Dimmable electrodeless light source

A dimmable electrodeless light source includes an electrodeless lamp, an electronic ballast and a dimming module. The light source further includes coupling transformers coupled to the electrodeless lamp for inductively coupling power to the lamp to generate light. An auxiliary winding electromagnetically coupled to the primary winding of at least one of the coupling transformers is driven by switching circuitry in the dimming module. The switching circuitry is pulse width modulated to control the average brightness of the light generated by the electrodeless lamp. An exemplary application for the dimmable electrodeless light source is as a backlight for a video display device, such as an liquid crystal display unit.

BACKGROUND OF THE INVENTION
 1. Field Of The Invention
 The present invention relates generally to electrodeless light sources and,
 more particularly, to a method and apparatus for dimming an electrodeless
 fluorescent light source.
 2. Background Of The Related Art
 This section is intended to introduce the reader to various aspects of art
 which may be related to various aspects of the present invention which are
 described and/or claimed below. This discussion is believed to be helpful
 in providing the reader with background information to facilitate a better
 understanding of the various aspects of the present invention.
 Accordingly, it should be understood that these statements are to be read
 in this light, and not as admissions of prior art.
 Conventional fluorescent lamps are driven with an electronic ballast which
 powers the lamps via electrodes disposed at each end of the lamp. The
 electrodes, however, are major life-limiting components of the fluorescent
 lamp. Electrodeless fluorescent lamps also are known. An electrodeless
 lamp is configured as a closed loop tube around which one or more coupling
 transformers are positioned. As with conventional fluorescent tubes, the
 electrodeless lamp is energized by an electronic ballast. However, rather
 than applying power to electrodes disposed at each end of a lamp tube, the
 ballast drives the coupling transformers, which, in turn, inductively
 couple the power to the lamp. The elimination of electrodes from the
 fluorescent lamp is particularly advantageous as it increases the life and
 reliability of the lamp and systems incorporating such lamps. Thus,
 electrodeless lamps are particularly useful in applications in which
 access to the lamps is restricted such that replacement of the lamps
 becomes difficult or expensive.
 Backlit video display devices are one type of application in which the
 access to the lamp is not readily available. Such video displays may be
 found in computer systems, automatic teller machines, information kiosks,
 gas pumps, shipboard controls, etc. To enhance viewing of displayed
 images, such video displays commonly include a backlight source to provide
 a brightly lit background that contrasts with the displayed image.
 However, such video displays often are located in environments in which
 the ambient lighting conditions vary considerably, interfering with vivid
 viewing of the displayed image. For example, in a dimly lit environment
 (e.g., a cloudy day, the enclosed interior of a ship, etc.), a brightly
 lit background provides for the best viewing of a displayed image.
 However, in a brightly lit environment (e.g., a sunny day, a well-lit
 office, etc.), a dimly lit background provides for better viewing.
 Accordingly, it would be desirable to provide the capability to control
 the brightness of the backlighting to compensate for variations in ambient
 lighting to enhance the viewing capabilities of the video display unit
 further. Unfortunately, electrodeless lamps rarely are used in such
 displays due to the lack of suitable means for dimming such lamps.
 The use of electrodeless fluorescent lamps is not limited to backlight
 sources for video display units or applications in which the lamp is not
 readily accessible. Electrodeless lamps also may be used in other types of
 applications requiring a light source, such as office or home lighting
 systems, desk lamps, etc. Moreover, if the control of the brightness of
 the light generated by the lamps in these other applications also is
 desirable, it would be advantageous to provide the capability to vary the
 brightness of the generated light in any type of lighting application in
 which an electrodeless fluorescent lamp is incorporated. Still further, it
 would be advantageous to provide a dimming module for electrodeless lamps
 that can be easily installed in existing electrodeless lighting systems to
 retrofit such systems with a brightness control capability
 The present invention may address one or more of the problems set forth
 above.
 SUMMARY OF THE INVENTION
 Certain aspects commensurate in scope with the originally claimed invention
 are set forth below. It should be understood that these aspects are
 presented merely to provide the reader with a brief summary of certain
 forms the invention might take and that these aspects are not intended to
 limit the scope of the invention. Indeed, the invention may encompass a
 variety of aspects that may not be set forth below.
