WO2009040705A2

WO2009040705A2 - Method and apparatus for light intensity control with drive current modulation

Info

Publication number: WO2009040705A2
Application number: PCT/IB2008/053783
Authority: WO
Inventors: Ian Ashdown
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2007-09-28
Filing date: 2008-09-17
Publication date: 2009-04-02
Also published as: WO2009040705A3

Abstract

Disclosed are a method and apparatus for optical feedback control for a lighting unit (10) for generating light having a desired luminous flux and chromaticity. The control signals (110, 210) for the drive currents of each array of one or more light sources are independently configured using a suitable modification signal (111, 211) for each array. In this manner, upon detection of the output light of the arrays, which will have encoded therein a respective modification signal, a controller can be configured to separate each array's contribution based on the respective modification signal. The modification signal can be configured to modulate the pulse widths of drive currents supplied to each array.

Description

METHOD AND APPARATUS FOR LIGHT INTENSITY CONTROL WITH DRIVE

CURRENT MODULATION

Technical Field

[0001] The present invention is directed generally to illumination systems. More particularly, various inventive methods and apparatus disclosed herein relate to light intensity control employing drive current modulation for feedback purposes.

Background

[0002] Digital lighting technologies, i.e. illumination based on semiconductor light sources, such as light-emitting diodes (LEDs), offer a viable alternative to traditional fluorescent, HID, and incandescent lamps. Functional advantages and benefits of LEDs include high energy conversion and optical efficiency, durability, lower operating costs, and many others. Recent advances in LED technology have provided efficient and robust lighting sources that enable a variety of lighting effects in many applications, such as ambient lighting, signage, advertising, display lighting, and backlit lighting applications. Some of the fixtures embodying these sources feature a lighting module, including one or more LEDs capable of producing different colors, e.g. red, green, and blue, as well as a processor for independently controlling the output of the LEDs in order to generate a variety of colors and color-changing lighting effects, for example, as discussed in detail in U.S. Patent Nos. 6,016,038 and 6,211,626, incorporated herein by reference.

[0003] It is well known that light of a desired spectral composition or, in photometric terms, a desired chromaticity and luminous flux, can be generated by intermixing adequate amounts of light from different color light sources. When light from, for example, LEDs generating light of different colors is intermixed, the chromaticity of the mixed light can be sufficiently accurately determined by characteristics such as the intensities, center wavelengths and spectral bandwidths of the LEDs.

[0004] The characteristics of LEDs can vary for a number of reasons, for example, aging and/or fluctuations in operating temperature. These variations can cause undesirable effects under operating conditions of the LEDs. Possible solutions include optical feedback control to monitor the luminous flux output of the different color LEDs and to adjust the drive currents of the LEDs such that the luminous flux output and chromaticity of the light emitted by each LED or at least the mixed light generated by a group of LEDs remains substantially constant. Monitoring the emitted light requires some means of measuring, for example, the luminous flux output per LED color or per LED.

[0005] To date, a number of optical feedback solutions have been proposed to detect and evaluate the luminous flux output and chromaticity of the output light of a lighting device in order to monitor these characteristics. For instance, one approach teaches an array of photosensors each having a selected color filter responsive to light of a selected color. These photosensors however, are prone to optical crosstalk due to the overlap in the spectral radiant power distribution of the light emitted by various colors of LEDs. This optical crosstalk can reduce the accuracy of the light information collected by the photosensors. For example, one such system comprises a LED lighting fixture with multi-channel color sensors for optical feedback, wherein each channel is comprised of a broadband photosensor and a color filter with transmittances that approximate that of the red, green and blue LED spectral radiant power distributions. Since the spectral radiant power distributions of the LEDs tend to overlap for the different colors, channel crosstalk is inevitable and can limit the performance of the optical feedback system.

[0006] A partial solution to this optical crosstalk problem is to select bandpass filters with narrow bandwidths and steep cutoff characteristics. Although satisfactory performance levels for such filters can be achieved using multilayer interference filters, these interference filters can be expensive and typically require further optics for collimating the emitted light, as the interference filter characteristics depend on the incidence angle at which the light impinges on these filters.

[0007] Another problem associated with interference filters is that the center wavelength of an LED depends on the LED junction temperature and this center wavelength can vary significantly with temperature, depending on the type of LED. In addition, the bandpass transmittance spectra of interference filters are also temperature dependent. The output signal of the photosensor therefore depends on the spectral radiant power distribution of the LED as modified by the bandpass characteristics of the interference filter associated therewith. Hence there exist situations where the output signal of the photosensor may change with ambient temperature even if the LED spectral radiant power distribution remains constant, which can further limit the performance of the sensor system.

[0008] In a proposed solution to this problem, radiation from each LED color is controlled by an electronic control circuit, which can selectively turn off the LEDs, which are the colors not being measured, in a sequence of time pulses and uses a single broadband optical sensor for detection. A problem with this approach is that color balance is periodically and potentially drastically altered each time the LEDs are de-energized, thereby possibly causing noticeable flicker. Since the optical sensor requires a minimum amount of time to sense the radiant flux of the energized LEDs accurately and with an acceptable signal-to-noise ratio, the choice of sampling frequencies can be limited by the sensitivity and noise characteristics of the optical sensor. A limited sampling frequency can result in lower sampling resolution and longer response times for the optical feedback loop. Since light from no more than one LED color is measured at a time, this approach for optical data collection can increase the feedback loop response time by about the number of different LED colors used in the system. For example, for a system with red, green, and blue LED clusters the response time can be multiplied by factor of about three, and for a system with red, green, blue, and amber LED clusters the response time can be multiplied by a factor of about four.

[0009] In a proposed variation of the above solution, the average light output during the measuring period is made substantially equal to the nominal continuous light output during the ordinary operation to avoid visible flicker. Another proposed variation seeks to alleviate the flicker by selectively measuring the light output of the LEDs in a sequence of time pulses, whereby the current for the color being measured is turned off. Neither of these proposed solutions, however, addresses periodic and potentially drastic changes in color balance or degradation in feedback loop response time due to the deactivation sequences required for light sampling.

[0010] In another proposed system, the light output of the LEDs is sampled by a broadband optical sensor during PWM drive current pulses whenever the drive current has reached full magnitude. This procedure can avoid the effect of the rise and fall times of the PWM pulse. The average drive current can then be determined by low pass filtering. A difficulty associated with this approach can be that the PWM pulses must be synchronized such that at least one LED color is de-energized for a finite period of time during the PWM period. This requirement can prevent operation of all different color LEDs at full power at 100% duty factor. Another disadvantage associated with the average light sensing method is that the sampling period typically must provide sufficient time for the optical sensor to reliably measure the radiant flux of the energized LEDs. In addition this light sensing method requires that the LED colors are to be measured sequentially, which can limit the feedback loop response time.

[0011] Another proposed solution comprises an optical feedback method and apparatus for modulation of PWM, PCM or analog drive currents for light sources in order to simultaneously measure the spectral radiant flux output of each light source color without the need for tristimulus color sensors. However, analog modulation requires analog circuitry with power transistors for drive current control as well as operating the transistors in linear mode which can affect efficiency. In addition, known PWM or PCM drive current modulation methods require different PWM/PCM drive frequencies for different colors which can affect complexity and cost of respective apparatus.

[0012] Thus, there is a need in the art to provide a new method and apparatus for light intensity feedback control for a lighting fixture.

[0013] This background information is provided to reveal information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention. Summarv

[0014] The present disclosure is directed to inventive methods and apparatus for light intensity control. For example, one or more light sources can be driven with a controllable drive current waveform such as the one comprising pulses of controllable width, the waveform being modulated so as to convey information while providing for a desired lighting effect. The methods and apparatus disclosed herein can be advantageous in providing new methods and apparatus for optical feedback for lighting control.

[0015] Generally, in one aspect, the invention features a multi-channel lighting unit for generating light having a desired luminous flux and chromaticity. The lighting unit includes one or more first arrays of one or more light sources, which are adapted to generate first light in response to a first drive current. The lighting unit also includes one or more second arrays of one or more light sources adapted to generate second light in response to a second drive current. Also provided is a first current driver operatively coupled to the one or more first arrays, the first current driver being configured to supply the first drive current to the one or more first arrays based on a first pulsed signal. Also provided is a second current driver operatively coupled to the one or more second arrays, the second current driver being configured to supply the second drive current to the one or more second arrays based on a second pulsed signal. Also provided is an optical sensor for sensing a portion of mixed light, the mixed light being a combination of the first light and second light. The optical sensor is configured to generate a sensor signal representative of the mixed light. Also provided is a controller operatively connected to the first current driver, second current driver, and the optical sensor. The controller is configured to generate the first pulsed signal and second pulsed signal based at least in part on characteristics of the first light and second light respectively, and based at least in part on the desired luminous flux and chromaticity. The first pulsed signal comprises pulses modulated according to a first modification signal. The second pulsed signal comprises pulses modulated according to a second modification signal. The controller is configured to electronically filter the sensor signal based on the first modification signal and second modification signal, thereby determining optical characteristics of the first light and the second light. [0016] In many embodiments of the present invention, the first modification signal has a first frequency, the second modification signal has a second frequency different from the first frequency, and the first pulse widths and the second pulse widths are modulated according to the first modulation frequency and the second modulation frequency, respectively. Also, the first modification signal and the second modification signal can be configured via time division multiplexing, time division multiple access, carrier sense multiple access, and/or code division multiple access.

[0017] In accordance with another aspect of the present invention, there is provided a method for modulating light having a desired luminous flux and chromaticity. The method comprises providing a light source for generating light in response to a drive current. The method further comprises operating the light source with a pulsed drive current and controlling the pulsed drive current by modulating the pulses thereof while maintaining the desired luminous flux and chromaticity of the light.

