Electrodeless lamps and methods

An electrodeless plasma lamp and a method of generating light are described. The lamp may comprise a lamp body including a dielectric material. The bulb is positioned proximate the lamp body and contains a fill that forms a plasma when radio frequency (RF) power is coupled to the fill. The conductive element is located within the lamp body and configured to enhance coupling of the RF power to the fill. The lamp may include a feed coupled to the RF power source and configured to radiate power into the lamp body. The at least one conductive element is configured to enhance the coupling of radiated power from the feed to the fill. In an example, two spaced apart conductive elements may be located within the lamp body. The bulb may be an elongated bulb having opposed ends, each opposed end of the bulb being proximate a corresponding conductive element.

The field relates to systems and methods for generating light, and more particularly to electrodeless plasma lamps.

Electrodeless plasma lamps may be used to provide bright, white light sources. Because electrodes are not used, they may have longer useful lifetimes than other lamps. In projection display systems, it is desirable to have a lamp capable of high light collection efficiency. Collection efficiency can be expressed as the percentage of light that can be collected from a source into a given etendue, compared to the total light emitted by that source. High collection efficiency means that most of the power consumed by the lamp is going toward delivering light where it needs to be. In microwave energized electrodeless plasma lamps, the need for high collection efficiency is elevated due to the losses incurred by converting d.c. power to RF power.

Example methods, electrodeless plasma lamps and systems are described.

In one example embodiment, an electrodeless plasma lamp comprises a source of radio frequency (RF) power, a bulb containing a fill that forms a plasma when the RF power is coupled to the fill, and a dipole antenna proximate the bulb. The dipole antenna may comprise a first dipole arm and a second dipole arm spaced apart from the first dipole arm. The source of RF power may be configured to couple the RF power to the dipole antenna such that an electric field is formed between the first dipole arm and the second dipole arm. The dipole antenna may be configured such that a portion of the electric field extends into the bulb and the RF power is coupled from the dipole antenna to the plasma.

In one example embodiment, a method of generating light is described. The method may comprise providing a bulb containing a fill that forms a plasma when the RF power is coupled to the fill, and providing a dipole antenna proximate the bulb, the dipole antenna comprising a first dipole arm and a second dipole arm spaced apart from the first dipole arm. The RF power may be coupled to the dipole antenna such that an electric field is formed between the first dipole arm and the second dipole arm, and RF power is coupled from the dipole antenna to the plasma.

Some example embodiments provide systems and methods for increasing the amount of collectable light into a given etendue from an electrodeless plasma lamp, such as a plasma lamp using a solid dielectric lamp body. A maximum (or substantially maximum) electric field may be deliberately transferred off center to a side (or proximate a side) of a dielectric structure that serves as the body of the lamp. A bulb of the electrodeless lamp may be maintained at the side (or proximate the side) of the body, coinciding with the offset electric field maximum. In an example embodiment, a portion of the bulb is inside the body, and the rest of the bulb protrudes out the side in such a way that an entire (or substantially entire) plasma arc is visible to an outside half-space.

In some example embodiments, the electric field is substantially parallel to the length of a bulb and/or the length of a plasma arc formed in the bulb. In some example embodiments, 40% to 100% (or any rang subsumed therein) of the bulb length and/or arc length is visible from outside the lamp and is in line of sight of collection optics. In some example embodiments, the collected lumens from the collection optics is 20% to 50% (or any range subsumed therein) or more of the total lumens output by the bulb.

In some examples, the orientation of the bulb allows a thicker bulb wall to be used while allowing light to be efficiently transmitted out of the bulb. In one example, the thickness of the side wall of the lamp is in the range of about 2 mm to 10 mm or any range subsumed therein. In some examples, the thicker walls allow a higher power to be used without damaging the bulb walls. In one example, a power of greater than 150 watts may be used to drive the lamp body. In one example, a fill of a noble gas, metal halide and Mercury is used at a power of 150 watts or more with a bulb wall thickness of about 3-5 mm.

In some examples, a reflector or reflective surface is provided on one side of an elongated bulb. In some examples, the reflector may be a specular reflector. In some embodiments, the reflector may be provided by a thin film, multi-layer dielectric coating. In some examples, the other side of the bulb is exposed to the outside of the lamp. In some embodiments, substantial light is transmitted through the exposed side without internal reflection and substantial light is reflected from the other side and out of the exposed side with only one internal reflection. In example embodiments, light with a minimal number (e.g., one or no internal reflections) comprises the majority of the light output from the bulb. In some embodiments, the total light output from the bulb is in the range of about 5,000 to 20,000 lumens or any range subsumed therein.

In some examples, power is provided to the lamp at or near a resonant frequency for the lamp. In some examples, the resonant frequency is determined primarily by the resonant structure formed by electrically conductive surfaces in the lamp body rather than being determined primarily by the shape, dimensions and relative permittivity of the dielectric lamp body. In some examples, the resonant frequency is determined primarily by the structure formed by electrically conductive field concentrating and shaping elements in the lamp body. In some examples, the field concentrating and shaping elements substantially change the resonant waveform in the lamp body from the waveform that would resonate in the body in the absence of the field concentrating and shaping elements. In some embodiments, an electric field maxima would be positioned along a central axis of the lamp body in the absence of the electrically conductive elements. In some examples, the electrically conductive elements move the electric field maxima from a central region of the lamp body to a position adjacent to a surface (e.g., a front or upper surface) of the lamp body. In some examples, the position of the electric field maxima is moved by 20-50% of the diameter or width of the lamp body or any range subsumed therein. In some examples, the position of the electric field maxima is moved by 3-50 mm (or any range subsumed therein) or more relative to the position of the electric field maxima in the absence of the conductive elements. In some examples, the orientation of the primary electric field at the bulb is substantially different than the orientation in the absence of the electrically conductive elements. In one example, a fundamental resonant frequency in a dielectric body without the electrically conductive elements would be oriented substantially orthogonal to the length of the bulb. In the example embodiments described herein, a fundamental resonant frequency for the resonant structure formed by the electrically conductive elements in the lamp body results in an electric field at the bulb that is substantially parallel to the length of the bulb.

