Patent Publication Number: US-6657206-B2

Title: Ultraviolet lamp system and methods

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation-in-Part of commonly assigned, co-pending application Ser. No. 09/702,519, filed Oct. 31, 2000 and entitled ULTRAVIOLET LAMP SYSTEM AND METHODS, naming Patrick G. Keogh and James W. Schmitkons as inventors, the disclosure of which is hereby incorporated by reference herein in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to ultraviolet lamp systems and, more particularly, to microwave-excited ultraviolet lamp systems configured to irradiate a substrate with ultraviolet radiation. 
     BACKGROUND OF THE INVENTION 
     Ultraviolet lamp systems are commonly used for heating and curing materials such as adhesives, sealants, inks, and coatings. Certain ultraviolet lamp systems have electrodeless light sources and operate by exciting an electrodeless plasma lamp with either radiofrequency energy or microwave energy. In an electrodeless ultraviolet lamp system that relies upon excitation with microwave energy, the electrodeless plasma lamp is mounted within a metallic microwave cavity or chamber. One or more microwave generators are coupled via waveguides with the interior of the microwave chamber. The microwave generators supply microwave energy to initiate and sustain a plasma from a gas mixture enclosed in the plasma lamp. The plasma emits a characteristic spectrum of electromagnetic radiation strongly weighted with spectral lines or photons having ultraviolet and infrared wavelengths. To irradiate a substrate, the radiation is directed from the microwave chamber through a chamber outlet to an external location. The chamber outlet is capable of blocking emission of microwave energy but allows electromagnetic radiation to be transmitted outside the microwave chamber. A fine-meshed metal screen covers the chamber outlet of many conventional ultraviolet lamp systems. The openings in the metal screen transmit electromagnetic radiation for irradiating a substrate positioned outside the microwave chamber, yet substantially block the emission of microwave energy. 
     The electrodeless plasma lamp emits a characteristic spectrum isotropically outward along its cylindrical length. Part of the emitted radiation moves directly from the plasma lamp toward the substrate without reflection. However, a significant portion of the emitted radiation must undergo one or more reflections to reach the substrate. To capture this indirect radiation, a reflector can be provided that is mounted within the microwave chamber in which the plasma lamp is positioned. The reflector includes surfaces capable of redirecting incident radiation in a predetermined pattern toward the chamber outlet and to the substrate positioned outside the microwave chamber. 
     A major shortcoming of conventional systems is the inability to accurately predict the focal point or focal plane outside the microwave chamber at which the reflected ultraviolet radiation will be delivered. Another shortcoming is the reflector of the lamp system cannot be easily modified to adjust the focal point or focal plane, if known, so that the substrate can be repositioned relative to the lamp system. Further, the inability to accurately predict the focal point or focal plane limits the ability to mass produce lamp systems capable of delivering predictable radiation patterns to a substrate. A further limitation is that conventional ultraviolet lamp systems are designed to irradiate a flat surface on large-area substrates and cannot be easily adapted to uniformly irradiate substrates in a surrounding fashion. For example, conventional ultraviolet lamp systems cannot uniformly irradiate the entire circumference of round substrates. 
     If the plasma lamp is considered a line source of radiation, the intensity of ultraviolet radiation striking the substrate is inversely proportional to the separation between the plasma lamp and the substrate. As a result, the ultraviolet radiation is significantly attenuated when traveling from the plasma lamp on the interior of the microwave chamber to the substrate positioned outside the microwave chamber. To compensate for this loss in intensity, the microwave power must be elevated to increase the output of the plasma lamp. However, the amount of infrared radiation will likewise increase with the output of the plasma lamp. The excess infrared energy heats the substrate, the microwave chamber, and the plasma lamp. The elevation in temperature associated with the excess infrared energy can significantly reduce the lifetime of the plasma lamp and can produce additional undesirable effects. 
     Thus, a microwave-excited ultraviolet lamp system is needed with a configuration capable of uniformly irradiating a substrate positioned within the microwave chamber with ultraviolet radiation and that can do so without emitting significant amounts of microwave energy. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the foregoing and other deficiencies of conventional microwave-excited ultraviolet lamp systems. While the invention will be described in connection with certain embodiments, the invention is not limited to these embodiments. On the contrary, the invention includes all alternatives, modifications and equivalents as may be included within the spirit and scope of the present invention. 
     According to the present invention, an ultraviolet radiation generating system for treating a coating on a substrate, such as a coating on a cable or, more specifically, a coating on a fiber optic cable, comprises a microwave chamber having an inlet port capable of permitting the cable to be positioned within or to travel within a processing space of the microwave chamber. During operation, the microwave chamber is substantially closed to emission of microwave energy and the emission of ultraviolet radiation. A microwave generator is coupled to the microwave chamber for exciting a longitudinally-extending plasma lamp mounted within the processing space of the microwave chamber. The plasma lamp emits ultraviolet radiation for irradiating the substrate. A first portion of the ultraviolet radiation directly irradiates the frontside of the substrate. Mounted within the microwave chamber is a pair of reflectors which substantially surround the processing space. The reflectors are capable of reflecting a portion of the ultraviolet radiation for indirectly irradiating the backside of the substrate with reflected ultraviolet radiation. 