[0018] In accordance with yet another aspect of the present invention, there is provided a method for modulating light having a desired luminous flux and chromaticity. The method comprises providing a first light source for generating first light in response to a first drive current and operating the first light source with a first pulsed drive current. The method further comprises modulating pulses of the first pulsed drive current using a first modification signal. The method further comprises providing a second light source for generating second light in response to a second drive current and operating the second light source with a second pulsed drive current. The method further comprises modulating pulses of the second pulsed drive current using a second modification signal.

[0019] In accordance with still another aspect of the present invention, there is provided a computer program product comprising a computer readable medium. The computer readable medium has recorded thereon statements and instructions for execution by a processor to carry out one or more methods for carrying out the present invention.

[0020] As used herein for purposes of the present disclosure, the term "LED" should be understood to include any electroluminescent diode or other type of carrier injection/junction- based system that is capable of generating radiation in response to an electric signal. Thus, the term LED includes, but is not limited to, various semiconductor-based structures that emit light in response to current, light emitting polymers, organic light emitting diodes (OLEDs), electroluminescent strips, and the like. In particular, the term LED refers to light emitting diodes of all types (including semi-conductor and organic light emitting diodes) that may be configured to generate radiation in one or more of the infrared spectrum, ultraviolet spectrum, and various portions of the visible spectrum (generally including radiation wavelengths from approximately 400 nanometers to approximately 700 nanometers). Some examples of LEDs include, but are not limited to, various types of infrared LEDs, ultraviolet LEDs, red LEDs, blue LEDs, green LEDs, yellow LEDs, amber LEDs, orange LEDs, and white LEDs (discussed further below). It also should be appreciated that LEDs may be configured and/or controlled to generate radiation having various bandwidths (e.g., full widths at half maximum, or FWHM) for a given spectrum (e.g., narrow bandwidth, broad bandwidth), and a variety of dominant wavelengths within a given general color categorization.

[0021] For example, one implementation of an LED configured to generate essentially white light (e.g., a white LED) may include a number of dies which respectively emit different spectra of electroluminescence that, in combination, mix to form essentially white light. In another implementation, a white light LED may be associated with a phosphor material that converts electroluminescence having a first spectrum to a different second spectrum. In one example of this implementation, electroluminescence having a relatively short wavelength and narrow bandwidth spectrum "pumps" the phosphor material, which in turn radiates longer wavelength radiation having a somewhat broader spectrum.

[0022] It should also be understood that the term LED does not limit the physical and/or electrical package type of an LED. For example, as discussed above, an LED may refer to a single light emitting device having multiple dies that are configured to respectively emit different spectra of radiation (e.g., that may or may not be individually controllable). Also, an LED may be associated with a phosphor that is considered as an integral part of the LED (e.g., some types of white LEDs). In general, the term LED may refer to packaged LEDs, non-packaged LEDs, surface mount LEDs, chip-on-board LEDs, T-package mount LEDs, radial package LEDs, power package LEDs, LEDs including some type of encasement and/or optical element (e.g., a diffusing lens), etc.

[0023] The term "light source" should be understood to refer to any one or more of a variety of radiation sources, including, but not limited to, LED-based sources (including one or more LEDs as defined above), incandescent sources (e.g., filament lamps, halogen lamps), fluorescent sources, phosphorescent sources, high-intensity discharge sources (e.g., sodium vapor, mercury vapor, and metal halide lamps), lasers, other types of electroluminescent sources, pyro-luminescent sources (e.g., flames), candle-luminescent sources (e.g., gas mantles, carbon arc radiation sources), photo-luminescent sources (e.g., gaseous discharge sources), cathode luminescent sources using electronic satiation, galvano-luminescent sources, crystallo- luminescent sources, kine-luminescent sources, thermo-luminescent sources, triboluminescent sources, sonoluminescent sources, radioluminescent sources, and luminescent polymers.

[0024] A given light source may be configured to generate electromagnetic radiation within the visible spectrum, outside the visible spectrum, or a combination of both. Hence, the terms "light" and "radiation" are used interchangeably herein. Additionally, a light source may include as an integral component one or more filters (e.g., color filters), lenses, or other optical components. Also, it should be understood that light sources may be configured for a variety of applications, including, but not limited to, indication, display, and/or illumination. An "illumination source" is a light source that is particularly configured to generate radiation having a sufficient intensity to effectively illuminate an interior or exterior space. In this context, "sufficient intensity" refers to sufficient radiant power in the visible spectrum generated in the space or environment (the unit "lumens" often is employed to represent the total light output from a light source in all directions, in terms of radiant power or "luminous flux") to provide ambient illumination (i.e., light that may be perceived indirectly and that may be, for example, reflected off of one or more of a variety of intervening surfaces before being perceived in whole or in part).

[0025] The term "spectrum" should be understood to refer to any one or more frequencies (or wavelengths) of radiation produced by one or more light sources. Accordingly, the term "spectrum" refers to frequencies (or wavelengths) not only in the visible range, but also frequencies (or wavelengths) in the infrared, ultraviolet, and other areas of the overall electromagnetic spectrum. Also, a given spectrum may have a relatively narrow bandwidth (e.g., a FWHM having essentially few frequency or wavelength components) or a relatively wide bandwidth (several frequency or wavelength components having various relative strengths). It should also be appreciated that a given spectrum may be the result of a mixing of two or more other spectra (e.g., mixing radiation respectively emitted from multiple light sources).

[0026] For purposes of this disclosure, the term "color" is used interchangeably with the term "spectrum." However, the term "color" generally is used to refer primarily to a property of radiation that is perceivable by an observer (although this usage is not intended to limit the scope of this term). Accordingly, the terms "different colors" implicitly refer to multiple spectra having different wavelength components and/or bandwidths. It also should be appreciated that the term "color" may be used in connection with both white and non-white light.

[0027] The term "color temperature" generally is used herein in connection with white light, although this usage is not intended to limit the scope of this term. Color temperature essentially refers to a particular color content or shade (e.g., reddish, bluish) of white light. The color temperature of a given radiation sample conventionally is characterized according to the temperature in degrees Kelvin (K) of a black body radiator that radiates essentially the same spectrum as the radiation sample in question. Black body radiator color temperatures generally fall within a range of from approximately 700 degrees K (typically considered the first visible to the human eye) to over 10,000 degrees K; white light generally is perceived at color temperatures above 1500-2000 degrees K.

[0028] Lower color temperatures generally indicate white light having a more significant red component or a "warmer feel," while higher color temperatures generally indicate white light having a more significant blue component or a "cooler feel." By way of example, fire has a color temperature of approximately 1,800 degrees K, a conventional incandescent bulb has a color temperature of approximately 2848 degrees K, early morning daylight has a color temperature of approximately 3,000 degrees K, and overcast midday skies have a color temperature of approximately 10,000 degrees K. A color image viewed under white light having a color temperature of approximately 3,000 degree K has a relatively reddish tone, whereas the same color image viewed under white light having a color temperature of approximately 10,000 degrees K has a relatively bluish tone.

[0029] The term "lighting fixture" is used herein to refer to an implementation or arrangement of one or more lighting units in a particular form factor, assembly, or package. The term "lighting unit" is used herein to refer to an apparatus including one or more light sources of same or different types. A given lighting unit may have any one of a variety of mounting arrangements for the light source(s), enclosure/housing arrangements and shapes, and/or electrical and mechanical connection configurations. Additionally, a given lighting unit optionally may be associated with (e.g., include, be coupled to and/or packaged together with) various other components (e.g., control circuitry) relating to the operation of the light source(s). An "LED-based lighting unit" refers to a lighting unit that includes one or more LED- based light sources as discussed above, alone or in combination with other non LED-based light sources. A "multi-channel" lighting unit refers to an LED-based or non LED-based lighting unit that includes at least two light sources configured to respectively generate different spectrums of radiation, wherein each different source spectrum may be referred to as a "channel" of the multi-channel lighting unit.

[0030] The term "controller" is used herein generally to describe various apparatus relating to the operation of one or more light sources. A controller can be implemented in numerous ways (e.g., such as with dedicated hardware) to perform various functions discussed herein. A "processor" is one example of a controller which employs one or more microprocessors that may be programmed using software (e.g., microcode) to perform various functions discussed herein. A controller may be implemented with or without employing a processor, and also may be implemented as a combination of dedicated hardware to perform some functions and a processor (e.g., one or more programmed microprocessors and associated circuitry) to perform other functions. Examples of controller components that may be employed in various embodiments of the present disclosure include, but are not limited to, conventional microprocessors, application specific integrated circuits (ASICs), and field-programmable gate arrays (FPGAs). [0031] In various implementations, a processor or controller may be associated with one or more storage media (generically referred to herein as "memory," e.g., volatile and non-volatile computer memory such as RAM, PROM, EPROM, and EEPROM, floppy disks, compact disks, optical disks, magnetic tape, etc.). In some implementations, the storage media may be encoded with one or more programs that, when executed on one or more processors and/or controllers, perform at least some of the functions discussed herein. Various storage media may be fixed within a processor or controller or may be transportable, such that the one or more programs stored thereon can be loaded into a processor or controller so as to implement various aspects of the present invention discussed herein. The terms "program" or "computer program" are used herein in a generic sense to refer to any type of computer code (e.g., software or microcode) that can be employed to program one or more processors or controllers.

[0032] The term "addressable" is used herein to refer to a device (e.g., a light source in general, a lighting unit or fixture, a controller or processor associated with one or more light sources or lighting units, other non-lighting related devices, etc.) that is configured to receive information (e.g., data) intended for multiple devices, including itself, and to selectively respond to particular information intended for it. The term "addressable" often is used in connection with a networked environment (or a "network," discussed further below), in which multiple devices are coupled together via some communications medium or media.