In some examples, the length of the bulb is substantially parallel to a front surface of the lamp body. In some embodiments, the bulb may be positioned within a cavity formed in the lamp body or may protrude outside of the lamp body. In some examples, the bulb is positioned in a recess formed in the front surface of the lamp body. In some examples, a portion of the bulb is below the plane defined by the front surface of the lamp body and a portion protrudes outside the lamp body. In some examples, the portion below the front surface is a cross section along the length of the bulb. In some examples, the portion of the front surface adjacent to the bulb defines a cross section through the bulb along the length of the bulb. In some examples, the cross-section substantially bisects the bulb along its length. In other examples 30%-70% (or any range subsumed therein) of the interior of the bulb may be below this cross section and 30%-70% (or any range subsumed therein) of the interior of the bulb may be above this cross section.

In example embodiments, the volume of lamp body may be less than those achieved with the same dielectric lamp bodies without conductive elements in the lamp body, where the resonant frequency is determined primarily by the shape, dimensions and relative permittivity of the dielectric body. In some examples, a resonant frequency for a lamp with the electrically conductive resonant structure according to an example embodiment is lower than a fundamental resonant frequency for a dielectric lamp body of the same shape, dimensions and relative permittivity. In example embodiments, it is believed that a lamp body using electrically conductive elements according to example embodiments with a dielectric material having a relative permittivity of 10 or less may have a volume less than about 3 cm3for operating frequencies less than about 2.3 GHz, less than about 4 cm3for operating frequencies less than about 2 GHz, less than about 8 cm3for operating frequencies less than about 1.5 GHz, less than about 11 cm3for operating frequencies less than about 1 GHz, less than about 20 cm3for operating frequencies less than about 900 MHz, less than about 30 cm3for operating frequencies less than about 750 MHz, less than about 50 cm3for operating frequencies less than about 650 MHz, and less than about 100 cm3for operating frequencies less than about 650 MHz. In one example embodiment, a volume of about 13.824 cm3was used at an operating frequency of about 880 MHz. It is believed that similar sizes may be used even at lower frequencies below 500 MHz.

In some examples, the volume of the bulb may be less than the volume of the lamp body. In some examples, the volume of the lamp body may be 3-100 times (or any range subsumed therein) of the volume of the bulb.

In example embodiments, the field concentrating and shaping elements are spaced apart from the RF feed(s) that provide RF power to the lamp body. In example embodiments, the RF feed is a linear drive probe and is substantially parallel to the direction of the electric field at the bulb. In some examples, the shortest distance from the end of the RF feed to an end of the bulb traverses at least one metal surface in the body that is part of the field concentrating and shaping elements. In some examples, a second RF feed is used to obtain feedback from the lamp body. In some examples, the shortest distance from the end of the drive probe to an end of the feedback probe does not traverse an electrically conductive material in the lamp body. In some examples, the shortest distance from the end of the feedback probe to an end of the bulb traverses at least one metal surface in the body that is part of the field concentrating and shaping elements. In some examples, the RF feed for providing power to the lamp body is coupled to the lamp body through a first side surface and the RF feed for obtaining feedback from the lamp body is coupled to the lamp body through an opposing side surface. In example embodiments, the bulb is positioned adjacent to a different surface of the lamp body than the drive probe and feedback probe.

In some example embodiments, the field concentrating and shaping elements are formed by at least two conductive internal surfaces spaced apart from one another in the lamp body. In some examples, these electrically conductive surfaces form a dipole. In example embodiments, the closest distance between the first internal surface and the second internal surface is in the range of about 1-15 mm or any range subsumed therein. In one example, portions of these internal surfaces are spaced apart by about 3 mm. In one example, the internal surfaces are spaced apart from an outer front surface of the lamp body. The front surface of the lamp body may be coated with an electrically conductive material. In some example embodiments, the inner surfaces are spaced from the outer front surface by a distance of less than about 1-10 mm or any range subsumed therein. In one example, the inner surfaces are spaced from the outer front surface by a distance less than an outer diameter or width of the bulb. In some examples this distance is less than 2-5 mm or any range subsumed therein.

In some examples, the bulb is positioned adjacent to an uncoated surface (e.g., a portion without a conductive coating) of the lamp body. In example embodiments, power is coupled from the lamp body to the bulb through an uncoated dielectric surface adjacent to the bulb. In example embodiments, the surface area through which power is coupled to the bulb is relatively small. In some embodiments, the surface area is in the range of about 5%-100% of the outer surface area of the bulb or any range subsumed therein. In some examples, the surface area is less than 60% of the outer surface area of the bulb. In some example embodiments, the surface area is less than 200 mm2. In other examples, the surface area is less than 100 mm2, 75 mm2, 50 mm2or 35 mm2. In some embodiments, the surface area is disposed asymmetrically adjacent to one side of the bulb. In some embodiments, power is concentrated in the middle of the bulb and a small plasma arc length is formed that does not impinge on the ends of the bulb. In some examples, the plasma arc length is less than about 20% to 95% of the interior length of the bulb or any range subsumed therein. In some examples, the plasma arc length is within the range of 2 mm to 5 mm or any range subsumed therein.

It is understood that each of the above aspects of example embodiments may be used alone or in combination with other aspects described above or in the detailed description below. A more complete understanding of example embodiments and other aspects and advantages thereof will be gained from a consideration of the following description read in conjunction with the accompanying drawing figures provided herein. In the figures and description, numerals indicate the various features of example embodiments, like numerals referring to like features throughout both the drawings and description.

DETAILED DESCRIPTION

While the present invention is open to various modifications and alternative constructions, the example embodiments shown in the drawings will be described herein in detail. It is to be understood, however, there is no intention to limit the invention to the particular example forms disclosed. On the contrary, it is intended that the invention cover all modifications, equivalences and alternative constructions falling within the spirit and scope of the invention as expressed in the appended claims.

FIG. 1is a cross-section and schematic view of a plasma lamp100according to an example embodiment. The plasma lamp100may have a lamp body102formed from one or more solid dielectric materials and a bulb104positioned adjacent to the lamp body102. The bulb104contains a fill that is capable of forming a light emitting plasma. A lamp drive circuit (e.g., a lamp drive circuit106shown by way of example inFIG. 9) couples radio frequency (RF) power into the lamp body102which, in turn, is coupled into the fill in the bulb104to form the light emitting plasma. In example embodiments, the lamp body102forms a structure that contains and guides the radio frequency power.

