Patent Publication Number: US-2010123396-A1

Title: Replaceable lamp bodies for electrodeless plasma lamps

Description:
CLAIM OF PRIORITY 
     The present application claims the benefit under 35 U.S.C. 119(e) of U.S. Provisional Patent Application Ser. No. 61/104,014, filed Oct. 9, 2008, which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     I. Field 
     This disclosure relates to systems and methods for generating light, and more particularly to radio frequency powered discharge lamps. 
     II. Background 
     Electrodeless plasma lamps can offer long operating lifetimes but, nevertheless, still fail after prolonged use. These plasma lamps include a bulb that includes a light emitting plasma when radio frequency power is coupled to the bulb. Bulbs typically wear out more quickly than any other components of a plasma lamp system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments are illustrated by way of example, and not limitation, in the figures of the accompanying drawings, in which like references indicate similar elements unless otherwise indicated. In the drawings: 
         FIG. 1  is a cross-section and schematic view of a plasma lamp according to an example embodiment; 
         FIG. 2A  shows an exploded cross-section view of a plasma lamp, in accordance with an example embodiment, including a lamp housing and replaceable lamp body; 
         FIG. 2B  shows an assembled cross-section view of the plasma lamp of  FIG. 2A ; 
         FIG. 3A  shows an exploded cross-section view of a further plasma lamp, in accordance with an example embodiment, including a lamp housing and replaceable lamp body; 
         FIG. 3B  shows an assembled cross-section view of the plasma lamp of  FIG. 3A ; 
         FIG. 4A  shows an exploded cross-section view of a yet further plasma lamp, in accordance with an example embodiment, including a lamp housing and replaceable lamp body; and 
         FIG. 4B  shows an assembled cross-section view of the plasma lamp of  FIG. 4A . 
     
    
    
     DETAILED DESCRIPTION 
     While the present invention is open to various modifications and alternative constructions, the 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 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. 
     In an example embodiment, an electrodeless plasma lamp comprises a lamp housing and a lamp body releasably received with the lamp housing. The lamp housing includes a first electrical connector operatively coupled a power source to provide radio frequency (RF) power The lamp body includes a second electrical connector to releasably engage with the first electrical connector of the lamp housing. In an example embodiment, the plasma lamp includes a retaining arrangement releasably to retain the lamp body at least partially within the lamp housing. As the lamp body is releasably mounted and connected to the lamp housing, removal and replacement of the lamp body by a service technician is facilitated. In an example embodiment, the retaining arrangement includes a simple mechanical fastener that may be easily undone. Thus, no soldering iron or other tools are required to replace, for example, a defective plasma lamp body. 
     Example embodiments provide for lamp body replacement in RF powered plasma lamps.  FIG. 1  is a cross-section and schematic view of electrodeless plasma lamp  100  in accordance with an example embodiment. It should be noted that the plasma lamp  100  is merely an example and other plasma lamps may be used with other embodiments, including microwave, capacitive or inductive plasma lamps or other high intensity discharge lamps. 
     In the example embodiment of  FIG. 1 , the plasma lamp  100  may have a lamp body  102  formed from one or more solid dielectric materials and a bulb  104  positioned adjacent to the lamp body  102 . The lamp body  102  includes a dielectric material having a dielectric constant (relative permittivity) greater than 2. The bulb  104  contains a fill that is capable of forming a light emitting plasma when radio frequency (RF) power is coupled to the bulb  104 . A lamp drive circuit  106  provides RF to the lamp body  102  which, in turn, is coupled to the fill in the bulb  104  to form the light emitting plasma. In example embodiments, the lamp body  102  forms a waveguide that contains and guides the radio frequency power. In example embodiments, the RF power may be provided at or near a frequency that resonates within the lamp body  102 . This is an example only and some embodiments may use a different electrodeless plasma lamp, such as a capacitively or inductively coupled plasma lamp, or other high intensity discharge lamp. 