[0033] In one network implementation, one or more devices coupled to a network may serve as a controller for one or more other devices coupled to the network (e.g., in a master/slave relationship). In another implementation, a networked environment may include one or more dedicated controllers that are configured to control one or more of the devices coupled to the network. Generally, multiple devices coupled to the network each may have access to data that is present on the communications medium or media; however, a given device may be "addressable" in that it is configured to selectively exchange data with (i.e., receive data from and/or transmit data to) the network, based, for example, on one or more particular identifiers (e.g., "addresses") assigned to it. [0034] The term "network" as used herein refers to any interconnection of two or more devices (including controllers or processors) that facilitates the transport of information (e.g. for device control, data storage, data exchange, etc.) between any two or more devices and/or among multiple devices coupled to the network. As should be readily appreciated, various implementations of networks suitable for interconnecting multiple devices may include any of a variety of network topologies and employ any of a variety of communication protocols. Additionally, in various networks according to the present disclosure, any one connection between two devices may represent a dedicated connection between the two systems, or alternatively a non-dedicated connection. In addition to carrying information intended for the two devices, such a non-dedicated connection may carry information not necessarily intended for either of the two devices (e.g., an open network connection). Furthermore, it should be readily appreciated that various networks of devices as discussed herein may employ one or more wireless, wire/cable, and/or fiber optic links to facilitate information transport throughout the network.

[0035] The term "user interface" as used herein refers to an interface between a human user or operator and one or more devices that enables communication between the user and the device(s). Examples of user interfaces that may be employed in various implementations of the present disclosure include, but are not limited to, switches, potentiometers, buttons, dials, sliders, a mouse, keyboard, keypad, various types of game controllers (e.g., joysticks), track balls, display screens, various types of graphical user interfaces (GUIs), touch screens, microphones and other types of sensors that may receive some form of human-generated stimulus and generate a signal in response thereto.

[0036] The term "optical sensor" is used to define an optical device having a measurable sensor parameter in response to a characteristic of incident light, such as its luminous flux or radiant flux.

[0037] The term "broadband optical sensor" is used to define an optical sensor that is responsive to light within a wide range of wavelengths, such as the visible spectrum or other wide range of wavelengths as would be readily understood by a worker skilled in the art. [0038] The term "narrowband optical sensor" is used to define an optical sensor that is responsive to light within a narrow range of wavelengths, such as the red region of the visible spectrum, or other narrow range of wavelengths as would be readily understood by a worker skilled in the art.

[0039] The term "chromaticity" is used to define the perceived color impression of light according to standards of the Illuminating Engineering Society of North America.

[0040] The term "luminous flux" is used to define the instantaneous quantity of visible light emitted by a light source according to standards of the Illuminating Engineering Society of North America.

[0041] The term "spectral radiant flux" is used to define the instantaneous quantity of electromagnetic power emitted by a light source at a specified wavelength according to standards of the Illuminating Engineering Society of North America.

[0042] The term "spectral radiant power distribution" is used to define the distribution of spectral radiant flux emitted by a light source over a range of wavelengths, such as the visible spectrum, for example. In some embodiments, properties of the spectral radiant power distribution can also be associated with spectrum and color of a light source.

[0043] The term "radiant flux" is used to define the sum of spectral radiant flux emitted by a light source over a specified range of wavelengths.

[0044] The term "illumination system" is used to refer to a collection of one or more lighting fixtures or lighting units. Components of an illumination system can in some embodiments be networked, arranged to provide a desired combined effect, or otherwise interact with each other.

[0045] As used herein, the term "about" refers to a +/-10% variation from the nominal value. It is to be understood that such a variation is always included in any given value provided herein, whether or not it is specifically referred to.

[0046] It should be appreciated that all combinations of the foregoing concepts and additional concepts discussed in greater detail below (provided such concepts are not mutually inconsistent) are contemplated as being part of the inventive subject matter disclosed herein. In particular, all combinations of claimed subject matter appearing at the end of this disclosure are contemplated as being part of the inventive subject matter disclosed herein. It should also be appreciated that terminology explicitly employed herein that also may appear in any disclosure incorporated by reference should be accorded a meaning most consistent with the particular concepts disclosed herein.

Brief Description of the Drawings

[0047] In the drawings, like reference characters generally refer to the same parts throughout the different views. Also, the drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention.

[0048] FIG. IA and IB illustrate signal diagrams with pulse-width modulated PWM drive current signals evaluated in accordance with an embodiment of the present invention.

[0049] FIG. 2 illustrates a block diagram of a multi-channel lighting unit according to an embodiment of the present invention.

[0050] FIG. 3 schematically illustrates a lighting fixture for indirect lighting according to one embodiment of the present invention.

[0051] FIG. 4 schematically illustrates a setup of two luminaries according to an embodiment of the present invention.

[0052] FIG. 5 illustrates a flow chart showing a sequence of steps for a control method according to an embodiment of the present invention.

[0053] FIG. 6A and 6B illustrate block diagrams of apparatus for providing a modulated pulsed waveform according to embodiments of the present invention. Detailed Description

[0054] The present invention arises from the realization that various characteristics and properties (such as luminous flux and chromaticity) of mixed light emitted by a combination of light sources under operating conditions, while being subject to certain other operating conditions, can be maintained at a desired level by adjusting the drive current of the light sources via optical feedback. The light sources can have nominal optical characteristics that are substantially equal or different in at least some aspect. For example, they can fall into groups of light sources that can emit light of different nominal color, such as may comprise a multichannel lighting unit. Maintaining adequately consistent characteristics of light using optical feedback control requires adequate controller design and consideration of certain effects that may affect optical feedback control systems when configuring the controller including, for example, the degree of potential crosstalk between narrowband optical sensors and feedback sampling frequencies. Inadequate design and configuration can have undesired effects on system characteristics including instabilities of system parameters caused by the control system due to, for example, slow feedback response time or inaccurate estimates of system parameters.

[0055] The present invention seeks to overcome certain limitations of known optical feedback control systems, whereby the control signals for the drive currents of each array of one or more light sources are independently configured using a suitable modification signal for each array. In this manner, upon detection of the output of the arrays, which will have encoded therein a respective modification signal, a controller can be configured to separate each array's contribution based on the respective modification signal. For this purpose, embodiments of the present invention employ pulsed drive currents for operating the light sources. It is noted that different arrays may, but do not have to, correspond to different light colors.

[0056] More generally, Applicants have recognized and appreciated that it would be beneficial to discriminate or separate properties of different color light sources of a mixed light, based on observing and discriminating identifiable time-varying aspects of light output by one or more component light sources providing the mixed light. By discriminating properties of the component light sources, optical feedback can be facilitated.

[0057] In view of the foregoing, various embodiments and implementations of the present invention are directed to configuring the drive currents of arrays of one or more light sources according to a pulsed waveform, the pulse widths or switching times of which can be controlled according to a modification signal. The pulse widths or intervals between switching times for an array can vary in time according to an identifiable modification signal for that array, resulting in a time-varying component of light from the array which can be detected and filtered to separate or discriminate light emitted substantially or primarily due to that array. Thus, optical feedback can be obtained for different arrays while providing a desired lighting effect.

[0058] In some embodiments of the present invention, as the drive currents for each array are pulsed, initially the pulse widths for the drive currents for each array are determined based on the desired luminous flux and chromaticity of the mixed light. According to embodiments of the present invention, initially, the drive current pulses may be determined according to a number of different pulse generation methods including pulse width modulation (PWM), pulse code modulation (PCM), pulse density modulation (PDM) or pulse amplitude modulation (PAM), for example. In order to be able to discriminate the amount of light contributed to the mixed light by each array, a modification signal is used to mark or uniquely identify the light contributed by each array.

[0059] According to various embodiments of the present invention, modification signals are used to adequately modulate the pulsed drive currents for each array so that the pulse widths of the respective drive current pulses of an array are modulated in accordance with the respective modification signal. Hence pulse widths of the drive currents provided to the light sources are determined by both the initial pulse generation method and the modulation prescribed by the modification signal. For example, the drive current pulse widths can be determined by a sum, difference, Boolean function, or other function of aspects of the initial pulses and the modification signal. [0060] The drive current pulse widths can be modulated by applying modulation to an initial waveform to induce periodic or quasi-periodic variation of one or more of the duty factor, pulse density factor, or modulation frequency of the initial waveform. Variation of the pulse widths may be primarily or solely due to the format of the modification signal. For example, this may occur when the initial pulses, before modulation, are determined according to a static PCM method which does not affect the shape of a pulse but rather shapes pulse sequences by omitting certain pulses from an otherwise uniform periodic sequence of pulses. In other embodiments, for other pulse generation methods, such as for PWM, the pulse widths of the drive currents may be determined by both the initial pulse widths as determined by the PWM method and the pulse width modulation due to the modification signal.

[0061] A modification signal can be used to modulate the drive current in such a way that the overall moving average radiant flux per array remains as desired. For example, the moving average can be an average over a finite characteristic amount of time. The characteristic time is of an adequate duration to be useful for encoding a modification signal that can be used for reliable feedback control as well as for providing a figure of merit that is useful for quantifying perceptible flicker. Alternatively, other averaging techniques as would be understood by a worker skilled in the art, such as weighted moving averages, exponential moving averages, trended, smoothed or censored averages, can be used in embodiments of the present invention.

