Abstract:
Systems and methods are provided for operating a fluorescent tube having a pair of filaments. The system comprises a transformer, an AC signal generator and a coupling device. The AC signal generator is connected to drive a primary AC signal onto the primary winding of the transformer and to thereby create a secondary AC signal on the secondary winding of the transformer. The secondary winding of the transformer is coupled between the filaments of the fluorescent tube at first terminals thereof. The coupling device is coupled between the filaments at second terminals thereof. The coupling device is switchable between a conducting state, wherein a majority of the current associated with the secondary AC signal is conducted by the coupling device, and a non-conducting state wherein a voltage drop across the coupling device is sufficient to cause a majority of the current associated with the secondary AC signal to arc between the filaments of the fluorescent tube.

Description:
RELATED APPLICATIONS 
   This Application claims the benefit of the priority date of U.S. application No. 60/494,812 Aug. 14, 2003 under 35 USC § 119(e). 

   TECHNICAL FIELD 
   The invention relates to fluorescent lighting. Particular embodiments of the invention comprise systems for controlling one or more fluorescent light tubes. 
   BACKGROUND OF THE INVENTION 
   Fluorescent lights are well known in the lighting industry as efficient light sources. Fluorescent lights have a wide variety of domestic and industrial applications, including lighting rooms, lighting workspaces and lighting signs, for example. In general, fluorescent lights comprise one or more fluorescent tubes, each tube providing a separate light source. Fluorescent tubes can vary in size, with larger tubes generally drawing more power and providing more light. 
   As is well known in the art, fluorescent tubes are a gas discharge type of light source. A typical prior art fluorescent tube  10  is shown schematically in  FIG. 1 , along with its ballast  12 , its power supply  14  and its starter switch  20 . Ballast  12  conventionally comprises at least one ferromagnetic inductor  13 . Fluorescent tube  10  comprises a pair of filaments  16 ,  18  which typically have some slight resistance on the order of approximately 0.5–30 Ω. Tube  10  also contains a small amount of mercury (initially a liquid) and one or more inert gases, such as argon, which are under low pressure. 
   Lighting tube  10  involves creating current flow or “arc” through tube  10  between filaments  16 ,  18 . In fluorescent tubes commonly referred to as the “hot cathode” type, creating the current in tube  10  typically involves preheating at least one of filaments  16 ,  18  to cause thermionic emission of electrons. Filaments  16 ,  18  may be coated with various types of materials well known in the art to increase the amount of thermionic emission. Preheating filaments  16 ,  18  may be said to “boil off” electrons. In addition to preheating filaments  16 ,  18 , creating a current arc through tube  10  also typically involves providing a relatively large “ignition voltage” across tube  10 . The ignition voltage induces ionization of the inert gas in tube  10  and ignites the current flow between filaments  16 ,  18 . The required ignition voltage for a given tube  10  varies depending on many factors. Typical commercial fluorescent tubes of the hot cathode type operate with an ignition voltage in a range between 300–800 V AC RMS. The thermionic emission of electrons into tube  10  during preheating of filaments  16 ,  18  tends to reduce the required ignition voltage. Typically, the ignition voltage is provided between filaments  16 ,  18  by ballast  12 , which works together with starter switch  20  as explained briefly below. 
   During preheating, starter switch  20  is closed and AC current flows through inductive ballast  12 , filament  16 , switch  20  and filament  18 . This current preheats filaments  16 ,  18 , resulting in thermionic emission of electrons, and also builds up a magnetic field in the inductor  13  of ballast  12 . During preheating, there may also be some ionization of the gas in tube  10 ; however, the voltage across tube  10  (i.e. between filaments  16 ,  18 ) is not sufficient to create a current arc through the gas in tube  10 . Consequently, almost all of the current flows through starter switch  20  and correspondingly little or no current flows through tube  10 . 
   When a sufficient number of electrons have been thermionically emitted from filaments  16 ,  18  and sufficient magnetic field has been established in inductor  13  of ballast  12 , starter switch  20  is opened, briefly cutting off current flow through ballast  12 . When the current is cut off from ballast  12 , the magnetic field in the inductor  13  of ballast  12  collapses, causing an inductive voltage spike. This inductive voltage spike provides the ignition voltage across tube  10  (i.e. between filaments  16 ,  18 ), which in turn ionizes the gas in tube  10  and creates an arc of current between filaments  16 ,  18 . 
   Instead of flowing through starter switch  20 , current now flows through tube  10 . Current flow is maintained through tube  10  by electrons emitted from hot filaments  16 ,  18  and by the electrons and ionized gas particles in tube  10 . Filaments  16 ,  18  remain hot because of the emission of electrons. These moving ions and electrons provide energy to the mercury contained in tube  10 , converting some of the mercury from liquid to gas. Collisions between electrons and gaseous mercury atoms cause electrons in the gaseous mercury atoms to occupy higher energy states. When these mercury electrons return to their ground energy states, they release ultra-violet photons. Tube  10  is typically coated with phosphors (not shown), which absorb the ultraviolet photons. Absorption of ultraviolet photons causes the electrons of the phosphors to occupy higher energy states. When these phosphor electrons return to their ground energy states, they release photons in the visible spectrum. 
