A HIGH PRESSURE SERIES ARC DISCHARGE LAMP CONSTRUCTION WITH SIMPLIFIED STARTING AID

Abstract: A high pressure gas discharge lamp includes first and second discharge devices electrically connected in series within an outer envelope. The discharge devices each include a discharge vessel enclosing a discharge space with an ionizable fill and first and second discharge electrode assemblies. The first discharge electrode assemblies of the discharge devices are connected so as to receive a starting pulse and lamp operating voltage. Each discharge vessel includes a first wall portion spaced from the first discharge electrode assembly and defining an ionizable gap therebetween. A conductive element bridges the discharge devices at the first wall portions and capacitively couples the first discharge electrode assemblies to induce ionization in one of the discharge devices in the ionizable gap between the first wall portion and first discharge electrode assembly.

Description: A high pressure series arc discharge lamp construction with simplified starting aid. The invention relates to a high pressure discharge lamp with first and second discharge devices connected electrically in series within an outer bulb, each discharge device including a discharge vessel enclosing a discharge space and an ionizable filling, first and second discharge electrode assemblies within the discharge space each including an electrode portion on which a discharge terminates during normal lamp operation and a current conductor portion extending to the exterior of the discharge vessel, and means for electrically connecting the first electrode assembly of each discharge device to a source of electric potential outside of the lamp envelope, and a starting aid facilitating ignition of the discharge devices. Such a lamp is known from U.S. Patent 4,751 ,432 (Van Delm). High pressure discharge lamps may have series connected discharge devices included within a single lamp envelope to decrease the overall size of the lamp or to achieve a blended light. Lumen output and consumed power of a high pressure discharge device are proportional to the physical separation between the discharge electrodes and consequently the overall length of the discharge device. Lamps rated at high power with a single arc tube therefore have a large overall length, which is generally undesirable from the optical as well as cost and handling standpoints. The overall length of the lamp can be significantly reduced, for example, by arranging two discharge devices within an outer envelope each operated at half the total desired power. Two discharge devices emitting different spectrums have also been employed to achieve an improved blended spectrum different from either device alone. A high pressure arc discharge device is ignited by providing an ignition pulse across the discharge electrodes with a prescribed voltage and bandwidth. This is typically accomplished with an external ignitor in a ballast contained in a lighting fixture. The ignition pulse(s) are applied through the lamp cap, usually in the form of a threaded base. Reliable ignition of such discharge lamps is frequently a problem, as the multiple discharge devices affect the igniting characteristics of each other, generally requiring an ignition pulse of much higher energy than that which reliably ignites one discharge device of the same total wattage. However, safety constraints place an upper limit on the voltage of the ignition pulse applied through the lamp cap. Furthermore, commercial viability does not permit a lamp designer to market a lamp which requires its own special ballast and/or ignitor. Rather, HID lamps with multiple discharge devices rated at a certain total wattage are operated with existing ballasts designed to operate a lamp with a single discharge device of corresponding rated wattage. The above-mentioned patent discloses a starting aid which sequentially ignites the two discharge devices. The starting aid is a bi-metal switch which shorts one of the discharge devices to permit the ignition pulse to be applied initially across one device only. After the one device ignites and supports an arc discharge, the heat therefrom causes the bi-metal switch to open. This permits the ignition pulse to be applied across both the first and second discharge devices. Since the impedance across the already-burning discharge device is low, the second discharge device sees essentially the entire energy of the ignition pulse, providing reliable ignition. One disadvantage of this construction is the long delay in igniting the second discharge device due to the time it takes for the first discharge device to heat the bimetal to its opening temperature, on the order of about 1-2 min. Additionally, bi- metal switches are cumbersome to install, usually requiring hand-mounting and/or adjustment. Accordingly, it is an object of the invention to provide a simplified starting aid for two discharge devices connected in series. This object is accomplished in that a lamp of the type described in the opening paragraph is characterized in that: said starting is formed by a conductive element which is bridging a first wall portion of each of said sicharge vessels, the first wall portion being spaced from said first discharge electrode assembly and defining an ionizable gap therebetween. The invention has an advantage that an ignition pulse is capacitively coupled by the starting aid between the first electrode assemblies and this induces ionization in the ionization gap between the first electrode assembly and the first wall portion of at least one of the discharge device. According to a favorable embodiment of the invention, the discharge vessels are ceramic, each having a central zone extending between each of the electrode portions and an end zone communicating with the central zone. The end zone includes the first wall portion, surrounds the respective first discharge electrode assembly, and has a