[0062] In one embodiment, for example, if a PWM drive current is used and the PWM drive frequency is n Hertz, where n may be 10³ to 10⁵ Hz, the PWM duty factor can be modulated by the modification signal at m Hz, where m is less than n, and m is desirably greater than, for example, 100 Hz to avoid perceptible flicker, or impressions of pumping or pulsation. If the drive current pulse widths in different arrays are modulated at different frequencies, then one bandpass filter per array each with a bandpass center frequency that substantially matches the respective modulation frequency can be employed to adequately and reliably determine how much radiant flux is emitted per array while only using a single broadband photosensor.

[0063] The present invention can be implemented in a partially or entirely digital fashion. This can provide a number of system design benefits including reduced heat dissipation in drive current circuitry, power transistors may be operated in saturated mode, a single drive current pulse frequency may be used substantially irrespective of the number of arrays and, for example, ready availability of microcontrollers with hardware PWM circuitry.

[0064] The modulation may be provided by a number of different types of modification signals including a sinusoidal harmonic wave, a rectangular wave form, a staircase wave form, a triangular or saw-tooth wave form and the like, for example. It is noted that digital or discrete control methods may require discretizing the modification signals at certain times and using discretized values of the modification signal in order to determine the modulation of the pulse width of the respective pulse. Discretization can be performed on signal values, switching times, or a combination thereof, for example to obtain a representation of an initial waveform by a piecewise constant waveform.

[0065] Figures IA and IB illustrate examples of modification signals according to some embodiments of the present invention. Figure IA illustrates single frequency sinusoidal modulation and Figure IB illustrates symmetric rectangular modulation. In the first row of Figures IA and IB the respective initial pulsed signals 110 and 210 and the respective modification signals 111 and 211 are illustrated. Figure IA also includes the discretized modification signal 112. In the second row, the respective modulated signals 113 and 213, and in the third row the respective difference signals 115 and 215, being the difference between the initial pulsed signal and the modulated signal, are illustrated.

[0066] In digitally controlled implementations according to the present invention, modulating an initial pulsed signal may require periodically increasing and decreasing the pulse widths or pulse densities by the equivalent of one or more bits as determined by a controller, for example. The pulse width of a PWM waveform, or the pulse density of a PCM waveform may be digitally modulated in this manner.

[0067] Various forms of modification signals other than described above may be used without departing from the scope and spirit of the invention. It is furthermore noted that the instances when a modification signal may be discretized, if necessary, may vary. As an example and as illustrated in Figure IA, the sinusoidal modification signal 111 is discretized at the beginning of each pulse to achieve the discretized modification signal 112. Discretization of a modification signal for the purpose of obtaining a modulation of a particular pulse may occur at the beginning, the center, the end or elsewhere of the initial pulse, for example. The center of a pulse may be determined in a number of different ways depending on the specific pulse generation method (PWM, PCM, PDM, PAM etc) used to generate the initial signal. It is noted that the period of the modulation signal may, but is not required to be an integer multiple of the period of the drive current pulses. Equally, the modulation signal may, but is not required to be in a fixed phase relationship with the drive current pulses. It is noted that a discretized sine wave or similar modification signal may be used in comparison to rectangular or binary modification signals to help reduce generation of potential undesired sideband harmonics of the light source drive currents or corresponding fluctuations in the emitted light.

[0068] The examples illustrated in Figures IA and IB are based on signals which are initially pulse width modulated (PWM) in order to achieve a desired time averaged drive current. Also, in the examples illustrated in Figures IA and IB, the pulse widths of all pulses of the initial pulsed signals 110 and 210 are the same.

[0069] In many embodiments, the pulse widths of the initial signal pulses are shaped in accordance with a PWM method, for example, as illustrated in Figures IA and IB and can vary during operation over time beyond the modulation introduced by the modification signals as, for PWM, the pulse widths are determined in accordance with desired drive current conditions. For example, the pulse widths of the pulses of the initial signals may need to be varied in order to change the luminous flux of the emitted light while dimming, or to change the chromaticity of the emitted light, or even when it is desired that the luminous flux and the chromaticity of the emitted light are to remain the same but in order to compensate for otherwise undesired changes to the light emitted by the one or more light sources. For example, these undesired changes may be caused by variations in certain operating conditions of the respective light sources, which may include, for example, the operating temperature of the light sources or other operational changes, such as failure of certain light sources, as would be readily understood by a worker skilled in the art. [0070] In one embodiment, the pulse widths of the initial signal may be dilated or contracted in accordance with the modification signal. According to the present invention the strength of the modulation provided by the modification signal may not need to be at the same scale as indicated in Figures IA or IB. It may be sufficient to modulate the pulse widths by merely a few percent of the nominal maximum pulse width, wherein the nominal maximum pulse width may be defined differently for different pulse generation methods. The magnitude of the pulse width modulation introduced by the modification signal may depend on the employed pulse generation method.

[0071] The shape and/or duty cycle of the pulsed drive current can vary over time in order to provide a desired average lighting effect. For example, if a discrete number of pulsed drive current waveforms can be produced, each having its own duty factor, an effective intermediate duty factor can be achieved by switching between two or more such pulsed drive current waveforms.

[0072] The strength of the modulation provided by the modification signal may be determined dynamically in accordance with a predetermined method depending on the desired average pulse width, and depending on the employed pulse generation method. It is noted that the strength of the modulation of the pulse widths due to the modification signal may affect or limit the range operating conditions. Modulating very small as well as very large pulse widths may in some instances require deviating from, for example, the desired operating conditions of the light sources in order to be able to provide feedback signals with adequate signal-to-noise ratios. Determining the strength of the modulation can comprise balancing several operational requirements, such as power consumption, flicker control, quality and range of feedback, signal-to-noise ratio of the feedback signals, amount of crosstalk, and the like. In some embodiments of the present invention, the strength of the modulation can be affected by changing the absolute or relative amount of temporal variation in all or a portion of the pulsed drive current waveforms. For example, if the duty cycle is being modulated, then the amount by which the duty cycle changes, either in absolute terms or in proportion to the average duty cycle, can be related to the modulation strength, which can then be varied during operation. As another example, the modulation strength can additionally or alternatively be related to the amount of time modulation is applied, for example modulation can be applied every n out of Λ/ cycles, or every s out of S seconds, for some predetermined values of n, N, s and S.

[0073] The contribution of the light emitted by each array under operating conditions to the mixed light may be determined by adequately filtering the mixed light, for example, by electronically filtering the output of one or more sensors in response to the mixed light. Based on the modification signal used to modulate the control signals of the one or more arrays of light sources, adequate filtering may include, for example, using electronic filters with center frequencies about equal or proximate to the respective modification signal frequencies of the respective control signals. Filtering may be used to discriminate the light contributions that correspond to each of the different arrays by sampling the mixed light using, for example, one or more broadband optical sensors. The outputs of adequately configured electronic filters are substantially directly proportional to or otherwise correlated with the respective light outputs of the light sources of the associated arrays. Consequently, these filter outputs indicate how much light each array contributes to the total mixed light. The filter outputs can subsequently be used by the controller together with the desired luminous flux and chromaticity of the mixed light, in order to generate new control signals for each array. Digital filters useful for embodiments of the present invention may include a number of different filter types including Fourier, Kalman, Extended Kalman, Hidden Markov Model, Particle, infinite impulse response (MR), finite impulse response (FIR) or cascaded integrator-comb (CIC) filters and the like, for example. Particulars of digital filters, including Kalman, HMM, Particle, MR, FIR or CIC filters, are well known to those skilled in this art.

[0074] For example, Kalman filters can be configured to extrapolate the radiant flux per array during times when the drive current for that array is not modulated based on previously acquired data. In a similar fashion, Kalman filters can be configured to predict the radiant flux of the mixed light in consequence of the extrapolated radiant flux contributions per array in accordance with a linear dynamic systems model, for example.

[0075] The bandpass filters may be implemented using synchronous detection, as described in "Adapting Radio Technology to LED Feedback Systems", Proc. Solid State Lighting VII Conference, SPIE 6669, 666912 (2007), by Ian Ashdown and Marc Salsbury. In an example embodiment, the output of the sensor is multiplied by a reference signal comprising a periodic waveform such as a sine wave, for example, that may be phase shifted but otherwise synchronized with the modification signal. Upon low-pass filtering, the resultant signal comprises a DC component that is representative of the amplitude of the modification signal.

[0076] In one embodiment of the present invention, the modification signals for each of the one or more arrays are independent, thereby facilitating discrimination or separation of light from each of the one or more arrays. For example, the fundamental frequencies of each modification signal may be substantially different, and may moreover be non-integer multiples of one another, and/or the signals may be sufficiently phase shifted, applied at different times, differently shaped or otherwise distinguishable from each other using signal processing. As another example, modification signals can be independent if they are applied at different time intervals, having different distinguishing features, or otherwise displaying characteristics which make light from different light sources distinguishable. The filter output signals can therefore be considered practically unaffected by overlaps in the spectral power distributions of the light emitted by light sources in different arrays.

[0077] In one embodiment of the present invention the modification signals can be applied continuously to respective control signals. In another embodiment, the modification signals can be applied intermittently or in a time-multiplexed fashion, or according to other applicable multiplexing or multiple access methods as would be understood by a worker skilled in the art. Applying modification signals to two or more arrays at the same time requires different modulation frequencies for different arrays in order to be able to discriminate the amounts of light contributed to the mixed light by each array, however time multiplexing enables modulating the control signal of only one array at a time but with the same modulation frequency for all other arrays while still being able to discriminate the amounts of light contributed by each array.

