Electrodeless discharge lamp

Abstract: An electrodeless discharge lamp includes in a lamp tube (121,131,141) a discharge gas which is a mixture gas of a relatively high pressure of xenon gas and a mixture of neodymium iodide (NdI₃) and cesium iodide (CsI), and the outer periphery of the lamp tube (121,131,141) is provided on at least part of the outer periphery with a film (123,133,143) of a light transmitting material which raises the interior temperature of the lamp tube (121,131,141).

Description: BACKGROUND OF THE INVENTION This invention relates generally to an electrodeless discharge lamp and, more particularly, to a discharge lamp having no electrode inside lamp tube and causing an excitation luminescence of discharging gases sealed within the lamp tube to be generated with a high frequency electromagnetic field externally applied to the gases. The electrodeless discharge lamp of the kind referred to has been subjected to researches and development for providing to the lamp such features as being small in size, still high in the output and long in the life, so as to be usefully employable as a high output point source of light. DESCRIPTION OF RELATED ART There have been known various electrodeless discharge lamps arranged for the luminescence with the discharging gases in the lamp tube excited by the high frequency electromagnetic field acted upon the gases, in which the high frequency electromagnetic field is generally caused to act by means of an induction coil wound around the tube. While an initial starting of such discharge lamp is made relatively easy by an addition of mercury to the discharging gases sealed in the tube, a re-starting is made rather difficult. Further, there has been a problem, in particular, that a temperature rise in the lamp tube upon its lighting causes vapour pressure of mercury to vary in a manner of exponential function so as to be difficult to take its matching with a high frequency power source for applying a high frequency current to the induction coil, and the discharge lamp is caused to flicker out when the matching cannot be taken. When the luminous substance like mercury is not added to the discharging gas, it becomes easier to take the matching with the high frequency power source, but the gas pressure has to be made higher for obtaining a sufficient quantity of light, and the initial starting is thereby made difficult. While an application of a relatively high voltage to the induction coil may result in a forcible starting of the lamp, this causes another problem to arise in that a high frequency power source capable of applying a high voltage is required therefor so that the high frequency power source as a lighting circuit will have to be enlarged in size to render the entire electrodeless discharge lamp fixture to be eventually larger. In order to eliminate the above problem, there have been suggested in, for example, U.S. Patents Nos. 4,894,590, 4,902,937 and 4,982,140 to H.L. Witting, U.S. Patent No. 5,057,750 to G.A. Farrall et al, and U.S. Patent No. 5,059,868 to S.A. El-Hamamsy et al various electrodeless discharge lamps having a starting means for executing a preliminary discharge in advance of and separately from a main discharge by means of a main induction coil. In these known electrodeless discharge lamps, in general, an induced electric field is to be produced within the lamp tube by the high frequency electromagnetic field so as to interlink with this electromagnetic field, and a discharge plasma is caused to run along this induced electric field. While in this case, a state in which a preliminary discharge is made to take place by a starting means is shifted to the state in which the discharge plasma runs along the induced electric field, there has been a problem that a relatively large energy is required for the shifting of the plasma arc discharge to the state of running along the induced electric field, and the discharge lamp starting has been practically uneasy to smoothly carry out. In Japanese Patent Laid-Open Publication No. 5-217561 based on U.S. Patent 5,479,072 as the priority basis (though laid-open later than the date of priority claimed for the present invention), further, it is suggested to employ in the electrodeless discharge lamp a halide of rare earth metal, in particular, neodymium, but this is effective to improve only the luminous colour but is insufficient for improving the startability and the restartability. EP-A-458544, which can be considered as disclosing the closest art published before the priority date of the present application, shows an electrodeless discharge lamp with an auxiliary electrode and a halide-rare gas filling exempt from Hg and including Xe. SUMMARY OF THE INVENTION Therefore, it is a primary object of the present invention to provide an electrodeless discharge lamp which has eliminated the foregoing problems and is capable of improving both the startability and restartability even when a discharging gas not including in particular any mercury is employed, without requiring any large size high frequency power source, to render the lamp to be compact. According to the present invention, this object can be realized by an electrodeless discharge lamp in which a high frequency current is supplied from a first high frequency power source to an induction coil disposed on the exterior of a lamp tube of a light-transmitting material and containing a discharge gas filled