STARTING AID THAT SURROUNDS AN ELECTRODE OF A LOW PRESSURE DISCHARGE LAMP

Abstract: A discharge lamp (10) having an envelope (12), a first electrode (26), a second electrode (28), a conductive starting aid (34) that is enclosed in the envelope and located around the inner end (30) of the first electrode, a fill material (50) located in an enclosed volume (16), and a photoemissive coating (64) formed on the inner surface of the envelope is disclosed; wherein, the starting aid (34) yields a more reliable lamp for starting.

Description: STARTING AID THAT SURROUNDS AN ELECTRODE OF A LOW PRESSURE DISCHARGE LAMP The Applicants hereby claim the benefit of their provisional application, Serial Number 60/040,757 filed March 14, 1997 for Starting Aid for Low Pressure Discharge Lamp . 1. Technical Field The invention relates to lamps and particularly to electric lamps. More particularly the invention is concerned with a starting aid for electric discharge lamps. 2. Background Art To start a discharge lamp, an ionized column extending between the lamp electrodes must be created. The likelihood of achieving this can be influenced by the applied voltage, current, electrode distance, fill composition and temperature, ambient ionizing conditions, local charge states and other factors. Some of these factors can be controlled in the lamp design, others cannot. There is then a possibility that ambient conditions may not be ideal for starting, resulting in a delayed, or even a failed start. Slow starting for general lighting is usually only an annoyance, but even occasional slow starting can be dangerous or unacceptable in vehicle lighting. There is then a need for discharge lamps in general and vehicle discharge lamps in particular with a high success rate at rapid starting. In larger discharge lamps, additional equipment can be included in the lamp to help assure starting. Heated electrodes, secondary electrodes, and other elements are known to help form and guide the development of the ionization column. In small discharge lamps, there is little or no room to include such additional hardware. There is then a particular need for small mechanisms that enhance starting in small discharge lamps. Disclosure of the Invention A discharge lamp with improved starting, may be formed from a light transmissive envelope with a wall defining an enclosed volume, a first electrode sealed through the envelope having and inner end extending into the enclosed volume, a second electrode sealed through the envelope to be in electrical communication with the enclosed volume, a conductive starting aid positioned around, and offset from the inner end, and not in direct electrical communication with the first electrode, and a fill material contained in the enclosed volume. Brief Description of the Drawings Like reference numbers designate like or corresponding parts throughout the drawings and specification. FIG. 1 shows a cross sectional view of a preferred embodiment of a discharge lamp with a starting aid. FIG. 2 shows a transverse cross sectional view of the lamp in FIG. 1, taken at A-A 1 . FIG.s 3A-E show alternative designs of a conductive starting aid. Best Mode for Carrying Out the Invention When a discharge lamp is started, it is usually easier for the electrode to build up a charge on the nearby envelope wall than to breakdown the whole length of the discharge volume. The charge transfer to the envelope wall then forms a discharge path that can be progressively extended along the envelope wall to the second electrode. The Applicants have found that since the envelope wall is substantially insulating, any charge transfer to the envelope wall does not easily migrate through or along the envelope wall thereby enabling more charge to be conducted. The envelope surface becomes locally saturated, at the point of discharge (discharge spot). Further charge transfer is then limited, at least until the discharge spot moves along the envelope. In summary, discharges to the insulating envelope are then slowed or restricted in size and duration, which then limits the likelihood of lamp starting. A solution to this problem has been found that greatly increases the likelihood of the lamp starting on the first try. FIG. 1 shows a preferred embodiment of a cold cathode discharge lamp 10 with a starting aid. The discharge lamp 10 with a starting aid is assembled from an envelope 12, a first electrode 26, a second electrode 28, a conductive starting aid 34, lamp fill material 50, a ground 64 and optionally a photo-emissive coating 64. The envelope 12 may be made out of light transmissive material