INCANDESCENT LAMP AND LAMP MOUNTING ASSEMBLY HAVING FILAMENT

SAG-LIMITING SAFETY FEATURE

This invention relates to an incandescent lamp and lamp mounting assembly having a filament sag-limiting feature, and more particularly relates to such a lamp and lamp mounting assembly in which the sag-limiting feature induces a passive end-of-life of the lamp.

Incandescent lamps with tungsten halogen filament capsules are specially designed to exhibit unique traits over standard incandescent bulbs, characterized by superior white light output resulting from higher filament temperatures enabled by the well-known tungsten halogen wall maintenance cycle. In this cycle, tungsten particles evaporate from the incandescent filament and are transported toward the cooler envelope wall, where some of the tungsten is deposited on the wall. If the wall of the capsule is above 250 degrees centigrade, the halogen combines with tungsten on the wall and evaporate into the circulating hot gas. When the circulating tungsten halide molecules approach sufficiently close to the much hotter incandescent filament (e.g., 2500 degree centigrade), they decompose to tungsten metal and halogen. The tungsten is randomly re-deposited back onto the filament, and the halogen is again available for the composition/decomposition process. The tungsten halogen regenerative cycle reduces the build-up of a darkening film on the wall, thus extending lumen output, while the random re-depositing of the tungsten molecules back onto the filament significantly extends the life of the filament. Nevertheless, some parts of the filament grow thinner than others over time, and eventually the filament fails, ending the life of the lamp.

In U. S. patent 6,639,364 Bl, issued to Joseph P. Woods et al, and assigned to Koninklijke Philips Electronics N.V., at least one end of the primary coil of the coiled coil tungsten filament of a hard-glass tungsten halogen incandescent lamp capsule is stretched out and extended into the adjacent pinch seal of the capsule, resulting in passive extinction of the electric arc occuring at end-of-life. The passive arc extinction occurs when the electric arc circuit is interrupted by the filament fuse extending into the pinch seal.

While such passive extinction is a significant safety feature, enabling, e.g., the use of thin-glass outer envelopes with hard-glass capsules, there is a need for further measures to assure passive extinction, such as the prevention of contact of a sagging filament with the wall of the capsule, which could lead to non-passive failure due to thermal shock.

While tungsten wire materials and processes have been developed to compensate for, reduce or eliminate coil sag over time, there is a corresponding trend to higher coil operating temperatures to further improve the quality of the lumen output. Such higher coil temperatures and/or longer times promote creep due to higher crystal mobility within a coil structure, leading to increased coil sag during lamp operation. Other factors that contribute to coil distortion/ sag are mechanical shocks that occurr during use or while the coil is hot, rough handling, and/or transportation.

Accordingly, an object of the invention is to provide incandescent lamps having a filament sag-limiting feature which passively induces end-of-life. The sag-limiting feature includes an electrically conducting restrainer which surrounds a central portion of the filament and is electrically connected to one of a pair of electrically conducting filament supporting or mounting elements, and preferably also includes a fuse between one end of the filament and the other supporting element. The fuse may be in the form of a stretched portion of the filament or filament coil, which stretched portion fails upon sag-induced contact between the filament and the restrainer. The sag-limiting feature provides an additional measure of safety over prior lamp designs which may fail non passively by coil sag and thermal shock.

According to one aspect of the invention, a mounting assembly for an incandescent lamp is provided, the mounting assembly comprising: an incandescent filament; first and second electrically conducting mounting elements connected to opposing ends of the filament; an internal fuse interconnecting one of the opposing ends of the filament and the first mounting element; and an electrically conducting restrainer connected to the second mounting element and surrounding a central portion of filament.

The restrainer is preferably ring-shaped, and is connected to the mounting element at a point adjacent to a central portion of the filament, e.g., by a spot weld. The fuse is preferably a portion of the filament or filament coil having a reduced cross-section, achieved, e.g., by stretching.

The restrainer may also be in the form of two or more rings spaced along the central portion of the filament, or a helical coil or transparent screen or grid. The mounting

assembly may also include a second internal fuse interconnecting the filament with the second mounting element.

The invention also includes a halogen lamp capsule an incandescent lamp having a light-transmissive envelope with a gas-tight enclosure, and having the mounting assembly with the above-described sag-limiting feature mounted within the enclosure. Such a lamp may include one or more external fuses arranged, e.g., in the stem and/or base of the lamp.

According to a preferred embodiment of the invention, the mounting assembly is mounted in a halogen filament capsule, such as a tungsten halogen filament capsule, and the halogen filament capsule is surrounded by an outer envelope.

According to another embodiment of the invention, a halogen filament capsule is provided, the lamp capsule comprising: a gas-tight enclosure defined by a light- transmissive envelope; the enclosure containing a halogen gas and an incandescent filament; the filament supported by a long electrically conducting supporting element and a short electrically conducting supporting element; wherein an electrically conducting preferably ring-shaped restrainer connected to the long supporting element surrounds a central portion of filament.