     In certain embodiments, the microwave chamber may further include an outlet port so that the substrate travels between the inlet and outlet ports through the microwave chamber at least partially within the processing space. In other embodiments, the lamp system may also include an ultraviolet-transmissive conduit positioned within the microwave chamber generally between the inlet and outlet ports. The conduit encloses the substrate when it is positioned within the processing space of the microwave chamber. In still other embodiments, the lamp system may also include microwave chokes which are capable of reducing the emission of microwave energy from the inlet and outlet ports. 
     According to methods of the present invention, a substrate is positionable within a processing space of a microwave and a plasma lamp is excited with microwave energy to emit ultraviolet radiation for irradiating the substrate. While the substrate is positioned within or traveling through the processing space, the frontside of the substrate is irradiated with direct ultraviolet radiation emitted from the plasma lamp and the backside of the substrate is irradiated with indirect ultraviolet radiation emanating from the plasma lamp which is reflected from a pair of reflectors. The substrate is removed from the processing space after irradiating. 
     The present invention permits the substrate to be positioned directly within the microwave chamber for treatment with ultraviolet radiation. As a result, the chamber may be completely sealed to prohibit the emission of microwave energy and to eliminate the need to emit ultraviolet radiation from the microwave chamber. Because the substrate, the plasma lamp, and the reflector have well-defined relative positions within the microwave chamber, the plasma lamp and reflector can be precisely located relative to the substrate for purposes of providing a predictable, reproducible and substantially uniform pattern of radiation at and distributed about or surrounding the substrate. Furthermore, because the substrate is positioned within the microwave chamber and because the ultraviolet radiation does not have to be transmitted through a screen to a location outside of the microwave chamber, a greater intensity of ultraviolet radiation per unit measure of microwave energy can be delivered to the substrate. As a result, the microwave energy can be reduced to deliver a given intensity of ultraviolet radiation to the substrate or the ultraviolet intensity can be optimized for improving the treatment throughput of the lamp system. 
     The above and other advantages of the present invention shall be made apparent from the accompanying drawings and the description thereof. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with a general description of the invention given above, and the detailed description of the embodiments given below, serve to explain the principles of the invention. 
     FIG. 1 is a perspective side view of an ultraviolet lamp system of the present invention; 
     FIG. 2 is a partial longitudinal cross-sectional view of an ultraviolet lamp system taken along line  2 — 2  of FIG. 1; 
     FIG. 3 is a cross-sectional view of the ultraviolet lamp system of FIG. 1 taken along line  3 — 3  of FIG. 2, showing one embodiment of a reflector for use in the lamp system of FIG. 1; 
     FIG. 3A is a cross-sectional view similar to FIG. 3 of an alternative embodiment of a reflector of the present invention for use in the lamp system of FIG. 1; and 
     FIG. 3B is a cross-sectional view similar to FIG. 3 of an alternative embodiment of a pair of reflectors according to the present invention for use in the lamp system of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention relates to microwave-excited ultraviolet lamp systems configured to uniformly irradiate with ultraviolet radiation a substrate positioned within or traveling within a processing space of the microwave chamber. According to present invention, the lamp system is configured such that the substrate is capable of being positioned in the processing space near a microwave-excited plasma lamp, thereby increasing the intensity of the ultraviolet radiation irradiating the substrate. Further, the positioning of the substrate within the processing space eliminates the need to transmit the ultraviolet radiation outside of the microwave chamber for treating the substrate. Further, the present invention incorporates a reflector or a pair of reflectors that, along with the direct ultraviolet radiation from the plasma lamp, participate in providing a substantially uniformly irradiance of ultraviolet radiation in a surrounding relationship relative to, or about the circumference of, the substrate. Further, the present invention isolates the substrate with an ultraviolet-transmissive conduit such that fragile substrates can be accommodated and yet a sufficient air flow can be provided to cool the microwave generators and the plasma lamp of the system. Further, the present invention permits the substrate to enter the microwave chamber and to travel within or be positioned within the processing space without substantial microwave leakage from the chamber. Further, the reflector or reflectors, the substrate, and the plasma lamp are positioned within the processing space of the microwave chamber so as to provide a precise, reproducible and substantially uniform pattern of ultraviolet radiation that surrounds the substrate. As used herein, treatment encompasses curing, heating, or any other process that alters a physical property of a substrate or a coating on a substrate as a result of exposure to ultraviolet radiation. 