such as quartz or hard glass to have the general form of an elongated tube. The preferred envelope 12 has a wall 14 defining an enclosed volume 16, an inside diameter 18, and an outside diameter. The enclosed volume 16 may have any convenient form sufficient to contain an arc discharge. The preferred enclosed volume 16 is an elongated tube, with an axis 20 extending from a first seal 22 to a second seal 24. The preferred first seal 22 is a press seal capturing the first electrode 26. The second seal 24 may be formed to have substantially the same structure as the first seal 22, capturing a similarly formed second electrode 28. It is understood that lamp 10 is to be operated as a positive column, so the electrodes 26, 28 are sufficiently axially separated to allow formation of a positive discharge column therebetween. Electrode efficiency, and electrode durability are important to overall lamp performance. The preferred electrode is a cold cathode with a material chosen for long life operation high temperature. The first electrode 26 may be made out of an electrically conductive high temperature material such as steel, nickel, molybdenum, tungsten or other conductive metal to have any of numerous electrode forms as known in the art. The preferred first electrode 26 has an inner end 30, a seal portion, and an exterior end. The inner end 30 projects into the enclosed volume 16, and is designed to durably support the end of an arc discharge. The preferred inner end 30 is a metal rod supporting a metal cup 32 structure. The cup 32 may be formed from nickel rolled in the shape of a cylinder. The rolled nickel cup 32 may be crimped or otherwise attached to the end of the rod. A getter emitter or other emission enhancing material may be positioned in the cup 32 as is known in the art. The envelope 12 seals around the seal portion, and the exterior end extends outwards from the envelope 12 for electrical connection. The second electrode 28 may be similarly formed with the same general form as the first electrode 26. Intermediate the envelope wall 14 and the inner end 30, is a conductive metal starting aid 34. FIG. 2 shows a transverse cross sectional view of the lamp in FIG. 1, taken at A-A'. The conductive starting aid 34 may be made out of metal such as steel, nickel, tungsten, or others to have the general form of a ring, a hollow tube, a split hollow tube, or a spiral to provide an intermediate, conductive layer positioned in contact with the inner wall of the envelope, and between the envelope wall 14 and the inner end 30. Alternatively, a conductive coating, such as an aluminization layer, may be formed on the inside surface of the envelope wall 14. The conductive starting aid 34 may have the form of a metal cylinder, with an outer diameter 36 slightly less than the inside diameter 18 to enable mechanical insertion of the starting aid 34. The inner diameter 38 of the conductive starting aid 34 is sufficiently large to offset the inner end 30 from the conductive starting aid 34. The preferred conductive starting aid 34 extends substantially around the inner end, intersecting a plane transverse to the lamp axis and extends axially for a distance. The capacitive value of the conductive starting aid is what is of interest, and the axial extension of the conductive starting aid determines in part the capacitance of the conductive starting aid. The greater the capacitance the better, so the longer the conductive starting aid the better. However the conductive starting aid also blocks light, so in this regard, the smaller the axial extension the better. There is then a need to balance the needs for improved starting, against the loss of blocked light. The Applicants prefer an axial extension approximately equal to the length of the cup 32 portion of the inner end 30, or similarly the radius of the enclosed lamp cavity at the electrode tip. The conductive starting aid 34 in the ring form, establishes a zone of constant electrical potential around the inner end 30, and provides a substantially larger capacitance than does the immediate discharge spot of the envelope. The larger and more rapidly filled capacitance of the conductive starting aid 34 enables greater and longer charge transfers with respect to the electrode 26. By including the conductive starting aid, a charge transfer induced by the electrode can quickly spread through the metal starting aid, thereby enabling further charge to be immediately transferred. A larger and longer charge transfer can then occur, thereby holding the discharge path through the fill material open longer. The longer, and