Arc discharge lamp containing mechanism for extinguishing arc at end-of-life

Abstract: An arc discharge lamp having an end-of-life structure and including a light-transmissive envelope having a tubular-shaped main body and a press seal disposed at each end of the main body. An electrode filament having a pair of ends is located in each end of the envelope. Each electrode filament end is in a contiguous relationship with an inside surface of the lamp envelope. When one of the electrode filaments is depleted of electron-emissive material, increased heating of the electrode filament caused by an increase in cathode fall voltage, produces a localized hot spot on the internal surface of the envelope. This localized heating causes a puncture in the lamp envelope and evacuation of the lamp. The introduction of external atmosphere into the lamp extinguishes the arc discharge and renders the lamp inoperable.

Description: This invention relates to the field of low-pressure arc discharge lamps. More particularly, the invention relates to low-pressure arc discharge lamps, such as subminiature fluorescent lamps, having a structure or device which renders the lamp inoperable at the end of its useful life. Herein, the terms "end-of-life" and "end of the useful life" of a low-pressure arc discharge lamp are defined as that time when the electron-emissive material on one electrode filament has been depleted causing the arc discharge to destroy the filament and/or strike other parts of the electrode mount structure. Low-pressure arc discharge lamps, such as fluorescent lamps, are well known in the art and typically include a pair of electrodes made of a coil of tungsten wire upon which is deposited a coating of an electron-emissive material consisting of alkaline metal oxides (BaO, CaO, SrO) to lower the work function of the cathode and thus improve lamp efficiency. With electron-emissive material disposed on the electrode filament, the cathode fall voltage is typically about 13 volts. However, at the end of the useful life of the lamp when the electron-emissive material on one of the electrode filaments becomes depleted, the cathode fall voltage rises by 100 volts or more. If the external circuitry fails to limit the open circuit voltage across the lamp, the lamp may continue to operate with the additional power being deposited at the lamp electrode region. By way of example, a lamp which normally operates at 0.1 amp would deposit 0.65 watt at each electrode during normal operation. At end-of-life, the depleted electrode may consume 7.5 watts due to the increase in cathode fall voltage. This extra power can lead to excessive local heating of the lamp and fixture. Low-pressure arc discharge lamps, especially those designed for operation at high current (1.5 amp) loading, such as very high output (VHO) lamps, sometimes fail by causing the fracture of the glass envelope. It is believed the sequence of events leading to such failures is as follows. At the end of the useful life of the lamp, the electron-emissive material on one of the electrode filaments becomes depleted. When such depletion occurs, the arc discharge strikes other components of the electrode structure and, in particular, the arc strikes the electrical leads supporting the electrode structure. The electrical leads are heated by the arc to the point where the wires soften and bend. Subsequently, the electrical leads and the electrode structure sag and come in contact with the glass envelope. The severe heat generated by the arc and the heated electrode structure cause the glass envelope to fracture. Various internal structures for low-pressure arc discharge lamps have been proposed which cause the lamp to fail without fracture of the glass envelope. Such structures are suggested in the following references. US-A-3,265,917, discloses a structure comprising a wire or conductive coating electrically connected to the inside portion of the electrode structure and extending to a thin-walled portion of the glass stem press. Upon depletion of the electron-emissive material on the electrode filament, the arc strikes and follows the conductive path reaching the thin-walled portion of the stem press. The heat generated by the arc and the heated conductor softens and melts the thin wall of the stem press to the point where the hermetic seal is lost. The introduction of the external atmosphere into the lamp extinguishes the arc discharge and renders the lamp inoperable. US-A-4,105,910, discloses a structure providing for an auxiliary source of amalgam and for end-of-life extinguishment of the arc. This structure comprises a coating of a suitable amalgamative metal on portions of the stem press and the inside lead-in wire about the point where the lead-in wire emerges from the stem