     With reference to FIGS. 1 and 2, a microwave-excited ultraviolet lamp system of the present invention is indicated generally by reference numeral  10 . Lamp system  10  includes a pair of microwave generators  12  and  14 , illustrated as magnetrons, mechanically mounted by a respective one of a pair longitudinally-spaced waveguides  16  and  18  to a longitudinally-extending microwave chamber, indicated generally by reference numeral  20 . A pair of transformers  32  and  33  (FIG. 2 shows only transformer  33 ) are electrically coupled to a respective one of the microwave generators  12  and  14  for energizing filaments of the microwave generators  12  and  14  as understood by those of ordinary skill in the art. To prevent cross-coupling when the lamp system  10  is operating, the operating frequencies of the two microwave generators  12  and  14  should be offset by a small amount. By way of specific example but not limitation, the two microwave generators  12  and  14  may operate at respective frequencies of about 2470 MHz and about 2445 MHz, which represents a frequency offset of 25 MHz, and may have individual power ratings of about 3 kW. 
     While a pair of microwave generators  12  and  14  is illustrated and described herein, the lamp system  10  may include only a single microwave generator without departing from the spirit and scope of the present invention. Waveguide  16  includes an inlet port  21  coupled with microwave generator  12  and an outlet port  22  which is aligned and coupled for microwave transmission with an opening  24  provided in the microwave chamber  20 . Similarly, waveguide  18  includes an inlet port  26  coupled with microwave generator  14  and an outlet port  27  which is aligned and coupled for microwave transmission with an opening  28  provided in the microwave chamber  20 . Microwave energy from the microwave generators  12  and  14  is directed via waveguides  16  and  18  to an interior space  15  of the microwave chamber  20  through the openings  24  and  28 . Microwave energy is deposited with a three-dimensional density distribution within the microwave chamber  20  as understood by those of ordinary skill in the art. 
     A plasma lamp  34  is positioned longitudinally within the microwave chamber  20 . Opposite ends  36  of plasma lamp  34  are supported within the microwave chamber  20  as understood by those of ordinary skill in the art. Plasma lamp  34  comprises a hermetically sealed, longitudinally-extending envelope or tube filled with a gas mixture. Plasma lamp  34  does not require either electrical connections or electrodes for its operation. The plasma lamp  34  is formed of an ultraviolet-transmissive material that is an electrical insulator, such as vitreous silica or quartz, so that the plasma lamp  34  is electrically isolated from other structures in the microwave chamber  20 . Microwave energy provided by the microwave generators  12  and  14  guides excited atoms in the gas mixture within plasma lamp  34  to initiate and, thereafter, sustain the plasma therein. A starter bulb  30  is provided to assist in initiating a plasma within plasma lamp  34  as understood by those of ordinary skill in the art. By adjusting the shape of microwave chamber  20  and the power level of microwave generators  12  and  14 , the density distribution of the microwave energy is selected to excite atoms in the gas mixture along the entire longitudinal dimension of the plasma lamp  34 . Once the plasma is initiated, the intensity of the radiation output by the plasma lamp  34  depends upon the microwave power provided to microwave chamber  20  by microwave generators  12  and  14 . 
     The gas mixture inside plasma lamp  34  has an elemental composition selected to produce photons having a predetermined distribution of wavelengths of radiation when the gas atoms are excited to a plasma state. For ultraviolet treating applications, the gas mixture may comprise a mercury vapor and an inert gas, such as argon, and may include trace amounts of one or more elements such as iron, gallium, or indium. The mercury vapor is provided by the vaporization of a small quantity of mercury that is solid at room temperature. The spectrum of radiation output by a plasma excited from such a gas mixture includes highly intense ultraviolet and infrared spectral components. As used herein, radiation is defined as photons having wavelengths ranging between about 200 nm to about 2000 nm, ultraviolet radiation is defined as photons having wavelengths ranging between about 200 nm to about 400 nm, and infrared radiation is defined as photons having wavelengths ranging between about 750 nm to about 2000 nm. 
     As best understood with reference to FIG. 1, microwave chamber  20  includes a pair of generally vertical opposite end walls  38  and a pair of generally vertical opposite side walls  40  extending longitudinally between the end walls  38  and on opposite sides of the plasma lamp  34 . A segmented, domed wall  42  connects intermediate portions of the side walls  40  between openings  24  and  28 . Walls  38 ,  40 , and  42  are each perforated with a plurality of openings  44  that permit the free flow of air. It is understood that the walls of microwave chamber  20  can be configured differently without departing from the spirit and scope of the present invention. In particular, the configuration of the domed wall  42  can be varied to alter or tune the density distribution of microwave energy within microwave chamber  20 . Microwave chamber  20  is constructed of a suitable metal, such as a stainless steel, that confines the microwave energy to the interior space  15  of the microwave chamber  20 . 