largest external diameter smaller than the smallest external diameter of the central zone. The use of such a narrow diameter end zone permits of a close spacing between the conductive element and the electrode assembly to facilitate ionization of the fill present between the first wall portion of the end zone and the electrode assembly. As opposed to quartz glass, the use of a ceramic material permits a discharge vessel with a narrow diameter end zone and a wider diameter central zone. Much tighter tolerance in the spacing between the electrode assembly and the adjacent wall are also possible with a ceramic rather than quartz glass discharge vessel. In one embodiment, the conductive element has a maximum spacing from the discharge electrodes of about 0.9 mm. A simple, low cost construction is obtained when the conductive element consists of a length of conductive metal, such as wire or sheet strip, having end portions engaging each first wall portion of the discharge devices. Favorably, the conductive element has end portions each bent around a respective first wall portion to mechanically secure the conductive element to each discharge device. In this way, no additional fastening elements are required. These and other aspects, features and advantages of the invention will become apparent with reference to the drawings and the following detailed description. Figure 1 is a side view of a high pressure discharge lamp having a pair of discharge devices electrically connected in series and having a conductive bridging element; Figure 2 is a cross section of the arc tube illustrating an ionization gap between the electrode assembly and the bridging element; and Figure 3 is a perspective view of the bridging element and one discharge device. Figure 1 shows a metal halide high pressure discharge lamp with first and second discharge devices 2, 3 connected electrically in series within an outer bulb, or lamp envelope, 1. The discharge devices are nominally identical. Figure 2 further illustrates the discharge device 3, which includes a discharge vessel 30 enclosing a discharge space 1 1 and containing an ionizable filling of mercury, a metal halide and a rare gas. Discharge vessel 30 has a circular cylindrical wall 31 with end walls 32, 33 which together define a central zone of the discharge vessel. Circular cylindrical walls 34, 35 define first and second end zones which communicate with the central zone and enclose respective first and second discharge electrode assemblies 4, 5. The first end zone 34 has a first wall portion 340. Each end zone has a largest external diameter "d e " smaller than the smallest external diameter "d c " of the central zone. The walls 31-35 are ceramic. As used herein, "ceramic" means a refractory material such as monocrystalline metal oxide (for example, sapphire), polycrystalline metal oxide (for example, polycrystalline densely sintered aluminium oxide; yttrium-aluminium garnet, or yttrium oxide), and polycrystalline non-oxidic material (for example, aluminium nitride). Such materials allow for high wall temperatures up to 1500-1600 K and are satisfactorily resistant to chemical attacks by halides and by Na. Each discharge electrode assembly 4,5 includes (i) an electrode portion with an electrode rod 4a, 5a and a winding 4b, 5b on which a discharge terminates during normal lamp operation and (ii) a current conductor portion extending to the exterior of the discharge vessel. Each current conductor portion includes a first, halide resistant portion 41, 51 made of, for example, molybdenum, and a second portion 40, 50 which is sealed in a gas-tight manner to the respective wall 34, 35 with a ceramic frit 10. The second portions 40, 50 are of a conductive material which has a coefficient of thermal expansion which is close to that of the ceramic wall, for example niobium. The discharge device is further described in U.S. Patent 5,424,609. A conductive frame supports the discharge devices within the outer lamp envelope. (Fig. 1) The frame includes first and second conductive support rods 12, 13 extending from the lamp stem 14, each connected to a respective lamp contact on the lamp base 15 in a known manner. Respective C-shaped connectors 16, 17 electrically connect each of the first electrode assemblies 4 to a respective first and second support rod 12, 13, and consequently to a source of electric potential provided at the contacts at the lamp base 15. The connectors 16, 17, the first and second support rod and the lamp 14 together form means for electrically connecting the first electrode assembly of each discharge device to a source of electric potential outside the lamp envelope. The second electrode assemblies 5 are connected in series via conductive cross member 18. An insulative support 19 is connected between the cross member 18 and the upper end of the support rod 12 to provide further mechanical support to the discharge vessels. A conductive bridging element 20 forming a starting aid in the form of a planar metallic strap bridges the two discharge devices surrounding the electrode assemblies 4. As shown in Figure 3, the strap has opposing end portions 20a, 20b each bent around a respective first end zone to mechanically secure the strap to the discharge devices in a simple manner. In the embodiment shown, the resulting looped end portions were welded together and the straps stayed fixed on the end portions. Alternatively, the strap could be made of a memory metal which retains its shape to hold the strap on the discharge devices despite the large difference in temperature between the "on" and "off" state of the lamp. As shown in Figures 2 and 3, the strap lies against the wall portion 340 of the wall 34 of each discharge vessel in the first end zone, which wall portion is an area located immediately adjacent the end walls 32. In this area, the electrode assembly, in particular the halide resistant portion 41, is spaced from the inner surface of this