press. US-A-4,495,440, discloses an arc-extinguishing ampul mounted on each electrode structure. The ampul comprises a thin-walled glass body enclosing an arc-extinguishing gas, at least one electrically conductive support wire, and a heat-conductive coating covering the outer surface of the ampul and portions of the support wire. Upon depletion of the electron-emissive coating on one electrode filament at the end of the useful life of the lamp, the arc discharge is attracted to the ampul by the support wire. The heat of the arc softens and melts the ampul to the point where the arc-extinguishing gas within the ampul escapes and renders the lamp inoperable without loss of the lamp's hermetic seal. Although the above-described end-of-life structures have been employed with varying degrees of success, it has been found that certain disadvantages do exist and such structures do leave something to be desired. More specifically, the above-described wire, conductive coating or amalgamative metal require a stem press mount configuration to implement. On the other hand, the arc-extinguishing ampul is rather expensive from a lamp-manufacturing standpoint because it adds additional manufacturing steps to the lamp process. Therefore, it would be very desirable and advantageous to have an end-of-life structure that does not require a stem press mount and which is more economical to utilize. US-A-2,424,457 discloses an arc discharge lamp comprising: a light-transmissive envelope having a tubular-shaped main body, the main body of the envelope having an internal diameter and containing a fill material for supporting a low pressure discharge; a layer of phosphor disposed on a surface within the main body of the envelope; and an electrode filament located in each end of the main body and including a pair of electrode filament ends. An electrical lead-in wire is provided with a bent V-shaped portion which engages the wall of the envelope. If the filament and the lead-in wire overheat, the envelope is punctured at a point allowing air to be admitted to extinguish the lamp. In the present invention as claimed, however, the arc discharge lamp is characterised in that the lamp further includes a press seal disposed at each end of the main body of the envelope and a pair of electrical leads attached to each electrode filament and sealed within a respective press seal, and in that each electrode filament end is in a contiguous relationship with an inside surface of the envelope. Thus, in the present invention as claimed it is the ends of the filament which puncture the lamp envelope at the end of the lamp's useful life. In a preferred embodiment of the present invention, each electrode filament has an axial length greater than the internal diameter of the envelope. In one embodiment, the axial length of the electrode filament is equal to about 7 mm and the internal diameter of the envelope is equal to about 5 mm. Preferably, each pair of electrical leads includes a glass bead formed thereon for maintaining separation of said electrical leads. In accordance with further preferred aspects of the present invention, the arc discharge lamp further includes an insulative base member disposed at each end of the lamp. Each of the base members surrounds a respective press seal and a portion of the tubular-shaped main body of the envelope. Preferably, the base member extends axially from the press seal and beyond the center of a respective electrode filament. Some preferred embodiments of the present invention will now be described by way of example only and with reference to the accompanying drawings, in which: FIG. 1 is a plan view, partially in cross section, of one embodiment of an arc discharge lamp in accordance with the present invention; FIG. 2 is a partial view, partially in cross section, of the arc discharge lamp of FIG. 1 showing one of the press seals; FIG. 3 is an end view of the fluorescent lamp of FIG. 1; FIG. 4 is a cross-sectional view of the fluorescent lamp of FIG. 1 taken along the lines 4--4 of FIG. 1; FIG. 5 is a plan view of a mount structure for use in the arc discharge lamp of FIG. 1; FIG. 6 is a plan view, partially in cross section, of another embodiment of an arc discharge lamp in accordance with the present invention; FIG. 7 is an end view of the arc discharge lamp of FIG. 6; FIG. 8 is a partial view of the arc discharge lamp of FIG. 6 showing one of the lamp base members surrounding one of the press seals and a portion of the main body of the lamp; and FIG. 9 is a cross-sectional view of the fluorescent lamp taken along the lines 9--9 in FIG. 8. With particular attention to FIGS. 1-4, there is illustrated an improved low-pressure arc discharge lamp 10 (i.e., a fluorescent lamp) in accordance with the teachings of the invention. Lamp 10 