therein for an excitation luminescence of the gas with a high frequency electromagnetic field made to act upon the gas, wherein an auxiliary electrode formed with a metal foil is provided at a position on one end side of an axial line of the induction coil to be electromagnetically coupled to the interior space of the lamp tube for causing a preliminary discharge of the discharge gas in the lamp tube to take place prior to the excitation luminescence by means of the induction coil, with a power supplied from a second high frequency power source to the auxiliary electrode separately from said first high frequency power source for the high frequency current supply to the induction coil, characterized in that the discharge gas is a mixture of xenon, neodymium iodide and cesium iodide, and that the lamp tube is provided on at least part of its outer periphery with a film of light transmitting material effective to raise the interior temperature of the lamp tube. All other objects and advantages of the present invention shall be made clear in following description of the invention detailed with reference to preferred embodiments of the invention shown in accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 shows in a schematic diagram a basic arrangement of an electrodeless discharge lamp according to the preamble of claim 1, in which the discharge gas includes a halide of rare earth metal and, in addition to the induction coil and first high frequency power source for the coil, an auxiliary electrode and second high frequency power source for the electrode are provided; FIGS. 2A to 2D are explanatory views for the operation of the auxiliary electrode provided in the electrodeless discharge lamp of FIG. 1; FIGS. 3 through 8 are schematic diagrams showing respective modified examples of an electrodeless discharge lamp according to the preamble of claim 1; FIG. 9 is an explanatory view for the operation of the electrodeless discharge lamp in the example of FIG. 8; FIG. 10 is a schematic diagram of an arrangement of the electrodeless discharge lamp in a first embodiment of the present invention; FIGS. 11A and 11B are diagrams to graphically show output light spectra in relation to the electrodeless discharge lamp of FIG. 10; FIG. 12 shows in an schematic diagram an arrangement of the electrodeless discharge lamp in a second embodiment of the present invention; FIGS. 13A and 13B are diagrams for graphically showing output light spectra in relation to the electrodeless discharge lamp om FIG 12; FIG. 14 is a schematic diagram showing the electrodeless discharge lamp in a third embodiment of the present invention; FIG. 15 is a schematic, fragmentary sectioned view of the lamp in the embodiment of FIG. 14; While the present invention will now be described in detail with reference to the respective embodiments shown in the drawings, it will be appreciated that the intention is not to limit the present invention only to these embodiments shown but rather to include all alterations, modifications and equivalent arrangements possible within the scope of appended claims. DETAILLED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, there is shown a basic arrangement of an electrodeless discharge lamp according to the preamble of claim 1, in which the electrodeless discharge lamp comprises a lamp tube 11 formed into a spherical shape preferably with such light-transmitting material as a silica glass or the like, and a discharge gas including a halide of rare earth metal, preferably a mixture gas of 13.332 kPa (100 Torr) of xenon gas as a rare gas and 20mg of neodymium iodide as a halide of neodymium is sealed within the tube 11. Peripherally around the lamp tube 11, there is wound an induction coil 12, and a single type auxiliary electrode 13 is provided to be adjacent to outer surface of the lamp tube 11. While the induction coil 12 is shown in FIG. 1 as wound in three turns, the number of coil turn is not required to be particularly limited but may only be required to be more than one turn. The auxiliary electrode 13 is formed with a metal foil into a square shape of each 10mm side, for example, and is disposed in the present instance on one end side of axial line of the induction coil 12. First high frequency power source 14 is provided for supplying a high frequency current to the induction coil 12, so that a high frequency electromagnetic field will be thereby applied from the coil 12 to act upon the discharge gas within the lamp tube 11 for causing an excitation luminescence of the discharge gas to take place inside the lamp tube 11, upon which an induction electric field is generated within the lamp tube 11 by the action of the high frequency electromagnetic field, and a discharge plasma occurring in the tube 11 due to this induction electric field is formed into a toroidal shape. To the auxiliary electrode 13, on the other hand, there is applied a high frequency voltage from a second high frequency power source, and there occurs a string-shape preliminary discharge due to a high frequency electric field generated around the auxiliary electrode 13. In this case, the preliminary discharge is to be generated as the result of ionization of electrons accelerated by the high frequency electric field occurring around the auxiliary electrode 13 and caused to collide with atoms of the discharge gas. Since the auxiliary electrode 13 is of the single type, the thus generated preliminary