[0078] For example, in one embodiment, the drive current signals fory arrays can be periodically frequency modulated for duration of T/j seconds every 7 seconds using the same modulation frequency for each array. An adequate combination of a single broadband photosensor and a single narrow-band bandpass filter may then be used to synchronously sample, per each time slice T/j, the strength of the modulation of the radiant flux for the respective array. In this example, multiple narrow bandpass electronic filters are not required, as the broadband photosensor output can be connected, for example, to a l-to-\ analog multiplexer and then low-pass filtered to obtain j time-averaged outputs corresponding to the measured signal components of they^' arrays.

[0079] More generally, the drive current for each of they^' arrays can be modulated for a duration of t, seconds, where ^t < T. Furthermore, in various embodiments, modulation of j some arrays may be overlapping or simultaneous, provided that the modulations can be separated using signal processing techniques as would be understood in the art. For example, by viewing the optical channel as a collision channel or collection of collision channels, various multi-access techniques can be used to configure sequential or simultaneous modulation of drive currents for multiple arrays.

[0080] In one embodiment, the control system can be configured to improve robustness against cross talk with other ambient lighting units that may cause interference. This may be relevant when two or more lighting units need to be used together, for example, when illuminating a space with a single lighting unit would otherwise be inadequate, and the feedback systems in one lighting unit may pick up light from other lighting units. According to an embodiment of the present invention an illumination system may be formed from a plurality of lighting units, wherein each lighting unit comprises an optical feedback control system whereby the control signal for each array of light sources corresponding to a particular array in a particular lighting unit is independently configured using a modification signal whose modulation frequency is different for each array and each lighting unit.

[0081] In one embodiment, modification signals may be pseudo-randomly applied to different arrays when, for example, time-multiplexed modulation is used or the frequencies for the pulse width modulation may be pseudo-randomly chosen within a predetermined range of frequencies. Furthermore, pseudo-random durations of modulation at proximate or substantially equal frequencies of the light generated by the light sources in different arrays and different lighting units, when using time-multiplexed modulation, may be used to discriminate between light originating from different arrays by correspondingly processing or filtering the sensed signal. Adequate signal processing methods, also using digital codes, for example, are known in the art. Randomization of modulation frequencies of modification signals and their application to arrays can reduce the chances for interference between different lighting units. For this purpose, embodiments may employ one or more electronic bandpass filters whose center frequencies are substantially equal to the modification signal frequencies of the drive currents for the light sources at respective times of modulation to discriminate between the radiant flux corresponding to each of the different arrays, for example different colors, of light sources of each lighting unit, from the sample of the mixed radiant flux output collected by a broadband optical sensor. The output of an individual bandpass filter is substantially directly proportional to the radiant flux output of the light sources of the associated array and lighting unit. This information can subsequently be used by the controller in each lighting unit together with the desired luminous flux and chromaticity of the emitted light from that lighting unit in order to generate subsequent control signals for each array of light source array.

[0082] Channel access techniques such as frequency hopping spread spectrum, carrier sense multiple access, TDMA, FDMA, CDMA and the like, can be used to configure random, pseudorandom or deterministic application of modification signals to different arrays, to allow multiple feedback channels over the optical channel.

[0083] For signal processing purposes, it may be necessary to detect certain harmonics in a sensed signal, for example. As is widely known, superimposed harmonics of input signals can be reliably extracted and separated from quasi random signals such as carrier or noise signals by decomposing the input signal into a series of harmonic frequency signals using various transformation techniques such as a Fourier Transformation, for example. As is known in the art, Fourier Transformations typically employing sinusoidal basis functions may be useful for an analysis depending on the nature of the to-be-investigated signal and the type of information to be extracted from the signal. Consequently, the strength of the pulse width modulations introduced by the modification signal can be made relatively small while remaining useful for feedback control purposes and also retaining desired utility of the emitted light for illumination purposes. In embodiments of the present invention, therefore, Fourier Transformations, Fast Fourier Transformations, or processing based on Fourier series analysis can be performed in support of discriminating, separating and measuring light. Such processing can be performed using a spectral filter, for example.

[0084] In one embodiment of the present invention, an adequate sensor and signal processing system for sensing light and processing the sensed signal can comprise a broadband photosensor and a predetermined number of bandpass filters for determining the modulated intensities and average continuous intensities for the light emitted by the different arrays of light sources. The optical sensor and signal processing system can comprise any type of passive or active analog or digital, discrete-time (sampled) or continuous-time, linear or non-linear, infinite impulse response (NR type) or finite impulse response (FIR type) digital or analog subsystem, or the like as would be readily understood by a person skilled in the art. In one embodiment of the present invention, filtering of the optical signal from the optical sensor can be performed using combinations of optical filters, electronic filter circuitry and digital filtering.

[0085] A lighting unit can be configured such that under operating conditions the one or more sensors receive light that substantially only originates from the lighting unit itself i.e. the lighting unit that the sensor(s) belong(s) to. Alternatively, the lighting unit can be configured such that under operating conditions its sensor(s) can also receive amounts of ambient light, for example, from other ambient light sources such as from a nearby second lighting unit or daylight, for example. The lighting unit can also be configured such that its sensors substantially primarily receive a portion of the light that it provides to illuminate objects and which is reflected back to the sensor(s). So configured lighting units can be designed, for example, to mix light effectively and provide a desired illumination pattern at predetermined distances from the lighting unit. As would be apparent to a worker skilled in the art, similar configurations can be applied to lighting fixtures.

Lighting Unit with Optical Feedback Apparatus

[0086] Figure 2 illustrates a block diagram of a lighting unit 10 according to an embodiment of the present invention. As illustrated, the lighting unit 10 includes arrays 20, 30 and 40 each array having one or more light sources 22, 32 and 42. In this embodiment the light sources 22, 32 and 42 can generate radiation in the red, green, and blue regions of the visible spectrum, and thus the lighting unit is multi-channel. It is noted that in other embodiments different arrays may comprise nominally equal color light sources. Alternative embodiments of the present invention can employ light sources with other than three different colors possibly including light sources of colors such as amber, pink, cyan or white, for example. The light sources 22, 32 and 42 can be thermally connected to a common heat sink or alternatively to separate heat sinks (not shown) for improved thermal management of certain operating conditions of the light sources 22, 32 and 42. Other thermal management apparatus, for example heat pipes, thermosyphons or other passive or active thermal management systems would be readily understood by a worker skilled in the art. Embodiments of the multi-channel lighting unit can also include mixing optics (not shown) for intermixing the light emitted by the different color light sources.

[0087] It is noted that when differently colored light sources emit light which is adequately mixed, controlling color and intensity of the mixed light is then a matter of controlling the amount of light provided by each of the same color light sources. The color of the mixed light can thus be controlled within a range of colors defined by the color gamut of the lighting unit. The color gamut is defined by the different color light sources within the multi-channel lighting unit subject to achievable operating conditions.

[0088] Continuing with respect to Figure 2, current drivers 28, 38 and 48 are coupled to arrays 20, 30 and 40, respectively, and are configured to separately supply current to the red light sources 22, green light sources 32, and blue light sources 42 in arrays 20, 30 and 40. A power supply 54 coupled to the current drivers 28, 38 and 48 can provide electrical power. The current drivers 28, 38 and 48 control the amount of drive current supplied to and hence the amount of light emitted by light sources 22, 32 and 42. The current drivers 28, 38 and 48 are configured to regulate the supply of current to each array 20, 30 and 40 separately so as to control properties of the combined mixed light, such as luminous flux and chromaticity.

[0089] In one embodiment, the current drivers 28, 38 and 48 provide a pulsed drive current, for example, a PWM, PCM or the like for controlling the luminous flux and chromaticity of the combined emitted light of the red light sources 22, green light sources 32, and blue light sources 42. The average drive current through light sources 22, 32 or 42 is determined by the amplitude of the pulses and the duty factor of the PWM control signal or pulse density in case of a PCM control signal, for example. Therefore, it is possible to control the amount of light generated by light sources 22, 32 or 42 by adjusting the duty factor or pulse density for each array 20, 30 and 40, respectively. Dimming or adjusting of the red light sources 22, green light sources 32, or blue light sources 42 affects the mixed radiant flux output of the multi-channel lighting unit. The current drivers can be current regulators, switches or other similar devices as would be known in the art. Alternate control techniques for controlling the activation of the light sources would be readily understood by a worker skilled in the art.

[0090] Those having skill in the art will recognize that the PWM or PCM or the like control signals generated by the controller can be implemented using computer software or firmware provided by a computer readable medium having instructions for determining the pulse generation control signal sequence. For example, computer readable media such as optical or magnetic storage media, RAM, ROM or the like can carry instructions readable by a generic or special-purpose computing device configured to carry out drive control. It will be readily understood that similarly configured computer software can be used to enable other aspects of the invention, such as processing optical signals and performing other methods and algorithms in accordance with various aspects of the present invention.

[0091] In one embodiment, current sensors 29, 39 and 49 are coupled to the output of current drivers 28, 38 and 48 and continuously sense the drive current supplied to the arrays 20, 30 and 40. The current sensors 29, 39 and 49 can comprise a fixed resistor, a variable resistor, an inductor, a Hall Effect current sensor, or other element which has a known voltage- current relationship and can provide an adequately accurate indication of the drive current.

[0092] In one embodiment, the instantaneous forward currents supplied to the arrays 20, 30 and 40 are measured by the current sensors 29, 39 and 49 which can communicate the sensed signals to a signal processing system 52 coupled to the controller 50. The signal processing system 52 can pre-process the drive current signals from the sensors 29, 39 and 49 and provide respective information to the controller 50. The signal processing system 52 can include analog- to-digital (A/D) converters, amplifiers, filters, microprocessors, signal processors or other signal processing devices as would be readily understood by a person skilled in the art.

