Arc lamp assembly with containment means

Abstract: A metal halide arc lamp assembly (10) has a configuration which provides improved containment performance in the event that the arc tube (14) bursts. The lamp assembly (10) includes a metal halide arc tube (14), a cylindrical quartz shroud (20) surrounding the arc tube (14) and a sealed outer envelope (12) enclosing the shroud (20) and the arc tube (14). The arc tube (14) has press seals (52,54) with a generally I-shaped cross section. The maximum dimension (D1) of the press seals (52,54) perpendicular to the arc tube (14) axis (2) is not substantially greater than the maximum dimension (D2) of the bulb portion of the arc tube. The spacing (70) between the outside surface of the bulb portion (50) and the inside surface of the shroud (20) is not greater than about three millimeters.

Description: Field of the Invention This invention relates to electric lamps and, more particularly, to double-enveloped lamps which can be safely operated without the need for enclosing the lamp within a protective fixture, even in the event of a burst of the arc tube. Background of the Invention Electric lamps known as double-enveloped lamps include a light-source capsule, or arc tube, and an outer envelope surrounding the light-source capsule. Metal halide arc lamps are examples of double-enveloped lamps. In such double-enveloped lamps, there is a small probability that the light-source capsule will burst. When such an event occurs, hot fragments of glass, or shards, and other capsule parts emanating from the burst capsule are forcibly propelled against the outer envelope. If the outer envelope also shatters, there is a potential safety hazard to persons or property in the immediate surroundings. Failure of the outer envelope is known as a containment failure. One way to avoid the safety hazard of containment failure is to operate the lamp within a protective fixture that is capable of containing such a failure. However, a protective fixture usually incurs additional cost, particularly when an existing fixture must be modified or replaced. Furthermore, a protective fixture reduces the light output of the lamp, and it may be more difficult and expensive to replace the lamp in a protective fixture. A preferred solution to the containment failure problem is a lamp assembly capable of self-containment. One known self-containment technique is to make the outer envelope sufficiently strong to contain the shattered light-source capsule. An outer envelope having a relatively thick wall in combination with a light-source capsule having a relatively thin wall is disclosed in U.S. Patent No. 4,598,225 issued July 1, 1986 to Gagnon. Another prior art technique is to shield the outer envelope from the effects of a burst light-source capsule. In U.S. Patent No. 4,580,989 issued April 8, 1986 to Fohl et al, a light-transmissive enclosure, or shield, located within an outer envelope surrounds a light-source capsule and shields the outer envelope. See also U.S. Patent No. 4,281,274 issued July 28, 1981 to Bechard et al. Still another technique for containment is to reinforce the shield or the light-source capsule. In U.S. Patent No. 4,721,876 issued January 26, 1988 to White et al, a light-transmissive shield is reinforced by a cloth-like wire mesh. Wire mesh reinforcement of a light-source capsule is disclosed in U.S. Patent No. 4,625,140 issued November 25, 1986 to Gagnon. Containment techniques are also disclosed in U.S. Patent No. 4,888,517 issued December 19, 1989, U.S. Patent No. 4,942,330 issued July 17, 1990 and in pending application Serial No. 07/468,042 filed January 20, 1990, all assigned to the assignee of the present application. Guards for protecting incandescent lamps are disclosed in U.S. Patent Nos. 314,208 issued March 18, 1885 to White, 765,568 issued July 19, 1904 to Eisenmann and 781,391 issued January 31, 1905 to Blake. While the above-referenced containment techniques are highly effective for some lamp types and sizes, they may have disadvantages when applied to other lamp types and sizes. For example, the use of a thick-walled outer envelope is effective for relatively small lamps but adds to the cost of the lamp. Lamps of greater than about 400 watts having a thick-walled outer envelope are so heavy that there is a possibility that the lamp base will fail, leading to the lamp falling out of the light fixture. Furthermore, thick-walled outer envelopes of large physical size are difficult to fabricate. Various wire mesh containment devices are effective in achieving containment but add to the cost of the lamp and block a portion of its light output. Quartz shields, or shrouds, between the arc tube and the outer envelope are also effective in achieving containment. However, the