According to a preferred embodiment, the filament has a central barrel portion and leg portions connected to the long and short supporting elements, respectively, and one of the leg portions has a modified portion having a reduced linear wire density.

According to another preferred embodiment, the envelope has a pinch seal at one end of the enclosure, and the modified leg portion is enclosed within the pinch seal.

According to a still further preferred embodiment, the filament has a coiled coil configuration of a primary coil and a secondary coil, and the modified portion of leg comprises a stretched portion of the primary coil of the filament.

According to yet another preferred embodiment, the envelope comprises hard glass, e.g., alumino-silicate glass, the halogen gas comprises bromine, the filament comprises tungsten, and the long and short supporting elements and the restrainer comprise molybdenum, tungsten, tantalum or their alloys and similar electrically conducting refractory lamp grade materials.

According to another preferred embodiment, the restrainer comprises a getter material such as a tungsten/tantalum alloy, and thus functions as a getter as well as a restrainer, eliminating the need for a separate getter in the filament capsule. Alternatively, a

separate getter may be provided in a known form such as a gas, a strip, and/or a slurry deposit or the like.

According to another aspect of the invention, a halogen lamp capsule is provided comprising: a gas-tight enclosure defined by a light-transmissive envelope containing a halogen gas and an incandescent filament supported by a long electrically conducting supporting element and a short electrically conducting supporting element; wherein an electrically conducting restrainer connected to the long supporting element surrounds a central portion of filament; and wherein filament comprises a central portion and leg portions connected to the long and short supporting elements, respectively; and wherein the leg portion connected to the short supporting element has a modified portion having a reduced linear wire density, which modified leg portion is enclosed within the enclosure.

The invention is useful in a variety of lighting applications, but is particularly useful in applications requiring a high degree of safety and reliability. These and other aspects of the invention will be further elucidated with reference to the Figures, in which:

Fig. 1 shows one embodiment of a halogen incandescent filament capsule of the prior art;

Fig. 2 shows a halogen incandescent filament capsule incorporating one embodiment of the sag-induced passive life-ending feature of the invention;

Fig. 3 shows a halogen incandescent filament capsule incorporating another embodiment of the sag-induced passive life-ending feature of the invention;

Fig. 4 shows one embodiment of an incandescent lamp mounting assembly of the invention having a ring-shaped restrainer; Fig. 5 shows another embodiment of an incandescent lamp mounting assembly of the invention having a second internal fuse;

Fig. 6 shows another embodiment of an incandescent lamp mounting assembly of the invention having two ring-shaped restrainers;

Fig. 7 shows another embodiment of an incandescent lamp mounting assembly of the invention having a helical restrainer;

Fig. 8 shows another embodiment of an incandescent lamp mounting assembly of the invention having a mesh restrainer;

Fig. 9 shows one embodiment of a halogen incandescent lamp of the invention incorporating a filament capsule similar to the embodiment of Fig. 2; and

Fig. 10 is a detailed view of the prior art lamp of Fig. 1, showing a filament having a coiled coil configuration. The Figures are diagrammatic and not drawn to scale. The same reference numbers in different Figures refer to like parts.

Referring now to Fig. 1, there is shown a conventional halogen incandescent lamp capsule Jj) of the prior art, being of the single-ended hard-glass type having a sealed enclosure 11 defined by a light-transmissive hard-glass (e.g., alumino silicate) envelope 12 having a gas-tight pinch seal 14 at one end. Contained within enclosure 11 is a fill of halogen gas and a tungsten filament in the form of a wire coil 16 having a central barrel portion 17 and legs 18 and 20. Coil 16 is supported within enclosure 11 by long (22) and short (24) electrically conducting support elements (e.g., molybdenum), which at their proximal ends are clamped to legs 18 and 20 by clamping loops 26 and 28. A portion of leg 20 extends into pinch seal 14, where its distal end is clamped to the proximal end of short support element 24. The distal ends of long and short support elements 22 and 24 extend through pinch seal 14 to provide external connection to an electrical power source.

Coil 16 is typically of the coiled coil type having a primary coil and a secondary coil. In accordance with the teachings of the above-referenced U.S. patent 6,639,364, the entire specification of which is incorporated herein by reference, the primary coil of leg 18 is modified, e.g., stretched so that portion 21 is straight or has an increased pitch. The thus modified portion 21 reduces the extinction time and electric arc energy at end- o f- life due to reduced linear wire density near the pinch seal 14.

While the above prior art arrangement provides a means for safely ending the life of a tungsten halogen lamp in the event of filament burn out, a further concern is non- passive failure of the capsule due to sagging of the coil 16 to an extent which results in contact of the coil 16 with the envelope 12. In accordance with the teachings of the invention, a further safety feature is provided which triggers a passive end-of-life before such sag- induced contact can occur. This safety feature is embodied in an electrically conducting restrainer which surrounds the central or barrel portion 17 of coil 16, and is electrically connected to the long support element 22, so that sag-induced contact of the coil 16 with the restrainer causes an electrical short, fusing the stretched portion 21 of coil leg 20.