[0093] In another embodiment of the present invention, the output signals from the current sensors 29, 39 and 49 are directly forwarded (not illustrated) to a controller for processing. In a further alternative embodiment, the peak forward currents for each array 20, 30 or 40 can be fixed to a pre-set value to avoid having to measure the instantaneous forward current supplied to arrays 20, 30 and 40. This forward current data may be useful, for example, for obtaining information about the current operative behavior of light sources, such as light output as a function of input current. Such information can be useful for feedback control.

[0094] The controller 50 is coupled to current drivers 28, 38 and 48. The controller 50 is configured to substantially independently adjust each average forward current by separately adjusting the duty cycles or pulse densities of each of current driver 28, 38 and 48. The controller 50 transmits control signals to each of current drivers 28, 38 and 48. The control signals determine the current generated by the current drivers 28, 38 and 48 which is supplied to red light sources 22, green light sources 32, and blue light sources 42, respectively. Pulsing of the drive current, which is intended to control the time-averaged amount of light emitted by the light sources, desirably is fast enough to avoid perceivable flicker.

[0095] The lighting unit 10 further includes a broadband optical sensor 60 for sensing the emitted light. In one embodiment, the output of the optical sensor 60 is coupled to the inputs of electronic bandpass filters 24, 34 and 44. The bandpass filters 24, 34 and 44 can be configured so that their nominal center frequencies are about equal to the frequencies of the modification signals used to modulate the pulse widths of the light source drive current pulses. The bandpass filters can be configured so that controller 50 can control their center frequencies. The optical sensor 60 provides a signal representative of the mixed radiant flux output of the emitted light. The optical sensor 60 can be responsive to the spectral radiant power distributions generated by the red light sources 22, green light sources 32, and blue light sources 42 so as to monitor the contributions of light sources 22, 32 and 42 to the mixed radiant flux output of the multi-channel lighting unit. The optical sensor can be a phototransistor, a photosensor integrated circuit, an adequately configured LED or a silicon photodiode with an optical filter etc.

[0096] In one embodiment of the present invention, a digital signal processing system can be integrated into the lighting unit. The digital signal processing system can be configured to evaluate the respective radiant flux contributions of each of the one or more arrays based on the modification signal used for each array.

[0097] In one embodiment of the present invention, the optical sensor is a silicon photodiode with an optical filter that has a substantially constant responsivity to spectral radiant flux for light within the practically relevant spectral range of light emitted by the light sources of the lighting unit.

[0098] Multilayer interference filters which may require substantially collimated light may optionally be used. In the present invention, the control signals for activation of the light sources are independently modified by the controller 50 with a modification signal as described, whose frequency may for example be different for different color light sources and which may be configured to be different from those used by other lighting units.

[0099] In another embodiment of the present invention, the optical signal representative of the radiant flux incident upon the optical sensor 60 can be electronically pre-processed with amplifier circuitry associated with the optical sensor or it can be processed by analog or digital means in the controller 50.

[0100] In one embodiment, a user interface 56 is operatively coupled to the controller 50 to obtain the desired values of luminous flux output and chromaticity of the output light from a user of the lighting unit. Alternately, the lighting unit can have predetermined luminous flux output and chromaticity values of the output light stored internally, for example, in some form of memory that is part of the lighting unit and which, for example, may be operatively coupled to the controller, for example through wired, wireless or network connectivity.

[0101] Figure 3 illustrates a lighting fixture including a lighting unit according to certain embodiment of the present invention. The lighting fixture 410 is used in an indirect illumination application in which an optical sensor 415 receives light which is reflected from an illuminated target surface 405, for example, a ceiling which is located at a certain distance. This setup can be useful in cases where, for example the light emitted by the lighting fixture is sufficiently mixed only once it reaches the target surface and it is therefore necessary to sense the light reflected from an illuminated surface. The lighting unit inside the lighting fixture can be calibrated, for example at installation, so that its controller can account for color shifts in the reflected light relative to the incident light which can arise from colored target surfaces. The optical sensor 415 can be combined with an adequate optical system (not illustrated), which can comprise a plastic lens mounted directly on top of a photosensor. The optical system can be used for imaging purposes such that the light from the target surface can be focused on the optical sensor 415. Depending on the optical system, it is possible to sense light within a certain solid angle and therefore a certain portion of the target surface. This application scenario allows for lighting fixture designs with reduced light mixing requirements within the lighting fixture.

[0102] Embodiments of the lighting unit that are suitable for direct illumination applications may be configured differently. In this case, the field of view of an optical system may include dynamic or moving objects including persons, for example. Different fractions of the total field of view may be occupied by dynamic objects depending on the size of the field of view. In such situations, the feedback control system of the lighting unit can be configured with adequate filter methods to separate changes in the sensed reflected light that are caused by moving objects from actual changes in the emitted light as it is sensed via the reflected light. Therefore, certain embodiments of the present invention can have control systems which can be configured to respond only to sensor signal changes of a certain speed such as caused by aging of the light sources while, for example, ignoring variations in light characteristics that occur on second or minute timescales.

[0103] Figure 4 schematically illustrates an illumination system including lighting fixture 11 and lighting fixture 12, each including a lighting unit according to various embodiments of the present invention. The light emitted by lighting fixtures 11, 12 may be reflected back from a surface towards the lighting units as indicated by arrows 13, such that light originating from one lighting fixture reaches the sensor(s) of the other lighting fixture or vice versa. This can potentially cause interference with the optical feedback system of the respective lighting fixture. According to one embodiment of the present invention a multi-channel lighting unit associated with lighting fixture 11 uses modification signal frequencies f_r>1, f_g>1 and f_b,i, that adequately differ from the modification signal frequencies f_{r 2}, f_t>,2 and f_gj2 used in a multichannel lighting unit associated with lighting fixture 12. This enables each lighting unit to discriminate light generated by it from light generated by another lighting unit.

Modification Signal

[0104] When adequately modulating the light source drive currents using a selected modification signal, radiant flux measurements can be performed without having to sequentially selectively turn ON or OFF different color light sources and without having to use different parameters for the initial pulse generation for different arrays. Accordingly, deviations in the luminous flux and chromaticity of the mixed light from the desired luminous flux and chromaticity, can be detected and compensated for by the controller. The measured radiant flux of the different arrays of light sources is substantially independent of practically relevant shifts in the center wavelengths of the light emitted by the light sources. Thus, while changes in the operating temperatures of the light sources may change the respective ratios of drive currents to radiant fluxes, changes in light source center wavelengths typically may not affect the measured radiant fluxes of the different color light sources in a way that is practically significant for purposes of the present invention.

[0105] The controller can control when and by how much the drive currents are modulated. In one embodiment, for example, the pulse widths of the drive currents may be modulated by a predetermined amount relative to a nominal pulse width. The controller can determine adequate pulse width modulation strengths for the modification signals based on instant pulses or time-averaged values of the drive currents supplied by the current drivers.

[0106] In one embodiment, accurately evaluating the radiant flux contributions from different arrays of light sources based on a single sensor signal obtained from a single broadband optical sensor sensing the mixed light can be achieved by processing respective components of the Fourier-transformed sensor signal. For a sensor with sufficient linear responsiveness across the range of operating conditions, the output signal is directly proportional to the input signal. In embodiments with such a sensor the strength of the output sensor signal modulations relative to the strength of the output sensor signal corresponds to the strength of the input signal modulations relative to the strength of the input signal. Therefore the strength of the output sensor signal can be inferred, for example, by dividing the strength of the output sensor signal modulations by the known ratio between the input signal modulations and the input signal. If, for practical purposes, the responsivity of the sensor is not sufficiently linear but still unambiguously correlates the output and the input signal, the correlation may be linearized, if necessary, which, for example, can be performed by a signal processing system or controller.

[0107] The bandpass filters can be implemented electronically or digitally in firmware based on, for example, the Goertzel algorithm or other efficient Discrete Fourier Transformation methods. Use of this technique for digital filtering is widely known in the art and described in, for example, "Digital Decoding Simplified," Eric Kiser, Circuit Cellar Issue 182, pp. 22-28, September 2005. In an embodiment of the present invention, the output of each bandpass filter may be sampled with a peak detector amplifier to determine the instantaneous radiant flux for the corresponding array. The output of each bandpass filter may also be further low- pass filtered to determine the time-averaged radiant flux output for each array or by way of Kalman filters to predict short-term changes in the radiant flux of the emitted light, for example.

[0108] The outputs of the bandpass filters are coupled to the controller, optionally via further processing and filtering means. The radiant flux or other aspects of light from each array of light sources can be inferred from these outputs. For example, the output of each bandpass filter can be in known or computable proportion to the total radiant flux of the light source modulated at the frequency to which the filter is tuned, as would be understood by a worker skilled in the art.

[0109] Based on the inferred radiant flux of each array of light sources upon filtering and processing, the controller can compensate for and adjust the amounts of drive current for the different arrays of light sources in order to maintain, for example, the luminous flux and chromaticity of the emitted light at desired levels. Also, the outputs of the bandpass filters can be operatively coupled to a proportional-integral-derivative (PID) or similar feedback loop circuit that can be implemented in firmware in the controller. Alternatively, the PID feedback loop circuitry (not shown) can be a separate component operatively connected to the controller.

[0110] Properly configured electronic bandpass filters with sufficiently narrow bandwidths can be effective in eliminating crosstalk between light from different color light sources, which can greatly improve responsiveness of the optical feedback loop. For example, filters with a sufficiently high Q-factor, representative of a ratio of filter center frequency to filter full-width half maximum bandwidth, can be effective for this purpose.

[0111] In a particular embodiment of the present invention, two or more of the modification signals may have the same modulation frequencies but are further modulated with orthogonal digital or analog codes such that complementary bandpass filters with the same center frequency are responsive to only one signal.