discharge is subjected to a restriction only at one end by the auxiliary electrode 13, and the other end of the discharge is kept to be a free end so as to be relative freely shiftable. The first and second high frequency power sources 14 and 15 comprise respectively a high frequency generating section for a high frequency output, an amplifier section for a power amplification of the high frequency output, a matching section for taking an impedance matching with the induction coil 12 or with the auxiliary electrode 13. In practice, the second high frequency power source 15 is to apply the high frequency voltage across the auxiliary electrode 13 and an earth. Now, in the electrodeless discharge lamp shown in FIG. 1, the high frequency voltage is applied from the second high frequency power source 15 across the auxiliary electrode 13 and the earth, and a preliminary discharge D p is thereby caused to occur inside the tube 11 nearby the auxiliary electrode 13, which discharge D P gradually grows to extent upward from the position of the auxiliary electrode 13 and reached the other end side of the tube 11, as shown in FIGS. 2A and 2B. Here, the high frequency current is fed to the induction coil 12 from the first high frequency power source 14, the extended free end of the preliminary discharge D P is induced to further extend along the induction electric field occurring due to the high frequency electromagnetic field generated around the induction coil 12, so as to form an annular discharge path as shown in FIG. 2C. As the annular discharge path is completed, the discharge is to shift to such toroidal arc discharge D A as shown in FIG. 2D, whereby the discharge plasma is caused to occur, a strong luminescence takes place as the result of the excitation of the discharge gas, and a lighting state is reached. After this shift to the lighting state, the application of the high frequency voltage to the auxiliary electrode 13 becomes unnecessary. While in the above the high frequency current has been referred to as being supplied to the induction coil 12 after the occurrence of the preliminary discharge D P , it is also possible to start sypplying the high frequency current to the induction coil 12 simultaneously with the application of the high frequency voltage to the auxiliary electrode 13 and to have the supplied high frequency current to the induction coil 12 increased after the occurrence of the preliminary discharge D P . For the discharge gas, it is also possible to use a mixture gas containing other halide of rare earth metal. While the auxiliary electrode 13 has been disclosed as being formed by the metal foil of square shape of each 10mm side, futher, the same is not required to be specifically limited in size and shape, as well as in the position of provision. It should be appreciated that, according to the foregoing electodeless discharge lamp, the annular or continuous string-shaped preliminary discharge can be generated with the application of the high frequency voltage to the single type auxiliary electrode 13, and its shift to the electrodeless discharge D A is rendered easier. In addition, the use of the mixture of xenon and neodymium iodide as the discharge gas in conjunction with the significant action of the preliminary discharge at the starting enables the lighting in an extremely short time to readily take place. Further, with the use of this discharge gas, mainly neodymium is attaining the excitation luminescence during the lighting while the vapour pressure of this neodymium is kept relatively low in the lighting state, and it is made possible to instantaneously light the lamp even upon the restarting immediately after the lighting-off. In another working aspect of an electrodeless discharge lamp according to the preamble of claim 1, such halide of cesium as cesium iodide is admixed further with the mixture of xenon and neodymium iodide, so that the relatively low vapour pressure of neodymium during the lighting can be properly raised, so as to be able to improve the luminescence efficiency. In the present working aspect, other constituents are the same as those in the embodiment of FIG. 1 except for the difference in the discharge gas. In a modified example of a discharge lamp according to the preamble of claim 1 as shown in FIG.3, there is utilized an advantage that required circuit designing work for the first and second high frequency power sources 24 and 25 can be made easier by the independent provision of the second high frequency powersource 25 for the auxiliary electrode 23 as separated from the first high frequency power source 24 for the induction coil 22 wound on the lamp tube 21. In the present instance, there is disposed at an output section of the second high frequency power source 25 a parallel resonance circuit of an inductor L and capacitor C connected in parallel to each other, while a series resonance circuit may alternatively employed. In this embodiment, all other constituents are the same as those in the embodiment of FIG. 1, except for the arrangement at the output section of the second high frequency power source 25. In another modification shown in FIG. 4 of an electrodeless discharge lamp according to the preamble of claim 1, the auxiliary electrode 53 is formed on the outer wall surface of the lamp tube 51 as a metal film by means of a deposition process. For this metal deposition, it is advantageous to employ, for example, platinum