Fig. 2 shows such an arrangement in accordance with one embodiment of the invention, which is a single-ended hard-glass type halogen incandescent lamp capsule 30, similar in all respects to capsule Jj) of Fig. 1, except that a sag-limiting ring-shaped restrainer is provided in the form of an electrically conducting ring 32 surrounding the central or barrel portion 17 of coil 16. Ring 32 is electrically connected to long support element 22, so that any sag-induced contact between coil 16 and ring 32 will cause a life-ending fusing in the leg 20 of coil 16.

While it is preferred to have a portion 21 of leg 20 contained within the pinch seal 14, such is not necessary to achieve the desired sag-induced passive end-of-life of the capsule. Fig. 3 shows another embodiment of the invention, which is another single-ended hard-glass type halogen incandescent lamp capsule 40, similar in all respects to capsule 30 of Fig. 2, except that the modified leg portion 21 is contained entirely within enclosure 11, instead of pinch seal 14.

Fig. 4 is a schematic diagram of another embodiment of the invention, a coil mounting assembly 50 for an incandescent lamp, in which the modified leg of the previous embodiments is replaced by an internal fuse 52 which electrically interconnects one end of an incandescent filament 54 and short mounting element 56. Internal fuse 52 may be an extension of filament 54 having a reduced cross section produced, e.g., by stretching. The other end of filament 54 is connected to a proximal end of long mounting element 58 via a connecting element 60, while restrainer ring 62 surrounds a central portion of filament 54 and is electrically connected (e.g., spot welded) at an adjacent point 58A of long mounting element 58. Mounting elements 56 and 58 may be supported in a stem such as a conventional stem containing one or more external fuses (not shown) which fuses provide an additional back-up safety feature by causing end-of-life in the unlikely event that the internal fuse 52 does not fail.

Fig. 5 is a schematic diagram similar to Fig. 4, showing another embodiment 70 of the invention in which a second internal fuse 72 is provided. As in the case of internal fuse 52, fuse 72 may comprise an extension of filament 54 having a reduced cross section produced, e.g., by stretching. Fig. 6 is a schematic diagram similar to Fig. 4, showing another embodiment

80 of the invention in which the restrainer comprises a pair of rings 82 and 84, spaced apart along the length of filament 54.

Fig. 7 is a schematic diagram similar to Fig. 4, showing another embodiment 90 of the invention in which the restrainer comprises a helical coil 92, surrounding filament 54.

Fig. 8 is a schematic diagram similar to Fig. 4, showing another embodiment 100 of the invention in which the restrainer comprises a transparent mesh 102, surrounding filament 54, and attached to lead 58 by spot welds at points 104, 106 and 108.

Fig. 9 shows one embodiment of a halogen incandescent lamp 200 of the invention, including tungsten halogen filament capsule 202 enclosing a cavity 204 and having a pinch seal 206. A filament 208 having a central portion 210 and legs 212 and 214, is suspended within the cavity 204 by electrically conducting support elements 216 and 218, and ring-shaped restrainer 220, electrically connected to support element 216, surrounds central portion 210 of filament 208. A stretched portion 222 of leg 212 extends into pinch seal 206. A pair of electrically conducting mounting elements 224 and 226, composed, e.g., of NiFe, are electrically connected to support elements 216 and 218, and extend out of the capsule 202 via the pinch seal 206. Capsule 202 is enclosed within an outer glass envelope 228, having a gas-tight enclosure 229, a stem 230 and a base 232. A pair of mounting leads 234 and 236 are electrically connected to the base 232 and extend through stem 230 into the interior of envelope 228, and are electrically connected to mounting elements 224 and 226, respectively. Leads 234 and 236 have external fuse portions 240 and 242 located in stem 230. Fig. 10 is a detailed view of a portion of the prior art lamp capsule (Jj)) of Fig.

1, in which the filament (16) comprises tungsten wire (17) having a coiled coil configuration including a primary coil (27) and a secondary coil (29), the secondary coil forming the central or barrel portion (17) of the filament. Leg (20) is formed by stretching out one end of the primary coil (27) of the tungsten wire (17). The invention has necessarily been described in terms of a limited number of embodiments. From this description, other embodiments and variations of embodiments will become apparent to those skilled in the art, and are intended to be fully encompassed within the scope of the invention and the appended claims. For example, while the invention has mainly been described in terms of tungsten halogen incandescent capsules, it will be appreciated that the benefits of the sag-limiting safety feature can be obtained in any incandescent lamp structure in which uncontrolled sag of the incandescent filament could result in non-passive failure of the lamp. Moreover, while the restrainer has been described in terms of a limited number of shapes, any electrically conducting structure which serves to

limit the extent of a sagging filament falls within the meaning of the term 'restrainer' as used herein.