 In accordance with one aspect of the present invention, a dimmable light
 source includes an electrodeless lamp to generate light, a ballast coupled
 to the lamp to energize the lamp, and a dimming circuit coupled to the
 lamp. The dimming circuit controls the amount of energy provided to the
 lamp to vary the brightness of the generated light.
 In accordance with another aspect of the present invention, there is
 provided a dimming circuit for an electrodeless lamp that is inductively
 coupled to a coupling transformer. The coupling transformer provides
 electromagnetic energy to cause the lamp to generate light. The dimming
 circuit includes an auxiliary winding electromagnetically coupled to the
 coupling transformer, a switch coupled to the auxiliary winding, and a
 drive circuit coupled to the switch. The drive circuit is configured to
 transition the switch between a conductive state and a non-conductive
 state during a time interval to control the brightness of the generated
 light. During the conductive state, a current-carrying path is established
 through the auxiliary winding and the switch.
 In accordance with still another aspect of the present invention, a
 dimmable display device includes an electrodeless lamp to generate light,
 a display unit to display an image and to use the light to enhance viewing
 of the image, and a dimming module coupled to the lamp. The dimming module
 controls the brightness of the generated light in response to a control
 signal, such as a signal representative of a detected amount of ambient
 light or a signal representative of a user-selected dimming setting.
 In accordance with yet another aspect of the present invention, there is
 provided a method for dimming an electrodeless lamp. The method includes
 inductively coupling energy to an electrodeless lamp to energize the lamp
 and generate light and restricting the coupling of the energy during a
 first time interval of a repetitive time period to dim the generated
 light.
 In accordance with a further aspect of the present invention, there is
 provided a method for making a dimmable electrodeless light source. The
 method includes providing an electrodeless lamp, attaching a coupling
 transformer to the lamp, and disposing an auxiliary winding on the
 coupling transformer. The method further includes coupling the auxiliary
 winding to a drive circuit that is configured to establish a
 current-carrying path through the auxiliary winding for a first time
 interval of a repetitive time period and to interrupt the current-carrying
 path through the auxiliary winding for a second time interval of the
 repetitive time period.

DESCRIPTION OF SPECIFIC EMBODIMENTS
 One or more specific embodiments of the present invention will be described
 below. In an effort to provide a concise description of these embodiments,
 not all features of an actual implementation are described in the
 specification. It should be appreciated that in the development of any
 such actual implementation, as in any engineering or design project,
 numerous implementation-specific decisions must be made to achieve the
 developers' specific goals, such as compliance with system-related and
 business-related constraints, which may vary from one implementation to
 another. Moreover, it should be appreciated that such a development effort
 might be complex and time consuming, but would nevertheless be a routine
 undertaking of design, fabrication, and manufacture for those of ordinary
 skill having the benefit of this disclosure.
 Turning now to FIG. 1, a block diagram of a dimmable light source 10 is
 illustrated. The dimmable light source 10 includes an electrodeless lamp
 12, a ballast 14, and a dimming module 16. The ballast 14 includes
 circuitry configured to provide energy to the electrodeless lamp 12 to
 cause the lamp 12 to generate light. The dimming module 16 includes
 circuitry configured to control the amount of energy provided to the lamp
 12 to control the brightness of the generated light.
 By way of example, the electrodeless lamp 12 can be an inductively coupled
 electrodeless fluorescent lamp, such as a lamp included in a lamp assembly
 available from OSRAM SYLVANIA Products, Inc., located in Danvers, Mass.,
 under one of the product names, ICETRON.TM. 100 and ICETRON.TM. 150, which
 are described in the SYLVANIA ICETRON Design Guide, Document No. FL022
 07/98. Such lamps are configured as closed loop vessels which use
 electromagnetic-induction technology to energize the lamp and generate
 light.
