Filament support for incandescent lamps

Abstract: An incandescent lamp (54) containing a coiled filament (28) within a vitreous envelope wherein the filament is supported at at least one point along its coil length by a refractory metal support wire (10) one end of which is welded to a coil (28) and the other end secured to the envelope wall by a glass bead (24). The support prevents filament sag during lamp operation and is especially useful with double-ended high intensity tungsten-halogen lamps having an elliptically shaped filament chamber with an infrared reflecting and visible light transmitting optical interference coating on the chamber.

Description: This invention relates to a filament support for an incandescent lamp. More particularly this invention relates to an incandescent lamp containing a coiled filament which is supported within a vitreous filament chamber by at least one refractory metal support wire one end of which is welded to the filament and the other end attached to the vitreous chamber. Various types of supports for supporting a filament in an incandescent lamp are well known to those skilled in the art. These supports are invariably made of refractory metal wire such as tungsten, molybdenum, tantalum and the like which can withstand the hot temperature (i.e., 3000°K) reached by the lamp filament without melting. Examples of typical prior art supports include wire loops or spirals adjacent the inner surface of the filament chamber and wrapped around the filament as is disclosed, for example, in U.S. Patents 3,392,299; 3,538,374; 3,736,455 and 3,784,865. In U.S. 4,613,787 and in 2,032,791 a tubular incandescent lamp is disclosed having a linear filament coil supported by wire supports having one end wrapped around the filament and the other end embedded in a glass or quartz rod secured within the lamp. U.S. 3,188,513 is similar except that one end of each support is connected to a longitudinal support wire by a globule of glass. All of these patents disclose supports which are used for supporting a filament in a tubular incandescent lamp and are not suitable for use with incandescent lamps wherein the entrance to the filament chamber is much smaller than the maximum diameter of the chamber. One example of such lamps are the well known double ended high intensity incandescent lamps having a spherical, elliptical or other shape filament chamber formed from a single piece of lamp tubing, wherein the maximum inner diameter of the filament chamber is greater than the inner diameter of the tubular portions which extend outwardly from each end of the filament chamber as is disclosed, for example, in U.S. 4,942,331. In manufacturing such lamps the filament support is inserted through one of the relatively small diameter tubular portions and into the larger filament chamber as part of the filament and foil seal assembly as is disclosed in U.S. 4,810,932 and 5,045,798. Filament sag is particularly serious in high voltage lamps of such construction and which use thin filament wire and it is possible for the filament to sag and touch the wall of the chamber shorting the filament and/or melting the vitreous chamber wall. Further, double ended halogen-incandescent lamps having an optical interference coating or filter on the surface of the filament chamber for transmitting visible light radiation and reflecting infrared radiation back to the filament require precise radial alignment of the longitudinal axis of the filament coincident with the optical axis of the filament chamber in order to achieve maximum conversion of the infrared radiation reflected by the coating back to the filament to visible light radiation which is transmitted by the filter (i.e., see R. S. Bergman, Halogen-IR Lamp Development: A System Approach, J. of the IES, p. 10-16, Summer, 1991). Thus, there has been a need for a filament support for such double ended lamps that will prevent sagging, precisely align the filament and which can be inserted through the small diameter tubing into the larger filament chamber without breaking the filament. US-A-4839559 discloses an electric incandescent lamp for selective radiant energy distribution which employs a reflective film for improved efficiency of the lamp operation. Said lamp further employs a tungsten filament extending the length of its elongated lamp envelope with particular support means for said incandescent filament also being disclosed to precisely position the filament along the principal lamp axis. Various structural configurations for said particular support means are also disclosed so that such filament position is maintained regardless of the lamp spatial orientation. EP-A-0373632 discloses a method of