In the plasma lamp100the bulb104is positioned or orientated so that a length of a plasma arc108generally faces a lamp opening110(as opposed to facing side walls112) to increase an amount of collectable light emitted from the plasma arc108in a given etendue. Since the length of plasma arc108orients in a direction of an applied electric field, the lamp body102and the coupled RF power are configured to provide an electric field114that is aligned or substantially parallel to the length of the bulb104and a front or upper surface116of the lamp body102. Thus, in an example embodiment, the length of the plasma arc108may be substantially (if not completely) visible from outside the lamp body102. In example embodiments, collection optics118may be in the line of sight of the full length of the bulb104and plasma arc108. In other examples, about 40%-100%, or any range subsumed therein, of the plasma arc108may be visible to the collection optics118in front of the lamp100. Accordingly, the amount of light emitted from the bulb104and received by the collection optics118may be enhanced. In example embodiments, a substantial amount of light may be emitted out of the lamp100from the plasma arc108through a front side wall of the lamp100without any internal reflection.

As described herein, the lamp body102is configured to realize the necessary resonator structure such that the light emission of the lamp100is enabled while satisfying Maxwell's equations.

InFIG. 1, the lamp100is shown to include a lamp body102including a solid dielectric body and an electrically conductive coating120which extends to the front or upper surface116. The lamp100is also shown to include dipole arms122and conductive elements124,126(e.g., metallized cylindrical holes bored into the body102) to concentrate the electric field present in the lamp body102. The dipole arms122may thus define an internal dipole. In an example embodiment, a resonant frequency applied to a lamp body102without dipole arms122and conductive elements124,126would result in a high electric field at the center of the solid dielectric lamp body102. This is based on the intrinsic resonant frequency response of the lamp body due to its shape, dimensions and relative permittivity. However, in the example embodiment ofFIG. 1, the shape of the standing waveform inside the lamp body102is substantially modified by the presence of the dipole arms122and conductive elements124,126and the electric field maxima is brought out to ends portions128,130of the bulb104using the internal dipole structure. This results in the electric field114near the upper surface116of the lamp100that is substantially parallel to the length of the bulb104. In some example embodiments, this electric field is also substantially parallel to a drive probe170and feedback probe172(seeFIG. 9below).

The fact that the plasma arc108in lamp100is oriented such that it presents a long side to the lamp exit aperture or opening110may provide several advantages. The basic physical difference relative to an “end-facing” orientation of the plasma arc is that much of the light can exit the lamp100without suffering multiple reflections within the lamp body102. Therefore, a specular reflector may show a significant improvement in light collection performance over a diffuse reflector that may be utilized in a lamp with an end facing orientation. An example embodiment of a specular reflector geometry that may be used in some embodiments is a parabolic line reflector, positioned such that the plasma arc lies in the focal-line of the reflector.

Another advantage may lie in that the side wall of the bulb104can be relatively thick, without unduly inhibiting light collection performance. Again, this is because the geometry of the plasma arc108with respect to the lamp opening110is such that the most of the light emanating from the plasma arc108will traverse thicker walls at angles closer to normal, and will traverse them only once or twice (or at least a reduced number of times). In example embodiments, the side wall of the bulb104may have a thickness in the range of about 1 mm to 10 mm or any range subsumed therein. In one example, a wall thickness greater than the interior diameter or width of the bulb may be used (e.g., 2-4 mm in some examples). Thicker walls may allow higher power to be coupled to the bulb104without damaging the wall of the bulb104. This is an example only and other embodiments may use other bulbs. It will be appreciated that the bulb is not restricted to a circular cylindrical shape and may have more than one side wall.

FIGS. 2-4show more detailed diagrams of the example plasma lamp100shown inFIG. 1. The lamp100is shown in exploded view and includes the electrically conductive coating120(seeFIG. 4) provided on an internal solid dielectric132defining the lamp body102. The oblong bulb104and surrounding interface material134(seeFIG. 2) are also shown. Power is fed into the lamp100with an electric monopole probe closely received within a drive probe passage136. The two opposing conductive elements124,126are formed electrically by the metallization of the bore138(seeFIG. 4), which extend toward the center of the lamp body102(see alsoFIG. 1) to concentrate the electric field, and build up a high voltage to energize the lamp104. The dipole arms122connected to the conductive elements124,126by conductive surfaces transfer the voltage out towards the bulb104. The cup-shaped terminations or end portions140on the dipole arms122partially enclose the bulb104. A feedback probe passage142is provided in the lamp body102to snugly receive a feedback probe that connects to a drive circuit (e.g. a lamp drive circuit106shown by way of example inFIG. 9). In an example embodiment the interface material134may be selected so as to act as a specular reflector to reflect light emitted by the plasma arc108.

In an example embodiment, the lamp body102is shown to include three body portions144,146and148. The body portions144and148are mirror images of each other and may each have a thickness150of about 11.2 mm, a height152of about 25.4 mm and width154of about 25.4 mm. The inner portion146may have a thickness155of about 3 mm. The lamp opening110in the upper surface116may be partly circular cylindrical in shape having a diameter156of about 7 mm and have a bulbous end portions with a radius158of about 3.5 mm. The drive probe passage136and the feedback probe passage142may have a diameter160of about 1.32 mm. A recess162with a diameter164is provided in the body portion148. The bores138of the conductive elements124,126may have a diameter166of about 7 mm.

An example analysis of the lamp100using 3-D electromagnetic simulation based on the finite-integral-time-domain (FITD) method is described below with reference toFIGS. 5-7. The electric (E) field (seeFIG. 7), the magnetic (H) field (seeFIG. 6), and the power flow (which is the vectoral product of the E and H fields—seeFIG. 5), are separately displayed for insight, although they are simply three aspects of the total electromagnetic behavior of the lamp100. In the example embodiment simulated in the three figures, a drive probe170couples power into the lamp body102and a feedback probe172is placed on the same side of the body102as the drive probe170. This is an alternative embodiment representing only a superficial difference from the configuration of drive and feedback probes for use in the example embodiment shown inFIGS. 2-4.

FIG. 5shows a simulation180of power transfer to the bulb104in an example embodiment. Input power is provided via the drive probe170(not shown inFIG. 1) and is incident onto the bulb104utilizing the dipole arms122. It should be noted that power is concentrated near the bulb104. In an example embodiment the power proximate the ends portions bulb128and130may be about 39063-45313 W/m2. Power along the parallel central portions182of the dipole arms122104may vary from about 10938-35938 W/m2. It should be noted that power near the electrically conductive coating120and proximate the bulb104is minimal in the example simulation180.

As shown in a simulation190ofFIG. 6, the conductive elements124,126shape the magnetic field such that it is concentrated near the elements themselves, rather than near the walls as is the case if RF power was provided to the lamp body102at a resonant frequency without the embedded conductive elements124,126. Regions of high magnetic field concentration correspond to regions of high AC current. Therefore, the current flow near the outer walls of the present example embodiments is small compared to a lamp without the embedded conductive elements. The significance of this will be discussed below. The simulation190ofFIG. 6shows at every point the magnitude of the H-field only, ignoring the vectoral nature of the field.