 Referring to FIG. 2, the lamp 12 is excited by an electromagnetic field
 produced by a pair of coupling transformers 18 and 20. The coupling
 transformers 18 and 20 are driven by the electronic ballast 14, such as
 the QUICKTRONIC.RTM. I.C.E. ballast available from OSRAM SYLVANIA which
 operates at a frequency of 250 kHz, or any other suitable electronic
 ballast. The ballast 14 receives input power from a conventional 120 VAC
 power line via a power plug 22.
 The coupling transformers 18 and 20 are substantially identical
 transformers, each of which include a respective ferrite core 24 and 26, a
 respective primary winding 28 and 30 and a respective secondary lamp
 winding 32 and 34. The secondary lamp windings 32 and 34 comprise the
 sections of the lamp vessel in the regions where the lamp vessel threads
 through the cores 24 and 26. The SYLVANIA ICETRON lamp assembly, for
 instance, includes both the lamp and the coupling transforners. The
 coupling transforners 18 and 20 advantageously have split cores, so that
 the transformers 18 and 20 may be disposed about the lamp tube and
 retained by clamps which secure the two halves of each core together, as
 will be discussed in further detail below. The interconnections of the
 windings of transformers 18 and 20 are illustrated in FIG. 3.
 Referring to FIG. 3, the primary winding 28 of the transformer 18 and the
 primary winding 30 of the transformer 20 are driven by the ballast 14. The
 primary winding 28 is connected in parallel with the series combination of
 a resistor 36 and the primary winding 30. The secondary lamp winding 32 of
 the transformer 18 and the secondary winding 34 of the transformer 20 are
 connected in series. The primary windings 28 and 30 are driven by the
 ballast 14 and electromagnetically couple energy from the ballast 14 to
 the secondary lamp windings 32 and 34, respectively. The secondary
 windings 32 and 34, which are provided by the lamp vessel, couple the
 energy to electrodeless lamp 12 to cause the lamp 12 to generate light. In
 this exemplary embodiment, the primary windings 28 and 30 each are
 eighteen turns of magnet wire, and each secondary winding is one turn of
 the lamp vessel. Accordingly, the turns ratio of the overall magnetic
 circuit is 18:2 (i.e., 9:1) in this exemplary embodiment.
 Referring again to FIGS. 2 and 3, the resistor 36 is connected in series
 with the primary winding 30 of the coupling transforners 20. The resistor
 36 is sized to present a minimum load impedance to the ballast 14 and, in
 one embodiment, has a value of 50 ohms. A minimum load impedance is
 desirable because conventional ballasts typically include protection
 circuitry which interrupt operation of the ballast upon detection of load
 changes. For example, a ballast may include a protection circuit to
 interrupt operation if a "no load" condition is detected. Further, the
 ballast electronics may include a protection circuit to interrupt
 operation if a short circuit condition on the ballast output is detected.
 Accordingly, the connection of the resistor 36 in series with the output
 of the ballast 14 ensures that the operation of the ballast electronics
 shall not be disturbed by the inclusion and/or operation of the dimming
 circuitry.
 Referring to FIGS. 2 and 3, the transformer 20 also includes an auxiliary
 winding 38. In the disclosed embodiment, the auxiliary winding 38 is made
 of four turns of magnet wire disposed about the core 26 of the transformer
 20. The auxiliary winding 38 is connected to a switch 40 in the dimming
 module 16. The module 16 further includes a drive device 42 for
 transitioning switch 40 between alternating conducting and non-conducting
 states. In the conducting state, a current-carrying path is established
 through the auxiliary winding 38 and the switch 40. In the non-conducting
 state, the current-carrying path is interrupted.
 The switch 40 can be any type of switching device capable of alternating
 between conductive and non-conductive states when driven by a drive
 device. For example, the switch 40 can be a cam-driven switch that is
 mechanically operated by a multi-lobed cam driven by a rotating shaft. The
 cam-driven switch can include mechanical provisions for varying the
 percentage of time that the switch is closed during each rotation cycle
 (i.e., the duty cycle). Alternatively, the switch 40 can be one or more
 switching transistors which are driven by appropriate electronic drive
 circuitry at a selected switching frequency. The electronic drive
 circuitry can include electrical provisions for varying the percentage of
 time that the transistor or transistors are closed during each frequency
 cycle.