constructing a filament support member from a single-piece preformed refractory metal wire having at least one loop protrusion formed in the wire and positioned between the ends of the wire. A section of glass tubing is positioned over and supported by the loop protrusion with a head portion of the loop extending beyond the section of tubing. Heat is applied to melt the section of tubing to form an insulating bead at the base of said loop protrusion. The head portion of the loop protrusion is then trimmed to interrupt electrical continuity in the wire and thus electrically isolate the respective ends of the wire. GB-A-9615/1910 discloses an incandescent electric lamp having a divided straight filament in a glass tube and a support for the filament including a link seated to the glass tube. EP-A-0397422 discloses a filament support for incandescent lamps. The support is of refractory metal wire in the shape of a ring with a loop on each side. It radially aligns and provides electricity to a filament in e.g. a double ended tungsten halogen lamp. The support has a leg extending from the end of each loop, one of which is attached to the filament and the other forming part of an inlead assembly. According to the invention there is provided a filament assembly having the features recited in claim 1. In one embodiment a filament is supported within a vitreous envelope by a refractory metal wire support one end of which is welded to the filament and the other end attached to the envelope. The support may be attached to the vitreous envelope by a vitreous bead melted to the support and fused to the interior surface of the envelope. Thus, in an incandescent lamp according to an embodiment of the invention having a vitreous envelope which includes a filament supported within a filament chamber, one part of each filament support will be welded directly to one of the coils of a filament coil and another part embedded in a vitreous bead fused to the interior wall of the filament chamber. The support itself will generally be a wire made of a suitable refractory metal such as tungsten, molybdenum, tantalum, rhenium or alloys thereof. This invention is useful for supporting linear filament coils in double ended incandescent lamps, such as high intensity halogen-incandescent lamps, wherein the maximum diameter of the filament chamber is substantially greater than the diameter at the end or ends of the chamber. This invention has been found to be particularly useful for high intensity incandescent halogen lamps of the double ended type having an elliptical, spherical or other shaped filament chamber the opposite ends of which terminate in tubular portions of a diameter substantially smaller than that of the filament chamber and wherein a visible light transmitting and infrared reflecting coating is disposed on the surface of the filament chamber. Such lamps have been made according to the invention with the longitudinal axis of the filament concentric within a radial distance of 20% of the filament diameter. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 schematically illustrates a double ended incandescent lamp having a filament support according to the invention. Figure 2 illustrates an embodiment of the invention wherein a filament support wire is welded to a coil of a filament. Figures 3(a) and 3(b) schematically illustrate two different methods used to secure a vitreous bead to the end of a filament support. Figure 4(a) schematically illustrates a filament, molybdenum foil seal and inlead assembly having a filament support of the invention welded to the filament coil and Figure 4(b) illustrates the assembly positioned in a double ended quartz lamp envelope prior to flushing, filling and sealing the lamp and fusing the vitreous bead at the end of the filament support to the interior wall surface of the filament chamber. Figure 5 schematically illustrates an incandescent lamp mounted in an parabolic reflector and having a filament supported according to the invention. DETAILED DESCRIPTION Figure 1 schematically illustrates a double ended lamp 54 having a vitreous envelope of fused quartz which comprises filament chamber 49 and shrink sealed tubular end portions 56, 56'. The elliptical filament chamber 49 contains a coiled-coil linear filament 28. Both of the tubular end portions 56, 56' are shrink sealed over molybdenum foil members 40 and 40' to form a hermetic seal. Outer leads 42 and 42' extend past end portions 56 and 56'. Linear filament coil 28 is attached at each end to a respective filament alignment spud 34 and 34' of the type disclosed in U.S. Patent 4,942,331, and which will be explained in greater detail below. In the embodiment shown, linear