[0112] In some embodiments, a modification signal is configured using a digital code. In addition, a first digital code for configuration of a first modification signal can be orthogonal to a second digital code for configuration of a second modification signal. The first modification signal can be associated with a first array of light sources and the second modification signal can be associated with a second array of light sources.

Modulation Frequency Selection

[0113] In various embodiments of the present invention, implementation is based on an assumption of independence of modification signals. In one embodiment, to facilitate this independence, a lighting unit can perform a configuration operation. During the configuration the lighting unit can, for example, generate light and modulate light of an array of light sources as described at one or more predetermined modulation frequencies and subsequently process all sensed responses at the modulation frequencies and the corresponding harmonics. If the control system detects no response in the sensed signal other than that originating from the modulation of its own light, it can use these frequencies for subsequent additional modulation of the pulse widths of the already pulsed drive currents for its light sources. If it receives a response that does not correlate with the modulation of its own light sources, the lighting unit can change the one or more frequencies at which it modulates its own light sources and repeats the above operation until a sufficient number of available frequencies has been determined.

[0114] The configuration operation may include waiting for a random or predetermined period before modulation of light occurs, to allow fair sharing of modulation frequencies between lighting units. For example, "listen before talk" schemes such as carrier sense multiple access protocols, including variants such as collision detect and collision avoidance, can be adapted for this purpose.

[0115] In some embodiments of the present invention, an illumination system includes a plurality of lighting units, wherein each lighting unit may scan for signals at frequencies in a predetermined sequence, accumulate an adequate number of free frequencies and use these free frequencies for subsequent modulation of its light sources. The modulation frequencies of a particular lighting unit can have a predetermined relationship that clearly identifies all other modulation frequencies used by that lighting unit, for example when only one frequency is known. Thus, other lighting units can predict and subsequently avoid frequencies known to be in use.

[0116] If the optical sensor of one lighting unit receives enough light from another lighting unit, that lighting unit can also be configured to detect the carrier signal frequencies of the other lighting unit and reconfigure itself as described if this is necessary to avoid interference. It is noted, however, that two or more lighting units may also be used to communicate additional information with each other via the emitted light and received light.

[0117] In another embodiment, two or more lighting units may be interconnected, via wired, wireless or networked connections, for control purposes, and signals may be passed between the lighting units to communicate information about, for example, used modification signal frequencies. The controller may be adapted to selectively turn OFF all but one array and monitor the output signals from the electronic filters to process the light emitted from that array in order to assign a unique center frequency to that array and the respective electronic filters during configuration.

[0118] Embodiments of the present invention can be configured to continuously, frequently, or intermittently evaluate usable modulation frequencies during a self-configuration procedure in order to avoid potential interference with other lighting units or with other light sources within a lighting unit, possibly due to sharing the same or similar modulation frequencies. For this purpose the control system of the lighting unit can be configured to include switching the lighting unit into a passive scan mode while sensing and scanning for a sufficient number of free available modulation frequencies. The control system can configure the lighting unit to enter the scan mode for a brief period of time, for example, during an initial phase subsequent to a switch ON of the lighting unit or during an OFF period. The control system can scan a predetermined frequency range for modulations in the sensed light according to a predetermined scheme until a sufficient number of free frequencies or bands of frequencies have been determined. The control system can retain freely available frequencies in a suitable memory device within the lighting unit. The controller can subsequently assign a free modulation frequency to each light source array, and use these frequencies to modulate the respective light source drive currents.

[0119] Figure 5 illustrates a flow chart comprising a sequence of steps of a control method for the controller to maintain the luminous flux and chromaticity of the light generated by a lighting unit according to an embodiment of the present invention. As illustrated a user of the lighting unit communicates the desired luminous flux and chromaticity of the emitted light to the controller by way of a user interface as shown in Step SIl. The user preference values are subsequently obtained by the controller in Step S12. At Step S14, the controller assesses whether any new desired luminous flux and chromaticity information for the emitted light have been input by the user. This assessment can be based on a comparison between the current values with the new values input by the user. If the user preference values have changed, the controller uses the new luminous flux and chromaticity obtained in Step S12. In the negative, the controller obtains the amount of instantaneous or time-averaged forward current supplied to each array from current sensors as shown in Step S16. During Step S18, the controller determines the pulse width modulation strength for each array, for example the absolute or relative temporal variation of portions of the pulse waveform. On the basis of these values and in accordance with the type of modification signal and the respective modulation frequencies, the controller determines in Step S20 the instant modulated pulse widths for each of the arrays. The control signals are subsequently modulated according to the respective pulse widths as indicated by Step S22. For computational efficiency, this may done, for example, by determining respective sequences of pulse widths in advance and which can be periodically repeated by respective current drivers unless the sequences are updated.

[0120] In Step S24 the controller processes the luminous flux and chromaticity of the emitted light as measured by the optical sensor. The signals received from the optical sensor, which are representative of the emitted light, are electronically filtered based on the modulation frequencies used for each array, thereby determining radiant flux of each array of light sources. The controller determines whether the measured luminous flux and chromaticity correspond to the desired luminous flux and chromaticity preferred by the user (Step S26). In the event that the user preference values match those measured by the optical sensor, the controller continues operating with the current parameters. However, if there exist discrepancies between the user preference values and the luminous flux and chromaticity of the emitted light measured by the optical sensor, the controller verifies whether new user preference values have been entered (Step S14) and provides a correction factor in the PWM control signals and modification signals based on the user preference values and the operating conditions, and repeats Step S16 to Step S24 as outlined.

Exemplary Modulation Method and Apparatus

[0121] Figures 6A and 6B illustrate two example apparatus, each corresponding to a method for modulating a pulsed waveform according to a modification signal. These examples arise from the realization that pulse widths of a pulsed waveform can be modulated in a variety of ways. For example, according to one method, portions of the pulsed waveform, for example a sequence of "on" portions, can be varied in their temporal widths. This can correspond to varying the duty cycle of the pulsed waveform. In this method, temporal variations can be restricted to having a local effect. For example, if the length of an "on" pulse is increased by an amount x, the cumulative length of the surrounding "off" pulses can be decreased by the same amount x. According to another method, the temporal widths of contiguous portions of the pulsed waveform, for example including portions of adjacent "on" and "off" pulses, can be varied together. This can correspond to varying the overall switching frequency of the pulsed waveform.

[0122] Figure 6A illustrates a pulse generator 610 which generates at its output 616 a pulsed waveform for driving a light source, the pulsed waveform having a variable period and duty cycle controllable by inputs 612 and 614, respectively. Construction of such a pulse generator would be understood by a worker skilled in the art. A signal 640 indicative of the desired duty cycle φ is provided at input 614 by the controller, for example in accordance with a desired intensity of light of the driven light source. The controller also provides a signal 620 indicative of the desired pulse waveform period T, which is provided as input 602 to a transformation module 600. The transformation module 600 also accepts as input 604 a time-varying modification signal 630 M(t) for modifying the period of the pulsed waveform. The transformation module 600 provides an output 606 that is a function of at least its inputs 602 and 604. The output 606 can be a weighted sum of inputs, a combination of weighted sums and products of inputs, or other function. Two examples are a weighted sum of inputs yielding output T+gM(t), and a weighted sum-product function yielding output T(l+gM(t)), where in each case g is an appropriate weighting factor based for example on the desired modulation strength. It is noted that, in each of these examples, if M(t) is a zero-mean signal, the output 606 has mean T. Output 606 of the transformation module is provided at input 612 of the pulse generator, thereby providing a time-varying period of the output waveform at output 616 in accordance with the desired period land the modification signal M(t). For example, portions 650a and 650b of the output waveform are illustrated, both portions having substantially the same duty cycle or average value, but different temporal lengths.

[0123] Figure 6B illustrates an alternative embodiment to Figure 6A, wherein the input 614 of pulse generator 610 is modulated according to the modification signal 630 M(t). A signal 620 indicative of the desired period 7^"is provided at input 612 by the controller. The controller also provides a signal 640 indicative of the desired duty cycle φ as input 602 to the transformation module 600. The transformation module 600 also accepts as input 604 a time-varying modification signal 630 M(t) for modifying the duty cycle of the pulsed waveform. The transformation module 600 provides an output 606 as described above with respect to Figure 6A, that is a function of at least its inputs 602 and 604. Two examples are a weighted sum of inputs yielding output φfgM(t), and a weighted sum-product function yielding output φ(l+gM(t)). Output 606 of the transformation module is provided at input 614 of the pulse generator, thereby providing a time-varying duty cycle of the output waveform at output 616 in accordance with the desired duty cycle φ and the modification signal M(t). For example, portions 655a and 655b of the output waveform are illustrated, both portions having substantially the same temporal length, but different duty cycles or average values.

[0124] The present invention may also be useful, for example, in embodiments where the initial pulse widths of pulse sequences per array are already modulated for the purpose of improving pulse width resolution as defined by adequate time averaging over sequences of pulse widths that otherwise only provide low instant pulse width resolution, such as described in International Patent Application Publication No. WO2006/039790, incorporated herein by reference. Moreover, the present invention may also be useful in embodiments where each initial drive current pulse of a pulse sequence that is shaped, for example, according to a PWM pulse generation method, is split into a number of shorter pulses. The shorter pulses can be distributed desirably more equally over the respective initial PWM period to desirably average the instant power load for arrays of light sources that may otherwise cause high power requirements at the beginning and low power loads at the end of respective synchronous pulses as described in International Patent Application No. PCT/CA2007/000408, incorporated herein by reference.