 Regardless of whether the switch is mechanically driven or electrically
 driven, when the switch is closed, current flows through the switch and
 the auxiliary winding to create a short circuit. The short circuit
 condition is reflected onto the primary winding of the coupling
 transformer and prohibits, or substantially restricts, the inductive
 coupling of energy to the lamp. The average brightness of light generated
 by the lamp during each switching cycle (or shaft rotation) can thus be
 varied by adjusting the duty cycle of the switch. In the embodiments
 disclosed, the average brightness increases as the duty cycle of the
 switch (i.e., the percentage on-time) is decreased. Conversely, the
 average brightness decreases as the duty cycle of the switch is increased.
 In the embodiment disclosed in FIG. 2, the duty cycle of the switch 40 can
 be adjusted via a brightness adjustment device 44 coupled to the dimming
 module 16. The device 44 can be a potentiometer having a variable
 impedance, for instance. The brightness adjustment device 44
 advantageously is accessible to a user of the dimmable light source 10 and
 can include a panel-mounted control device, such as an adjustment knob,
 dial, or the like. In other embodiments, the brightness adjustment device
 44 may operate without user action by including a photodetector which
 detects ambient lighting conditions and provides an electrical signal
 representative of the lighting condition for example. The dimming module
 16 can be configured to adjust the duty cycle of the switch 40 in response
 to the electrical signal. The duty cycle may be adjusted, for instance, in
 discrete steps to provide for discrete brightness levels within a dimming
 range. Alternatively, the duty cycle may be continuously adjusted to
 provide for continuous variation of the brightness of the light over the
 dimming range. The relationship between an exemplary brightness adjustment
 device 44 and the electronic circuitry of the dimming module 16 will be
 explained in further detail below with reference to the schematic diagram
 of FIG. 4.
 Referring again to FIG. 2, the dimming module 16, which may be powered by a
 conventional auxiliary power supply 46 that converts a 120 VAC input to a
 12 VDC output, may be connected to a frequency adjustment device 48.
 Device 48 can include a potentiometer having a panel-mounted,
 user-accessible control device (e.g., an adjustment knob or dial) that
 provides for adjustment of the switching frequency of the switch 40. The
 frequency adjustment feature is particularly advantageous when the
 lighting source 10 is used as a backlight for a video display because
 adjustment of the switching frequency of the switch 40 can eliminate
 visual artifacts that may be visible on the video display due to
 electromechanical, electrical and/or optical coupling effects caused by
 the switching of the dimming circuitry. For example, the user of a
 dimmable light source or of a backlit video display device may perceive a
 flicker effect in the lighting that is caused by the interruption of
 generated light by the dimming circuitry. To eliminate the flicker, the
 switching frequency may be adjusted to a rate that is sufficiently fast
 such that the flicker cannot be perceived by a user. It has been found
 that a switching frequency of approximately 120 Hz is particularly
 suitable to avoid flicker.
 Adjustment of the switching frequency of the switch 40 may also be
 desirable to synchronize the switching frequency with the frequency of the
 vertical refresh video signal of a video display unit. In some instances,
 if the switching frequency is not synchronized with the vertical refresh
 rate, the user may perceive visual artifacts on the display, such as
 scrolling lines. A panel-mounted control device for varying the switching
 frequency can allow the user to eliminate the undesirable video effects.
 A manually controlled, user-accessible switching frequency adjustment
 device is optional. For example, the switching frequency adjustment device
 48 can be an automatic device or circuit that automatically adjusts the
 switching frequency in response to a particular parameter. For instance,
 in an embodiment of the invention in which the dimmable light source 10 is
 installed as a backlight in a video display device, the frequency
 adjustment device 48 can be replaced with a frequency synchronization
 circuit, such as a conventional phase locked loop, that has an input for
 receiving the vertical refresh video signal of the display unit. In such
 an embodiment, the frequency synchronization circuit can automatically
 synchronize the switching frequency to the vertical refresh rate.