so that the auxiliary electrode 53 is improved in the degree of adhesion with respect to the lamp tube 51, better than in the case of the arrangement of FIG. 1. That is, according to the arrangement of FIG. 1, the metal foil is employed as the auxiliary electrode so that there will arise certain complicated factors when a sufficient contact of the metal foil with the spherical outer wall surface of the lamp tube, whereby the eventual contact is caused to be limited to be of the one at multiple points on the wall surface of the lamp tube, and it may happen that the action of the high frequency electric field occurring around the auxiliary electrode with respect to the discharge gas is insufficient. In the present modification on the other hand, the degree of adhesion of the auxiliary electrode 53 with respect to the lamp tube 51 can be sufficiently elevated, and the action of the high frequency electric field occurring around the auxiliary electrode 53 upon the discharge gas can be made sufficient. In accompaniment to this, it is made possible to have the preliminary discharge D p generated by a relatively low energy, and the discharge lamp can be improved in the startability. Further, the lamp tube 51 is improved in the heat retaining properties so that, in the event where the luminous substance is mixed in the discharge gas, the vapour pressure of the luminous substance is thereby elevated to increase the amount of luminescence, and the discharge lamp can be improved in the input/output efficiency. Including the induction coil and first and second high frequency power sources, all other constituents in this modification are the same as those in the foregoing arrangement of FIG. 1. According to another modification shown in FIG. 5 of an electrodeless discharge lamp according to the preamble of claim 1, the lamp tube 71 is of a cylindrical member, the induction coil 72 is wound on cylindrical periphery of the member, and the auxiliary electrode 73 is provided on one of substantially flat axial en faces of the cilindrical member, while the other end face functions as a main luminscent light radiating surface 76 which is substantially flat. In such case as the arrangement of FIG. 1 where the lamp tube is spherical, there remains a possibility that the induced electric field due to the high frequency electromagnetic field occuring around the induction coil cannot act sufficiently upon the free end of the preliminary discharge D p extended so as to be out of the zone surrounded by the coil as shown in FIG. 2B. In the present instance, on the other hand, the cylindrical lamp tube 71 renders the distance from the auxiliary electrode 73 to the extended free end of the preliminary discharge D p to be shorter to render the action of the electric field sufficient, the discharge shift from the preliminary discharge D p to the arc discharge D A is made thereby to be easier, and the discharge lamp can be improved in the startability. In the instant modification, all other constituents including the first and second high frequency power sources 74 and 75 are the same as those in the arrangement of FIG. 1. In another modification shown in FIG. 6 of an electrodeless discharge lamp according to the preamble of claim 1, the lamp tube 81 is formed to be substantially hemispherical, so as to have a substantially cylindrical central part on which the induction coil 82 is wound, a spherical axial end surface on which the auxiliary electrode 83 is provided, and the other axial end surface substantially flat and acting as the main luminescent light radiating surface 86. In this modification all other constituents including the first and second high frequency power sources 84 and 85 are the same as those in the arrangement of FIG. 1 or 5. In another modification shown in FIG. 7 of an electrodeless discharge lamp according to the preamble of claim 1, the lamp tube 91 is of a half-compressed ball shape having a swelling periphery on which the induction coil 92 is wound, andd two concave axial end surfaces on one of which the auxiliary electrode 93 is provided and the other of which is to act as the main luminescent surface 96. In this modification, all other constituents are the same as those in the arrangement of FIG. 1. In a further modification shown in FIG. 8 of an electrodeless discharge lamp according to the preamble of claim 1, the arrangement is similar to that in FIG. 5, but the lamp tube 101 in cylindrical shape having on one axial end surface the auxiliary electrode 103 is so disposed within the induction coil 102 that the other axial end surface acting as the main luminescent light radiating surface 106 is substantially in match with the central plane intersecting at right angles the axial line of the coil 102. Since in this case the intensity of the induction electric field due to the thigh frequency electromagnetic field generated around the induction coil 102 is made to be the largest in the central area of the axial line of the induction coil 102 and to be smaller at both sides of the axial line, as shown in FIG. 9, the disposition of the main luminescent light radiating surface 106 of the lamp tube 101 substantially in match with the central plane 107 intersecting at right angles the axial line of the induction coil 102 is effective to have the strongest induction electric field acted upon the free end of the preliminary discharge D p . Consequently, the shift of the discharge