filament coil 28 is a coiled-coil tungsten filament having two tungsten filament supports 10 and 10' each welded directly to a separate secondary coil of the filament by means of a molybdenum weld explained in greater detail below, with the other end of each support 10, 10' being securely anchored to the inner wall of filament chamber 48 by means of a vitreous bead 24, 24' which has been melted around the end of each filament support 10, 10' and fused to the inner surface of the filament chamber wall 48. Coating 50 is an optical interference filter disposed on the outer surface of filament chamber 48 and reflects infrared radiation emitted by the filament back to the filament and transmits visible light radiation. In a preferred embodiment, coating 50 is preferably made up of alternating layers of a low refractory index material such as silica and a high refractory index material such as tantala, titania, niobia and the like for selectively reflecting infrared radiation and transmitting visible light radiation. However, if desired, coating 50 may be designed to selectively reflect and transmit other portions of the electromagnetic spectrum. Such optical interference filters and their use as coatings for lamps are known to those skilled in the art and may be found, for example, in U.S. Patents 4,229,066 and 4,587,923. The interior of filament chamber 48 contains an inert gas such as argon, xenon, or krypton along with minor (i.e., <12%) amounts of nitrogen, and one or more halogen compounds such as methylbromide, dibromomethane, dichlorobromomethane and the like and, optionally, a getter. In a typical halogen incandescent or lamp of a type described above having an elliptical filament chamber whose dimensions are 10 mm OD and 22 mm long containing a coiled-coil tungsten filament 18 mm long, an unsupported 70 to 150 watt, 240 V filament coil will sag to the wall after about 24 hours of operation. This can result in melting of the wall and/or shorting and rupturing of the filament coil. This problem increases as the operating voltage of the lamp increases for a given size filament chamber and coil length, because as one increases the operating voltage of the lamp for a given operating wattage, the diameter of the filament wire is smaller than that for a lower voltage, while the overall length of the wire from which the filament coil is fabricated is greater. For example, for a typical lamp rated at 50 watts and 120 volts a 10 mm long coiled-coil tungsten filament will be made from tungsten wire having a diameter of about 48,3 µm (1.9 mils) and a total length of 600 mm. For the same type of lamp rated at 100 watts and 120 volts, the filament wire diameter will be about 76,2 µm (3 mils), but with a total wire length of about 800 mm. In marked contrast, a lamp rated at 100 watts and 245 volts will have a wire diameter of 48,3 µm (1.9 mils) and a total wire length of 1400 millimeters. Coiling this length of wire in the same manner as above gives a coil 18 mm long. Thus, going from 100 watts and 120 volts to 100 watts and 245 volts results in a total wire length increase of 75% and a decrease in the wire diameter by almost 40%. One can therefore appreciate why a higher voltage rated filament coil will tend to sag more than one rated for operation at a lower voltage. Hence a filament support is essential for the successful manufacture and operation of such high voltage lamps. Halogen-incandescent lamps were made of the type illustrated in Figure 1 and described above were fabricated from fused quartz lamp tubing and had an elliptical filament chamber 10 mm x 22 mm containing a coiled-coil tungsten filament 18 mm long radially aligned along the longitudinal optical axis of the elliptical filament chamber. That is, the longitudinal axis of the filament was substantially coincident with the longitudinal optical axis of the filament chamber. These lamps were made rated at 70 and 100 watts and operating voltage of 240 V. The filament in each lamp was supported by two filament supports each made of tungsten wire 127 µm (5 mils) diameter welded with molybdenum to a separate secondary coil turn at one end of the support and the other end of the support wire enclosed in a vitreous bead melted to that end and fused to the filament chamber inner wall. Turning now to Figure 2, refractory metal support assembly 10 is schematically illustrated as refractory metal wire 12 having a 127 µm (5 mil) diameter with support portion 14 being 4 mm long and terminating at one end in leg 16 which is about 0.5 mm long and offset at an angle of about 110° to the leg 14. This portion of the support assembly 10 is placed adjacent to a secondary coil turn of a linear tungsten filament