generally includes a tubular-shaped light-transmissive envelope 12 having a tubular-shaped main body. Envelope 12 is typically fabricated of soda lime glass and, by the way of example, can have an outside diameter on the order of about 4.6 mm (0.18 inch) to 6.7 mm (0.27 inch) and a length in the range of 102-508 mm (4-20 inches). An electrode filament 14 is mounted in each end of envelope 12. Electrical leads 16 and 18 are connected to filament 14 and extend through a press seal 20. A glass bead 38 formed on electrical leads 16 and 18 insures that a predetermined separation between the electrical leads is maintained during formation of the press seal. The opposite end of the lamp 10 is constructed in the same manner and includes a press seal 22. A mercury dispenser 24 at one end of the lamp is attached to electrical lead 16. The lamp 10 contains a fill material including mercury supplied from dispenser 24 and a rare gas such as argon at a low pressure (e.g., 670 Pa (5 torr)). As illustrated in FIGS. 1 and 4, a coating 30 is applied to the inside surface of envelope 12. Lamp 10 may include an aperture 34 which is formed in layer 30 to direct light from lamp 10 in a preferred direction. As best shown in FIG. 1, the aperture 34 extends axially along a major portion of length of envelope 12 and has a uniform width. The width of aperture 34 depends on the desired radiation pattern from lamp 10. In the case of an aperture fluorescent lamp, layer 30 comprises a reflective layer and a phosphor layer. The reflective layer is first applied to the inside surface of envelope 12 and then the phosphor layer is applied over the reflective layer. The reflective layer has a reflective inside surface. The reflective layer insures that light emitted from the lamp 10 is directed through aperture 34. In an alternative configuration, the reflective layer is removed in aperture 34 but a phosphor layer is applied to the entire inner surface of envelope 12. In another alternative, aperture 34 and the reflective layer are omitted. In this case, the phosphor layer is uniformly applied to the inside surface of envelope 12, and the lamp provides a uniform cylindrical radiation pattern. A preferred technique for scraping aperture 34 is described in detail in EP-A-0,464,723. The press seals 20 and 22 each include a tubulation 40 generally positioned on an axis 42 of envelope 12 and flattened regions 44 and 46 on opposite sides of tubulation 40. Electrical lead 16 extends through and is sealed into flattened region 44, and electrical lead 18 extends through and is sealed into flattened region 46. Flattened regions 44 and 46 include generally flat surfaces 44a and 46a (FIG. 3), respectively, which are used for orientation of aperture 34. In a preferred embodiment, the surfaces 44a and 46a are oriented at 90° with respect to a line drawn through the center of aperture 34 and the axis 42 of the envelope 12. Electrical leads 16 and 18 extend from the end of press seal 20 for connection of the electrode filament 14 to a source of electrical energy. In one configuration as shown, electrical leads 16 and 18 are bent on opposite sides of press seal 20 and extend along the surfaces of flattened regions 44 and 46 respectively. In another configuration, the leads 16 and 18 extend from the end of lamp 10 parallel to axis 42 and can be connected to leads from the electrical source in any convenient manner, such as by crimping. The press seals 20 and 22 can be provided with means for positioning the baseless lamp 10 of FIG. 1 along axis 42. The positioning means can comprise one or more detents 50 formed in the press seals 20 and 22. In the example shown in FIGS. 1 and 2, the detents 50 comprise a depression or groove in flattened regions 44 and 46. The groove is oriented with its sides perpendicular to the axis 42 of envelope 12. Thus, when detent 50 engages a projection in the lamp mounting hardware (not shown), the lamp 10 is prevented from moving along axis 42. In a preferred embodiment, one detent 50 is provided on each side of each press seal. Thus, with respect to press seal 20, a detent 50 is located on the front of flattened region 46 and a detent (not shown) is located on the back of flattened region 44. Alternatively, the detents 50 can be replaced with projections which engage corresponding detents in the mounting hardware. That portion of each electrode filament 14 located between electrical leads 16 and 18 is coated with a quantity of electron-emissive material in order to lower the work function of the cathode and thus improve lamp efficiency. As stated earlier, at the end of the useful life of the lamp, the electron-emissive material on one of the electrode filaments becomes depleted causing the cathode fall voltage to rise by 100 volts or more. If the external