from the preliminary discharge D p to the toroidal arc discharge D A can be easily attained, and the startability of the discharge lamp can be further improved. In the present modification all other constituents including the auxiliary electrode 103 and first and second high frequency power source 104 and 105 are the same as those on the arrangement of FIG. 1. Referring now to a first embodiment of the electrodeless discharge lamp according to the present invention as shown in FIG. 10, there are provided heat insulating films 123 and 123a on the outer periphery of the lamp tube 121 at its portions other than the zone around which the induction coil 122 is wound, if required, all over such other portions. In the present instance, the high frequency power is supplied from the high frequency power source 124 to the induction coil 122, and the excitation luminescence is caused to take place with the discharge gas affected by the high frequency electromagnetic field generated around the induction coil 122, whereas heat radiation of the lamp tube 121 is restrained by the presence of the heat insulating films 123 and 123a, consequent upon which the coldest portion of the lamp tube 121 will have a higher temperature as compared with a case where having no heat insulating film is provided, whereby a vaporization amount of the luminous substance is increased to raise the vapour pressure, and the operating property of the lamp upon the re-lighting can be thereby improved. It has been found that, when, for example, the lamp tube 121 is made to have an outer diameter of 27 mm and to contain 13.332 kPa (100 Torr) of xenon gas with 15 mg of NdI 3 and 5 mg of CsI added, attainable efficiency and colour temperature with an input of 200 W have been 40 lm/W and 10,500 K, respectively, in an event where no heat insulating film is provided, but have been 38 lm/W and 5,500 K in the other event where the heat insulating films of platinum are provided, and thus that the colour temperature can be remarkably lowered without substantial loss in the efficiency by the provision of the heat insulating films. In FIG. 11A, an optical output spectrum with respect to wavelength in the case of the lamp tube 121 having the heat insulating films 123 and 123a is shown, whereas in FIG. 11B the optical output spectrum with respect to the wavelength in the case where the lamp tube 121 has no heat insulating film is shown. It will be appreciated when these figures are compared with each other, the provision of the platinum made heat insulating films 123 and 123a is effective to reduce the output quantity of light on short wavelength side to lower the colour temperature. In a second embodiment of the electrodeless discharge lamp according to the present invention as shown in FIG. 12, the lamp tube 131 is provided at the other portions than that where the induction coil 132 is wound on the outer periphery of the tube with electrically conducting films 133 and 133a, which are formed with a transparent, electrically conducting film such as indium tin oxide (ITO) generally In 2 O 3 -S n O 2 , or electrically conducting ceramic film. In the present instance, the high frequency power is supplied from the high frequency power source 134 to the induction film 132, the luminous substances are affected by the high frequency electromagnetic field generated around the induction coil 132 to cause the excitation luminescence to take place, and also to generate an induced current at the conducting films 133 and 133a, which films are heated due to a current loss occurring therein, whereby the lamp tube 131 is heated to raise the temperature at the coldest portion of the tube, and the luminous efficiency can be improved with the vaporization amount of the luminous substances increased. When, for example, the lamp tube 131 was made to be 18 mm in the outer diameter and 13.332 kPa (100 Torr) of xenon with 15 mg of NdI 3 and 5 mg of CsI added was charged therein, the efficiency attained with an input of 150 W was 35 lm/W in an event where no electrically conducting films 133 and 133a were provided, whereas the efficiency attained with the same input in the other event where the platinum made conducting films 133 and 133a were provided has been improved to be 45 lm/W. In FIG. 13A, there is shown output spectrum with to wavelength in the case were the conducting films 133 and 133a are provided while FIG. 13B shows the output spectrum with respect to the wavelength in the case where no conducting film is provided. As will be clear when both drawings are compared with each other, it has been found that the provision of the platinum made electrically conducting films enables it possible to lower the quantity of output light on the lower wavelength side. In a third embodiment of the electrodeless discharge lamp according to the present invention as shown in FIGS. 14 and 15, the lamp tube 141 is covered with a light transmitting and heat conducting film 143 showing a high thermal conductivity, preferably substantially all over the outer peripheral surface of the tube, as specifically shown in FIG. 15. In the present instance, the induction coil 142 is supplied with the high frequency power from the high frequency power source 144, the luminous substances affected by the high frequency electromagnetic field generated around the coil 142 cause the excitation luminescence to take place within the tube, while generated heat adjacent to the