 Regardless whether the switching frequency is automatically or manually
 adjusted, the adjustment may be performed in discrete steps over a range
 of frequencies, or the adjustment may be continuous over the range. In
 other embodiments of the invention, the switching frequency can be set at
 a fixed frequency (e.g., approximately 120 Hz) which is known to eliminate
 or minimize the occurrence of most visual artifacts.
 Turning now to FIG. 4, a schematic of an exemplary electronic embodiment of
 the dimming module 16 is illustrated. It should be noted that the
 following description focuses on the functions of the primary components
 of the dimming module and does not discuss in detail the interconnections
 or the specific function of each individual electrical component
 illustrated in the schematic, as such details are conventional and would
 be clearly understood by any person of ordinary skill in the art who
 reviews this description and the accompanying FIGURES. Further, it should
 be understood that the specific circuitry illustrated is merely one
 example of a dimming module for adjusting the brightness of light
 generated by an electrodeless lamp. It is currently believed that the
 functions performed by the various electrical devices could be performed
 by other conventional devices arranged in other configurations, as would
 be well known by any person of ordinary skill in the art.
 The dimming module 16 includes a voltage regulator 50 (e.g., a conventional
 regulator, such as a MIC5205 available from Micrel) to regulate the 12 VDC
 input from the auxiliary power supply 46 (input via a connector 52) to a
 DC level (e.g., 10 VDC) appropriate for use by the other electrical
 components in the dimming module. The dimming module further includes a
 timer 54 (e.g., a MIC1555 available from Micrel), a pulse width modulator
 56 (e.g., a MIC502 available from Micrel), a driver 58 (e.g., a MAX4429
 available from Maxim), and a switch assembly 60 which includes a pair of
 switching transistors 62 and 64 (e.g., n-channel MOSFETS, such as
 IXFT26N50 available from IXFT) coupled to the auxiliary winding 38 via a
 connector 66.
 The pulse width modulator 56 provides a pulse width modulated signal to the
 driver 58, which provides the power to drive the MOSFET switches 62 and 64
 between conducting and non-conducting states. The gates 68 and 70 of the
 MOSFET switches 62 and 64 are connected to resistors 72 and 74,
 respectively, which prevent undesired oscillation of the switches 62 and
 64. The other ends of the resistors 72 and 74 are connected to an output
 76 of the driver 58. Diodes 78 and 80 are connected from the source to the
 drain of switches 62 and 64, respectively. The sources of the FET switches
 62 and 64 are connected together and to signal ground through a resistor
 82.
 The driver 58 provides a pulse width modulated waveform at its output to
 transition switches 62 and 64 between conductive (i.e., the driver output
 is at a HIGH level which is at or exceeds the turn-on threshold voltage of
 the switches 62 and 64) and non-conductive states (i.e., the driver output
 is at a LOW level which is at or below the threshold voltage to turn off
 the switches 62 and 64). The MOSFET 62 or 64 which is switched to a
 conducting state upon application of a HIGH level signal is determined by
 the polarity of the voltage reflected across the auxiliary winding 38 by
 the primary winding of the coupling transformer to which the auxiliary
 winding is coupled. That is, when the polarity of the voltage across the
 auxiliary winding 38 is such that the voltage at the drain 82 of the
 MOSFET 62 is positive with respect to the voltage at the drain 84 of the
 MOSFET 64 and a HIGH level signal is applied to the gates 72 and 74, the
 MOSFET 62 will transition to a conducting state. In this state, the MOSFET
 64 is in a non-conducting state and a current carrying path is established
 through the auxiliary winding 38, through the MOSFET 62, and through the
 diode 80 and the internal parasitic diode (not shown) of MOSFET 64.
 Conversely, when the polarity of the voltage across the auxiliary winding
 38 is such that the voltage at the drain 84 of the MOSFET 64 is positive
 with respect to the voltage at the drain 82 of the MOSFET 62 and a HIGH
 level signal is applied to the gates 72 and 74, the MOSFET 64 will
 transition to a conducting state. In this state, the MOSFET 62 is in a
 non-conducting state and a current carrying path is established through
 the auxiliary winding, through the MOSFET 64, and through the diode 78 and
 the internal parasitic diode (not shown) of MOSFET 62.