     As best shown in FIG. 3, a cover  46  is mounted to a pair of generally horizontal flanges  48  that extend inwardly from the chamber side walls  40 . Cover  46  is removable to reveal an access opening  47  for entry into interior space  15  of the microwave chamber  20 . Interior space  15  must be accessed for maintenance purposes, such as servicing or replacing plasma lamp  34  or other objects within the interior space  15  of the microwave chamber  20 . Cover  46  has a sealing engagement with access opening  47  that prevents significant amounts of either radiation or microwave energy from being emitted through access opening  47 . 
     With reference to FIG. 2, lamp system  10  is mounted within an enclosure  50 , shown in phantom, having a configuration as recognized by those of ordinary skill in the art. The housing  50  includes an air inlet  51  and an air outlet  52  provided in cover  46 . A flow of a pressurized gas, such as air, into air inlet  51  is used to regulate the operating temperature of the microwave generators  12  and  14  and the operating temperature of the plasma lamp  34 . Microwave generators  12  and  14  each include a plurality of circumferential fins  53 . The fins  53  are operable for increasing the efficiency for conducting heat away from the microwave generators  12  and  14  and enhance the available surface area for convective cooling by the flow of air. A fan (not shown) is generally provided as a means for forcing a pressurized flow of air into enclosure  50 , over microwave generators  12  and  14 , through openings  44  into the microwave chamber  20 , and out of enclosure  50  through outlet  52 . The pressurized flow of air provides a constant exchange of cool air for heated air within the enclosure  50  and reduces maintenance caused by overheated components. Those skilled in the art would recognize that microwave-excited ultraviolet lamp systems, such as lamp system  10 , generate significant amounts of heat that must be eliminated to avoid unacceptably high operating temperatures. 
     A microwave choke  54  is attached to an inlet port  55  provided in one of the end walls  38  of the microwave chamber  20 . A microwave choke  56  is attached to an outlet port  57  provided in the opposite end wall  38 . The ports  55  and  57  and the interior passageways  58  of microwave chokes  54 ,  56  are gene rally aligned longitudinally. Microwave chokes  54  and  56  are hollow, tubular members with a length and diameter chosen, as would be familiar to those of ordinary skill in the art, for preventing a significant amount of microwave energy from leaking outwardly from the interior space  15  of the microwave chamber  20  through ports  55  and  57 . By way of example, and not by way of limitation, microwave chokes  54  and  56  may have a length of about 1 inch and an inner diameter of about 0.75 inches. 
     Microwave chokes  54  and  56  are attached flush with the ports  55  and  57 , respectively, such that no portion of either microwave choke  54  and  56  protrudes a significant distance into the interior space  15  of the microwave chamber  20 . Suitable microwave chokes  54  and  56  are constructed of a metal alloy, such as a stainless steel, and include, but are not limited to, waveguide chokes, quarter-wave stub chokes, or corrugated chokes in combination with a resistive choke. In certain embodiments of the present invention, microwave chokes  54  and  56  may be omitted from ports  55  and  57  without departing from the spirit and scope of the present invention. 
     Lamp system  10  is used for the treatment of a non-conductive substrate  60  which is at least partially covered by a coating or surface layer sensitive to treatment by ultraviolet radiation, such as an ultraviolet-curable coating. Substrate  60  may comprise one or more cables or ribbons which are at least partially covered by a coating or surface layer sensitive to treatment by ultraviolet radiation or, more specifically, one or more fiber optic cables or ribbons which are at least partially covered by a coating or surface layer sensitive to treatment by ultraviolet radiation. Multiple cables or ribbons would be arranged accordingly within the microwave chamber  20  to permit simultaneous treatment. 
     Substrate  60  travels within or through the interior space  15  via inlet port  55  and outlet port  57  of the microwave chamber  20 . Those of ordinary skill will appreciate that substrate  60  may both enter and exit the interior space  15  through one of either the inlet port  55  or the outlet port  57  such that microwave chamber  20  can include only one of inlet port  55  or outlet port  57  without departing from the spirit and scope of the present invention. During transfer within or through the interior space  15  of the microwave chamber  20 , the substrate  60  is continuously irradiated with ultraviolet radiation while positioned in a longitudinally-extending processing space  61 . Processing space  61  comprises a portion of the interior space  15  having an irradiance or flux density of ultraviolet radiation. Because substrate  60  is positioned directly within the processing space  61  of the microwave chamber  20 , the separation distance between the plasma bulb  34  and the substrate  60  is minimized. Because the intensity of ultraviolet radiation per unit measure of microwave energy delivered to the substrate  60  is optimized by the proximity of the plasma bulb  34  to substrate  60  and by the elimination of the need to transmit the ultraviolet radiation externally of the microwave chamber  20 , the microwave generators  12  and  14  can be operated at a reduced power level for exciting plasma lamp  34  to deliver a given intensity of ultraviolet energy. Alternatively, the intensity of the ultraviolet radiation can be optimized such that substrate  60  may be transferred through or within the microwave chamber  20  at a higher rate for enhancing the treatment throughput of the lamp system  10 . 