   When the arc is created through tube  10 , the resistance between filament  16  and filament  18  decreases. More specifically, the flow of electrons and ions through tube  10  creates collisions with other atoms, liberating more ions and electrons and facilitating the flow of more current. Inductive ballast  12  prevents damage to filaments  16 ,  18  and tube  10  by limiting the total current through tube  10 . Since power supply  14  typically provides a known AC signal, the inductance of inductor  13  of ballast  12  may be selected appropriately to limit the current through tube  10  to a desired level. 
   More recently designed fluorescent tubes, known as “rapid start” tubes, incorporate the same basic principles as the classical tubes described above. Other modern fluorescent tubes, known as “instant start” tubes, eliminate the preheating stage and ignite current flow through the tube with a corona discharge. The corona discharge associated with instant start tubes causes stress on tube components, particularly the filaments, and reduces the service life of the tube. Still other types of fluorescent tubes, known as “cold cathode” tubes incorporate relatively large, typically iron, electrodes. Cold cathode tubes require extremely large voltage drops between their electrodes to generate electrons through the impact of accelerated ions, which is referred to as “secondary emission”. These large voltages are a safety concern, particularly in multi-tube applications. Modern fluorescent tubes may also utilize more complex solid state electronic ballasts, which use high frequency switching techniques to provide ignition voltage and current regulation in a manner similar to classical inductive ballasts. 
   For some applications, such as industrial signage for example, there remains a general need for low cost control systems capable of independently controlling a plurality of fluorescent tubes. Because the ignition voltages of fluorescent tubes can be quite high, it is desirable to generate such voltages in close proximity to the tubes, and to thereby minimize the required voltage(s) on exposed wiring connections. 
   SUMMARY OF INVENTION 
   One aspect of the invention provides a system for controlling a light having a plurality of fluorescent tubes. The system comprises a plurality of transformers located in the light. Each transformer has a primary winding and a secondary winding coupled between the filaments of a corresponding one of the fluorescent tubes at first terminals thereof. The system also comprises a plurality of sensors. Each sensor is connected to detect signal information indicative of a signal between the filaments of a corresponding one of the fluorescent tubes. One or more AC signal generators are located remotely from the light. The one or more AC signal generators are capable of generating a plurality of low voltage AC signals. Each low voltage AC signal is associated with a corresponding one of the fluorescent tubes. The system also comprises a plurality of low voltage AC links. Each low voltage AC link is connected to conduct a corresponding low voltage AC signal between the one or more AC signal generators and the primary winding of a corresponding transformer of a corresponding one of the fluorescent tubes. A controller is located remotely from the light and is connected to receive signal information from the plurality of sensors. The controller configured, on the basis of the signal information, to independently control one or more characteristics of each of the low voltage AC signals. 
   The system may also comprise a plurality of coupling devices located in the light. Each coupling device may be associated with a corresponding one of the fluorescent tubes and may be coupled between the filaments of the corresponding fluorescent tube at second terminals thereof. Each coupling device may be changeable between a conducting state, wherein the coupling device is capable of conducting sufficient current to prevent arcing between the filaments of the corresponding fluorescent tube, and a non-conducting state wherein a voltage drop across the coupling device is sufficient to cause current to arc between the filaments of the corresponding fluorescent tube. Each coupling device may comprise a capacitor to form an LCR resonant circuit with the secondary winding of its corresponding transformer and the filaments of its corresponding fluorescent tube. 
   Each signal sensor may be connected to sense signal information relating to the low voltage signal associated with the corresponding fluorescent tube at a location remote from the light. For each signal sensor, the controller may be configured to estimate signal information related to the signal between the filaments of the corresponding fluorescent tube. The only electrical connections necessary to independently control operation of the plurality of fluorescent tubes may be the plurality of low voltage AC links. 
   Each signal sensor may be located in the light and may be connected to sense signal information relating to a secondary AC signal in the secondary winding of the corresponding transformer. The controller may be configured to estimate signal information related to the signal between the filaments of the corresponding fluorescent tube. The only connections necessary to independently control operation of the plurality of fluorescent tubes may be the plurality of low voltage AC links and a communication link for providing signal information from the sensors to the controller. 
   The controller may be configured to independently control operation of the plurality of fluorescent tubes using time division multiplexing. 
   Another aspect of the invention provides a system for controlling a light having a plurality of fluorescent tubes. The system comprises a plurality of transformers located in the light. Each transformer has a primary winding and a secondary winding coupled between the filaments of a corresponding one of the fluorescent tubes at first terminals thereof. The system also comprises a plurality of sensors located in the light. Each sensor is connected to detect signal information related to a signal between the filaments of a corresponding one of the fluorescent tubes. One or more AC signal generators are located in the light. The one or more AC signal generators are capable of generating a plurality of low voltage AC signals and driving a corresponding one of the low voltage AC signals onto the primary winding of a corresponding transformer of a corresponding one of the fluorescent tubes. A low voltage DC link provides a low voltage DC signal to the one or more AC signal generators. The system also comprises a controller located remotely from the light. The controller is connected to receive signal information from the plurality of sensors over a communication link. The controller is configured, on the basis of the signal information, to generate control signals and to communicate the control signals to the one or more AC signal generators over the communications link so as to independently control one or more characteristics of each of the low voltage AC signals. 