greater discharge path through the fill material then results in a more likely lamp start on the initial try. By further being in effective capacitive range of an exterior ground, the conductive starting aid's function can be further improved. FIG.s 3A-E show alternative designs of a conductive starting aid. This may be done with a conductive tube offset and insulated from the conductive portion of the inner end 30. In a first design, FIG. 3A, the conductive starting aid 34 is a solid metal tube, that may be held in place against the inside of the enclosed volume by friction, melting and pressing the adjacent glass envelope, forming a mechanical limit 40 (lip, depression, track, bumps or other) in the material of the envelope 12 to engage the conductive starting aid 34. Other mechanical methods to use the lamp wall 14 to support the conductive starting aid 34 may be devised, such as springs, interference fit pieces, pastes or embedded supports. It is only relevant the conductive starting aid 34 be held in place offset from the conductive portion of the inner end 30. In one example, the inner diameter of the conductive starting aid was -greater than or equal to 1.73 times the outer diameter of the electrode cup 32. The conductive tube FIG. 3A, may present mechanical problems in the lamp manufacture, such as thermal expansion and contraction. As an alternative, the tube may be split axially, FIG. 3B, to provide an expansion or compression joint. The split tube 42 may be compressed to fit into the envelope 12 with a spring tension providing sufficient force to lock the split tube 42 in place with reference to the envelope wall 14. The ends of the split tube 42 may be overlapping or not. In another alternative, FIG. 3C, the conductive starting aid may be formed as a metal spiral 44, or as a tube with multiple axial reliefs 46, FIG. 3D. In these cases the conductive starting aid 44, 46 can again provide a radial locating force, so the conductive starting aid is held in spring tension against the inner surface of the envelope wall. In a further alternative, FIG. 3E, a band of the inside surface of the envelope wall may be coated with a metal layer 48, such as an aluminization, a sintered metal slurry, chemical deposition, or other coating method. In general, in lamps with relatively large envelope diameters, it may be easier to mount the conductive starting aid on the envelope wall. The envelope wall may then be heated and deformed slightly around the conductive starting aid to entrain a portion thereof, to pin or hold the conductive starting aid in place. This is may be achieved as a part of the sealing procedure. The volume between the inner end 30 and the metal conductive starting aid 34 then is then filled with a material with a lower electrical conductivity to form an insulating starting aid to conduction. This electrical separation forces any conduction or charge build up to occur through the fill material. For example, given the gap between the inner end 30 and the conductive starting aid 34, the lamp fill material 50 may provide a moderate but sufficient insulating starting aid. In the preferred embodiment, adjacent the conductive starting aid 34, on the exterior side of the lamp envelope 12 is a ground plane. The ground plane may be provided by a metal foil 52, for example a band of copper foil that is held or cemented to the envelope exterior 14, and is wired to a ground. Alternatively the lamp holder (for example a bracket, clip or other support or lamp holding means) or an adjacent reflector or other conductive piece may contact the exterior of the envelope wall 14 in the area adjacent the conductive starting aid 34 and additionally function as a ground contact. The best results are achieved when a ground is used; however, it is believed that some of advantages of the inner conductive starting aid 34 are provided even if there is no specific outer ground element. The lamp fill material 50 may be made out of an inert gas or gases, photo-emissive gas such as mercury vapor, metal halides, and any of the numerous fill material combinations known in the art of discharge lighting. Applicants believe there is no limitation in the effective use of the conductive starting aid and optional ground plane regarding various discharge lamp fills, or lamp sizes. Optionally, a photo-emissive coating 64 may be formed on the inner surface of the envelope 12. The coating may be any of numerous fluorescent or phosphorescent materials as known in the art. These coatings generally have the form of an a thin layer formed on the inner surface -of the envelope wall 14. The