     Because substrate  60  is physically positioned inside the microwave chamber  20  during irradiation, a chamber outlet covered by a metallic mesh screen is not required in one of the walls  38 ,  40  and  42  of the microwave chamber  20  for transmitting ultraviolet radiation to an externally-positioned substrate and for confining the microwave energy to the interior of the microwave chamber  20 . As a result, the microwave chamber  20  is robust, tightly sealed against microwave and ultraviolet leakage, and does require special structure to prevent microwave leakage while irradiating substrate  60  with ultraviolet radiation. 
     In an aspect of the present invention, the passageways  58  of the inlet port  55  and the outlet port  57  in end walls  38  are generally aligned with an ultraviolet-transmissive conduit  62  positioned within the microwave chamber  20 . Conduit  62  extends longitudinally between the end walls  38  and is supported at opposite ends by the interior of passageways  58  of ports  55  and  57 . Conduit  62  encloses the substrate  60  during the longitudinal transfer of substrate  60  within the interior space  15  of the microwave chamber  20 . Conduit  62  is formed of an electrically-insulating material that is highly transmissive of ultraviolet radiation, such as a quartz or a vitreous silica. Conduit  62  prevents extraneous forces from acting on substrate  60 , such as the forced air currents directed into the microwave chamber  20  for cooling the plasma lamp  34 . This isolation ability is particularly important if substrate  60  is fragile or otherwise prone to damage. However, the conduit  62  may be omitted, such that substrate  60  is not enclosed while in interior space  15 , without departing from the spirit and scope of the present invention. 
     A longitudinally-extending reflector, indicated generally by reference numeral  64 , is positioned within the microwave chamber  20 . As best shown in FIG. 3, reflector  64  includes a quartet of longitudinally-extending, rectangular reflector panels  66 ,  68 ,  70 , and  72 . The reflector panels  66 ,  68 ,  70 , and  72  are mounted in a spaced rectangular arrangement via a pair of brackets  74  attached to opposed end walls  38  of the microwave chamber  20 . Brackets  74  are preferably formed of an electrically-insulating material, such as a thermally-stable polymer and, more specifically, a fluoropolymer. Opposite ends of each reflector panel  66 ,  68 ,  70 , and  72  are received by slots (not shown) in each bracket  74 . Reflector panels  66 ,  68 ,  70 , and  72  have a spaced relationship relative to the plasma lamp  34  and a spaced relationship relative to the ultraviolet-transmissive conduit  62  enclosing substrate  60  such that the portion of interior space  15  between the reflector panels  66 ,  68 ,  70 , and  72  at least partially defines the processing space  61 . Microwave energy provided by microwave generators  12  and  14  is readily transmitted through the reflector panels  66  and  68  for initiating a plasma from the gas mixture in plasma lamp  34  and for sustaining the plasma for the duration of a heating or curing operation. Gaps  76 ,  77  and  78  are provided between the reflector panels  66 ,  68 ,  70 , and  72  for permitting a flow of relatively cool air to cool the plasma lamp  34 . Diverter baffle  75  is provided to preferentially direct a flow of relatively cool air through gap  76  toward plasma lamp  34 . 
     The reflector panels  66 ,  68 ,  70 , and  72  are configured with an inclined arrangement relative to the side walls  40  of the microwave chamber  20  so that the plasma lamp  34  can be physically accessed from access opening  47  when cover  46  is removed. As best shown in FIGS. 2 and 3, each bracket  74  includes a removable portion  79  that is attached by fasteners  83 . The fasteners  83  are preferably formed of an electrically insulating material, such as a ceramic. To remove reflector panel  72 , fasteners  83  are loosened to free the removable portion  79  for detachment from each bracket  74  and reflector panel  72  is slidingly removed from the corresponding slots in brackets  74 . With reflector panel  72  removed, the path is unobstructed from the access opening  47  to objects, such as the plasma lamp bulb  34 , specifically within the processing space  61  and from the access opening  47  to objects generally within the interior space  15  and within the processing space  61 . 
     The reflector panels  66 ,  68 ,  70 , and  72  are preferably formed of a radiation-transmissive material, such as a borosilicate glass or, more specifically, a Pyrex® glass. Flat plates of Pyrex® glass suitable for use as reflector panels  66 ,  68 ,  70 , and  72  are commercially available from Corning Inc. (Corning, N.Y.). Alternatively, reflector panels  66 ,  68 ,  70 , and  72  may be formed of any material having suitable reflective and thermal properties and, in particular, reflector panels  66 ,  68 ,  70 , and  72  may be constructed of a metal and need not be radiation-transmissive or infrared-transmissive if integrally formed as a portion of the microwave chamber  20 . 
