 wherein the controller is further configured to:
adjust the brightness of the first and second LEDs to compensate for at one of (a) LED heating and (b) light output changes over time.

9. The apparatus of claim 5 wherein at least one of the sensors is a broad spectrum sensor.

10. The apparatus of claim 9 wherein a single, broad spectrum sensor provides the signals indicating brightness at the first and second times.

11. The apparatus of claim 5 wherein the controller is further configured to:
modulate current to the first and second LEDs so that the relative contribution to the brightness of the light received by the one or more sensors is different for the first and second times.

12. The apparatus of claim 11 wherein to modulate current to the first and second LEDs comprises:
reducing current to the first LED to zero while providing current to the second LED during the first time; and
reducing current to the second LED to zero while providing current to the first LED during the second time.

13. The apparatus of claim 11 wherein to modulate current to the first and second LEDs comprises:
providing less average current to the first LED than the second LED during the first time.

14. The apparatus of claim 11 wherein to modulate current to the first and second LEDs comprises:
modulating current to the first and second LEDs during sequential times.

15. The apparatus of claim 11 wherein to modulate current to the first and second LEDs comprises:
interspersing reductions in current to the first and second LEDs over time.

16. The apparatus of claim 1 wherein to reduce the power to a first LED comprises to reduce the power to the first LED from a first brightness level and the controller is further configured to:
increase the power to the first brightness level with compensation for ambient light after receiving the signal from the sensor.

17. The apparatus of claim 1 wherein the controller comprises:
a time division module configured to facilitate time division power modulation to (i) reduce the power to the LED for no more than 0.02 seconds to sense the brightness of light from the LED and ambient light and then (ii) increase the power to the LED.

18. The apparatus of claim 17 wherein the time division module is further configured to repeat the time division power modulation at least every 1 second for multiple cycles of the time division power modulation.

19. An apparatus comprising:
a lamp having at least a first light emitting diode (LED);
a sensor to sense brightness of received light; and
a controller coupled to the lamp and the sensor, wherein the controller is configured to:
i. reduce power to the first LED to a value greater than zero;
ii. receive a signal from the sensor indicating the brightness of light received by the sensor while the power to the first LED is reduced; and
iii. adjust the brightness of the first LED in accordance with a brightness related target value and the brightness indicated by the signal from the sensor.

20. The apparatus of claim 19 wherein the controller is configured to receive a sense signal from a light sensor indicating a brightness of ambient light and adjust the brightness of the LED to complement the ambient light.

21. The apparatus of claim 19 wherein reducing power to the first LED comprises reducing current to the first LED.

22. The apparatus of claim 19 wherein the controller is further configured to synchronize multiple controllers, wherein each of the multiple controllers is configured to:
i. reduce power to an LED coupled to the controller during a same period of time as a reduction in power to LEDs coupled to each of the other controllers;
ii. receive a signal from a sensor indicating a brightness of light received by the sensor while the power to the LED coupled to the controller is reduced; and
iii. adjust the brightness of the LED coupled to the controller in accordance with a brightness related target value.

23. The apparatus of claim 19 wherein the controller is further configured to:
i. receive a first signal indicating a brightness of received light at a first time;
ii. receive a second signal indicating a brightness of the received light at a second time, wherein a relative contribution to the brightness from the first LED and a second LED is different for the first and second times and wherein the light emitted from the first LED has a different spectrum than the light emitted from the second LED;
iii. determine the brightness of light emitted from the first LED and the brightness of light emitted from the second LED using information from the signals; and
iv. adjust the brightness of the light emitted from the first LED and the brightness of the light emitted from the second LED in accordance with one or more brightness related target values.

24. The apparatus of claim 19 wherein to reduce the power to a first LED comprises to reduce the power to the first LED from a first brightness level and the controller is further configured to:
increase the power to the first brightness level with compensation for ambient light after receiving the signal from the sensor.

25. The apparatus of claim 19 wherein the controller comprises:
a time division module configured to facilitate time division power modulation to (i) reduce the power to the LED for no more than 0.02 seconds to sense the brightness of light from the LED and ambient light and then (ii) increase the power to the LED.