coil which is schematically shown as a secondary coil 28 consisting of a multiple number of primary coils 30. The outer diameter of secondary coil 28 is 1.47 mm (58 mils) and the tungsten filament wire employed to make the coils has a diameter of 50.8 µm (2 mils). A plasma torch 18 having a tip ID of 812.8 µm (32 mils) is placed approximately 1 mm from the free end of leg 16 as shown in Figure 2 and molybdenum wire 26 being 127 µm (5 mils) in diameter is placed adjacent coil 28 at that point where leg 14 is adjacent to and touches the coil. An inert cover gas such as argon is discharged from the end of plasma torch 18 and preferably containing a small percentage (5%) of hydrogen to provide a reducing atmosphere and plasma torch 18 is energized for one hundred to four hundred milliseconds in time duration and at a current of 1½ to 3 amps to create a discharge between the plasma torch and free end of tip 16 of tungsten support wire 12. This causes leg 16 to melt and form a ball at that end of leg 14 and, at the same time, provides enough heat to melt a small portion (i.e., 1-2 mm) of a lower melting refractory metal such as molybdenum wire 26 which melts and welds leg 14 to coil 28. The molybdenum wire used as welding material is brought into contact with the coil and support before the plasma torch is energized. In this example, which is an actual example used in making a lamp of the invention, between two and eight primary coils are shorted due to the molybdenum filling the space in between these coils by capillary action. This results in welding filament support assembly 10 directly to one of the secondary filament coil turns. By increasing the energy pulses from the plasma torch, the tungsten filament supports have been welded to secondary coil turns without the use of a lower melting refractory metal such as molybdenum. Similar supports made of molybdenum wire have also been brazed by melting the molybdenum directly onto the tungsten filament coil and have performed satisfactorily in lamps. Support wires of alloys of (i) tungsten and molybdenum and (ii) tungsten and rhenium have also been satisfactorily welded to tungsten filament coils. The other end of filament support assembly 10 terminates in leg 20 containing a bead 24 made of a high silica content vitreous material which can withstand the chemical environment in the lamp and the high operating temperatures and which preferably has a melting point lower than the wall of the filament chamber. One example is a sealing glass which melts at a temperature slightly lower than the temperature of the vitreous material forming the filament chamber wall of the lamp and which has a thermal coefficient of expansion as close to the filament chamber wall material as possible to avoid or minimize thermal mismatch. In the case of a filament chamber or lamp envelope made of fused quartz, bead 24 can also be and has been made of fused quartz, but a lower melting seal glass is preferable. One particular seal glass composition that has been found particularly efficacious for use with the present invention in lamps having fused quartz filament chambers contains 82.5% SiO 2 , 13.0% B 2 O 3 and 4.5% Al 2 O 3 . This material is available from GE Lighting in Willoughby, Ohio, as a GS-3 graded seal glass and has a softening point of about 1100°C as compared to the softening point of fused quartz which is about 1700°C. In the illustration shown, seal glass bead 24 is in the form of a cylinder having inner and outer diameters of 762 to 1016 and 177.8 to 330.2 µm (30 to 40 and 7 to 13 mils), respectively, and is 762 to 1524 µm (30 to 60 mils) long. The end of tungsten leg 20 has been melted by means of a plasma torch or TIG welder (not shown) to form tungsten ball 22 which merely serves to hold bead 24 in place and keep it from slipping off leg 20 until assembly of the lamp is complete. In all cases it has been found convenient to assemble the bead onto the support wire prior to welding the support wire to a filament coil. Figure 3 schematically illustrates two additional methods by which the vitreous bead may be secured at that end of the filament support which contacts and is ultimately fused to the interior wall surface of the filament chamber. In Figure 3(a) a plasma torch has been used to melt the end of a vitreous bead such as that shown in Figure 2 over the end of leg 12. In Figure 3(b) leg 20 terminates in leg 23 which is merely bent at roughly a right angle in order to secure vitreous bead 24 to the end of the filament support. In both of these figures, 17 represents a globule of metal as a result of the plasma welding and 31 represents two primary coil turns of a secondary filament coil filled with metal