 Thus, whenever either one of the MOSFETS 62 and 64 are in a conducting
 state, a short circuit is established across the auxiliary winding 38
 which is reflected onto the primary winding of the coupling transformer 18
 or 20 to which the auxiliary winding is coupled. As a result, the
 inductive coupling of energy to the electrodeless lamp 12 is interrupted,
 which substantially interrupts the generation of light. Accordingly, the
 average light generated by the lamp 12 during one cycle of the switching
 frequency of the switch assembly 60 can be adjusted by varying the time
 that the switches 62, 64 are in a conducting state (i.e., the duty cycle)
 during that cycle.
 The switching frequency of the switch assembly 60 is set by a capacitor 86
 connected to an input 88 of the pulse width modulator 56 ("PWM"). The
 capacitor 86 cooperates with internal components of the PWM 56 to create
 an oscillator which generates a repetitive ramp-shaped voltage signal at
 the input 88 of the PWM. The repetition rate of the ramp-shaped voltage
 signal corresponds to the switching frequency of the switch assembly 60.
 The duty cycle at which the switches 62 and 64 are driven is determined by
 the PWM's comparison of a variable amplitude voltage signal applied at an
 input 90 of the PWM 56 with the ramp at the input 88. In one embodiment,
 the amplitude of the voltage signal is determined by a voltage divider
 circuit that includes a brightness adjustment device (i.e., a
 potentiometer 92) which is connected to the dimming module via a connector
 94. The value of the potentiometer 92 is selected such that the amplitude
 of the voltage signal at the input 90 of the PWM can be varied within a
 range that results in a duty cycle that is fully adjustable between 0% and
 100%. The brightness adjustment device advantageously is accessible to a
 user of the dimmable light source such that the user can select a desired
 brightness level of the light generated by the electrodeless lamp. For
 example, the brightness adjustment device can include a control device
 (e.g., an adjustment knob) mounted on a panel of an enclosure containing
 the lamp system or mounted in any location accessible by a user of the
 light source. Alternatively, the brightness adjustment device may be an
 automatic device that automatically adjusts the brightness in response to
 a detected parameter, such as detection of ambient lighting conditions.
 In certain embodiments, it may be desirable to limit the minimum duty cycle
 produced at the output of the PWM 56 to ensure a minimum time interval
 during which the switches 62 and 64 are not in a conducting state. In one
 embodiment, a voltage divider comprising resistors 96 and 98 is connected
 to an input 100 of the PWM 56 to ensure that the minimum duty cycle is
 limited to approximately 1%.
 It may also be desirable to disable operation of the dimming module 16 for
 a brief time period (e.g., a few seconds) after the electrodeless lamp 12
 is initially energized. To disable operation temporarily, the dimming
 module 16 includes the timer 54. Energization of the electrodeless lamp 12
 is accompanied by energization of the 12 VDC auxiliary power supply 46
 which provides power to the dimming module 16 via the connector 52. When
 auxiliary power is applied to the dimming module 16, the rate of rise of
 the voltage signal applied to the input of the timer 54 is controlled by
 the charging of a capacitor 102 (e.g., 4.4 .mu.F) through a resistor 104
 (e.g., 845 Kohms). During the time the voltage on the capacitor 102 is
 below a threshold level, the voltage produced at the output of the timer
 54 remains at a HIGH level. The output of the timer 54 is coupled to the
 input 100 of the PWM 56 through a diode 106. While the timer output is
 HIGH, the diode 106 is forward biased, thus allowing application of the
 HIGH level voltage to the input 100 of the PWM 56, which prevents
 switching of the transistors 62 and 64, thus disabling dimming of the
 light generated by the lamp 12. When the voltage on the capacitor 102
 exceeds the threshold level, the voltage at the output of the timer 54
 transitions to a LOW level, thus reverse biasing the diode 106 which
 enables operation of the PWM 56 and, consequently, dimming of the light
 generated by the lamp.