   Another aspect of the invention provides a method for controlling a light having a plurality of fluorescent tubes. A plurality of transformers located in the light are provided. Each transformer has a primary winding and a secondary winding coupled between the filaments of a corresponding one of the fluorescent tubes at first terminals thereof. A plurality of capacitors are provided. The capacitors are located in the light and each capacitor is coupled between the filaments of a corresponding one of the fluorescent tubes at second terminals thereof. The secondary winding, the capacitor and the filaments of each fluorescent tube form an LCR resonant circuit. The method comprises generating a plurality of low voltage AC signals having a relatively high frequency at a location remote from the light; conducting the plurality of low voltage AC signals to the light over a corresponding plurality of low voltage AC links; and applying each low voltage AC signal to the primary winding of a corresponding transformer of a corresponding one of the fluorescent tubes. The emthod also comprises decreasing a frequency of the plurality of low voltage AC signals until resonance in the corresponding LCR circuits causes the corresponding fluorescent tubes to ignite. 
   Further features of specific embodiments of the invention, aspects of the invention and applications of the invention are described below. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
     In drawings which illustrate non-limiting embodiment of the invention: 
       FIG. 1  is a schematic diagram of a prior art fluorescent tube; 
       FIG. 2  is a schematic diagram of a fluorescent light, together with a distributed light controller according to a particular embodiment of the invention; 
       FIG. 3  is a schematic circuit diagram of the LCR circuit associated with a fluorescent tube of the  FIG. 2  light; 
       FIG. 4  is a schematic diagram of a fluorescent light, together with a light controller according to an alternative embodiment of the invention; 
       FIG. 5  is a schematic diagram of a fluorescent light together with a light controller according to another alternative embodiment of the invention; and 
       FIG. 6  is a schematic block diagram of a method of preheating, igniting and controlling a fluorescent tube in accordance with a particular embodiment of the present invention 
   

   DETAILED DESCRIPTION 
   Throughout the following description, specific details are set forth in order to provide a more thorough understanding of the invention. However, the invention may be practiced without these particulars. In other instances, well known elements have not been shown or described in detail to avoid unnecessarily obscuring the invention. Accordingly, the specification and drawings are to be regarded in an illustrative, rather than a restrictive, sense. 
     FIG. 2  schematically depicts a multi-tube fluorescent light  50 , which comprises a distributed lighting control system  52  according to a particular embodiment of the invention. For clarity, light  50  is shown in  FIG. 2  to have only two fluorescent tubes  54 A,  54 B, it being understood that light  50  may generally include any practical number of tubes  54 . In addition, tubes  54  in light  50  may have different sizes, shapes, intensities and/or operating characteristics. Control system  52  may be located at a remote location, away from light  50 . In the illustrated embodiment, control system  52  is independently connected to each tube  54 A,  54 B of light  50  by a corresponding low voltage AC link  56 A,  56 B. In this description and the accompanying claims, the terms “low voltage AC link”, “low voltage AC connector” and similar terms are understood to mean an electrical connection rated to carry a maximum of 60V AC RMS and the term “low voltage AC signal” and similar terms are understood to mean an AC electrical signal having a voltage of less than or equal to 60V AC RMS. In preferred embodiments, the low voltage AC links and low voltage AC connectors of all of the systems described herein are rated to carry a maximum of 30 V AC RMS and the low voltage AC signals used by such systems have a voltage less than or equal to 30 V AC RMS. These connections and signals allow users to avoid the need for licensed electricians in order to install the systems of the invention. 
   In alternative embodiments explained in more detail below, control system  52  is also connected to light  50  by one or more communications links. Such communications links may be provided using wireless components and may carry information related to the control of light  50  by control system  52 . In other alternative embodiments explained in more detail below, control system  52  is connected to light  50  by one or more low voltage DC links. In this description and the accompanying claims, the term “low voltage DC link” and similar terms are understood to mean an electrical connection rated to carry a maximum of 84.8V DC and the term “low voltage DC signal” and similar terms are understood to mean a DC electrical signal having a voltage of less than or equal to 84.8V DC. In preferred embodiments, the low voltage DC links and low voltage DC connectors of all of the systems described herein are rated to carry a maximum of 42.4V DC and the low voltage DC signals used by such systems have a voltage less than or equal to 42.4V DC. These connections and signals allow users to avoid the need for licensed electricians in order to install the systems of the invention. 
   Each fluorescent tube  54 A,  54 B comprises an associated transformer  58 A,  58 B and an associated capacitor  60 A,  60 B, which, as explained further below, provide the drive signal to their respective tubes  54 A,  54 B. As shown in  FIG. 2 , transformer  58 A is connected between filaments  66 ,  68  of tube  54 A at first terminals thereof. Capacitor  60 A is connected between filaments  66 ,  68  at the other two terminals thereof. Transformer  58 B and capacitor  60 B are similarly connected to the terminals of tube  54 B. 