from the welding operation. Also, in some embodiments the longitudinal axis of the bead will be parallel to the longitudinal axis of the filament instead of perpendicular as illustrated in Figure 3(b). As previously stated, the filament support is be made of a suitable refractory metal such as tungsten, molybdenum, tantalum, rhenium, and alloys thereof and the like which can withstand the relatively hot temperatures of 3000°K or more reached by the lamp filament during operation of the lamp without melting the support. The embodiments previously described are actual embodiments from which lamps have been successfully made and are meant to be illustrative, but non-limiting. Thus, the filament may be a single coil filament, a double coil or coiled-coil filament, or a triple coil filament. As set forth above, tungsten filament support wires have also been welded directly to the filament coil using the procedure described above and as illustrated in Figure 2 wherein tungsten support wire leg 16 is welded directly to the filament coil. However, it is more difficult to do this with the present state of technology than to employ another refractory metal of lower melting temperature, such as molybdenum, as a welding metal. Alternately, the support wire has also been made of molybdenum in which case leg 16 will be somewhat longer (i.e., 1.5 mm) and will melt down into one or two primary turns of coil 28. Lamps have also been successfully made employing a molybdenum support using this technique. Also, lamps using support wires made of a 74% tungsten - 26% rhenium alloy have been successfully made. As used in the context of the invention, the term "welded" is meant to include all of these embodiments. Turning next to Figures 4(a) and 4(b), there is illustrated a filament-molybdenum foil inlead assembly 32 containing two refractory metal supports 10, 10' welded to one of the secondary turns of linear coiled-coil filament 28. Filament supports 10, 10' each contain a vitreous seal glass bead 24, 24' secured to the other end. Filament 28 is secured by welding at each end to an alignment spud 34, 34' made of a suitable refractory metal (such as tungsten or molybdenum) as is disclosed in U.S. 4,942,311 as having one or more turns 35 from which extend leads 36 and 36'. The other legs 38 and 38' of the refractory metal spuds are welded to one end of respective molybdenum foils 40 and 40' with outer leads 42 and 42' being connected to the other end of sealing foils 40 and 40'. Thus, the spuds also function as leads and end supports for the filament. Figure 4(b) illustrates assembly 32 after it has been inserted into an unsealed lamp assembly 52 which has been fabricated from fused quartz tubing as is disclosed, for example, in U.S. 4,810,932 and 5,045,798. In a typical example, an elliptical filament chamber whose dimensions are 10 mm OD and 22 mm long contain an 18 mm long coiled-coil tungsten filament 28. The inner diameter of tubular leg portions 46 is about 3 mm. Thus, it will be appreciated that the filament supports of the invention are able to be inserted through one of the tubular portions as part of the filament coil assembly and into the filament chamber without breaking the filament or support during this part of the lamp manufacturing process. After the filament is positioned in the filament chamber, the bead is melted onto the end of the support and fused to the interior surface of the filament chamber by laser fusion or heating with a torch. In the laser method, an infrared laser beam is pulsed on the bead through the fused quartz wall of the filament chamber which actually heats that portion of the support within the bead. In the torch method, two different approaches have been successfully used. In one approach the unfinished lamp containing the filament support and seal assembly is placed in a lamp lathe and rotated fast enough to insure that vitreous bead 24 is firmly adjacent the inner surface of filament chamber 48 and a torch is applied to the outside of the filament chamber in a position to heat that portion of the filament chamber sufficiently to partially melt and fuse bead 24 onto both filament support 10 and the interior wall surface of the filament chamber, thereby positively securing the filament support 10 to the wall of the filament chamber which, in turn, positively supports filament 28 with its linear axis substantially concentric with the linear optical axis of the filament chamber. In the other method, the lamp envelope is positioned so that the filament support or supports are vertical with the vitreous bead at the bottom adjacent the interior wall surface of the filament chamber. The torch is then applied to the outside surface of