As shown in a simulation200ofFIG. 7, the electric field is strongly concentrated between the dipole arms122, and between the dipole endcaps or end portions140. The weaker electric field in the remainder of the lamp body102is confined by the outer conductive coating or layer120(metallization), except near the discontinuity in the outer conductive coating120brought about by the opening110for the lamp104. LikeFIG. 6,FIG. 5shows at every point the magnitude of the E-field only, ignoring the vectoral nature of the field.

In addition to the improved light collection efficiency as a consequence of the orientation of the plasma arc108with respect to the lamp body102, the E and H field patterns may provide several advantages. The resonant frequency of the structure may be decoupled and be substantially independent of the physical extent or size of the lamp body102. This can be seen in two aspects. The concentration of the magnetic field near the conductive elements124and126indicates that the inductance of those elements, and to a lesser extent the connected dipole arms122, strongly influence the operational frequency (e.g., a resonant frequency). The concentration of the electric field between the dipole arms122indicates that the capacitance of those elements strongly influences the operational frequency (e.g., resonant frequency). Taken together, this means the lamp body102, can be reduced in size relative to a lamp with a lamp body of the same dimensions but without the conductive elements124and126and dipole arms122(even for a relatively low frequency of operation, and even compared to both simple and specially-shaped geometries of lamp bodies where the resonant frequency is determined primarily by the shape, dimensions and relative permittivity of the dielectric body). In example embodiments, the volume of lamp body102may be less than those achieved with the same dielectric lamp bodies without conductive elements124and126and dipole arms122, where the resonant frequency is determined primarily by the shape, dimensions and relative permittivity of the dielectric body. In example embodiments, it is believed that lamp body102with a relative permittivity of 10 or less may have a volume less than about 3 cm3for operating frequencies less than about 2.3 GHz, less than about 4 cm3for operating frequencies less than about 2 GHz, less than about 8 cm3for operating frequencies less than about 1.5 GHz, less than about 11 cm3for operating frequencies less than about 1 GHz, less than about 20 cm3for operating frequencies less than about 900 MHz, less than about 30 cm3for operating frequencies less than about 750 MHz, less than about 50 cm3for operating frequencies less than about 650 MHz, and less than about 100 cm3for operating frequencies less than about 650 MHz. In one example embodiment, lamp body with a volume of about 13.824 cm3was used at an operating frequency of about 880 MHz. It is believed that similar sizes may be used even at lower frequencies below 500 MHz.

Low frequency operation may provide several advantages in some example embodiments. For example, at low frequencies, especially below 500 MHz, very high power amplifier efficiencies are relatively easily attained. For example, in silicon LDMOS transistors, typical efficiencies at 450 MHz are about 75% or higher, while at 900 MHz they are about 60% or lower. In one example embodiment, a lamp body is used with a relative permittivity less than 15 and volume of less than 30 cm3at a resonant frequency for the lamp structure of less than 500 MHz and the lamp drive circuit uses an LDMOS amplifier with an efficiency of greater than 70%. High amplifier efficiency enables smaller heat sinks, since less d.c. power is required to generate a given quantity of RF power. Smaller heat sinks mean smaller overall packages, so the net effect of the example embodiment is to enable more compact lamp designs at lower frequencies. For example, compact lamps may be more affordable and more easily integrated into projection systems, such as front projectors and rear projection televisions.

A second possible advantage in some example embodiments is the relative immunity to electromagnetic interference (EMI). Again, this effect can be appreciated from the point of view of examining either the E or H field. Loosely, EMI is created when disturbances in the current flow force the current to radiate (“jump off”) from the structure supporting it. Because the magnetic field is concentrated at conductive structures (e.g., the dipole arms122) inside the lamp body102, current flow near the surface of the lamp body102and, most significantly, near the disturbance represented by the lamp opening110, is minimized, thereby also minimizing EMI. The E-field point of view is more subtle.FIG. 8shows a line drawing adaptation of the electric fields of the simulation200shown inFIG. 7, indicating electric dipole moments202,203of the field omitted for the sake of clarity in the magnitude-only depiction ofFIG. 7. The dipole moment202of the main input field delivered by the dipole arms122has the opposite sign as the dipole moments203of the parasitic field induced on the outer electrically conductive coating120of the lamp body102. By “opposite sign,” we mean that the vector of the electric fields for each dipole arm extend in opposing directions (e.g., the Right Hand Rule as applied to dipole moment202yields, in this example, a vector pointing out of the page, where as the Right Hand Rule as applied to dipole moments203yields, in this example, a vector pointing into the page). The net effect is that the field201radiated by the main-field dipole moment202cancels out the field204radiated by the parasitic dipole moments203in the far-field region205, thus minimizing EMI.

A further possible advantage in some example embodiments is increased resistance to the dielectric breakdown of air near the bulb104. As shown inFIG. 7, the peak of the electric field distribution in this example design is contained within the body102, which has a higher breakdown voltage than air.

In an example embodiment, the lamp100is fabricated from alumina ceramic and metallized to provide the electrically conductive coating108using a silver paint fired onto the ceramic components or body portions144-148. In this example embodiment, the resonant frequency was close to the predicted value of about 880 MHz for an external dimension of about 25.4×25.4×25.4 mm, or 1 cubic inch (seeFIG. 3). The bulb fill in this example embodiment is a mixture of mercury, metal halide, and argon gas. Ray-tracing simulations indicate that collection ratios of about 50% are achievable with minimal modifications to this example embodiment.

In example embodiments, the lamp body102has a relative permittivity greater than air. In an example embodiment, the lamp body102is formed from solid alumina having a relative permittivity of about 9.2. In some embodiments, the dielectric material may have a relative permittivity in the range of from 2 to 100 or any range subsumed therein, or an even higher relative permittivity. In some embodiments, the lamp body102may include more than one such dielectric material resulting in an effective relative permittivity for the lamp body102within any of the ranges described above. The lamp body102may be rectangular, cylindrical or other shape.

As mentioned above, in example embodiments, the outer surfaces of the lamp body102may be coated with the electrically conductive coating120, such as electroplating or a silver paint or other metallic paint which may be fired onto an outer surface of the lamp body102. The electrically conductive coating120may be grounded to form a boundary condition for radio frequency power applied to the lamp body102. The electrically conductive coating120may help contain the radio frequency power in the lamp body102. Regions of the lamp body102may remain uncoated to allow power to be transferred to or from the lamp body102. For example, the bulb104may be positioned adjacent to an uncoated portion of the lamp body102to receive radio frequency power from the lamp body102.