     The plasma lamp  100  is shown to have a drive probe  120  inserted into the lamp body  102  to provide RF power to the lamp body  102 . The lamp drive circuit  106  including a power supply, such as a voltage controlled oscillator  130  and an amplifier  124 , may be coupled to the drive probe  120  through a low pass filter  126  to provide the RF power. In an example embodiment, the lamp drive circuit  106  is matched to the load (formed by the lamp body  102 , bulb  104  and plasma) for the steady state operating conditions of the plasma lamp  100 . In an example embodiment, the lamp drive circuit  106  is matched to the load at the drive probe  120  using a matching network  126 . A photodetector  134 , a microprocessor  132 , and a current sensor  136  may be used to control the drive circuit  106  during operation of the plasma lamp  100 . 
     In example embodiments, the RF 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 RF power may be provided to the drive probe  120  at or near a resonant frequency for the lamp body  102 . The frequency may be selected based on the dimensions, shape and relative permittivity of the lamp body  102  to provide resonance in the lamp body  102 . In example embodiments, the frequency is selected for a fundamental resonant mode of the lamp body  102 , 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 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 embodiment, the RF 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 some example embodiments, the RF power is provided by an RF wave coupling. The RF power may be coupled at a frequency that forms a standing wave in the lamp body  102  (sometimes referred to as a sustained waveform discharge or microwave discharge when using microwave frequencies). In other embodiments, a capacitively coupled or inductively coupled electrodeless plasma lamp may be used. Other high intensity discharge lamps may be used in other embodiments. 
       FIG. 2A  shows an exploded cross-section view of an electrodeless plasma lamp  200 .  FIG. 2B  depicts an assembled view of the plasma lamp  200  shown in  FIG. 2A . 
     The plasma lamp  200  comprises an assembly of a replaceable lamp body  202  and lamp housing  204  with associated parts that enable replacement. The lamp housing  204  and its associated components constitute the portion of the electrodeless plasma lamp  200  that is a fixed part of a lighting product or apparatus (e.g., a stage lighting installation, street and area lighting installations, or the like). The lamp body  202  and its associated components constitute a replaceable portion of the plasma lamp  200 . The plasma lamp  200  further includes a securing arrangement in the form of a retaining component  206  and fasteners  208  to secure or hold captive the replaceable lamp body  202  at least partially within the lamp housing  204  (see  FIG. 2B ). In some example embodiments, the lamp body is almost fully received within the lamp housing when the plasma lamp is in its assembled form and operating. 
     The lamp housing  204  may be constructed out of cast metal or forged aluminum. The lamp housing  204  may have one or more air cavities  210  which may or may not be contiguous. The air cavity  210  may form the space that the other components, including the lamp body  202  will occupy. Accordingly, the lamp housing  204  may be shaped and dimensioned to receive the lamp body  202 . In an example embodiment, the lamp housing includes a closure member such as a lid  212 . The lid  212  may be a stamped sheet of aluminum, and serve to cover over an open top of the housing  204 . In some embodiments, the housing  204  and the lid  212  form part of an electromagnetic shield or barrier to electromagnetic interference (EMI) emitted by the plasma lamp  200  during operation. Accordingly, the lid  212  may be coated with a compressible EMI gasket material to at least reduce or ideally eliminate RF energy leaks. 
     In an example embodiment, a light tunnel  214  is defined by a hole in the housing  204  to carry light from the bulb  216  back to a lamp control circuit (e.g., the lamp drive circuit  106  shown in  FIG. 1 ). The lamp control circuit may monitor and control light output in a closed-loop fashion using a light-monitoring element, such as a photodiode (see the photodetector  134  shown in  FIG. 1 ). A printed circuit board (PCB)  218  is provided to connect a high-power RF output of the lamp control circuit to the lamp body  202 . In some example embodiments, the PCB  218  may also contain an EMI low-pass filter that effectively passes through power at the resonant frequency for lamp body  204 , but rejects selected harmonics of that frequency. A trace  220  in the form of a strip of etched copper metal cladding may be provided on the PCB  218  to carry the RF power from the high-power RF output of the lamp control circuit to the lamp body  202 . A socket or receptacle  222  is mounted and electrically coupled to the trace  220 , and transfers the RF power from the trace  220  to an RF feed  224  (e.g., a drive probe  120 ), which is inserted into the receptacle  222 . The receptacle  222  grasps the RF feed with some a spring force or bias, which arises from one or more spring clips internal to the receptacle  222 . The spring clips may be deflected when the RF feed  224  is inserted. It is however to be noted that any removable coupling or socket may be provided which allows electrical coupling between the RF feed and the trace  220 . 