wall portion by a distance "S". Since the end zone communicates with the central zone, the ionizable fill is present in the space between the electrode assembly and the wall forming an ionizable gap therebetween. The wall has a thickness "t" , so the strap is spaced from the electrode assembly by a total distance D = S + t. The distance D is selected such that upon application of a predetermined ignition pulse across the first discharge electrode assemblies 4 (via base 15 and conductive supports 12, 13) the conductive element 20 capacitively couples the first discharge electrode assemblies 4 to each other and induces ionization in the ionizable gap of one of the discharge devices. The ionization provides protons to ensure initial electron emission from the electrode. This leads to further breakdown of the ionizable fill, proceeding to a gas discharge being maintained between the discharge electrodes of that discharge device. Once a gas discharge is supported, the impedance of that discharge device is drastically reduced so that the other discharge device sees substantially the entire energy of subsequent ignition pulses. Consequently, the discharge devices start sequentially and reliably. The predetermined ignition pulse for which a lamp is designed will vary depending on lamp wattage and type. The ignition pulse is limited in magnitude by the maximum voltage which the lamp cap and frame can take without arcing. The ignition pulse is also typically limited to that which is supplied by commercially available ignitors\ballasts with which the lamp will be used. In a practical embodiment, the discharge vessel walls were made from polycrystalline densely sintered aluminum oxide. The electrode rods, winding were made of tungsten and free from emitter. Each discharge device had a rated power of 100 W. The filling of the discharge vessel was 6.7 mg Hg and 7 mg of the metal halides sodium iodide/thalium iodide/dysprosium iodide in a weight ratio 91 :8: 1 , and argon as a starter gas at 200 Torr cold pressure. Each discharge vessel had an internal diameter of 7.2 mm and an internal length of 10 mm. The width S of the ionizable gap was 35 μm and the wall thickness t of wall 34 was 850 μm, providing a total distance D between the electrode assembly and the conductive bridging element of about 0.9 mm. A comparison test was conducted between six of the above described lamps and ten otherwise identical lamps without the conductive strap 20. The lamps were operated in a dark environment on an 100 W metal halide ballast (make advance transformer) with the ballast ignitor replaced with the standard ANSI pulse generator circuit (type Velonix 350 High Voltage Pulse Generator or an Equivalent), according to the ANSI standard: C78.387-1995 for metal halide lamps. The pulse width was 1 μs, and the pulse was increased in amplitude until the lamp started. The ANSI specified maximum is 4000V for the pulse amplitude. All lamps with the conductive strap 20 ignited in the voltage range of 2400 to 2800 V, with the average being 2600V. Lamps without the starting aid would only randomly ignite, in all cases above 3500V and typically not below the ANSI specified maximum of 4000 V.

Claims: CLAIMS: 1. A high pressure discharge lamp comprising an outer bulb (1), first and second discharge devices (2, 3) within said outer bulb connected electrically in series, each discharge device including a discharge vessel (30) enclosing a discharge space (11) and an ionizable filling, first and second discharge electrode assemblies (4, 5) within said discharge space each including an electrode portion (4a, 4b, 5a, 5b) on which a discharge arc terminates during normal lamp operation and a current conductor portion (40, 41 , 50, 51) extending to the exterior of said discharge vessel, means (16, 12, 17, 13, 15) for electrically connecting said first electrode assembly of each discharge device to a source of electric potential outside of said lamp envelope, and a starting aid within said outer bulb facilitating ignition of said discharge devices upon application of an ignition pulse to said discharge lamp, characterized in that: said starting aid is formed by a conductive element (20) which is bridging a first wall portion (340) of each of said discharge vessels (2, 3), the first wall portion being spaced from said first discharge electrode assembly (4) and defining an ionizable gap (D) therebetween. 2. A high pressure discharge lamp according to claim 1 , wherein each of said discharge devices (2, 3) comprises a ceramic discharge vessel (30) having a central zone (31) extending between said electrode portions and an end zone (34) communicating with said central zone, said end zone comprising the first wall portion (340) and surrounding said first discharge electrode assembly, said end zone having a largest external diameter smaller than the smallest external diameter of said central zone. 3. A high pressure discharge lamp according to claim 1 or 2, wherein said conductive element (20) has a maximum spacing from the said first discharge assembly (4) electrodes of about 0.9 mm. 4. A high pressure discharge lamp according to claim 1 , 2 or 3, characterized in that said conductive element (20) consists of a length of planar metal strap having end portions engaging said first wall portion (340) of each discharge vessel. 5. A high pressure discharge lamp according to claim 1 , 2, 3 or 4, wherein said length of planar metal strap is bent around each of said wall portions to mechanically secure said wire to said discharge devices. 6. A high pressure discharge lamp according to claim 1 , 2, 3, 4 or 5, wherein said ionizable filling comprises mercury, a metal halide and a rare gas.

Inventors: FELLOWS MARK W, JACKSON ANDREW D, SHUMWAY DANIEL, FELLOWS, MARK, W., JACKSON, ANDREW, D., SHUMWAY, DANIEL
Applicants: PHILIPS ELECTRONICS NV, PHILIPS NORDEN AB, PHILIPS ELECTRONICS N.V.
Application year: 1997
Main classification: H01J61
IPC codes: H01J61