lamp 10 may be operated by a continuous power or a pulsed power generator 50. One suitable glass for neon lamps is an alumina silicate glass, a "hard glass" available from Corning Glass Works, and known as type 1724. Applicants have found that the 1724 hard glass stops nearly all neon loss. The 1724 glass may be baked at 900 degrees Celsius to drive out water and hydrocarbons. The hot bake out improves the cleanliness that helps standardize the color produced, and improves lamp life. Narrow neon lamps have been made with an inside diameters that varied from 2.0 to 10.0 millimeters, with the preferred inside diameter being about 3.0 to 5.0 millimeters. Narrow neon discharge lamps have been found to work marginally well at 9 or 10 millimeters inside diameter. Better results occur at 5 millimeters, and 3 millimeters appears to be the best inside diameter. The preferred narrow neon envelope elongated tube thickness may vary from 1.0 to 3.0 millimeters with a preferred elongated tube thickness of about 1.0 millimeter. The outside diameter then may vary from 4.0 millimeters to 16.0 millimeters with a preferred outside diameter of 5.0 to 7.0 millimeters. Tubular envelopes have been made with overall lengths from 7.62 centimeters to 127 centimeters (3 to 50 inches). The overall length for a positive column emission is thought to be a matter of designer choice. The first electrode was made of electrically conductive nickel coated steel outer end, welded to a molybdenum rod supporting a cup made from a nickel foil crimped to the molybdenum support rod. The cup had an axial extension of 12.7 to 15.9 millimeters (1/2 to 5/8 inches). The cup had an inner shell of 1.52 millimeters (0.06 inches) radius, covered by a 0.254 millimeter (0.01 inches) thick mica layer, that was in turn covered by an outer metal sleeve 0.254 millimeters (0.01 inches) thick, thereby providing a cup with an overall exterior diameter of 2.54 millimeters (0.1 inches) . The inside of the cup was coated with an aluminum and zirconium getter/emitter material, known as Sylvania 8488 that is spun deposited and baked on to provide an even interior coating of the inner end. The cup surrounds the emitter tip, and extends slightly farther, perhaps 2.0 millimeters, into the tubular envelope than the inner most part of the electrode rod, and the emitter material extend. The second electrode was similarly formed. The preferred neon fill is substantially pure, research quality neon. The photo-emissive coating may be made of any of numerous phosphorescent materials as known in the art. Six lamps were made in a standard fashion without a starting aid. There were variations in the electrode structures, but no starting aid was used in these six lamps. Each lamp was started ten times and the starting time recorded. The starting time averages were 279.4, 285.9, 262.3, 273.7, 262.6, and 249.1 microseconds. The group average was 268.8 microseconds. Four similar lamps were made with the starting aid and each was tested ten times, again measuring the starting time in microseconds. One of the starting aid lamps was tested with a ballast that had a low starting voltage. As a group the four starting aid lamps averaged 204.3 microseconds to start. This was a 24% increase in starting time. Discounting the low voltage ballast as defective, the three remaining lamps averaged 142.1 microseconds to start. This was a 53% increase in starting time. The disclosed dimensions, configurations and embodiments are as examples only, and other suitable configurations and relations may be used to implement the invention. The preferred dimensions of the conductive starting aid diameters (OD, ID) , length, thickness in comparison to the lamp voltage, envelope and electrode sizes are expected to be worked out by testing. The use of the inner conductive starting aid is believed to not be restricted to neon, or even low pressure lamps. The conductive starting aid and ground concept is expected to have general application in other pressure ranges, other chemistries, and electronic operating, understanding that shape, size, position, and materials may need to be adjusted for each particular lamp conditions. While there have been shown and described what are at present considered to be the preferred embodiments of the invention, it will be apparent to those skilled in the art that various changes and modifications can be made herein without departing from the scope of the invention defined by the appended claims. It is understood that hot cathode lamps may possibly be made to operate using the disclosed structure and method of operation.