     In the example plasma lamp  200 , the receptacle  222  provides a first electrical connector forming part of the housing  204  and the RF feed  224  provides a second electrical connector that forms part of the lamp body  202 . The first and second electrical connectors are releasable electrical connectors held together, for example with a friction fit. Accordingly, the first and second electrical connectors can be disengaged with relative ease without the use of a soldering iron or the like required for fixed connectors. In an example embodiment any releasable plug and socket arrangement suitable for coupling power between two conductors. 
     Several features shown in the plasma lamp  200  of  FIG. 2A  may ensure the proper thermo-mechanical interface between the lamp body  202  and the lamp housing  204 . These include a thermally conductive material such as a thermal pad  226 , and an alignment formation including, for example, an alignment pin  228 . The thermal pad  226  may be made of a material with a thermal conductivity of approximately 1 W/m-K to 20 W/m-K or any value subsumed therein. The thermal pad  226  may provide uniform contact between the lamp body  202  and the housing  204 , filling any small air gaps that would otherwise be present. The thermal pad  226  may ensure a predictable amount of heat transfer from the lamp body  202  to the lamp housing  204 , regardless of the level of contact otherwise conferred by their local surface geometry. In an example embodiment, the thermal pad  226  may be advantageous for enabling replaceable lamp bodies to be used with plasma lamps. As shown by way of example in  FIG. 2A , the thermal pad  226  is fixedly attached to the lamp housing  204  and snugly abuts the lamp body  202  when the lamp body  202  is received within the lamp housing  204 . 
     The metal alignment pin  228  may facilitate locating the lamp body  202  with respect to the housing  204 . The alignment pin  228  is received within an alignment aperture or hole  230  provided in the lamp body  202 . This alignment formation, along with the RF feed  224  receivable within the receptacle  222 , may provide proper alignment and faciliate that the following spatial relationships may be maintained. The lamp body  202  is centered relative to the housing cavity  210 ; this may facilitate the alignment of the bulb  216  to any optical elements (such as lenses or reflectors) in a lighting apparatus or product. When the bulb  216  has a tail  232 , the tail  232  may be aligned with the light tunnel  214 , so that light emitted from the tail  232  can illuminate the photodiode or other light measurement device (not shown) at the end of the light tunnel  214 . 
     In an example embodiment, the lamp body  202  comprises a dielectric waveguide resonator that couples RF power to the bulb  216 . The tail  232  of the bulb  216  may be an extension of its body and the tail  232  may not include any light emitting material. In an example embodiment, the tail  232  is a transparent piece of quartz rod. The tail  232  may transmit a small portion of the total light generated in the bulb  216  along to light tunnel  214  a photodetector connected to the lamp control circuit. In an example embodiment, the RF feed  224  couples power from the lamp control circuit in a resonant mode of the waveguide resonator that is suitable for delivering power to the bulb  216 . The alignment hole  230  may be a relatively small hole in the lamp body  202  so as not to substantially perturb the operation of the waveguide resonator electric or magnetic fields. The alignment hole  230 , together with the RF feed  224 , may form the alignment mechanism or formation that locates the lamp body  202  properly with respect to the housing  204  when replacing the lamp body  202 . Further, the alignment formation may be configured to align the lamp body  202  with the lamp housing  204  to facilitate engaging of the first and second electrical connectors. 