the chamber just under the bead to melt it to the chamber wall. After vitreous bead 24 has been secured to the interior surface of the filament chamber, the lamp assembly 52 is then evacuated, pumped, flushed and filled with the desired fill and sealed off by forming shrink seals 56, 56' (Figure 1) over molybdenum foil seals 40 and 40'. In some cases it has been found necessary to employ more than one filament support for the filament in such a lamp. Lamps according to this invention may be made with as many filament supports as are necessary and lamps have already been made with one and two filament supports securing the filament within the filament chamber. This also prevents the filament from sagging during operation of the lamp, particularly one that is rated at a high voltage. The support or supports also provide a more robust lamp in terms of vibration or shock sensitivity irrespective of whether the lamp operates at a low or high voltage and irrespective of whether or not an optical interference coating is on the filament chamber wall. This is an important benefit of the invention. Lamp 54 containing filament support 10 as illustrated in Figure 1 is shown assembled into a parabolic reflector lamp 60 illustrated in Figure 5. Thus, turning to Figure 5, reflector lamp 60 contains lamp 54 mounted into the bottom portion of parabolic glass reflector 62 by means of conductive mounting legs 64 and 66 which project through seals (not shown) at the bottom portion 72 of glass reflector 62. Lamp base 80 is crimped onto the bottom portion of the glass reflector by means not shown at neck portion 82. Screw base 84 is a standard screw base for screwing the completed assembly 60 into a suitable socket. Glass or plastic lens or cover 86 is attached or hermetically sealed by adhesive or other suitable means to the other end of reflector 62 to complete the lamp assembly. The foregoing have been illustrative, but non-limiting examples of the practice of the invention. As will be appreciated by those skilled in the art, other configurations and lamp applications may be practiced, including tubular lamps such as conventional heat lamps and single-ended lamps of the type illustrated, for example in U.S. 4,918,353, the choice being left to the practitioner. Those skilled in the art will also appreciate that the invention is also applicable to lamps made of glass, including lower silica content glass.

Claims: A filament assembly comprising a continuous coiled coil filament constructed as a coiled coil having a plurality of first coils (30) formed along a length of refractory metal and a plurality of larger second coils (28) formed by coiling said length of first coils (30), and at least one refractory metal support (10) intermediate the ends of the filament (28), characterized in that said support (10) is welded to a portion of said plurality of first coils (30) on one of said second coils (28) of said filament. The assembly of claim 1, wherein said filament comprises tungsten. The lamp or assembly of claim 1 or 2, wherein said support (10) comprises tungsten, molybdenum, tantalum, rhenium or alloys thereof. An incandescent lamp (52) comprising a light transmissive, vitreous envelope, said vitreous envelope having a filament chamber (48) formed therein enclosing a filament assembly according to any one of claims 1 to 3. The lamp of claim 4, wherein said filament support (10) is also secured to said envelope. The lamp of claim 5, wherein said filament support (10) is secured to said envelope by means of a vitreous bead (24). The lamp of claim 6, wherein said bead (24) is melted to said support (10) and fused to said envelope. The lamp of claim 6 or 7, wherein said bead (24) has a melting point lower than that of said vitreous envelope. The lamp of any one of claims 4 to 8, wherein said filament chamber (48) has two ends disposed apart from each other and wherein each of said ends terminates in an elongated tube (46) having a diameter substantially smaller than the maximum diameter of said filament chamber (48). The lamp of claim 9, being a double-ended halogen-incandescent lamp wherein said light-transmissive envelope has a generally elliptical shape and is coated with an infrared-reflecting and visible light-transmitting coating (50).

Inventors: AHIGREN FREDERICK FRANCIS, BERGMAN ROLF SVERRE, OLWERT RONALD JAMES, PRICE GARY LYNN, SCHINDLER DONALD ROY, SCOTT CURTIS EDWARD, WALSH LAVERNE E, AHIGREN, FREDERICK FRANCIS, BERGMAN, ROLF SVERRE, OLWERT, RONALD JAMES, PRICE, GARY LYNN, SCHINDLER, DONALD ROY, SCOTT, CURTIS EDWARD, WALSH, LAVERNE E.
Applicants: GEN ELECTRIC, GENERAL ELECTRIC COMPANY
Application year: 1993
Main classification: H01K1
IPC codes: H01K3, H01K1