induction coil 142 and reaching the highest temperature at the inner surface of the lamp tube 141 is transmitted through the heat conducting film 143 to other lower temperature portions of the tube, whereby the temperature on the outer periphery of the lamp tube 141 is relatively raised to increase the vaporization amount of the luminous substances, so as to raise the vapour pressure, and the lamp is improved in the efficiency of light output. When, for example, the lamp tube 141 was made to be 23 mm in the outer diameter, and 13.332 kPa (100 Torr) of xenon gas with 20 mg of NdI 3 -CsI added as the luminous substances was filled in the tube. In the case where no such heat conducting film as in the above was provided, the efficiency with the input of 250 W was 63 lm/W, whereas in the case where a diamond film of 2 µm thick was formed as the heat conducting film 143 on the tube, the efficiency with the same input of 250 W was improved to be 76 lm/W. In this case, the heat conductivity of diamond is 2,000 W/m·K, which is more than 10 times as high as that of the silica glass as the material for the lamp tube 141, and the diamond film is subtantially transparent involving almost none of attenuation of the flux of light, so as to be excellent as the material for forming the heat conducting film 143. For such material of the heat conducting film 143, it is also possible to employ such one showing charecteristics approximating those of diamond as beryllium oxide, aluminum nitride or silicon carbide. In providing the heat conducting film 143 which covering the tube, it may be possible to employ one of such various methods as ionization matallizing method, hot filament CVD method and plasma CVD method. Here the lamp tube 141 covered with the diamond film as the heat conducting film 143 was subjected to measurement of wall temperature, which has resulted in that the temperature at a portion close to the induction coil 142 and where plasma is generated has been lowered by about 150°C as compared with a case having no heat conducting film, while the temperature at the coldest portion has risen by abouw 120°C in contrast to the case where the heat conducting film has been absent. With the rise in the temperature at the colder portions, the luminous efficiency is improved, while any thermal load applied to the lamp tube is reduced by the fall of the temperature at the hotter portions. Further, when the heat conducting film 143 was made by beryllium oxide, the luminous efficiency was 70 lm/W with the input of 250 W, the temperature at the portion close to the induction coil 142 where plasma would be generated was lowered by about 90°C while the temperature at the coldest portion was raised by about 80°C. It has been found, accordingly, that a function close to that of the diamond film can be obtained. In the foregoing embodiments of the electrodeless discharge lamp as shown in FIGS. 10, 12 and 14, while not specifically described, there is provided a preliminary discharge means including the auxiliary electrode to which the second high frequency power source supplies the electric power, and the preliminary discharge for rendering the start to be easy is executed in similar manner to the earlier described embodiments. It will be also appreciated that all other constituents of the embodiments shown in FIGS. 10, 12 and 14 than those referred to are the same as those in the earlier described embodiments, and the same functions are attainable. In the electrodeless discharge lamp according to the present invention in general, further, the concurrent use of the halide of rare earth metal as filled in the lamp tube and the preliminary discharge means including the auxiliary electrode secured to the lamp tube has brought about such remarkable distiction as shown in a following table from conventional electrode less discharge lamps not provided with the preliminary discharge means though employing the halide of rare earth metal: Fill Starting time Restarting time Present invention NdI 3 -CsI, Xe 2 ms 2 ms NaI-TlI-InI, Xe 2 ms 35 s No prelim. dis. means employed NdI 3 -CsI, Xe Not started Not started NaI-TlI-InI, Xe Not started Not started For the starting and restarting time in the above table, the voltage across the induction coil has been measured. Here, the term "starting " means to start the discharge lamp after more than ten hours from previous lighting-off of the lamp, while the term "restarting" is to light the discharge lamp immediately after the lighting-off of stably lighted discharge lamp. Further, "Not Started" indicates that the discharge lamp has not started even upon application of the voltage of 3.0 kV across the induction coil. Further, the present invention allows a variety of design modifications. While, for example, the auxiliary electrode of the preliminary discharge means has been referred to as being single in the foregoing embodiments, it is possible to provide a pair of the preliminary electrodes opposing each other on the outer periphery of the lamp tube along the zone around which the induction coil is wound. It is also possible to employ three or more of the auxiliary electrodes as disposed on the lamp tube. Instead of providing the second high frequency power source for use with the auxiliary electrode, the power feeding can be performed with the first high frequency power source only but adapted to be used in common to the induction coil and the auxiliary electrode.