shroud disturbs the thermal environment of the arc tube. Thus, the arc tube and the structures surrounding it must be carefully designed for proper lamp operation. It is a general object of the present invention to provide improved double-enveloped lamps. It is another object of the present invention to provide improved double-enveloped lamps which can be safely operated without a protective fixture. It is a further object of the present invention to provide self-containing, double-enveloped lamps which have a high luminous output. It is still another object of the present invention to provide improved metal halide arc discharge lamps. It is a further object of the present invention to provide self-containing, double-enveloped lamps which are light in weight and low in cost. Summary of the Invention According to the present invention as defined in claim 1, these and other objects and advantages are achieved in a double-enveloped lamp assembly comprising a light-source capsule subject to burst on rare occasions, the light-source capsule including a bulb portion that encloses a discharge region and press seals at opposite ends of the bulb portion, containment means for absorbing and dissipating a portion of the energy when the light-source capsule bursts, the containment means comprising a generally cylindrical light-transmissive shroud which is spaced no more than about three millimeters from the bulb portion of the light-source capsule, a light-transmissive outer envelope enclosing the light-source capsule and the shroud, and means for coupling electrical energy to the light-source capsule. The maximum dimension of each press seal perpendicular to a longitudinal axis of the arc tube is not substantially greater than the maximum dimension of the bulb portion perpendicular to the arc tube axis. The light-source capsule is typically a metal halide are tube. The press seals preferably have an I-shaped cross section. The press seal configuration of the present invention provides improved containment and permits use of a shroud with a relatively small inside diameter and a relatively thick wall. The lamp assembly of the present invention provides improved containment performance in comparison with prior art lamp assemblies. In addition, the lamp assembly of the present invention reduces the possibility that the arc tube will burst when the arc tube material expands or bulges, since the bulge contacts the shroud, and the arc tube fails in a passive mode. Brief Description of the Drawings For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the accompanying drawings which are incorporated herein by reference and in which: FIG. 1 is a perspective view of a lamp assembly in accordance with a preferred embodiment of the invention; FIG. 2 is an enlarged, partial cross-sectional view showing the arc tube, shroud and mounting arrangement, taken along the line 2-2 of FIG. 1; and FIG. 3 is a cross-sectional view showing the arc tube and shroud, taken along the line 3-3 of FIG. 2. Detailed Description of the Invention An electric lamp assembly 10 in accordance with a preferred embodiment of the present invention is shown in FIGS. 1-3. The lamp assembly 10 includes an outer envelope 12 and an arc tube, or light-source capsule 14, mounted within outer envelope 12 by a mounting means 16. The arc tube 14 is positioned within a shroud 20. The shroud 20 is supported in the lamp assembly 10 by the mounting means 16. Electrical energy is coupled to arc tube 14 through a base 22, a stem 24 and electrical leads 26 and 28. Outer envelope 12 is typically formed from blow-molded hard glass having a thickness in the range of 0.64 mm (0.025 inch) to 1.52 mm (0.060 inch). The lamp capsule 14 can be a metal halide arc tube, a tungsten halogen incandescent capsule or any other light-source capsule which is advantageously operated within a shroud. The shroud 20 comprises a cylindrical tube of light-transmissive, heat resistant material such as quartz. The mounting means 16 mechanically supports both the light-source capsule 14 and the shroud 20 within outer envelope 12. The mounting means 16 secures light-source capsule 14 and shroud 20 in fixed positions so that they cannot move axially or laterally relative to the remainder of the assembly during shipping and handling or during operation. The mounting means 16 includes a metal support rod 30 attached to stem 24 by a strap 31 and attached to a dimple 32 in the upper end of the outer envelope 12. The support rod 30 in its central portion is parallel to a central axis 34 of light-source capsule 14 and shroud 20. The mounting means 16 further includes an upper clip 40 and a lower clip 42 which secure both light-source capsule 14 and shroud 20 to support rod 30. Preferred embodiments of the clips 