26. The apparatus of claim 25 wherein the time division module is further configured to repeat the time division power modulation at least every 1 second for multiple cycles of the time division power modulation.

27. A method for light harvesting, the method comprising:
reducing power to a first light emitting diode (LED) to a value greater than zero;
receiving a signal indicating a brightness of light while the power to the first LED is reduced; and
adjusting the brightness of the first LED in accordance with a brightness related target value and the brightness indicated by the signal.

28. The method of claim 27 wherein the method further comprises:
receiving a sense signal from a light sensor indicating a brightness of ambient light; and
adjusting the brightness of the LED to complement the ambient light.

29. The method of claim 27 wherein reducing power to the first LED comprises reducing current to the first LED.

30. The method of claim 27 further comprising:
operating multiple controllers in synchronization, wherein operating multiple controllers in synchronization comprises:
reducing power to an LED coupled to the controller during a same period of time as a reduction in power to LEDs coupled to each of the other controllers;
receiving a signal from a sensor indicating a brightness of light received by the sensor while the power to the LED coupled to the controller is reduced; and
adjusting the brightness of the LED coupled to the controller in accordance with a brightness related target value.

31. The method of claim 27 further comprising:
receiving a first signal indicating a brightness of received light at a first time;
receiving a second signal indicating a brightness of the received light at a second time, wherein a relative contribution to the brightness from the first LED and a second LED is different for the first and second times and wherein light emitted from the first LED has a different spectrum than the light emitted from the second LED;
determining the brightness of light emitted from the first LED and the brightness of light emitted from the second LED using information from the signals; and
adjusting the brightness of the light emitted from the first LED and the brightness of the light emitted from the second LED in accordance with one or more brightness related target values.

32. The method of claim 31 wherein the first and second LEDs are members of groups consisting of: red and green, red and yellow, amber and blue, green and blue, and red and blue.

33. The method of claim 31 wherein the first LED is a member of a first set of multiple LEDs having approximately identical spectra and the second LED is a member of a second set of multiple LEDs having approximately identical spectra.

34. The method of claim 31 further comprising:
adjusting the brightness of the first and second LEDs to compensate for at one of (a) LED heating and (b) light output changes over time.

35. The method of claim 31 wherein at least one of the sensors is a broad spectrum sensor.

36. The method of claim 35 wherein a single, broad spectrum sensor provides the signals indicating brightness at the first and second times.

37. The method of claim 31 further comprising:
modulating current to the first and second LEDs so that the relative contribution to the brightness of the light received by the one or more sensors is different for the first and second times.

38. The method of claim 37 wherein modulating current to the first and second LEDs comprises:
reducing current to the first LED to zero while providing current to the second LED during the first time; and
reducing current to the second LED to zero while providing current to the first LED during the second time.

39. The method of claim 37 wherein modulating current to the first and second LEDs comprises:
providing less average current to the first LED than the second LED during the first time.

40. The method of claim 37 wherein modulating current to the first and second LEDs comprises:
modulating current to the first and second LEDs during sequential times.

41. The method of claim 37 wherein to modulate current to the first and second LEDs comprises:
interspersing reductions in current to the first and second LEDs over time.

42. The method of claim 27 wherein reducing power to a first LED comprises reducing the power to the first LED from a first brightness level, and the method further comprises:
increasing the power to the first brightness level with compensation for ambient light after receiving the signal indicating the brightness of light while the power to the first LED is reduced.

43. The method of claim 27 wherein the method further comprises:
reducing power for no more than 0.02 seconds and then increasing power to the first LED using time division power modulation to respectively (i) reduce the power to the LED to sense the brightness of light from the LED and ambient light and (ii) increase the power to the LED.

44. The method of claim 43 further comprising:
repeating the time division power modulation at least every 1 second for multiple cycles of the time division power modulation.

45. An apparatus for light harvesting comprising:
means for reduce power to a first light emitting diode (LED) to a value greater than zero;
means for receiving a signal indicating a brightness of light while the power to the first LED is reduced; and
means for adjusting the brightness of the first LED in accordance with a brightness related target value and the brightness indicated by the signal from the sensor.