circuitry fails to limit the open circuit voltage across the lamp, the lamp may continue to operate with the additional power being deposited at the lamp cathode region. This extra power can lead to excessive local heating of the lamp and fixture. In accordance with the teachings of the invention as claimed, each electrode filament 14 is provided with a pair of ends 26 and 28 (FIG. 1), which extend toward and are in a contiguous relationship with an inside surface of the main body of envelope 12. During normal operation, the heating of electrode filament 14 is confined to those regions of the electrode filament located between electrical leads 16 and 18. Accordingly, little heat is conducted to the glass wall and operation of lamp 10 is unaffected. It has been discovered that at end-of-life, the heating of an electrode filament depleted of electron-emissive material increases by up to a factor of ten and extends uniformly to the coil ends beyond electrical leads 16 and 18. As a result, a localized hot spot is produced on the glass wall where the electrode filament end touches. This localized heating causes a puncture in the lamp envelope and evacuation of the lamp. The introduction of the external atmosphere into the lamp extinguishes the arc discharge and renders the lamp inoperable. FIG. 5 illustrates a mount 36 used in fabricating lamp 10. Mount 36 includes filament 14 supported by electrical leads 16 and 18 which are maintained separated by a conventionally-known glass bead 38. A mercury dispenser 24 is shown secured to electrical lead 16. It is understood that if a dispenser is employed as a means of introducing mercury into the lamp, only one dispenser is required per lamp. In order to insure that ends 26 and 28 of electrode filament 14 firmly contact the inside surface of envelope 12, the axial length D1 (FIG. 5) of electrode filament 14 is slightly greater than the internal diameter off envelope 12. In a typical example, a subminiature fluorescent lamp having an envelope with an internal diameter of about 5 mm, would typically have a length D1 equal to about 7 mm. The creation of a puncture in the lamp envelope may, in some instances, result in fracture of the lamp envelope and loss of structural integrity. Referring next to FIGS. 6-8, there is shown an arc discharge lamp 10' according to another embodiment of the present invention, wherein similar constituent members as those in FIG. 1 are denoted by the same reference numerals. Lamp 10' includes an electrically insulating (e.g., plastic) base member 60 secured to each end of the lamp. Base member 60 is of unitary construction and includes a first portion 62 having a tubular outer shape and a second portion 64 having a flattened shape and protruding from first portion 62. As shown in FIG. 7, a pair of apertures 66 are located within flattened portion 64, each of which extends through flattened portion 64 and is designed for having a respective exteriorly projecting portion of an electrical lead 16, 18 pass therethrough. As further illustrated in FIG. 7, a groove 76 is formed in the terminal end of flattened portion 64 for each electrical lead 16, 18. Grooves 76 provide positioning and allow the electrical leads to be flush with the end of the base member. One end 84 of first base member portion 62 located adjacent flattened portion 64 defines therein an opening 68 (FIG. 9) which is designed for having press seal 20 securedly positioned therein. As shown in FIG. 6, the other end 86 of first portion 62 of the base member defines therein a circular opening 88. End 86 is designed to surround that portion of the lamp envelope adjacent electrode mount structure 36 and, preferably, to extend along the lamp axis and beyond the center of electrode filament 14 by a length D2 (FIG. 6). Typically, distance D2 is equal to about 3/16 inch (4.75 mm). Extending the first base member portion 62 provides support to the lamp envelope in the event that the envelope cracks in the area immediately adjacent the electrode filament ends 26 and 28. To assist in retaining the base member on arc discharge lamp 10', at least one protruding segment 82 (as illustrated in FIGS. 8 and 9), projects from an internal wall of first base member portion 62. Preferably two such segments are utilized and positioned in a diagonally-opposing relationship as best illustrated in FIG. 9. Each of these protruding segments is designed for being aligned with and subsequently located within a corresponding detent 50 of press seal 20, 22. When one of the press seals of lamp 10' is inserted within a respective base member 60, each of the protruding segments 82 is inserted within a corresponding detent 50, the result being that the press seal is substantially "locked" in position. To assure positive positioning