   In the  FIG. 2  embodiment, transformers  58 A,  58 B and capacitors  60 A,  60 B associated with each tube  54 A,  54 B represent the only components located within (or in close proximity to) light  50 . In alternative embodiments, light  50  also includes control related components (e.g. sensors and their associated circuitry), drive components and/or communications circuitry (i.e. for sending and receiving control signals from control system  52 ). As will be explained further below, the low voltage AC drive signals provided by control system  52  on low voltage AC links  56 A,  56 B are preferably in a range between 35–100 kHz. As such, transformers  58 A,  58 B may be made relatively small. Transformers  58 A,  58 B may be adjustable ferrite core transformers, which are suitable for this range of frequencies. The minimal circuitry located within (or in close proximity to) light  50  represents an advantage over prior art lights, which include cumbersome and expensive inductive and/or electronic ballasts, active drive circuitry, electronic controllers and communications circuitry. 
   Control system  52  includes a controller  64 . Controller  64  may be embodied by a wide variety of components. For example, controller  64  may comprise one or more programmable processor(s) which may include, without limitation, embedded microprocessors, dedicated computers, groups of data processors or the like. Some functions of controller  64  may be implemented in software, while others may be implemented with specific hardware devices. The operation of controller  64  may be governed by appropriate firmware/code residing and executing therein, as is well known in the art. Controller  64  may comprise memory or have access to external memory (not shown). 
   Control system  52  also includes a DC power supply  62 , which preferably outputs a DC signal between 15–90 V DC. DC power supply  62  may have a wide variety of embodiments, the particular form of which is not germane to the invention. For example, DC power supply  62  may be an off the shelf DC power supply which plugs into a wall outlet to convert AC power into a DC signal. Such a converter may be a switch mode power supply or a linear power supply. DC power supply  62  may comprise one or more batteries. DC power supply  62  may be controlled by controller  64 . Alternatively, DC power supply  62  may be controlled directly by a user. 
   DC power supply  62  provides DC power signals  66 A,  66 B to low voltage AC signal generators  63 A,  63 B, which are respectively associated with low voltage AC links  56 A,  56 B of tubes  54 A,  54 B. AC signal generators  63 A,  63 B may comprise any type of well known AC signal generator, such as half-bridge generators, full-bridge generators, single transistor chopper generators, thyristor-based generators, inverters and the like. AC signal generators  63 A,  63 B receive DC signals  66 A,  66 B from DC power supply  62  and receive control inputs  65 A,  65 B from controller  64 . In response to these inputs, AC signal generators  63 A,  63 B generate controllable primary AC signals  67 A,  67 B. Primary AC signals are low voltage AC signals which are provided to their associated low voltage AC links  56 A,  56 B. Using control inputs  65 A,  65 B, controller  64  can control AC signal generators  63 A,  63 B to independently adjust various characteristics of their respective primary AC signals  67 A,  67 B. Adjustable characteristics of primary AC signals  67 A,  67 B may include: the average value, RMS value, amplitude, waveform shape, frequency and/or duty cycle of the voltage and/or current associated with primary AC signals  67 A,  67 B. 
   Preferably, signal generators  63 A,  63 B generate primary AC signals  67 A,  67 B which are substantially sinusoidal in shape. However, primary AC signals  67 A,  67 B may vary in shape and may be square waves or saw tooth waves, for example. The waveform shape of primary AC signals  67 A,  67 B may be controlled by controller  64 . In general, the exact waveform shape of primary AC signals  67 A,  67 B (and the waveform shape of the signals across tubes  54 A,  54 B) will depend on the complex impedance of the load “seen” by these signals. 
   In alternative embodiments, some aspects of DC power supply  62  and AC signal generators  63 A,  63 B are combined. For example, control system  52  may include a combined DC power supply and AC signal generator for each fluorescent tube  54 A,  54 B. Alternatively, a single power unit may receive power from an external source and may provide a plurality of controllable low voltage AC signal outputs, with one such low voltage AC signal output corresponding to each fluorescent tube  54 A,  54 B. In yet another alternative embodiment, AC signal generators  63 A,  63 B may be combined into a single AC signal generator which receives DC power from a DC supply  62  and which provides a controllable low voltage AC signal for each fluorescent tube  54 A,  54 B. 
   In the illustrated embodiment of  FIG. 2 , control system  52  also comprises signal sensors  70 A,  70 B, which are respectively associated with AC signal generators  63 A,  63 B and primary AC signals  67 A,  67 B. Signal sensors  70 A,  70 B may sense one or more characteristics of their associated primary AC signals  67 A,  67 B and feed this signal information  72 A,  72 B back to controller  64 . In alternative embodiments explained in more detail below, signal sensors  70 A,  70 B are located in light  50  and are connected to sense one or more characteristics of secondary AC signals  71 A,  71 B on the secondary windings  53 A,  53 B of transformers  58 A,  58 B. 
   Those skilled in the art will appreciate that the measured characteristics of low voltage primary AC signals  67 A,  67 B are related to characteristics of corresponding high voltage signals through the terminals of fluorescent tubes  54 A,  54 B. Controller  64  may use measured signal information  72 A,  72 B to estimate characteristics of the high voltage signals through the terminals of fluorescent tubes  54 A,  54 B and to determine suitable control signals  65 A,  65 B for adjustment of low voltage primary AC signals  67 A,  67 B. Signal information  72 A,  72 B may comprise information about the average value, RMS value, amplitude, waveform shape, frequency and/or duty cycle of the voltage and/or current of primary AC signals  67 A,  67 B. Preferably, signal sensors  70 A,  70 B have current sensing capabilities and signal information  72 A,  72 B includes information related to the current of signals  67 A,  67 B, which in turn is related to the current through the terminals of fluorescent tubes  54 A,  54 B. 