40 and 42 are disclosed in application Serial No. 07/539,753, filed June 19, 1990 and assigned to the assignee of the present application, which application is hereby incorporated by reference. The lamp assembly shown in FIGS. 1-3 has a configuration in accordance with the invention which has been found to provide improved containment performance in comparison with conventional lamp assemblies and which provides other advantages as described below. The light-source capsule or arc tube 14 includes a bulb portion 50 and press seals 52 and 54. The bulb portion 50 encloses a sealed discharge region which contains a suitable fill material for maintaining an arc discharge. Electrodes 56 and 58 are positioned at opposite ends of the discharge region. The press seals 52 and 54 are located at opposite ends of the bulb portion 50 and provide sealed electrical feedthroughs from leads 26 and 28 to electrodes 58 and 56, respectively. In the lamp assembly 10 of the present invention, the arc tube 14 is fabricated such that a maximum dimension D1 of each press seal 52, 54 in a direction perpendicular to arc tube axis 34 is not substantially greater than a maximum dimension D2 of bulb portion 50 perpendicular to arc tube axis 34. The press seals 52 and 54 are preferably formed using a four-jawed press tool. With reference to FIG. 3, the press tool includes a first pair of jaws, shown schematically at 60 and 61, for pressing the sides of the heated arc tube 14 together in a first press seal step. The arc tube is then pressed in a second press seal step by a second pair of press jaws, shown schematically at 62 and 63, which press the arc tube 14 in a direction perpendicular to the first press seal step. The press jaws 62 and 63 control the dimension D1 of each of the press seals 52 and 54. As noted above, the dimension D1 of press seal 52 should not be substantially greater than the dimension D2 of bulb portion 50. The press jaws 60-63 are configured to provide a press seals 52 and 54 having cross sections in a plane perpendicular to axis 34 which are generally I-shaped, as shown in FIG. 3. In practice, one pair of press jaws may be utilized for the first and second press seal steps. In this case, the arc tube 14 is rotated by 90° after the first press seal step, with the travel of the press jaws suitably controlled. Techniques for forming press seals in quartz arc tubes are well known in the art. In accordance with another feature of the invention, a spacing 70 between the outside surface of bulb portion 50 and the inside surface of shroud 20 is limited to no more than about 3 millimeters. The spacing 70 is measured at the widest part of bulb portion 50 away from the arc tube tipoff 72. In prior art lamp assemblies, the spacing 70 between the bulb portion 50 and the shroud 20 was typically about 5 mm. It would normally be expected that containment performance would be degraded by positioning the shroud 20 closer to the bulb portion 50 of the arc tube 14, since fragments of a burst arc tube have higher velocities closer to the arc tube. However, it has unexpectedly been found that this configuration provides improved performance, as described below. It can be seen that press seals 52 and 54, having controlled dimensions as described above, permit the shroud 20 to be located closer to the bulb portion 50 than in prior art lamp assemblies wherein the width of the press seal was substantially wider than the bulb portion. However, it has been found that the press seal configuration described herein provides improved containment performance independently of alterations to the shroud and shroud spacing. A further advantage of the controlled width press seals and the reduced spacing between the arc tube and the shroud is that the wall thickness of the shroud 20 can be increased, thereby further improving containment performance. In prior art lamp assemblies with relatively wide press seals and a relatively large spacing between the arc tube and the shroud, it was necessary to limit the shroud wall thickness in order to permit the assembly to be installed through the neck of outer envelope 12. In summary, the lamp assembly of the present invention includes an arc tube having press seals with a dimension D1 which is not substantially greater than the maximum dimension D2 of the bulb portion of the arc tube. The press seals 52 and 54 have a generally I-shaped cross section. The spacing 70 between the outside surface of bulb portion 50 and the inside surface of shroud 20 is not greater than about 3 millimeters. Preferably, the spacing 70 is not less than about 1 millimeter. The shroud 20 preferably has a wall thickness in the range of about 2.5 to 3.0 millimeters. In a preferred embodiment of a 100 watt metal halide lamp, the dimension D1 of