     For use in the ultraviolet lamp system  10 , reflector  64  is operable for at least partially transmitting, reflecting or absorbing photons of specific wavelengths. Specifically, reflector  64  is capable of preferentially reflecting photons of ultraviolet radiation, indicated diagrammatically by arrows  80 , from the spectrum of emitted radiation, indicated diagrammatically by arrows  81 , emanating from the plasma lamp  34  and preferentially transmitting absorbing photons of infrared radiation, where transmission of infrared radiation is indicated diagrammatically by arrows  82 . The preferential transmission and reflection of emitted radiation  81  can be provided by methods known to those of ordinary skill, such as applying a dichroic coating to reflector panels  66 ,  68 ,  70 , and  72 . Due to the nature of the reflections and multiple reflections, the reflector  64  (FIG. 3) provides a flood pattern of ultraviolet radiation  80  reflected to substrate  60 , rather than a focused pattern and, in particular, provides a substantially uniform flood pattern of ultraviolet irradiation  80  reflected about the circumference of, or in a surrounding relationship relative to, the substrate  60 . 
     As shown in FIG. 3, a significant portion of the infrared radiation  82  is transmitted through the reflector  64  and channeled to the peripheries of the microwave chamber  20  away from the vicinity of the reflector  64 . As a result, the ultraviolet radiation  80  reflected by reflector  64  toward the substrate  60  is not accompanied by a significant intensity of infrared radiation  82 . Therefore, substrate  60  remains at a relatively low temperature despite being exposed to a significant intensity of ultraviolet radiation  82 . Chamber walls  38 ,  40  and  42  are capable of absorbing the photons of infrared radiation  82  and dissipating the energy thermally. 
     Using like reference numerals for like elements discussed with reference to FIGS. 1,  2  and  3 , an alternative embodiment of a reflector, indicated generally by reference numeral  86 , in accordance with the present invention, is shown in FIG.  3 A. Reflector  86  includes a pair of longitudinally extending reflector panels  88  and  89  that are mounted within the microwave chamber  20  as understood by those of ordinary skill in the art on brackets (not shown) similar to brackets  74  (FIGS.  1  and  2 ). Each reflector panel  88  and  89  has a concave inner surface  90  and  91 , respectively, which is generally shaped as a portion of an ellipse having two spaced-foci. The concave inner surfaces  90  and  91  of reflector panels  88  and  89  have an opposing and facing relationship and are positioned with a spaced relationship relative to the plasma lamp  34  and relative to the ultraviolet-transmissive conduit  62  housing the substrate  60 . A processing space  96  is at least partially defined between reflector panels  88  and  89  and defines a portion of interior space  15  operable for irradiating substrate  60  with ultraviolet radiation. The reflector panels  88  and  89  are preferably formed of a radiation-transmissive material, such as a borosilicate glass and, more specifically, Pyrex® glass. Gaps  92  and  94  are provided between the reflector panels  88  and  89  for permitting a flow of air to cool the plasma lamp  34 . Diverter baffle  93  is provided to preferentially direct the flow of relatively cool air through gap  92  toward plasma lamp  34 . 
     The reflector panels  88  and  89  are arranged such that the respective concave surfaces  90  and  91  generally share common foci to effectively give reflector  86  a full elliptical geometrical shape. Reflector  86  operates in the same manner as discussed above with regard to reflector  64  (FIG. 3) for delivering a relatively uniform irradiance of ultraviolet radiation  80  about the circumference of, or in a surrounding relationship relative to, the substrate  60 . However, the ultraviolet radiation is focused about the substrate  60  as compared with the flood of radiation provided by reflector  64  (FIG.  3 ). Infrared radiation  82  is preferentially transmitted through the reflector  86  and absorbed by the walls  38 ,  40  and  42  of the microwave cavity  20  for subsequent thermal dissipation. Alternatively, infrared radiation  82  may be absorbed by the reflector  86  and thermally dissipated. 
     The reflector panels  88  and  89  have a spaced relationship with respect to the plasma lamp  34  and a spaced relationship relative to the substrate  60 . The substrate  60  is located near one focus of the ellipse defined by reflector panels  90  and  91 , and the plasma lamp  34  is located near the other focus of the ellipse. As a result of the arrangement of plasma lamp  34  and substrate  60 , a plurality of substantially focused longitudinal lines of ultraviolet radiation  82  from the plasma lamp  34  is delivered directly and indirectly by reflection from the reflector in a uniform fashion about the circumference of the substrate  60 . The lines of ultraviolet radiation  82  are also uniformly delivered along the entire longitudinal dimension of the portion of the substrate  60  positioned within the processing space  96 . 
     A known characteristic of an elliptical reflector is that a ray of radiation emitted from a source positioned at one focus will pass through the other focus after a single reflection. Thus, a light source that approximates a line source, such as plasma lamp  34 , that is positioned longitudinally at or near one focus of an elliptical reflector will deliver substantially focused lines of radiation about the circumference of a substrate, such as substrate  60 , positioned at or near the second focus. The radiation will be uniformly distributed along the length and about the circumference of the substrate  60  in a surrounding fashion. 