The bulb104may be quartz, sapphire, ceramic or other desired bulb material and may be cylindrical, pill shaped, spherical or other desired shape. In the example embodiment shown inFIGS. 1-4, the bulb104is cylindrical in the center and forms a hemisphere at each end. In one example, the outer length (from tip to tip) is about 11 mm and the outer diameter (at the center) is about 5 mm. In this example, the interior of the bulb104(which contains the fill) has an interior length of about 7 mm and an interior diameter (at the center) of about 3 mm. The wall thickness is about 1 mm along the sides of the cylindrical portion and about 2.25 mm on both ends. In other examples, a thicker wall may be used. In other examples, the wall may between 2-10 mm thick or any range subsumed therein. In other example embodiments, the bulb104may have an interior width or diameter in a range between about 2 and 30 mm or any range subsumed therein, a wall thickness in a range between about 0.5 and 4 mm or any range subsumed therein, and an interior length between about 2 and 30 mm or any range subsumed therein. In example embodiments, the interior of the bulb has a volume in the range of about 10 mm3to 750 mm3or any range subsumed therein. In some examples, the bulb has an interior volume of less than about 100 mm3or less than about 50 mm3. These dimensions are examples only and other embodiments may use bulbs having different dimensions.

In example embodiments, the bulb104contains a fill that forms a light emitting plasma when radio frequency power is received from the lamp body102. The fill may include a noble gas and a metal halide. Additives such as Mercury may also be used. An ignition enhancer may also be used. A small amount of an inert radioactive emitter such as Kr85may be used for this purpose. In other embodiments, different fills such as Sulfur, Selenium or Tellurium may also be used. In some example embodiments, a metal halide such as Cesium Bromide may be added to stabilize a discharge of Sulfur, Selenium or Tellurium.

In some example embodiments, a high pressure fill is used to increase the resistance of the gas at startup. This can be used to decrease the overall startup time required to reach full brightness for steady state operation. In one example embodiment, a noble gas such as Neon, Argon, Krypton or Xenon is provided at high pressures between 200 Torr to 3000 Torr or any range subsumed therein. Pressures less than or equal to 760 Torr may be desired in some embodiments to facilitate filling the bulb104at or below atmospheric pressure. In certain embodiments, pressures between 100 Torr and 600 Torr are used to enhance starting. Example high pressure fills may also include metal halide and Mercury which have a relatively low vapor pressure at room temperature. In example embodiments, the fill includes about 1 to 100 micrograms of metal halide per mm3of bulb volume, or any range subsumed therein, and 10 to 100 micrograms of Mercury per mm3of bulb volume, or any range subsumed therein. An ignition enhancer such as Kr85may also be used. In some embodiments, a radioactive ignition enhancer may be used in the range of from about 5 nanoCurie to 1 microCurie, or any range subsumed therein. In one example embodiment, the fill includes 1.608 mg Mercury, 0.1 mg Indium Bromide and about 10 nanoCurie of Kr85. In this example, Argon or Krypton is provided at a pressure in the range of about 100 Torr to 600 Torr, depending upon desired startup characteristics. Initial breakdown of the noble gas is more difficult at higher pressure, but the overall warm up time required for the fill to fully vaporize and reach peak brightness is reduced. The above pressures are measured at 22° C. (room temperature). It is understood that much higher pressures are achieved at operating temperatures after the plasma is formed. For example, the lamp may provide a high intensity discharge at high pressure during operation (e.g., much greater than 2 atmospheres and 10-80 atmospheres or more in example embodiments). These pressures and fills are examples only and other pressures and fills may be used in other embodiments.

The layer of interface material134may be placed between the bulb104and the dielectric material of lamp body102. In example embodiments, the interface material134may have a lower thermal conductivity than the lamp body102and may be used to optimize thermal conductivity between the bulb104and the lamp body102. In an example embodiment, the interface material134may have a thermal conductivity in the range of about 0.5 to 10 watts/meter-Kelvin (W/mK) or any range subsumed therein. For example, alumina powder with 55% packing density (45% fractional porosity) and thermal conductivity in a range of about 1 to 2 watts/meter-Kelvin (W/mK) may be used. In some embodiments, a centrifuge may be used to pack the alumina powder with high density. In an example embodiment, a layer of alumina powder is used with a thickness within the range of about ⅛ mm to 1 mm or any range subsumed therein. Alternatively, a thin layer of a ceramic-based adhesive or an admixture of such adhesives may be used. Depending on the formulation, a wide range of thermal conductivities is available. In practice, once a layer composition is selected having a thermal conductivity close to the desired value, fine-tuning may be accomplished by altering the layer thickness. Some example embodiments may not include a separate layer of material around the bulb104and may provide a direct conductive path to the lamp body102. Alternatively, the bulb104may be separated from the lamp body102by an air-gap (or other gas filled gap) or vacuum gap.

In example embodiments, a reflective material may be deposited on the inside or outside surface of the bulb104adjacent to the lamp body102, or a reflector may be positioned between the lamp and interface material134(seeFIG. 2) or a reflector may be embedded inside or positioned below interface material134(for example, if interface material134is transparent). Alternatively, the interface material134may be a reflective material or have a reflective surface. In some embodiments, the interface material134may be alumina or other ceramic material and have a polished surface for reflection. In other embodiments, a thin-film, multi-layer dielectric coating may be used. Other materials may be used in other embodiments. In some examples, the reflective surface is provided by a thin-film, multi-layer dielectric coating. In this example, the coating is made of a reflective material that would not prevent microwave power from heating the light-emitting plasma. In this example, tailored, broadband reflectivity over the emission range of the plasma is instead achieved by interference among electromagnetic waves propagating through thin-film layers presenting refractive index changes at length-scales on the order of their wavelength. The number of layers and their individual thicknesses are the primary design variables. See Chapters 5 and 7, H. A. McLeod, “Thin-Film Optical Filters,” 3rd edition, Institute of Physics Publishing (2001). For ruggedness in the harsh environment proximate to bulb104, example coatings may consist of layers of silicon dioxide (SiO.sub.2), which is transparent for wavelengths between 0.12 .mu.m and 4.5 .mu.m. Another example embodiment consists of layers of titanium dioxide (TiO.sub.2), which is transparent to wavelengths between 0.43 .mu.m and 6.2 .mu.m. Example coatings may have approximately 10 to 100 layers with each layer having a thickness in a range between 0.1 .mu.m and 10 .mu.m.