 In the illustrated embodiment, the switching frequency of the switch
 assembly 60 of the dimming module also can be adjusted via a frequency
 adjustment device 48 connected to the dimming module 16 via a connector
 108. In one embodiment, the frequency adjustment device can be a
 potentiometer which can be varied to adjust the amount of current drawn by
 a constant current source (sink) connected to the input 88 of the PWM 56.
 The constant current source (sink) includes transistor 110 (e.g., a PNP
 transistor), transistor 112 (e.g., a NPN transistor), and resistors 114
 and 116 (e.g., 100K ohms and 43K ohms, respectively). As the frequency
 adjustment device 48 is adjusted to increase the amount of current pulled
 by the transistor 112, the repetition rate of the ramp-shaped voltage
 signal at the input 88 of the PWM 56 decreases (i.e., the switching
 frequency of the dimming module decreases). Conversely, as the frequency
 adjustment device 48 is adjusted to decrease the amount of current sourced
 by transistor 112, the repetition rate of the rampshaped voltage signal
 increases. As discussed above, the frequency adjustment device 48
 advantageously is accessible to a user of the dimmable light source such
 that the user can adjust the switching frequency to eliminate undesirable
 visual artifacts perceived in the lighting or on a display.
 In an alternative embodiment, the frequency adjustment device 48 can be an
 electrical circuit configured to receive an electrical synchronization
 signal and to cooperate with the dimming module electronics to
 automatically synchronize the switching frequency to the received
 synchronization signal. For example, the frequency adjustment device 48,
 together with the constant current source (sink), can be configured as a
 phase-locked loop. That is, the frequency adjustment device 48 can be
 configured as a phase comparator that receives as an input the vertical
 video refresh signal from the video circuitry of a display unit which
 incorporates a dimmable electrodeless lamp system for backlighting. The
 frequency adjustment device 48 outputs a square wave voltage signal based
 on the comparison that causes the constant current source (sink) to
 synchronize the PWM oscillator. The phase-locked loop thus can synchronize
 the switching frequency of the switch assembly 60 to the frequency of the
 vertical refresh signal.
 Turning now to FIG. 5, the assembly of the dimmable light source 10 is
 illustrated, including the electrodeless lamp 12, the coupling
 transformers 18 and 20, the auxiliary winding 38, the resistor 36, the
 dimming module 16, and the ballast 14. The cores of the coupling
 transformers 18 and 20 are separable into halves such that the
 transformers 18 and 20 can be removably secured to the lamp 12 by
 retaining spring clamps 118 and 120. The spring clamps 118 and 120 are
 further coupled to mounting brackets 122 and 124 for mounting the
 electrodeless lamp 12 in an appropriate housing.
 A first end of the primary winding 30 (not shown in FIG. 5) of the coupling
 transformer 20 is connected in series with the resistor 36 via mating
 connectors 126a and 126b. The series combination of the resistor 36 and
 the primary winding 30 are connected in parallel with the primary winding
 28 (not shown in FIG. 5) of the coupling transformer 18. The parallel
 connection points are connected to the ballast 14 via the mating
 connectors 128a and 128b. The secondary windings 32 and 34 of the coupling
 transformers 18 and 20 are the glass lamp vessel itself.
 In one embodiment, the primary winding 30 of the coupling transformer 20 is
 wound within a 30.degree. sector on the core 26. To maximize the coupling
 between the auxiliary winding 38 and the primary winding 30, the auxiliary
 winding is wound such that two turns are adjacent one end of the sector
 wound primary winding and the remaining two turns are adjacent the other
 end of the sector. The auxiliary winding can be wound onto the core of the
 transformer 20 simply by threading the wire through the core while the
 transformer 20 is positioned on the electrodeless lamp 12. Alternatively,
 the transformer 20 can be removed from the lamp 12 by removing the
 retaining spring clamp 120. The auxiliary winding 38 can then be
 appropriately wound onto the core and the core and spring clamp replaced
 on the lamp. Each end of the auxiliary winding 38 is connected to
 connection terminals 66b.