     The retaining component  206  may apply an appropriate amount of force with which to hold the lamp body  202  within the housing  204 . The retaining component  206  may not only to ensure that the lamp body  202  does not inadvertently fall out of the housing  204 , but it also apply the correct amount of force to sufficiently compress the thermal pad  226  to enable adequate thermal coupling between the lamp body  202  and the lamp housing  204 . In an example embodiment, the retaining component  206  applies a spring force to the lamp body  202  through tightening of the fasteners  208 . In an example embodiment, these fasteners  208  are easily accessible on the outside of the plasma lamp  200  such that a lamp service technician could easily loosen the fasteners  208  by hand, possibly while wearing protective gloves, and replace the lamp body  202 . In order to do this, in some example embodiments the fasteners  208  include thumbscrews with head diameters of about 3 mm to 10 mm or any value subsumed therein. The retaining arrangement may also force the first and second electrical connectors to sufficiently engage so that a proper electrical connection is made. 
       FIG. 3A  shows an exploded cross-section view of a further plasma lamp  300 , in accordance with an example embodiment, including a lamp housing  304  and replaceable lamp body  302 .  FIG. 3B  shows an assembled cross-section view of the plasma lamp  300  of  FIG. 3A . In the plasma lamp  300  an RF contact or receptacle may be advantageously provided inside the lamp body itself (e.g., in the dielectric material), rather than inside the lamp housing, as shown by way of example in the plasma lamp  200  of in  FIGS. 2A  and B. In an example embodiment, a lamp body  302  is tethered at an end of a cable (e.g., a coaxial cable) that may originate from a lamp control circuit provided, for example, in the lamp housing  304 . 
     The cable supplying the lamp body  302  with RF power may have a metal center pin  308 , an insulating layer  310 , and a metal outer jacket  312 . In some example embodiments, the outer jacket  312  may comprise multiple layers. The cable terminates in a mounting plate  314 , which is grounded to the cable outer jacket  312 , typically through a standard crimp connection. The insulating layer  310  and the center pin  308  protrude through the mounting plate  314 , and the center pin  308  stands proud of the mounting plate  314 . An EMI gasket  316  is layered onto the mounting plate  314  and may provide a leak-free contact between the lamp body  302  and the grounded mounting plate  314  of the lamp housing  304  when sufficiently compressed. 
       FIG. 3A  clearly shows that the center pin  308  of the second connector being provided in the dielectric material cable defines an RF feed  318  to couple RF power into the lamp body  302 . By designing an example lamp body  302  replaceable system this way, it may not be necessary to have a separate RF feed integral with the lamp body  302  as is the case with the lamp body  202  (see  FIGS. 2A and 2B ). Thus, in an example embodiment, the RF feed may define a first connector that releasable connects to a second connector provided in the lamp body. In the example plasma lamp  300 , a receptacle  320  defines the second connector. An alignment pin  322  received within an aperture or hole  323  may perform the same function of the alignment pin  228  of the plasma lamp  200 . 
     The lamp body  302  of the plasma lamp  300  may not differ substantially from the lamp body  202  of the plasma lamp  200 . Example differences include the following. A bulb tail  324  of the plasma lamp  300  may or may not protrude entirely through the lamp body  302 , as is the case in the bulb tail  232  of the plasma lamp  200 . Instead of an RF feed  224 , in an example embodiment the plasma lamp  300  incorporates an oversized hole  326  to receive biased (spring loaded) socket or receptacle  320 . The function of the receptacle  320  may be identical to the function of the receptacle  222  of the plasma lamp  200 . In an example embodiment, the receptacle  320  receives a tip of the RF feed  318 , formed in the example embodiment by the center pin  308  of the coaxial cable, and holds it with some spring force. It is however to be noted that any socket or electrical contact that can electrically and releasable engage the RF feed  318  appropriately to ensure coupling of RF power into the lamp body  302  may be used in other example embodiments. 
     The plasma lamp  300  includes a retainer  306  that is similar in function to the retainer  206  of the plasma lamp  200 . Unlike the retainer  206  that includes fasteners  208 , the retainer  306  is a snap-fit. Accordingly, removal of the lamp body  302  from the lamp housing by, for example, a service technician may be greatly facilitated. 