   The operation of control system  52  and light  50  is now explained with reference to  FIG. 6 . It is assumed, for the purposes of this explanation, that control system  52  has received an indication that it is desired to light a particular fluorescent tube  54 A.  FIG. 6  schematically depicts a method  500  of preheating, lighting and controlling a particular fluorescent tube  54 A within light  50 . 
   The preheat phase is represented in  FIG. 6  by reference numeral  505 . As described above, DC power supply  62  provides DC signal  66 A to AC signal generator  63 A. DC signal  66 A is preferably in a range between 15–90 V. DC power supply  66 A may be controlled by a user or by controller  64 . At the outset of preheat phase  505 , in block  510 , controller  64  supplies AC signal generator  63 A with a control signal  65 A, which causes AC signal generator  63 A to output a low voltage primary AC signal  67 A over low voltage AC link  56 A. Initially, low voltage AC signal  67 A has a relatively low frequency. Preferably, at the outset of preheat phase  505 , primary AC signal  67 A has a frequency in a range of 85–100 kHz. 
   Low voltage AC link  56 A conducts primary AC signal  67 A to sign  50 , where primary AC signal is received on the primary winding  51 A of transformer  58 A. Transformer  58 A steps up primary AC signal  67 A to become a high voltage secondary AC signal  71 A on the secondary winding  53 A of transformer  58 A. Advantageously, primary AC signal  67 A is a low voltage AC signal and, because transformer  58 A is located within (or in close proximity to) light  50 , only low voltage AC connections are required between control system  52  and light  50 . The use of low-voltage AC connections(and low voltage AC signals) between control system  52  and light  50  (i.e. the omission of high voltage connections) substantially increases the operational safety of light  50 , particularly in applications where an electrical discharge may be of grave concern, such as in a petroleum refilling station for example. 
   The inductance L of the secondary winding  53 A of transformer  58 A, the resistances R 1 , R 2  of filaments  66 ,  68  and the capacitance C of capacitor  60 A form an LCR circuit  80 , best seen in  FIG. 3 . The capacitance C of capacitor  60 A and the inductance L of secondary winding  53 A are selected, such that during block  510  ( FIG. 6 ), the initial preheat phase low voltage primary AC signal  67 A and high voltage secondary AC signal  71 A are in a frequency range that is well above the resonant frequency of LCR circuit  80 . This selection of capacitor  60 A and transformer  58 A ensures that at the outset of preheat phase  505 , secondary AC signal  71 A signal creates a relatively small current flow through transformer  58 A, filaments  66 ,  68  and capacitor  60 A. In alternative embodiments, transformer  58 A and capacitor  60 A may be adjustable, such that capacitance C of capacitor  60 A and the inductance L of secondary winding  53 A may be adjusted to provide the characteristics described above. 
   In block  520 , controller  64  causes AC signal generator  63 A to controllably reduce the frequency of primary AC signal  67 A. As the frequency of primary AC signal  67 A decreases, the frequency of secondary AC signal  71 A also decreases and the current flow through LCR circuit  80  begins to increase. This increase in current flow is sensed by signal sensor  70 A, which feeds this current information back to controller  64  as part of signal information  72 A. Controller  64  continues to reduce the frequency of primary AC signal  67 A until it determines (in block  530 ) that the current flow through LCR circuit  80  has reached a desired preheat current level (as measured by sensor  70 A). The time required from start up until the desired preheat current level is reached may be on the order of 1–5 ms, for example. 
   In block  540 , controller  64  causes signal generator  63 A to maintain the frequency of primary AC signal  67 A, such that the desired preheat current level through LCR circuit  80  is maintained for a desired preheat period. The frequency of primary AC signal  67 A associated with the desired preheat current level may be in a range of 60–90 kHz. Controller  64  may stop reducing the frequency of primary AC signal  67 A as soon as it determines (on the basis of signal information  72 A obtained by sensor  70 A) that the current flow through LCR circuit  80  has reached the desired preheat current level. Alternatively, controller  64  may actively control the frequency of primary AC signal  67 A such that the current flow through LCR circuit  80  tracks the desired preheat current level. The preheat period of block  540  may have a duration in a range of 100–500 ms, for example. During the preheat period of block  540 , the desired preheat current flows through filaments  66 ,  68  causing filaments  66 ,  68  to heat up in an optimal and stress-free manner. 
   As discussed above, when filaments  66 ,  68  heat up to reach their thermionic emission temperature, which may be in the range of 800–2200 K, for example, electrons are thermionically emitted into tube  54 A. At the end of preheat phase  505 , a sufficient quantity of electrons has been emitted from filaments  66 ,  68  into tube  54 A and tube  54 A is ready for ignition. Because control system  52  is able to optimize the preheating of filaments  66 ,  68 , the ignition of tube  54 A is able to occur at a relatively low ignition voltage, with less stress on filaments  66 ,  68  and other parts of tube  54 A. 