     Reflector  86  is also positioned relative to the side walls  40  and domed wall  42  of the microwave chamber  20  to permit access through the access opening  47  to the plasma lamp  34  in the processing space  96  and other objects within the interior space  15  and the processing space  96  of the microwave chamber  20 . To that end, reflector panel  88  may be removably detached from the brackets (not shown) supporting panel  88  within the microwave chamber  20 . After cover  46  is removed, reflector panel  88  is repositioned so that it does not obstruct the path from the access opening  47  in the microwave chamber  20  to the plasma lamp  34 . 
     Using like reference numerals for like elements discussed with reference to FIGS. 1,  2  and  3 , a pair of reflectors, indicated generally by reference numerals  100  and  101 , in accordance with the present invention, is shown in cross-section in FIG.  3 B. Reflector  100  includes reflector panels  102  and  104  extending longitudinally within the microwave chamber  20  between the end walls  38 . Similarly, reflector  101  includes reflector panels  106  and  108  which extend longitudinally within the microwave chamber  20  between the end walls  38 . The portion of the interior space  15  substantially surrounded by the reflector panels  102 - 108  at least partially defines the processing space  61  in which the substrate  60  is exposed to ultraviolet radiation. The reflector panels  102 - 108  are mounted to opposed end walls  38  of the microwave chamber  20  by a pair of longitudinally-spaced brackets  110 , of which only one bracket  110  is shown in FIG.  3 B. Brackets  110  are formed of an electrically-insulating material, such as a ceramic or a thermally-stable polymer or, more specifically, a fluoropolymer such as those commercially available from E. I. du Pont de Nemours and Company (Wilmington) under the trade name of Teflon®. The brackets  110  are adapted to receive and hold the reflector panels  102 - 108  in any conventional manner, such as by an adhesive, fasteners, hangers, tabs and slots, or an array of curved grooves inscribed in the respective confronting faces of the brackets  110 . 
     The reflector panels  102 - 108  are preferably formed of a radiation-transmissive material, such as a borosilicate glass or, more specifically, a Pyrex® glass such as commercially available from Corning Inc. (Corning, N.Y.). Microwave energy provided to microwave chamber  20  by microwave generators  12  and  14  is readily transmitted through the reflector panels  102 - 108  for initiating a plasma from the gas mixture in plasma lamp  34  and for sustaining the plasma for the duration of the heating or curing operation. Alternatively, reflector panels  102 - 108  may be formed of any material having suitable reflective and thermal properties. In particular, panels  102 - 108  may be constructed of a metal and integrally formed as a portion of the microwave chamber or incorporated into or as part of the chamber walls  38 ,  40  and  42 , in which case the panels  102 - 108  need not transmit radiation of any wavelength. 
     Reflectors  100  and  101  are adapted to at least partially transmit, reflect or absorb photons of specific wavelengths. In particular and as illustrated in FIG. 3B, reflector panels  102 - 108  may be capable of preferentially reflecting photons of ultraviolet radiation  80  from the spectrum of emitted radiation  81  emanating from plasma lamp  34  and preferentially transmitting or absorbing photons of infrared radiation  82  therefrom. The preferential transmission, reflection and absorption can be provided by methods familiar to persons of ordinary skill in the art, such as by applying a dichroic coating to reflector panels  102 - 108  which is configured to selectively transmit infrared radiation  82  from emitted radiation  81  and selectively reflect ultraviolet radiation  81  from emitted radiation  81 . This selective transmission directs rays of infrared radiation  82  in optical paths toward the chamber walls  38 ,  40 ,  42  and, as a result, the flux of infrared radiation directed toward the substrate  60  is significantly reduced and the amount of infrared radiation irradiating substrate  60  is significantly attenuated. 
     Reflector panels  102 ,  104  of reflector  100  have a spaced relationship relative to the plasma lamp  34  and extend longitudinally substantially parallel to lamp  34 . Each of the reflector panels  102 ,  104  has an aspheric concave inner surface  112 ,  114 , respectively, which collectively form, and are arranged in, a common parabolic plane curve or conic section when viewed from a perspective parallel to the longitudinal axis of reflector  100 . Each infinitesimal planar cross-section of the reflector panels  102 ,  104  inherently includes a focal point mathematically representative of the parabolic shape. Because the reflector panels  102 ,  104  extend longitudinally substantially parallel to the plasma lamp  34 , the focal points of the parabolic conic sections collectively form a focal line with which the longitudinal centerline of the plasma lamp  34  is substantially collinear. Axial rays of emitted radiation  81  from the plasma lamp  34 , considered as a line source substantially aligned along the focal line, impinge on the inner surfaces  112 ,  114  of reflector panels  102 ,  104  and ultraviolet radiation  80  is reflected as substantially-parallel rays having optical paths directed toward the reflector  101 . 