the press seals is about 12.5 mm, and the dimension D2 of the bulb portion is about 12.0 mm. Spacing 70 is about 2.5 millimeters, and the wall thickness of shroud 20 is about 2.5 millimeters. A number of conventional lamp assemblies and lamp assemblies in accordance with the present invention were tested for containment performance. The results are summarized in Table 1. In each case, at least 25 lamps were tested. The arc tube was caused to fail by charging a 30 microfarad capacitor connected to the arc tube to between 1500 and 2000 volts. The containment criteria of Underwriters Laboratories Standard UL 1572, Section 57A.9 was used. Arc Tube Shroud Contained Within Outer Envelope Arc Tube - Shroud Spacing Standard 20 mm x 24 mm 55.0% 4.0 mm Standard 20 mm x 24 mm 55.0% 4.0 mm Four Jaw 20 mm x 22 mm 77.3% 4.0 mm Four Jaw 18 mm x 22 mm 87.0% 3.0 mm Four Jaw 18 mm x 22 mm 100.0% 3.0 mm Silicon Coated Bulb Four Jaw 17 mm x 22 mm 100.0% 2.5 mm Table 1 tabulates the results of experiments leading to the present invention. The shroud dimensions indicate the inside and outside diameters, respectively. The prior art lamps had a standard arc tube and a 20mm x 22mm shroud. The first and second trials in Table 1 were tested with the standard arc tube and a shroud with a 2mm wall thickness in place of a shroud with a 1mm wall thickness. The third trial used a four jaw press arc tube with the standard shroud having a 1 mm wall thickness. Since containment improvements were observed in the third trial, further studies were conducted with thicker wall shrouds, as indicated by trials four through six. In the fifth trial in Table 1, the outer jacket was coated with silicon rubber. It is seen from Table 1 that containment performance improved significantly for all embodiments of the present invention that were tested in comparison with prior art lamp assemblies. Another unexpected result of the reduced spacing between the arc tube outer diameter and the shroud inner diameter relates to a failure mode when the lamp assemblies are operated in a horizontal position. Lamp assemblies operated in a horizontal position sometimes result in the arc tube expanding or bulging due to high wall loading. The upper portion of the arc tube expands, thereby weakening the quartz to the point where it eventually bursts. It will be understood that the mode of failure wherein the arc tube bulges in a localized region and eventually bursts occurs more frequently in the horizontal position, but can also occur when the lamp is operated in a vertical position. When the spacing between the arc tube outside diameter and the shroud inside diameter is reduced as described above, the lamp assembly has been found to fail without bursting. When the arc tube expands but before the quartz is sufficiently weakened to burst, the arc tube contacts the shroud in the region of the bulge. The contact causes the arc tube to crack and to fail in a passive mode without bursting. The lamp configuration shown and described herein is considered most useful for metal halide arc lamps in a wattage range between about 75 and 400 watts, but is not limited to such a wattage range. It is believed that the change in failure mode wherein a bulge in the arc tube contacts the shroud and thereby produces a passive failure is applicable to any lamp wattage. 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: A lamp assembly comprising: a light-transmissive outer envelope (12); an arc tube (14) mounted within said outer envelope, said arc tube including a bulb portion (50) that encloses a discharge region and press seals (52,54) at opposite ends of the bulb portion; a generally cylindrical light-transmissive shroud (20) having an inside surface disposed around said arc tube; and means (56,58) for coupling electrical energy to said arc tube, characterised in that the maximum dimension (D1) of each seal in a direction perpendicular to the axis (34) of the arc tube is not substantially greater than a maximum dimension (D2) of the bulb portion, and in that the inside surface of said shroud (20) is spaced no more than about three millimetres from an outside surface of the bulb portion of said arc tube. A lamp assembly as claimed in claim 1, characterised in that said seals (52,54) each have a generally I-shaped cross section. A lamp assembly as claimed in claim 1 or 2, characterised in that said shroud (20) has a wall thickness in the range of about 2.5 mm to 3.0 mm. A lamp assembly as claimed in claim 1, 2 or 3, characterised in that said arc tube (14) comprises a metal halide arc tube.

Inventors: NORTRUP EDWARD H, PARROTT RICHARD A, NORTRUP, EDWARD H., PARROTT, RICHARD A.
Applicants: GTE PROD CORP, GTE PRODUCTS CORPORATION
Application year: 1991
Main classification: H01J61
IPC codes: H01J61