[0125] In another embodiment of the present invention, lighting units may also only comprise a single array of one or more similar light sources. In this configuration, the light sources can have nominally equal light emission characteristics for all lighting units. For example, the light sources can be white light LEDs employing photo-luminescent material such as certain phosphor materials, for example. The average intensities of each lighting unit can be maintained substantially constant despite changes in ambient temperature and/or possible light interference from other lighting units.

[0126] While several inventive embodiments have been described and illustrated herein, those of ordinary skill in the art will readily envision a variety of other means and/or structures for performing the function and/or obtaining the results and/or one or more of the advantages described herein, and each of such variations and/or modifications is deemed to be within the scope of the inventive embodiments described herein. More generally, those skilled in the art will readily appreciate that all parameters, dimensions, materials, and configurations described herein are meant to be exemplary and that the actual parameters, dimensions, materials, and/or configurations will depend upon the specific application or applications for which the inventive teachings is/are used. Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to the specific inventive embodiments described herein. It is, therefore, to be understood that the foregoing embodiments are presented by way of example only and that, within the scope of the appended claims and equivalents thereto, inventive embodiments may be practiced otherwise than as specifically described and claimed. Inventive embodiments of the present disclosure are directed to each individual feature, system, article, material, kit, and/or method described herein. In addition, any combination of two or more such features, systems, articles, materials, kits, and/or methods, if such features, systems, articles, materials, kits, and/or methods are not mutually inconsistent, is included within the inventive scope of the present disclosure.

[0127] All definitions, as defined and used herein, should be understood to control over dictionary definitions, definitions in documents incorporated by reference, and/or ordinary meanings of the defined terms.

[0128] The indefinite articles "a" and "an," as used herein in the specification and in the claims, unless clearly indicated to the contrary, should be understood to mean "at least one."

[0129] The phrase "and/or," as used herein in the specification and in the claims, should be understood to mean "either or both" of the elements so conjoined, i.e., elements that are conjunctively present in some cases and disjunctively present in other cases. Multiple elements listed with "and/or" should be construed in the same fashion, i.e., "one or more" of the elements so conjoined. Other elements may optionally be present other than the elements specifically identified by the "and/or" clause, whether related or unrelated to those elements specifically identified. Thus, as a non-limiting example, a reference to "A and/or B", when used in conjunction with open-ended language such as "comprising" can refer, in one embodiment, to A only (optionally including elements other than B); in another embodiment, to B only (optionally including elements other than A); in yet another embodiment, to both A and B (optionally including other elements); etc.

[0130] As used herein in the specification and in the claims, "or" should be understood to have the same meaning as "and/or" as defined above. For example, when separating items in a list, "or" or "and/or" shall be interpreted as being inclusive, i.e., the inclusion of at least one, but also including more than one, of a number or list of elements, and, optionally, additional unlisted items. Only terms clearly indicated to the contrary, such as "only one of" or "exactly one of," or, when used in the claims, "consisting of," will refer to the inclusion of exactly one element of a number or list of elements. In general, the term "or" as used herein shall only be interpreted as indicating exclusive alternatives (i.e. "one or the other but not both") when preceded by terms of exclusivity, such as "either," "one of," "only one of," or "exactly one of." "Consisting essentially of," when used in the claims, shall have its ordinary meaning as used in the field of patent law.

[0131] As used herein in the specification and in the claims, the phrase "at least one," in reference to a list of one or more elements, should be understood to mean at least one element selected from any one or more of the elements in the list of elements, but not necessarily including at least one of each and every element specifically listed within the list of elements and not excluding any combinations of elements in the list of elements. This definition also allows that elements may optionally be present other than the elements specifically identified within the list of elements to which the phrase "at least one" refers, whether related or unrelated to those elements specifically identified. Thus, as a non-limiting example, "at least one of A and B" (or, equivalently, "at least one of A or B," or, equivalently "at least one of A and/or B") can refer, in one embodiment, to at least one, optionally including more than one, A, with no B present (and optionally including elements other than B); in another embodiment, to at least one, optionally including more than one, B, with no A present (and optionally including elements other than A); in yet another embodiment, to at least one, optionally including more than one, A, and at least one, optionally including more than one, B (and optionally including other elements); etc.

[0132] It should also be understood that, unless clearly indicated to the contrary, in any methods claimed herein that include more than one step or act, the order of the steps or acts of the method is not necessarily limited to the order in which the steps or acts of the method are recited.

[0133] In the claims, as well as in the specification above, all transitional phrases such as "comprising," "including," "carrying," "having," "containing," "involving," "holding," "composed of," and the like are to be understood to be open-ended, i.e., to mean including but not limited to. Only the transitional phrases "consisting of" and "consisting essentially of" shall be closed or semi-closed transitional phrases, respectively.

What is claimed is:

Claims

1. A lighting unit (10) for generating a mixed light having a desired luminous flux and chromaticity, the lighting unit comprising: a. a first array (20) of light sources generating first light in response to a first drive current, b. a second array (30) of light sources generating second light in response to a second drive current; c. a first current driver (28) operatively coupled to the first array for supplying the first drive current thereto based on a first pulsed signal; d. a second current driver (38) operatively coupled to the second array for supplying the second drive current thereto based on a second pulsed signal; e. an optical sensor (60) for sensing the mixed light comprising a combination of the first light and second light and generating a sensor signal representative of the mixed light; and f. a controller (50) operatively connected to the first current driver, second current driver, and the optical sensor for generating the first pulsed signal and second pulsed signal (113, 213) based at least in part on (i) characteristics of the first light and second light respectively, and (ii) the desired luminous flux and chromaticity, the first pulsed signal comprising pulses modulated according to a first modification signal and the second pulsed signal comprising pulses modulated according to a second modification signal, the controller being configured to electronically filter the sensor signal based on the first modification signal and second modification signal, thereby determining optical characteristics of the first light and the second light.

2. The lighting unit of claim 1, wherein the controller is configured to generate the first pulsed signal using pulse width modulation or pulse code modulation.

3. The lighting unit of claim 1, wherein the first modification signal has a first frequency and the second modification signal has a second frequency different from the first frequency.

4. The lighting unit of claim 3, the controller further comprising two or more bandpass filters configured to electronically filter the sensor signal based on the first frequency and the second frequency.

5. The lighting unit of claim 1, wherein the controller is further configured to perform a configuration operation for configuring the first modification signal and/or the second modification signal so as to reduce interference from light external to the lighting unit.

6. The lighting unit of claim 1, wherein the first modification signal is representative of a discretized or a non-discretized waveform, said waveform selected from the group comprising sinusoidal, rectangular, triangular, staircase and saw-tooth waveforms.

7. A method for modulating light having a desired luminous flux and chromaticity, the method comprising: a. providing a light source (20) for generating light in response to a drive current; b. operating the light source with a pulsed drive current (113, 213); c. controlling the pulsed drive current by modulating pulses thereof (S22) while maintaining the desired luminous flux and chromaticity of the light (S26).

8. The method according to claim 7, wherein modulating pulses of the pulsed drive current comprises modulating pulse widths about a desired average pulse width.

9. A method for modulating light having a desired luminous flux and chromaticity, the method comprising: a. providing a first light source (20) for generating first light in response to a first drive current; b. operating the first light source with a first pulsed drive current; c. modulating pulses of the first pulsed drive current using a first modification signal; d. providing a second light source (30) for generating second light in response to a second drive current; e. operating the second light source with a second pulsed drive current; f. modulating pulses of the second pulsed drive current using a second modification signal; wherein said first pulsed drive current and said second pulsed drive current result in the first light source and second light source emitting a mixed light having the desired luminous flux and chromaticity.

10. The method according to claim 9, wherein modulating pulses of the first pulsed drive current comprises modulating pulse widths of the first pulsed drive current about a desired average pulse width.

11. The method according to claim 9, further comprising:

• mixing the first light and the second light into mixed light;

• sensing the mixed light and providing a sensor signal (S24);

• determining a first portion of the mixed light relative to the mixed light corresponding to the first light based on a correlation with the first modification signal detected in the sensor signal; and

• determining a second portion of the mixed light relative to the mixed light corresponding to the second light based on a correlation with the second modification signal detected in the sensor signal.

12. The method according to claim 11, wherein determining the first portion of the mixed light comprises the step of bandpass filtering of the sensor signal.

13. The method according to claim 11, further comprising selecting a modulation strength of the first modification signal to facilitate determination of the first portion of the mixed light.

14. The method according to claim 9, further comprising configuring the first pulsed drive current at least in part using pulse width modulation or pulse code modulation.

15. The method according to claim 9, wherein the first modification signal causes modulation at a first frequency and the second modification signal causes modulation at a second frequency, different from the first frequency.

16. The method according to claim 9, further comprising the step of configuring the first modification signal to be independent of the second modification signal.

17. The method according to claim 9, further comprising the step of configuring the first modification signal and/or the second modification signal so as to reduce interference from light external to the lighting unit.

18. The method according to claim 9, wherein the first modification signal and/or the second modification signal are configured according to a discretized or non-discretized waveform, said waveform selected from the group comprising sinusoidal, rectangular, triangular, staircase and saw-tooth waveforms.

19. A computer program product comprising a computer readable medium having recorded thereon statements and instructions for execution by a processor to carry out a method comprising the steps of: a. providing a first light source (20) for generating first light in response to a first drive current; b. operating the first light source with a first pulsed drive current; c. modulating pulses of the first pulsed drive current using a first modification signal; d. providing a second light source (30) for generating second light in response to a second drive current; e. operating the second light source with a second pulsed drive current; f. modulating pulses of the second pulsed drive current using a second modification signal; wherein said first pulsed drive current and said second pulsed drive current result in the first light source and second light source emitting a mixed light having the desired luminous flux and chromaticity.