     Reflector panels  106 ,  108  of reflector  101  have a spaced relationship relative to the ultraviolet-transmissive conduit  62  enclosing substrate  60  and extend longitudinally substantially parallel to conduit  62  and the substrate  60  contained therein. Each of the reflector panels  106 ,  108  has an aspheric concave inner surface  116 ,  118 , respectively, which collectively form, and are arranged as, a common parabolic plane curve or conic section when viewed from a perspective parallel to the longitudinal axis of reflector  101 . Each infinitesimal planar cross-section of the reflector panels  106 ,  108  inherently includes a focal point mathematically representative of the parabola. Because the reflector panels  106 ,  108  extend longitudinally substantially parallel to the conduit  62 , the focal points of the parabolic conic sections collectively form a focal line with which the longitudinal centerline of the substrate  60  is substantially collinear. A longitudinal axis of the conduit  62  is at least substantially parallel to the focal line and may be collinear therewith. Inner surfaces  116 ,  118  have a substantially confronting relationship with the inner surfaces  112 ,  114  of reflector  100 . Incident axial, parallel rays of ultraviolet radiation  80 , arriving at reflector  101  after reflection from reflector panels  102 ,  104  of reflector  100 , are re-reflected by the inner surfaces  116 ,  118  as rays of ultraviolet radiation  80   a  that converge or are focused at and about the focal line of the reflector  101 . 
     The substrate  60 , positioned longitudinally at or near the focal line, is irradiated by the ultraviolet radiation  80   a  reflected by reflector panels  106 ,  108 . In particular, due to the parabolic shape of the reflector panels  102 - 108  and their relative arrangement, the non-facing portion or backside of substrate  60 , remote from the plasma lamp  34  and shadowed by the facing portion or frontside of substrate  60 , is irradiated by the ultraviolet radiation  80   a  reflected by reflector panels  106 ,  108 . Preferably, the irradiation of the backside of substrate  60  by ultraviolet radiation  80   a  is substantially uniform about the circumference and along the length of substrate  60 , but the present invention is not so limited. For example, it is understood that the positioning of the plasma lamp  34  and the substrate  60  do not have to precisely coincide with the respective one of the pair of focal lines of reflectors  100  and  101 , respectively, and either of the plasma lamp  34  or the substrate  60  can be positioned slightly off-axis without departing from the spirit and scope of the present invention. The frontside of the substrate  60  is irradiated primarily by direct radiation  81   a , comprising both infrared and ultraviolet wavelengths, emanating from or emitted by the plasma lamp  34 . 
     The separation distance between the reflectors  100  and  101 , and more specifically the separation distance between the inner faces  112 ,  114  of reflector panels  102 ,  104  and the inner faces  116 ,  118  of reflector panels  106 ,  108 , can be adjusted within the confines of the microwave chamber  20 , provided that the respective focal lines remain substantially parallel to the centerline of the plasma lamp  34  and the substrate  60 , respectively. The relative insensitivity to the separation distance is due primarily to the parallelism of the rays of ultraviolet radiation  80  reflected from reflector panels  102 ,  104 . Likewise, the transverse position of reflector  101  can be varied slightly as long as the substrate  60  remains substantially positioned at the focal line of the parabola defined by panels  106 ,  108 . Furthermore, it is understood by persons of ordinary skill that the inner faces  112 ,  114  and the inner faces  116 ,  118  may deviate somewhat from a mathematically-precise parabolic shape such that the shape of each need only be substantially parabolic. 
     Provided between respective pairs of reflector panels  102 - 108  are longitudinally-extending gaps  120 ,  122 ,  124  and  126  that permit paths for a flow of air to cool the plasma lamp  34  and the conduit  62 . It will be appreciated that each of the pairs of reflector panels  102  and  104  and reflector panels  106  and  108  could be formed as a single or integral piece, which would eliminated at least gaps  120  and  126 , respectively. Further, the quartet of reflector panels  102 - 108  could be formed as a single piece and all of gaps  120 - 126  eliminated. However, suitable cooling for the plasma lamp  34  and the conduit  62  would have to be provided in an alternative manner, such as a sufficient flow of air directed axially between the reflectors  100 ,  101  or by plural openings (not shown) perforating the reflector panels  102 - 108  in a sufficient number and with a sufficient spacing to permit a sufficient flow of air adequate to cool the plasma lamp  34  and the conduit  62 . 
     While the present invention has been illustrated by a description of various embodiments and while these embodiments have been described in considerable detail, it is not the intention of the applicants to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. For example, the present invention could be used to irradiate fluids flowing within an ultraviolet-transmissive flow tube through the interior of the microwave chamber. In its broader aspects, the present invention is not limited to ultraviolet irradiation but could irradiate substrates positioned within the microwave chamber with radiation having visible wavelengths or infrared wavelengths. The invention in its broader aspects is therefore not limited to the specific details, representative apparatus and method, and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of applicants&#39; general inventive concept.