   As frequencies of low voltage primary AC signal  67 A and high voltage secondary AC signal  71 A decrease during preheat phase  505 , the voltage drop across capacitor  60 A tends to increase, which increases the potential difference across tube  54 A (i.e. between filament  66  and filament  68 ). The capacitance C of capacitor  60 A and the inductance L of transformer coil  58 A are preferably selected, such that throughout preheat phase  505 , the frequencies of low voltage primary AC signal  67 A and high voltage secondary AC signal  71 A are maintained above the resonant frequency of LCR circuit  80 . In particular, the selection of capacitor  60 A and transformer  58 A ensures that during preheat phase  505 , the current flow through transformer  58 A, filaments  66 ,  68  and capacitor  60 A is sufficient to preheat filaments  66 ,  68 , but the voltage across tube  54 A (i.e. between filament  66  and filament  68 ) is well below the voltage required to ignite an arc through tube  54 A. In alternative embodiments, transformer  58 A and capacitor  60 A may be adjustable, such that capacitance C of capacitor  60 A and the inductance L of secondary winding  53 A may be adjusted to provide the characteristics described above. 
   When the preheat period of block  540  is complete, tube  54 A enters ignition phase  545 . In block  550 , controller  64  causes AC signal generator  63 A to once again decrease the frequency of primary AC signal  67 A. As the frequency of low voltage primary AC signal  67 A decreases in block  550 , the frequency of high voltage secondary AC signal  71 A also decreases and LCR circuit  80  approaches its resonance frequency. This decrease in frequency causes the magnetic field built up in transformer coil  58 A to collapse relatively rapidly and the voltage across capacitor  60 A to increase relatively rapidly. As secondary AC signal  71 A in LCR circuit  80  approaches its resonance frequency, a point is reached in block  560 , where the voltage drop across capacitor  60 A is sufficiently high to ignite an arc through tube  54 A (i.e. between filament  66  and filament  68 ). The frequency of low voltage AC signal  67 A at which ignition occurs may be in the range of 1–10 kHz above the resonance frequency of LCR circuit  80 , which may be 40–60 kHz, for example. During ignition in block  560 , the typical voltage across tube  54 A (i.e. between filament  66  and filament  68 ) may be in a range between 650–2500 V peak to peak. In general, the minimum required ignition voltage for tube  54 A determines design limitations on the capacitance C of capacitor  60 A and the inductance L of secondary winding  53 A of transformer  58 A. 
   When ignition occurs in block  560 , the gas (not shown) in tube  54 A is ionized, current flows and photons are produced as discussed above and as is well known in the art of fluorescent lighting. Once ignition of tube  54 A has occurred, the current path through tube  54 A (i.e. between filament  66  and filament  68 ) has a very low resistance. As such, a majority of the current of high voltage secondary AC signal  71 A travels through filament  66 , tube  54 A and filament  68 . A correspondingly little amount of current travels on the current path through capacitor  60 A. 
   After ignition, controller  64  continues to reduce the frequency of primary AC signal  67 A in block  570 , as tube  54 A transitions from ignition phase  545  into burn phase  565 . During burn phase  565 , the frequency of low voltage primary AC signal  67 A may be in the range of 35–60 kHz. The decrease in frequency of primary AC signal  67 A between the ignition frequency and the burn frequency may take approximately 10–20 ms, for example. 
   Once the frequency has been decreased in block  570 , controller  64  may attempt to optimize the light produced by tube  54 A and the power consumed by tube  54 A in block  580 . Signal sensor  70 A detects one or more characteristics associated with low voltage primary AC signal  67 A and provides this measured information to controller  64  as a part of signal information  72 A. As mentioned above, signal information  72 A may include information about the average value, RMS value, amplitude, waveform shape, frequency and/or duty cycle of the voltage and/or current of primary AC signal  67 A. In alternative embodiments, signal sensor  70 A is located in light  50  and connected to sense one or more characteristics of secondary high voltage AC signal  71 A on secondary winding  53 A of transformer  58 A. Controller  64  then uses signal information  72 A to produce control signal  65 A, which is provided to signal generator  63 A. Control signal  65 A may cause signal generator  63 A to adjust various characteristics of low voltage primary AC signal  67 A, such as, for example, the average value, RMS value, amplitude, waveform shape, frequency and/or duty cycle of voltage and/or current of signal  67 A. 
   Throughout block  580  of burn phase  565 , control system  52  may continue to monitor and control low voltage primary AC signal  67 A in real time and in the manner discussed above. Preferably, the object of controlling primary AC signal  67 A is to optimize the light produced, efficiency and/or power consumed by tube  54 A. In block  580 , controller  64  may also make use of feedback information from other sensors (not shown) to provide information useful for controlling the light, efficiency and/or power consumed by tube  54 A. Such sensors may include light sensors, temperature sensors and/or power sensors for example. Variations in operating conditions can affect the operation of tube  54 A and control system  52 . Variable operating conditions may include, for example, variances in temperature, fluctuations in DC output signal  66 A from DC power supply  62 , degradation of tube  54 A, which may be caused by emitter breakdown, mercury pressure variations, pressure changes of the gas within tube  54 A, and the like. Control system  52  attempts to compensate for these variances over the duration of burn phase  565  of tube  54 A. Feedback-based control techniques are well known in the art and are not discussed further herein. 