One or more heat sinks may also be used around the sides and/or along the bottom surface of the lamp body102to manage temperature. Thermal modeling may be used to help select a lamp configuration providing a high peak plasma temperature resulting in high brightness, while remaining below the working temperature of the bulb material. Example thermal modeling software includes the TAS software package available commercially from Harvard Thermal, Inc. of Harvard, Mass.

An example lamp drive circuit106is shown by way of exampleFIG. 9. The circuit106is connected to the drive probe170inserted into the lamp body102to provide radio frequency power to the lamp body102. In the example ofFIG. 9, the lamp100is also shown to include the feedback probe172inserted into the lamp body102to sample power from the lamp body102and provide it as feedback to the lamp drive circuit106. In an example embodiment, the probes170and172may be brass rods glued into the lamp body102using silver paint. In other embodiments, a sheath or jacket of ceramic or other material may be used around the probes170,172, which may change the coupling to the lamp body102. In an example embodiment, a printed circuit board (PCB) may be positioned transverse to the lamp body102for the lamp drive circuit106. The probes170and172may be soldered to the PCB and extend off the edge of the PCB into the lamp body102(parallel to the PCB and orthogonal to the lamp body102). In other embodiments, the probes170,172may be orthogonal to the PCB or may be connected to the lamp drive circuit106through SMA connectors or other connectors. In an alternative embodiment, the probes170,172may be provided by a PCB trace and portions of the PCB containing the trace may extend into the lamp body102. Other radio frequency feeds may be used in other embodiments, such as microstrip lines or fin line antennas.

Various positions for the probes170,172are possible. The physical principle governing their position is the degree of desired power coupling versus the strength of the E-field in the lamp body102. For the drive probe170, the desire is for strong power coupling. Therefore, the drive probe170may be located near a field maximum in some embodiments. For the feedback probe172, the desire is for weak power coupling. Therefore, the feedback probe172may be located away from a field maximum in some embodiments.

The lamp drive circuit106including a power supply, such as amplifier210, may be coupled to the drive probe170to provide the radio frequency power. The amplifier210may be coupled to the drive probe170through a matching network212to provide impedance matching. In an example embodiment, the lamp drive circuit106is matched to the load (formed by the lamp body102, the bulb104and the plasma) for the steady state operating conditions of the lamp100.

A high efficiency amplifier may have some unstable regions of operation. The amplifier210and phase shift imposed by a feedback loop of the lamp circuit106should be configured so that the amplifier210operates in stable regions even as the load condition of the lamp100changes. The phase shift imposed by the feedback loop is determined by the length of the feedback loop (including the matching network212) and any phase shift imposed by circuit elements such as a phase shifter214. At initial startup before the noble gas in the bulb104is ignited, the load appears to the amplifier210as an open circuit. The load characteristics change as the noble gas ignites, the fill vaporizes and the plasma heats up to steady state operating conditions. The amplifier210and feedback loop may be designed so the amplifier210will operate within stable regions across the load conditions that may be presented by the lamp body102, bulb104and plasma. The amplifier210may include impedance matching elements such as resistive, capacitive and inductive circuit elements in series and/or in parallel. Similar elements may be used in the matching network. In one example embodiment, the matching network is formed from a selected length of PCB trace that is included in the lamp drive circuit106between the amplifier210and the drive probe170. These elements may be selected both for impedance matching and to provide a phase shift in the feedback loop that keeps the amplifier210within stable regions of its operation. The phase shifter214may be used to provide additional phase shifting as needed to keep the amplifier210in stable regions.

The amplifier210and phase shift in the feedback loop may be designed by looking at the reflection coefficient Γ, which is a measure of the changing load condition over the various phases of lamp operation, particularly the transition from cold gas at start-up to hot plasma at steady state. Γ, defined with respect to a reference plane at the amplifier output, is the ratio of the “reflected” electric field Einheading into the amplifier, to the “outgoing” electric field Eouttraveling out. Being a ratio of fields, Γis a complex number with a magnitude and phase. A useful way to depict changing conditions in a system is to use a “polar-chart” plot of Γ's behavior (termed a “load trajectory”) on the complex plane. Certain regions of the polar chart may represent unstable regions of operation for the amplifier210. The amplifier210and phase shift in the feedback loop should be designed so the load trajectory does not cross an unstable region. The load trajectory can be rotated on the polar chart by changing the phase shift of the feedback loop (by using the phase shifter214and/or adjusting the length of the circuit loop formed by the lamp drive circuit106to the extent permitted while maintaining the desired impedance matching). The load trajectory can be shifted radially by changing the magnitude (e.g., by using an attenuator).

In example embodiments, radio frequency power may be provided at a frequency in the range of between about 0.1 GHz and about 10 GHz or any range subsumed therein. The radio frequency power may be provided to the drive probe170at or near a resonant frequency for the overall lamp100. The resonant frequency is most strongly influenced by, and may be selected based on, the dimensions and shapes of all the field concentrating and shaping elements (e.g., the conductive elements124,126and the dipole arms122). High frequency simulation software may be used to help select the materials and shape of the field concentrating and shaping elements, as well as the lamp body102and the electrically conductive coating120to achieve desired resonant frequencies and field intensity distribution. Simulations may be performed using software tools such as HFSS, available from Ansoft, Inc. of Pittsburgh, Pa., and FEMLAB, available from COMSOL, Inc. of Burlington, Mass. The desired properties may then be fine-tuned empirically.

In example embodiments, radio frequency power may be provided at a frequency in the range of between about 50 MHz and about 10 GHz or any range subsumed therein. The radio frequency power may be provided to the drive probe170at or near a resonant frequency for the overall lamp. The frequency may be selected based primarily on the field concentrating and shaping elements to provide resonance in the lamp (as opposed to being selected primarily based on the dimensions, shape and relative permittivity of the lamp body). In example embodiments, the frequency is selected for a fundamental resonant mode of the lamp100, although higher order modes may also be used in some embodiments. In example embodiments, the RF power may be applied at a resonant frequency or in a range of from 0% to 10% above or below the resonant frequency or any range subsumed therein. In some embodiments, RF power may be applied in a range of from about 0% to 5% above or below the resonant frequency. In some embodiments, power may be provided at one or more frequencies within the range of about 0 to 50 MHz above or below the resonant frequency or any range subsumed therein. In another example, the power may be provided at one or more frequencies within the resonant bandwidth for at least one resonant mode. The resonant bandwidth is the full frequency width at half maximum of power on either side of the resonant frequency (on a plot of frequency versus power for the resonant cavity).