Claims: Claims What is claimed is: 1. A cold cathode discharge lamp with a starting aid comprising: a) a light transmissive envelope with a wall defining an enclosed volume, b) a first electrode sealed through the envelope having and inner end extending into the enclosed volume, c) a second electrode sealed through the envelope to be in electrical communication with the enclosed volume, having a nearest point that is closest to the inner end of the first electrode, and d) a starting aid in the form of a capacitive body having a plate formed from a material more conductive than the envelope material, the plate being in contact with the interior envelope wall and having an electrical contact point positioned to be offset through the enclosed volume from the first electrode but still closer to the first electrode than is the nearest point of the second electrode; the capacitive body being not in direct electrical communication with the first electrode or the second electrode; and e) a fill material contained in the enclosed volume. 2. The lamp in claim 1, further including a second capacitive plate approximately adjacent the first plate, and wherein the envelope is intermediate the first plate and the second plate. 3. A discharge lamp with a starting aid comprising: a) a light transmissive envelope with a wall defining an enclosed volume, b) a first electrode sealed through the envelope having and inner end extending into the enclosed volume, c) a second electrode sealed through the envelope to be in electrical communication with the enclosed volume, d) a starting aid in the form of a conductive body positioned around, and offset from the inner end, and not in direct electrical communication with the first electrode, and e) a fill material contained in the enclosed volume. 4. The lamp in claim 3, further including a ground plane adjacent the exterior of the lamp envelope near the conductive starting aid. 5. The lamp in claim 3, further including an insulation layer positioned intermediate the inner end and the conductive starting aid. 6. The lamp in claim 3, further including a photo-emissive layer formed on the inner surface of the envelope wall. 7. The lamp in claim 3, wherein the conductive starting aid is supported by the envelope wall. 8. The lamp in claim 3, wherein the first seal is a press seal the deformation region of which partially entrains the conductive starting aid to assist in holding the ring in place. 9. The lamp in claim 3, wherein the conductive starting aid is held in spring tension against the inner surface of the envelope wall. 10. The lamp in claim 3, wherein the axial extension of the conductive starting aid is approximately equal to or greater than the distance separating the inner end of the electrode and the conductive starting aid. 11. A discharge lamp with a starting aid comprising: a) an envelope having a wall with an inner surface defining an enclosed volume; a first seal, and second seal; b) a first electrode, sealed through the envelope having an inner end extending into the enclosed volume, c) a second electrode, sealed through the envelope having an inner end extending into the enclosed volume, d) a conductive starting aid, positioned in the enclosed volume, having an inner diameter, an outer diameter, and an axial extension; e) an insulation layer enclosed in the enclosed volume, intermediate the inner end and the conductive starting aid; f) a fill material including neon gas positioned in the enclosed volume; and g) a photo-emissive coating formed on the inner surface of the envelope wall. 12. The lamp in claim 11, wherein the envelope has an inner diameter of 3 millimeters. 13. The lamp in claim 11, wherein the envelope has an outer diameter of 5 millimeters. 14. The lamp in claim 11, wherein the first seal is a press seal the deformation region of which partially entrains the conductive starting aid to assist in holding the ring in place. 15. The lamp in claim 11, wherein the first electrode includes a cup portion formed on the inner end. 16. The lamp in claim 11, wherein the inner end is includes a cup portion including an emissive material. 17. The lamp in claim 11, wherein the second electrode is similarly surrounded by a similar conductive starting aid. 18. The lamp in claim 11, wherein the conductive starting aid is axially centered with respect to the inner end of the electrode. 19. The lamp in claim 11, wherein the inner diameter of the conductive starting aid is greater than or equal to 1.73 times the outer diameter of the electrode cup. 20. The lamp in claim 11, further including a photo-emissive coating responsive to ultraviolet emissions from the fill material.

Inventors: GARRISON ROBERT L, KACE DANIEL J, SIDWELL STEVE C, SMAGLICK RICHARD J, GARRISON, ROBERT, L., KACE, DANIEL, J., SIDWELL, STEVE, C., SMAGLICK, RICHARD, J.
Applicants: GARRISON ROBERT L, KACE DANIEL J, OSRAM SYLVANIA INC, SIDWELL STEVE C, SMAGLICK RICHARD J, GARRISON, ROBERT, L., KACE, DANIEL, J., OSRAM SYLVANIA INC., SIDWELL, STEVE, C., SMAGLICK, RICHARD, J.
Application year: 1998
Main classification: H01J1
IPC codes: H01J17, H01J61, H01J63