   An advantage of control system  52  and light  50  is that the total operational frequency range is preferably in a range between 35–100 kHz. This range of frequencies is in the portion of the RF/EMI spectrum allocated for unlicensed, unlimited power radiation levels. 
   As discussed above, fluorescent light  50  may comprise a plurality of fluorescent tubes  54 A,  54 B . . .  54 n. Fluorescent tubes  54 A,  54 B . . .  54 n may have different sizes, shapes, intensities and/or operating characteristics. Controller  64  preferably operates sufficiently fast that it can control the operation of all of tubes  54 A,  54 B . . .  54 n in real time. For example, controller  64  may make use of time division multiplexing techniques. Time division multiplexing is well known in the art and is not discussed further herein. 
   Control system  52  may also have other capabilities. For example, control system  52  may have a safety feature wherein signal sensors  70 A,  70 B . . .  70 n are capable of detecting if one of the filaments in a tube  54 A,  54 B . . .  54 n is broken or if one of tubes  54 A,  54 B . . .  54 n has been removed or unplugged. Using this information, controller  64  may ensure that no signal is provided to the low voltage AC link  56 A,  56 B . . .  56 n that is associated with the damaged or removed tube. 
   Control system  52  may also monitor the characteristics of individual fluorescent tubes  54 A,  54 B . . .  54 n over time. For example, control system  52  may individually monitor changes of ignition frequency, ignition current, burn frequency, burn current, power consumption, etc. of particular tubes  54 A,  54 B . . .  54 n over time. Such monitoring of tube characteristics may allow control system  52  to predict the end of the useful life of a tube. Control system  52  may provide diagnostic information including such tube characteristics to users over a suitable user interface (not shown). Such a user interface may be embodied in a wide variety of forms known to those skilled in the art. By way of non-limiting example, a user interface may include a suitably programmed computer, a visual interface, one or more LED&#39;s, a keypad input device and/or an ability to cause one or more of the fluorescent tubes  54 A,  54 B . . .  54 n to blink. 
   Because of its ability to independently monitor and control the current through individual tubes  54 A,  54 B . . .  54 n, control system  52  may permit additional “intelligent” functions, such as dimming and/or flashing of individual tubes  54 A,  54 B . . .  54 n and sequencing or pattern generation using individual tubes  54 A,  54 B . . .  54 n. 
     FIG. 4  schematically depicts a multi-tube fluorescent light  150 , which comprises a distributed lighting control system  152  according to an alternative embodiment of the invention. Light  150  comprises a plurality of fluorescent tubes  154 . Although only two such tubes  154 A,  154 B are shown in the illustrated embodiment, light  150  may generally comprise any practical number of fluorescent tubes  154 . Light  150  and distributed lighting control system  152  of  FIG. 4  are similar to light  50  and control system  52  of  FIG. 2 . Features of light  150  and control system  152  that are similar to features of light  50  and control system  52  are provided with similar reference numerals preceded by the digit “1”. Features of light  150  and control system  152  that are substantially the same as features of light  50  and control system  52  are not discussed further in this description. 
   The principal differences between light  150  of  FIG. 4  and light  50  of  FIG. 2 , are that signal sensors  170 A,  170 B are located within (or in close proximity to) light  150  and light  150  comprises extra communications links  157 A,  157 B, which provide for communication between signal sensors  170 A,  170 B and control system  152 . Signal sensors  170 A,  170 B may detect signal information  172 A,  172 B related low voltage primary AC signals  167 A,  167 B in the primary windings  151 A,  151 B and/or high voltage secondary AC signals  171 A,  171 B in the secondary windings  153 A,  153 B of their associated transformers  158 A,  158 B. Signal information  172 A,  172 B may include information about the average value, RMS value, amplitude, waveform shape, frequency and/or duty cycle of the voltage and/or current of low voltage primary AC signals  167 A,  167 B and/or high voltage secondary AC signals  171 A,  171 B. 
   In the  FIG. 4  embodiment, each signal sensor  170 A,  170 B communicates signal information  172 A,  172 B back to controller  164  via an associated communications link  157 A,  157 B. Communications links  157 A,  157 B are low power links and may be wireless connections. Control system  152  and each tube  154 A,  154 B may also comprise a small amount of suitably configured communications hardware (not shown) associated with communications links  157 A,  157 B. 
   Locating signal sensors  170 A,  170 B within (or in close proximity to) light  150  provides the possible advantage that the signal information  172 A,  172 B sensed may relate directly to high voltage secondary AC signals  171 A,  171 B and may therefore more closely approximate the actual signals through the terminals of tubes  154 A,  154 B. Consequently, signal information  172 A,  172 B may be independent of low voltage AC links  156 A,  156 B and transformers  158 A,  158 B. Such measurement may provide controller  164  with more accurate information about the actual current flow through the terminals of tubes  154 A,  154 B. 
   In other respects, light  150  and control system  152  comprise substantially the same components as light  50  and control system  52 , which function in a manner that is substantially similar to that described above. The characteristics of low voltage primary AC signals  167 A,  167 B may be controlled to individually preheat, ignite and burn each of tubes  154 A,  154 B in a manner similar to that described above for light  50  and control system  52 . Light  150  and control system  152  retain the important safety advantage that all electrical connections between control system  152  and light  150  are low voltage AC connections and low power communications connections, which improves the safety of light  150 . 