In example embodiments, the radio frequency power causes a light emitting plasma discharge in the bulb100. In example embodiments, power is provided by RF wave coupling. In example embodiments, RF power is coupled at a frequency that forms a standing wave in the lamp body102(sometimes referred to as a sustained waveform discharge or microwave discharge when using microwave frequencies), although the resonant condition is strongly influenced by the structure formed by the field concentrating and shaping elements in contrast to lamps where the resonant frequency is determined primarily by the shape, dimensions and relative permittivity of the microwave cavity.

In example embodiments, the amplifier210may be operated in multiple operating modes at different bias conditions to improve starting and then to improve overall amplifier efficiency during steady state operation. For example, the amplifier210may be biased to operate in Class A/B mode to provide better dynamic range during startup and in Class C mode during steady state operation to provide more efficiency. The amplifier210may also have a gain control that can be used to adjust the gain of the amplifier210. The amplifier210may include either a plurality of gain stages or a single stage.

The feedback probe172is shown to be coupled to an input of the amplifier210through an attenuator216and the phase shifter214. The attenuator216is used to adjust the power of the feedback signal to an appropriate level for input to the phase shifter214. In some example embodiments, a second attenuator may be used between the phase shifter214and the amplifier210to adjust the power of the signal to an appropriate level for amplification by the amplifier210. In some embodiments, the attenuator(s) may be variable attenuators controlled by control electronics218. In other embodiments, the attenuator(s) may be set to a fixed value. In some embodiments, the lamp drive circuit106may not include an attenuator. In an example embodiment, the phase shifter214may be a voltage-controlled phase shifter controlled by the control electronics218.

The feedback loop automatically oscillates at a frequency based on the load conditions and phase of the feedback signal. This feedback loop may be used to maintain a resonant condition in the lamp body102even though the load conditions change as the plasma is ignited and the temperature of the lamp100changes. If the phase is such that constructive interference occurs for waves of a particular frequency circulating through the loop, and if the total response of the loop (including the amplifier210, the lamp100, and all connecting elements) at that frequency is such that the wave is amplified rather than attenuated after traversing the loop, the loop will oscillate at that frequency. Whether a particular setting of the phase shifter214induces constructive or destructive feedback depends on frequency. The phase shifter214can be used to finely tune the frequency of oscillation within the range supported by the lamp's frequency response. In doing so, it also effectively tunes how well RF power is coupled into the lamp100because power absorption is frequency-dependent. Thus, the phase-shifter214may provide fast, finely-tunable control of the lamp output intensity. Both tuning and detuning may be useful. For example: tuning can be used to maximize intensity as component aging changes the overall loop phase; and detuning can be used to control lamp dimming. In some example embodiments, the phase selected for steady state operation may be slightly out of resonance, so maximum brightness is not achieved. This may be used to leave room for the brightness to be increased and/or decreased by the control electronics218.

In the example lamp drive circuit106shown inFIG. 9, the control electronics218is connected to the attenuator216, the phase shifter214and the amplifier210. The control electronics218provide signals to adjust the level of attenuation provided by the attenuator216, the phase of phase shifter214, the class in which the amplifier210operates (e.g., Class A/B, Class B or Class C mode) and/or the gain of the amplifier210to control the power provided to the lamp body102. In one example, the amplifier210has three stages, a pre-driver stage, a driver stage and an output stage, and the control electronics218provides a separate signal to each stage (drain voltage for the pre-driver stage and gate bias voltage of the driver stage and the output stage). The drain voltage of the pre-driver stage can be adjusted to adjust the gain of the amplifier210. The gate bias of the driver stage can be used to turn on or turn off the amplifier210. The gate bias of the output stage can be used to choose the operating mode of the amplifier210(e.g., Class A/B, Class B or Class C). The control electronics218can range from a simple analog feedback circuit to a microprocessor/microcontroller with embedded software or firmware that controls the operation of the lamp drive circuit106. The control electronics218may include a lookup table or other memory that contains control parameters (e.g., amount of phase shift or amplifier gain) to be used when certain operating conditions are detected. In example embodiments, feedback information regarding the lamp's light output intensity is provided either directly by the optical sensor220, e.g., a silicon photodiode sensitive in the visible wavelengths, or indirectly by the RF power sensor222, e.g., a rectifier. The RF power sensor222may be used to determine forward power, reflected power or net power at the drive probe170to determine the operating status of the lamp100. Matching network212may be designed to also include a directional coupler section, which may be used to tap a small portion of the power and feed it to the RF power sensor222. The RF power sensor222may also be coupled to the lamp drive circuit106at the feedback probe172to detect transmitted power for this purpose. In some example embodiments, the control electronics218may adjust the phase shifter214on an ongoing basis to automatically maintain desired operating conditions.

The phase of the phase shifter214and/or gain of the amplifier210may also be adjusted after startup to change the operating conditions of the lamp100. For example, the power input to the plasma in the bulb104may be modulated to modulate the intensity of light emitted by the plasma. This can be used for brightness adjustment or to modulate the light to adjust for video effects in a projection display. For example, a projection display system may use a microdisplay that controls intensity of the projected image using pulse-width modulation (PWM). PWM achieves proportional modulation of the intensity of any particular pixel by controlling, for each displayed frame, the fraction of time spent in either the “ON” or “OFF” state. By reducing the brightness of the lamp100during dark frames of video, a larger range of PWM values may be used to distinguish shades within the frame of video. The brightness of the lamp100may also be modulated during particular color segments of a color wheel for color balancing or to compensate for green snow effect in dark scenes by reducing the brightness of the lamp100during the green segment of the color wheel.

In another example embodiment, the phase shifter214can be modulated to spread the power provided by the lamp circuit106over a larger bandwidth. This can reduce ElectroMagnetic Interference (EMI) at any one frequency and thereby help with compliance with FCC regulations regarding EMI. In example embodiments, the degree of spectral spreading may be from 5-30% or any range subsumed therein. In one example embodiment, the control electronics218may include circuitry to generate a sawtooth voltage signal and sum it with the control voltage signal to be applied to the phase shifter214. In another example, the control electronics218may include a microcontroller that generates a Pulse Width Modulated (PWM) signal that is passed through an external low-pass filter to generate a modulated control voltage signal to be applied to the phase shifter214. In example embodiments, the modulation of the phase shifter214can be provided at a level that is effective in reducing EMI without any significant impact on the plasma in the bulb104.