therein without causing damage to the press seal, the first portion 62 of base member 60 further includes flexure means 70 therein as illustrated in FIGS. 8 and 9 to enable this part of the base member to expand a predetermined amount during said positioning. Flexure means 70 is preferably in the form of two elongated slots 72 formed within first portion 62 to thus enable the first portion to expand outwardly during positioning of the press seal within the base member. The press seal is thus firmly positioned within base member 60 without the need for cement or the like. To maintain the externally projecting portions of electrical leads 16, 18 in proper alignment, base member 60 further includes a plurality of channels 74 (FIGS. 6-8), each located on first portion 62 of the base member adjacent flattened portion 64. Each channel 74 is designed to have one of the terminal ends of a respective electrical lead 16, 18 inserted therein. As depicted in FIG. 7, each channel 74 includes a tapered portion 80 which facilitates positioning of the substantially annular (round) electrical leads therein. In addition, each channel further includes a narrowed portion formed by a pair of opposing bumps 78 separated a distance which is slightly smaller than the diameter of the electrical lead. Once the terminal end of the electrical lead is forced past bumps 78, the electrical lead is retained and prevented from springing back. In addition to retaining the terminal ends of the electrical leads, bumps 78 provide a reservoir of material which can be mechanically pushed towards the resting electrical lead to more positively retain the electrical lead. As a result of channels 74, each off the terminal ends of the electrical leads is secured to the base member in a locking relationship to thus prevent subsequent removal thereof during the positioning of each lamp end within a corresponding socket member (not shown). Such retention also serves to assist in maintaining the press seal of lamp 10' firmly within base member 60. A total of two channels are provided on each base member, one for each of the mentioned electrical leads. There has thus been shown and described an improved arc discharge lamp. The arc discharge lamp contains an end-of-life structure which does not require the lamp to have a stem press mount. The end-of-life structure is economical and does not add additional manufacturing steps to the lamp process. While there have been shown and described what are at present considered the preferred embodiments of the present invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims.

Claims: An arc discharge lamp comprising: a light-transmissive envelope (12) having a tubular-shaped main body, said main body of said envelope having an internal diameter and containing a fill material for supporting a low pressure discharge; a layer of phosphor (30) disposed on a surface within said main body of said envelope; and an electrode filament (14) located in each end of said main body and including a pair of electrode filament ends (26,28); characterised in that said lamp further has a press seal (20,22) disposed at each end of said main body of said envelope (12) and a pair of electrical leads (16,18) attached to each electrode filament (14) and sealed within a respective press seal, and in that each electrode filament end (26,28) is in a contiguous relationship with an inside surface of said envelope (12). A lamp as claimed in claim 1, wherein each electrode filament (14) has an axial length greater than said internal diameter of said envelope (12). A lamp as claimed in claim 2, wherein the axial length of said electrode filament (14) is equal to about 7 mm. A lamp as claimed in claim 3, wherein said internal diameter of said envelope (12) is about 5 mm. A lamp as claimed in any preceding claim, wherein each pair of electrical leads (16,18) includes a glass bead (38) formed thereon for maintaining separation of said electrical leads. A lamp as claimed in any preceding claim, wherein said lamp further comprises an insulative base member (60) disposed at each end of said lamp, each of said base members surrounding a respective press seal (20,22) and a portion of said tubular-shaped main body of said envelope (12). A lamp as claimed in claim 6, wherein said base member (60) extends axially from said press seal (20,22) and beyond the center of a respective electrode filament (14). A lamp as claimed in claim 7, wherein said base member (60) extends axially about 4.75 mm beyond the center of said electrode filament (14).

Inventors: HARADEN THOMAS, HOUGH HAROLD L, YANG PAI ROBERT, HARADEN, THOMAS, HOUGH, HAROLD L., YANG PAI, ROBERT
Applicants: GTE PROD CORP, GTE PRODUCTS CORPORATION
Application year: 1993
Main classification: H01J61
IPC codes: H01J5, H01J61