Electrodeless light source with reduced heat losses

#The present invention relates to an electrodeless light source excited by power in the microwave region of the electromagnetic spectrum.

#In view of the necessity of conserving natural resources, much effort has recently been directed to research in electrodeless light sources. An electrodeless light source requires less electrical power than the conventional incandescent light source which in turn reduces the demand for fossil fuels for power generating facilities.

#An electrodeless light source is described in the U.S. patent to Haugsjaa et al., No. 3,943,403 which is assigned to the same assignee as the present invention. This light source includes a source of power at a high frequency, such as in the range of 10 MHz to 300 GHz, and an electrodeless lamp having an envelope made of a light-transmitting material and a volatile fill material within the envelope. The fill material emits light upon breakdown and excitation. A termination fixture is coupled between the source and the lamp and has an inner conductor and an outer conductor disposed around the inner conductor, one pair of ends of the conductors being coupled to the lamp while the other pair is coupled to the source. The fixture has the capability of matching the impedance of the lamp during the state of excitation to the output impedance of the source to optimize the coupling of microwave power to the lamp. Lamps according to this arrangement have been operated with light outputs substantially greater than that of the conventional incandescent lamp for the same input electrical power. While an electrodeless light source according to the teachings of this patent has operated satisfactorily, there exists a need to further improve the efficacy of such light sources.

#In the drawing:

#FIG. 1 is a partial sectional view of an exemplary embodiment of an improved electrodeless light source according to the invention;

#FIG. 2 is an alternative embodiment according to the invention;

#In an exemplary embodiment of the present invention, as is illustrated in FIG. 1, there is provided an electrodeless light source, represented generally by the reference numeral 10. The light source 10 includes a source of power 12 at a high frequency. As used herein, the term high frequency is intended to include frequencies within the range of 10 MHz to 300 GHz. An electrodeless lamp 14 has an envelope made of a light-transmitting material, such as quartz, and a volatile fill material emitting light upon breakdown and excitation. One typical fill is 20 torr argon, 0.39 mg of sodium iodide, 0.36 mg of scandium iodide, and 0.2 .mu.l of mercury in a lamp envelope volume of 0.41 cm.sup.3. A termination fixture 16 is coupled to the source 12. The fixture 16 has an inner conductor 18 and an outer conductor 20 which is disposed around the inner conductor 18. The lamp 14 is disposed in the region of the first ends 22 and 24 of the inner and outer conductors 18 and 20, respectively. The source 12 is coupled to the second ends 26 and 28 of the conductors 18 and 20, respectively. The fixture includes a device for matching the impedance of the lamp during breakdown and excitation of the fill material to the output impedance of the source. This device includes a capacitor 30 coupled across the conductors at the source coupled end. For additional details on this impedance matching device, reference may be made to U.S. Pat. No. 3,943,403 which is incorporated by reference. A transparent dome 34 having a metallic mesh 36 encloses the upper end of the outer conductor. The mesh 36 is grounded at 32 to the outer conductor 20 to retain microwave energy inside the fixture.

#According to the invention, means are provided for restricting the flow of heat from the lamp 14 to the region 39 between the conductors 18 and 20 by producing a thermally non-conductive and non-convective region at least around the periphery of the envelope of the lamp 14 to restrict the flow of heat. In FIG. 1, this feature is accomplished by evacuating the entire region 39 by the provision of a sealed, evacuated light-transmitting chamber 40 surrounding the outer wall of the outer conductor 20 and the dome 34. In addition, there is provided a support member 42 sealingly affixed to the second ends 26 and 28 of the inner and outer conductors 18 and 20, respectively, and being formed with an aperture 44 which communicates with the region 39 between the conductors. A pair of hold-down flanges 48 and 50 and an O-ring seal 52 seal the interface between the chamber 40 and the support member 42. Means, such as a vacuum pump 60, are coupled to the aperture 44 to evacuate the region 39.

#FIG. 2 illustrates an alternative embodiment for defining an evacuated region around the lamp envelope and a portion of the inner conductor which is adjacent to the lamp. An evacuated glass envelope 70 is provided, the envelope having an aperture through which the inner conductor is positioned and a glass to metal seal 72 sealing the interface of the envelope 70 and a threaded member 74 forming a part of the inner conductor 18. In the fabrication of the envelope 70, the upper portion of the inner conductor including element 74, the lamp 14 and the envelope 70 are formed as a unitary assembly. The region is evacuated during fabrication by removing the gas with a vacuum pump which communicates with the internal region at a tip-off 76 of the envelope 70. In assembling the fixture, the lower threaded portion of the element 74 is inserted into a receiving threaded portion of the lower portion 78 of the inner conductor.