In example embodiments, the amplifier210may also be operated at different bias conditions during different modes of operation for the lamp100. The bias condition of the amplifier210may have a large impact on DC-RF efficiency. For example, an amplifier biased to operate in Class C mode is more efficient than an amplifier biased to operate in Class B mode, which in turn is more efficient than an amplifier biased to operate in Class A/B mode. However, an amplifier biased to operate in Class A/B mode has a better dynamic range than an amplifier biased to operate in Class B mode, which in turn has better dynamic range than an amplifier biased to operate in Class C mode.

In one example, when the lamp100is first turned on, the amplifier210is biased in a Class A/B mode. Class A/B provides better dynamic range and more gain to allow amplifier210to ignite the plasma and to follow the resonant frequency of the lamp100as it adjusts during startup. Once the lamp100reaches full brightness, amplifier bias is removed which puts amplifier210into a Class C mode. This may provide improved efficiency. However, the dynamic range in Class C mode may not be sufficient when the brightness of the lamp100is modulated below a certain level (e.g., less than about 70% of full brightness). When the brightness is lowered below the threshold, the amplifier210may be changed back to Class A/B mode. Alternatively, Class B mode may be used in some embodiments.

Further non-limiting example embodiments are shown inFIGS. 10 and 11. However, it should be noted that these embodiments are shown merely by way of example and that the invention is not limited to these example embodiments.

FIG. 10Ais a cross-section and schematic view of a plasma lamp300, according to an example embodiment, in which a bulb104of the lamp300is orientated to enhance an amount of collectable light into a given etendue. The lamp300includes a lamp body102including a solid dielectric resonator, and an electrically conductive coating120. In this example, an artificial magnetic wall302is used to modify orientation of the electric field. An ideal magnetic wall, made from an ideal magnetic conductor which does not exist in nature, would permit an electric field to point parallel to its surface, which is the desired configuration for this example embodiment. Approximations to an ideal magnetic conductor exist in the form of a planar surface patterned with periodic regions of varying conductivity. Such a structure, belonging to the family of periodically-patterned structures collectively known as Photonic Bandgap devices, permit among other things parallel attached electric fields when the relationship between the wavelength of the field and the periodicity of the structure is correctly designed. (see: F R Yang, K P Ma, Y Qian, T Itoh,A novel TEM waveguide using uniplanar compact photonic-bandgap(UC-PBG)structure, IEEE Transactions on Microwave Theory and Techniques, November. 1999, v47 #11, p 2092-8), which is hereby incorporated herein by reference in its entirety). For example, a unipolar compact photonic bandgap (UC-PBG) structure of the type described in this article may be used on a surface of the lamp body102in example embodiments to provide a magnetic boundary condition. A repeating unit used in an example photonic bandgap lattice has square pads and narrow lines with insets, as shown inFIG. 10B. The gaps between adjacent units provide capacitance. The branches and insets provide inductance. This forms a distributed LC circuit and has a particular frequency response. This structure can be tuned to provide an equivalent magnetic surface at particular frequencies, and can be scaled for different frequency bands. As a result, it is believed that a photonic bandgap lattice structure may be used to provide a magnetic boundary condition and adjust the orientation of the electric field to be substantially parallel to the length of the bulb adjacent to a front surface of the lamp body102. This is an example only and other structures may be used to provide a magnetic boundary condition in other embodiments.

FIG. 11is a cross-section and schematic view of a further example embodiment of a plasma lamp400, in which a bulb104of the lamp400is orientated to enhance an amount of collectable light into a given etendue. The lamp400is shown to include a lamp body102including a solid dielectric resonator and an electrically conductive coating120which extends to a front or upper surface116. The lamp body102is provided with the electrically conductive coating120such that there is a partial gap402in the electrically conductive coating120along a midplane of the bulb104. An internal cavity or chamber404extends into the lamp body102. The conductive coating120also extends into the cavity404. In this example embodiment, end portions128,130of the bulb104extend below the electrically conductive coating120on the upper surface116of the lamp body102. This lamp400operates in a manner similar to a vane resonator with a solid dielectric body.

FIG. 12is a cross-sectional view of a lamp1200according to another example embodiment. The lamp1200is similar to the lamp ofFIG. 9except that it does not have a feedback probe and uses a different power circuit. The lamp1200includes a bulb104, a lamp body102, conductive elements124and126, an electrically conductive layer120, dipole arms122, a drive probe170and a sensor220. As shown inFIG. 12, a lamp drive circuit1204is shown to include an oscillator1250and an amplifier1210(or other source of radio frequency (RF) power) may be used to provide RF power to the drive probe170. The drive probe170is embedded in the solid dielectric body of the lamp1200. Control electronics1218controls the frequency and power level provided to the drive probe170. Control electronics1218may include a microprocessor or microcontroller and memory or other circuitry to control the lamp drive circuit1206. The control electronics1218may cause power to be provided at a first frequency and power level for initial ignition, a second frequency and power level for startup after initial ignition and a third frequency and power level when the lamp1200reaches steady state operation. In some example embodiments, additional frequencies may be provided to match the changing conditions of the load during startup and heat up of the plasma. For example, in some embodiments, more than sixteen different frequencies may be stored in a lookup table and the lamp1200may cycle through the different frequencies at preset times to match the anticipated changes in the load conditions. In other embodiments, the frequency may be adjusted based on detected lamp operating conditions. The control electronics1218may include a lookup table or other memory that contains control parameters (e.g., frequency settings) to be used when certain operating conditions are detected. In example embodiments, feedback information regarding the lamp's light output intensity is provided either directly by an optical sensor220, e.g., a silicon photodiode sensitive in the visible wavelengths, or indirectly by an RF power sensor1222, e.g., a rectifier. The RF power sensor1222may be used to determine forward power, reflected power or net power at the drive probe170to determine the operating status of the lamp1200. A directional coupler1212may be used to tap a small portion of the power and feed it to the RF power sensor1222. In some embodiments, the control electronics1218may adjust the frequency of the oscillator1250on an ongoing basis to automatically maintain desired operating conditions. For example, reflected power may be minimized in some embodiments and the control electronics may rapidly toggle the frequency to determine whether an increase or decrease in frequency will decrease reflected power. In other examples, a brightness level may be maintained and the control electronics may rapidly toggle the frequency to determine whether the frequency should be increased or decreased to adjust for changes in brightness detected by sensor220.

The above circuits, dimensions, shapes, materials and operating parameters are examples only and other embodiments may use different circuits, dimensions, shapes, materials and operating parameters.