     The retainer  306  is shown by way of example as a spring-loaded clip which engages on its one side with a front surface  330  of the lamp body  302 , and on its other side to a back surface of  332  of the mounting plate  314  of the lamp housing  304 . The retainer  306  may be designed to be easily removed and replaced in the field by a service technician, for example, who is likely to experience reduced manual dexterity due to wearing protective gloves at the time. The retainer  306  applies sufficient force to the lamp body  302  to compress it against the EMI gasket  316  and the mounting plate  314 . 
       FIG. 4A  shows an exploded cross-section view of a yet further plasma lamp  400 , in accordance with an example embodiment, including a lamp housing  404  and replaceable lamp body  402 . A retaining component  406  is optionally provided to removably retain the lamp body  402  within the lamp housing  404 .  FIG. 4B  shows an assembled cross-section view of the plasma lamp  400  of  FIG. 4A . 
     In an example embodiment, the lamp body  402  substantially resembles the lamp body  102  and some features of the lamp body  102  are not shown in  FIGS. 4A and 4B  for the sake of clarity. The plasma lamp  400  may, functionally, substantially resemble to plasma lamps  200  and  300  in that it includes a replaceable lamp body  402 . For example, where the plasma lamp  400  is used in a lighting situation (e.g., street and area lighting, where a more permanent mounting of the lamp is provided), the lamp housing  404  is fixedly mounted to a support structure. In the event of the bulb  408  (or any part of the lamp body  402 ) failing, the lamp body  402  may be removed from the lamp housing  404  and replaced with a new lamp body. Accordingly, as in the case of the plasma lamps  200 ,  300 , the retaining component  406  is configured to be relatively easily removable to facilitate replacing of the lamp body  402 . Although the retaining component is shown as a clip secured to the lamp housing  404  using fasteners  410 , it should be understood that other retaining arrangements may be used in other embodiments. In fact, any retaining arrangement may be used that retains the lamp body  402  in the lamp housing  404 . For example a friction-fit retaining arrangement may be provides wherein the lamp housing  404  defines an opening to at least partially receive the lamp body  402 , walls of the opening cooperating (e.g., frictionally) with side walls of the lamp body  402  to define the alignment arrangement. 
     The retaining component  406  may be configured to facilitate mating of a complementary coupling arrangement  415  provided by a socket or receptacle  410  and a pin  412  to establish an electrical connection between drive circuitry and an RF feed  414 . The coupling arrangement  415  may comprise a first electrical connector defined, for example, by the pin  412  of the lamp housing  404 , and a second electrical connector defined, for example, by the socket or receptacle  410  of the lamp body  402 . In an example embodiment, the lamp housing receives RF power from the drive circuit via a coaxial cable  416  mounted to the lamp housing  404  by a coaxial cable connector  418 . A center conductor of the coaxial cable  416  may define the pin  412 . The receptacle  410  may include a biasing member (e.g., a spring-loaded contact) to electrically engage with the pin  412 . 
     It is however to be appreciated that the complementary coupling arrangements shown in  FIGS. 2A ,  2 B,  3 A,  3 B,  4 A, and  4 B may differ from one embodiment to another. It should thus be noted that any suitable releasable connector may be used to couple the RF power from the lamp housing  204 ,  304 ,  404  to the lamp body  202 ,  302 ,  402 . 
     It is to be noted that the plasma lamps  200 ,  300  and  400  may similar in design to the plasma lamp  100 . The various retaining formations to allow the plasma lamp  100  to function as a replacement lamp body  202 ,  302 ,  402  are not shown in  FIG. 1 . It should be noted that the plasma lamp  100  may include features of the plasma lamps  200 ,  330  and vice-versa. 