     FIG. 5  is a schematic diagram of a fluorescent light  250  together with its light controller  252  according to another alternative embodiment of the invention. Light  250  comprises a plurality of fluorescent tubes  254 . Although only two such tubes  254 A,  254 B are shown in the illustrated embodiment, light  250  may generally comprise any practical number of fluorescent tubes  254 . Light  250  and distributed lighting control system  252  of  FIG. 6  are similar to light  50  and control system  52  of  FIG. 1 . Features of light  250  and control system  252  that are substantially similar to features of light  50  and control system  52  are provided with similar reference numerals preceded by the digit “2”. Features of light  250  and control system  252  that are substantially the same as features of light  50  and control system  52  are not discussed further in this description. 
   The principal difference between light  250  and light control system  252  of  FIG. 5  and light  50  and light control system  52  of  FIG. 1 , is that both signal sensors  270 A,  270 B and AC signal generators  263 A,  263 B are located within (or in close proximity to) light  250 . Controller  264  provides control signals  265 A,  265 B to AC signal generators  263 A,  263 B over communication links  257 A,  257 B and signal sensors  270 A,  270 B use the same communication links  257 A,  257 B to provide signal information  272 A,  272 B to controller  264 . DC power supply  262  provides low voltage DC power to AC signal generators  263 A,  263 B via low voltage DC links  256 A,  256 B. 
   Locating sensors  270 A,  270 B within (or in close proximity to) light  250  allows sensors  270 A,  270 B to detect signal information  272 A,  272 B relating to primary AC signals  267 A,  267 B in the primary windings  251 A,  251 B and/or secondary AC signals  271 A,  271 B in the secondary windings  253 A,  253 B of their associated transformers  258 A,  258 B. Signal information  272 A,  272 B detected by sensors  270 A,  270 B may include information about the average value, RMS value, amplitude, waveform shape, frequency and/or duty cycle of the voltage and/or current of primary AC signals  267 A,  267 B and/or secondary AC signals  271 A,  271 B. 
   Communications links  257 A,  257 B are low power two-way communication links and may be wireless connections. Control system  252  and each tube  254 A,  254 B may also comprise a small amount of suitably configured communications hardware (not shown) associated with communications links  257 A,  257 B. 
   Locating signal sensors  270 A,  270 B within (or in close proximity to) light  250  provides the same advantages discussed above in relation to light  150  and control system  152  of  FIG. 4 . In other respects, light  250  and control system  252  comprise the same components and function in a manner that is substantially similar to light  50  and control system  52  described above. The characteristics of primary AC signals  267 A,  267 B may be controlled to individually preheat, ignite and burn each of tubes  254 A,  254 B in a manner similar to that described above for light  50  and control system  52 . Light  250  and control system  252  still retain the important safety advantage that all electrical connections between control system  252  and light  250  are low voltage DC connections and low powered communications connections, which improves the safety of light  250 . 
   As will be apparent to those skilled in the art in the light of the foregoing disclosure, many alterations and modifications are possible in the practice of this invention without departing from the spirit or scope thereof. For example:
         The embodiments described above comprise signal sensors, which sense signal information at various locations in the light or in the control system. Lights and/or control systems in accordance with the invention may include additional signal sensors. In some embodiments, signal sensors are provided in the light and in the control system. In addition, other types of sensors may be provided to sense other characteristics of the tubes within a light and feedback this information to the control system. For example, the control systems and lights described above could comprise power sensors, illumination sensors, temperature sensors and the like. Such sensors could provide additional information useful for feedback based control of fluorescent tubes.   In the embodiments described above, each fluorescent tube includes a transformer, which directly powers the tube. Other driver or interface electronics may be provided to power the tube. For example, a high voltage switch mode driver and fluorescent tube interface electronics may receive signals from the control system and may provide an output signal to the fluorescent tube.   Some of the embodiments described above include communications links associated with each tube. Those skilled in the art will appreciate that communication between each tube and the control system may be embodied with a single physical or wireless communications link and that information sent and/or received to/from each tube may be multiplexed on such a link.   While the embodiment of  FIG. 5  is depicted and described as having a low voltage DC link  256 A,  256 B associated with each of its fluorescent tubes  254 A,  254 B and their corresponding AC signal generators  263 A,  263 B, those skilled in the art will appreciate that only a single low voltage DC signal is required and such a low voltage DC signal is capable of powering a plurality of AC signal generators (e.g. AC signal generators  263 A,  263 B) or a single, multiple output AC signal generator (not shown). Accordingly, the  FIG. 5  embodiment only requires a single low voltage DC link  256  between control system  252  and light  250 .   All of the embodiments described above include a coupling capacitor between the filaments of the tube. For example, tube  54 A of light  50  comprises capacitor  60 A between two terminals of filaments  66 ,  68  (see  FIG. 2 ). Capacitor  60 A could be replaced with any suitably selected coupling device, capable of allowing current to pass during a preheat phase and then creating a large voltage drop during ignition and burn phases. Suitable coupling devices may include: a positive temperature coefficient thermistor, a bi-metallic switch, a bimetallic gas discharge switch, a controllable switching element or a resonant piezoelectric switch, for example. The switching action of the coupling device may be controlled by the controller.