 The connection terminals 66b mate with connection terminals 66a which are
 connected to the dimming module 16 via a wire harness 130. Wire harnesses
 132 and 134 also are connected to the dimming module 16 and terminate in
 connectors 94a and 108a. The connectors 94a and 108a are coupled to the
 brightness adjustment potentiometer 92 via the connector 94b and the
 frequency adjustment potentiometer 48 via the connector 108b,
 respectively.
 In an exemplary embodiment, the dimmable light source assembly 10
 illustrated in FIG. 5 can be incorporated in a display device assembly,
 such as the display device assembly 136 illustrated in FIG. 6. The display
 device assembly 136 includes a display unit 138 having a conventional
 liquid crystal display (LCD) element 140 that responds to appropriate
 electrical input signals to display an image. In some embodiments, the
 electrical input signals may be received from the processing and control
 elements of a computer. The front of the LCD element 140 is typically
 protected by a transparent screen 142 made of glass, plastic, or other
 suitable material. The screen 142 is mounted within an opening on a front
 frame 144 of the display unit 138 such that a user may clearly view the
 image displayed by the LCD element 140.
 The display device assembly 136 also includes a diffuser 146 disposed
 adjacent the LCD element 140. The diffuser 146 is arranged to receive
 light generated by the lamp 12 and to transmit the light to the LCD
 element 140 such that the LCD element 140 is substantially uniformly
 illuminated.
 The lamp 12 is mounted in a lamp housing 148 via mounting brackets (not
 shown), such as the mounting brackets 122 and 124 illustrated in FIG. 5.
 The lamp housing 148 includes a back reflective surface 150 and two end
 plates 152 and 154 which may be coated with a reflective material, such as
 an aluminum or silver-based material, or which may be made of a reflective
 material to form a reflective lamp lining. The lamp housing 148 can be
 formed, or stamped, from a suitable material in any number of well-known
 manufacturing processes. The reflective lamp lining may be configured to
 reflect light generated by the lamp 12 in a uniform manner toward the
 diffuser 146 and the LCD element 140.
 The ballast 14, the dimming module 16 (not shown in FIG. 6), and the
 resistor 36 (not shown in FIG. 6) can be mounted to the interior or
 exterior surfaces of the lamp housing 148 in an appropriate manner, such
 as by using mounting brackets, mounting bosses, etc. The wiring harnesses
 132 and 134 connecting the brightness adjustment potentiometer 92 and the
 frequency adjustment device 48 to the dimming module 16 can be
 appropriately routed such that devices 92 and 48 can be mounted to, for
 example, the front frame 144 of the display unit 138, or in any other
 location that is accessible to the user. Further, in some embodiments the
 dimming module may be connected to video display electronics to receive,
 for example, a video refresh signal to synchronize the switching
 frequency. Alternatively, the dimming module may be configured to receive
 input signals from a processor or other control elements of a computing
 device with which the display device assembly 136 is used.
 The display device assembly 136 further includes a fan 156 mounted to the
 lamp housing 148 to cool the assembly. To provide further cooling, a heat
 absorbing sheet of glass 158 can be disposed in front of the lamp 12.
 Because the lamp 12 provides a bright light, the glass 158 can be
 relatively inefficient at transmitting light. A back casing 160 attaches
 to the front frame 144 to enclose the various components.
 The dimmable video display assembly illustrated in FIG. 6 is merely an
 exemplary application of the dimmable light source. Other applications for
 the dimmable light source can be readily envisioned, such as lighting
 systems or light fixtures for the home or office. Further, it is
 envisioned that the assembly may include other components and mounting
 arrangements depending on the application and the intended use of the
 display, as would be obvious to a person of ordinary skill in the art.
 While the invention may be susceptible to various modifications and
 alternative forms, specific embodiments have been shown by way of example
 in the drawings and have been described in detail herein. However, it
 should be understood that the invention is not intended to be limited to
 the particular forms disclosed. Rather, the invention is to cover all
 modifications, equivalents, and alternatives falling within the spirit and
 scope of the invention as defined by the following appended claims.