     In an example embodiment, the lamp body  402  is circular cylindrical in cross sectional shape and the lamp housing  404  has a circular cylindrical opening to at least partially receive the lamp body. In another example embodiment, the lamp body  402  is rectangular in cross sectional shape and the lamp housing  404  has a rectangular opening to at least partially receive the lamp body  404 . As can be seen in  FIGS. 4A and 4B , in an example embodiment the lamp body  402  is almost fully received within the lamp housing  404 . It is however to ne noted that, in other example embodiments, the lamp body  402  may be fully received or only partially received within the lamp housing  404 . 
     High frequency simulation software may be used to help select the materials and the shape of the lamp body  102 ,  202 ,  302 ,  402  and electrically conductive coating (electrically coating  108  in  FIG. 1  and not shown in  FIGS. 2A ,  2 B,  3 A,  3 B,  4 A and  4 B) to achieve desired resonant frequencies and field intensity distribution in the lamp body  102 ,  202 ,  302 ,  402 . 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. to determine the desired shape and dimensions of the lamp body  102 ,  202 ,  302 ,  402 , resonant frequencies and field intensity distribution. The desired properties may then be fine-tuned empirically. 
     While a variety of materials, shapes and frequencies may be used, one example embodiment has a lamp body  102  designed to operate in a fundamental TM resonant mode at a frequency of about 880 MHz. The frequency may however be spread across a spectrum to reduce EMI and may also be adjusted based on load conditions or for brightness control. In this example, the plasma lamp  100  has an alumina lamp body  102  with a relative permittivity of 9.2. The lamp body  102  may have a cylindrical outer surface as shown with a recess  118  formed in the bottom surface. In an alternative embodiment, the lamp body  102  may have a rectangular outer surface. The outer diameter D 1  of the lamp body  102  may be about 40.75 mm and the diameter D 2  of the recess  118  may be about 8 mm. In an example embodiment, the lamp body  102  has a height H 1  of about 17 mm. A narrow or thin region  112  forms a shelf over the recess  118 . The thickness H 2  of the narrow region  112  is about 2 mm. As shown in  FIG. 1 , in the narrow region  112  of the lamp body  102 , the electrically conductive surfaces on the lamp body  102  are only separated by the narrow region  112  of a shelf in the lamp body  102 . This results in higher capacitance in this region of the lamp body  102  and higher electric field intensities. In an example embodiment, this shape may support a lower resonant frequency than a solid cylindrical body having the same overall diameter D 1  and height H 1  or a solid rectangular body having the same overall width and height. For example, in some embodiments, the relative permittivity is in the range of about 9-15 or any range subsumed therein, the frequency of the RF power is less than about 1 GHz and the volume of the lamp body is in the range of about 10 cm 3  to 30 cm 3  or any range subsumed therein. 
     In the plasma lamp  100 , a hole  110  is formed in the narrow region  112 . The hole  110  may have a diameter of about 5.5 mm and the bulb  104  has an outer diameter of about 5 mm. The shelf formed by the narrow region  112  extends radially from the edge of the hole  110  by a distance D 3  of about 1.25 mm. In an example embodiment, alumina powder is packed between the bulb  104  and the lamp body  102  and forms a layer having a thickness D 5  of about ¼ mm. The bulb  104  has an outer length of about 15 mm and an interior length of about 9 mm. The interior diameter of the bulb  104  at the center is about 2.2 mm and the sidewalls have a thickness of about 1.4 mm. The bulb  104  protrudes from the front surface of the lamp body  102  by about 4.7 mm. The bulb  104  has a fill of Argon, Kr 85 , Mercury and Indium Bromide. The pressure of the noble gas may be 400 Ton or more to reduce warm up times. This example pressure is 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  100 ,  200 ,  300  may provide a high intensity discharge at high pressure during operation (e.g., much greater than 2 atmospheres and 10-30 atmospheres or more in example embodiments). It will be noted that the bulbs  104 ,  216 ,  316 , and  408  are substantially smaller than the lamp housing  204 ,  304 ,  404  and the lamp body  202 ,  302 ,  402 . 
     The above dimensions, shape, materials and operating parameters are examples only and other embodiments may use different dimensions, shape, materials and operating parameters.