Claims: An electrodeless discharge lamp in which a high frequency current is supplied from a first high frequency power source (124, 134, 144) to an induction coil (122, 132, 142) disposed on the exterior of a lamp tube (121, 131, 141) of a light-transmitting material and containing a discharge gas filled therein for an excitation luminescence of the gas with a high frequency electromagnetic field made to act upon the gas, wherein an auxiliary electrode (13) formed with a metal foil is provided at a position on one end side of an axial line of the induction coil (122, 132, 142) to be electromagnetically coupled to the interior space of the lamp tube (121, 131, 141) for causing a preliminary discharge (Dp) of the discharge gas in the lamp tube (121, 131, 141) to take place prior to the excitation luminescence by means of the induction coil (122, 132, 142), with a power supplied from a second high frequency power source (15) to the auxiliary electrode (13) separately from said first high frequency power source (124, 134, 144) for the high frequency current supply to the induction coil (122, 132, 142), characterized in that the discharge gas is a mixture of xenon, neodynium iodide (NdI 3 ) and cesium iodide (CsI), and in that the lamp tube (121, 131, 141) is provided on at least part of its outer periphery with a film of a light transmitting material effective to raise the interior temperature of the lamp tube. A discharge lamp according to claim 1, characterized in that said discharge gas comprises 13.332 kPa (100 Torr) of xenon and 20 mg of mixture of NdI 3 and CsI. A discharge lamp according to claim 2, characterized in that said mixture of NdI 3 and CsI comprises 15 mg of NdI 3 and 5 mg of CsI. A discharge lamp according to claim 1, characterized in that said film is a heat insulating film (123, 123a) provided on other portions of the outer periphery of the lamp tube (121) than a portion around which said induction coil (122) is wound. A discharge lamp according to claim 1, characterized in that said film is an electrically conducting film (133, 133a) provided on other portions of the outer periphery of the lamp tube (131) than a portion around which said induction coil (132) is wound. A discharge lamp according to claim 5, characterized in that said electrically conducting film (133, 133a) is formed of one metal selected from the group consisting of gold, silver and platinum. A discharge lamp according to claim 1, characterized in that said film is a heat conducting film (143) of a good thermal conductivity and provided all over the outer periphery of the lamp tube (141).

Inventors: AOKI SHINICHI, HIGASHISAKA SHINGO, KOTANI MIKI, KURAMITU OSAMU, OKADA ATSUNORI, SAIMI MOTOHIRO, SUMITOMO TAKU, UKEGAWA SHIN, WADA SHIGEAKI, AOKI, SHINICHI, HIGASHISAKA, SHINGO, KOTANI, MIKI, KURAMITU, OSAMU, OKADA, ATSUNORI, SAIMI, MOTOHIRO, SUMITOMO, TAKU, UKEGAWA, SHIN, WADA, SHIGEAKI, Aoki, Shinichi, c/o Matsushita Electric, Higashisaka, Shingo, c/o Matsushita Electric, Kotani, Miki, c/o Matsushita Electric, Kuramitu, Osamu, c/o Matsushita Electric, Okada, Atsunori, c/o Matsushita Electric, Saimi, Motohiro, c/o Matsushita Electric, Sumitomo, Taku, c/o Matsushita Electric, Ukegawa, Shin, c/o Matsushita Electric, Wada, Shigeaki, c/o Matsushita Electric
Applicants: MATSUSHITA ELECTRIC WORKS LTD, MATSUSHITA ELECTRIC WORKS, LTD.
Application year: 1993
Main classification: H01J65
IPC codes: H01J65, H01J61