Incandescent lamp with improved filament-getter connection
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to electric incandescent lamps, and more particularly, to improvements in the fixation of a metal getter element to an incandescent filament. 
     2. Description of the Prior Art 
     Despite all precautions taken during the manufacture of incandescent lamps, small amounts of impurities are trapped within the lamp envelope after it is sealed. The impurities may remain on the filament, on the inner wall of the lamp envelope, or in residual gases within the envelope. After sealing, incandescent lamps are typically &#34;flashed&#34; by passing a current through the filament, to crystallize and set the filament. Without appropriate measures, the impurities will react with the hot filament and form vapors which condense on the coolest portions of the lamp envelope and form a black deposit. This deposit is not only cosmetically unsightly, but reduces lamp efficacy. Accordingly, it is common to provide a getter within the lamp envelope to clean-up, or absorb, any impurities which remain after sealing. 
     There are different types of getters for different types of incandescent lamps. For the typical incandescent lamp which is evacuated, the getter consists of a phosphate material deposited on the filament. When the filament is initially &#34;flashed&#34; the phosphate material deposited thereon is heated and reacts with, or cleans-up, the impurities within the lamp envelope. For evacuated lamps, the amount of impurities is typically small and the phosphate getter is sufficient to adequately clean-up the impurities during flashing, so that no further gettering over lamp life is needed. 
     Some incandescent lamps are not evacuated but contain a gaseous fill of argon at a pressure of about one atmosphere. With such a fill, the typical phosphate getter used for evacuated lamps is not sufficient to clean up all of the impurities within the lamp. Instead, a getter element consisting of a piece of metal which will clean-up impurities when heated by the filament is secured within the lamp envelope to continuously getter impurities over lamp life. Suitable metals include tantalum, as known from U.S. Pat. No. 3,317,264, and in the case of halogen lamps, copper, as known from U.S. Pat. No. 4,451,760 (Griffin et al). 
     Some non-evacuated lamps include filament support structure to which the metal getter element can be secured. Other lamps, however, such as radiant heat lamps, have an elongate filament extending between opposing sealed ends of a tubular envelope. In these lamps, there is no frame structure, and the metal getter element must be secured directly to the filament. 
     U.S. Pat. No. 3,317,264 (Martin et al.) disclose such a lamp in which the getter element is a disc which also supports the filament on the lamp axis. The disc includes a slot that receives the filament and has a thickness selected such that adjacent coil turns of the filament are spread apart and clamp the disc therebetween. Instead of the many discs used in Martin, many commercial lamps employ only one or two of the getter discs and support the filament on the axis with conventional spiral wound wire supports. It has been found, however, that for many combinations of filament pitch and wire diameter, the discs are not adequately secured on the filament, but fall off during the lamp manufacturing process, in shipment, or during use if subject to vibration. 
     If the getter falls off the filament during or after sealing, the lamp must be scrapped by the manufacturer because the getter will not function unless it is secured to and heated sufficiently by the filament. If the disc falls off after the lamp leaves the factory, the lamp will have a reduced efficiency and a shortened life. It has been found that even if the discs include tooth-like burrs as shown in U.S. Pat. No. 2,980,820 (Brundige et al.), or are manually bent against the coil turns by peening, the scrap rate is still unacceptable. Furthermore, the additional step of peening is labor intensive and stretches the filament coil, forming a cold spot which adversely impacts filament life. 
     Additionally, the outer edge of the discs are disposed against or very near the inner wall of the lamp envelope. It has been found that reaction of these tantalum getter discs with the lamp impurities upon initial flashing of the filament forms dark rings on the lamp envelope adjacent these edges, which further blacken over lamp life. The dark circles are not cosmetically attractive. 
     Accordingly, it is the object of the invention to provide an improved metal getter element which reduces lamp cost, through labor savings in installation on the filament and by reducing the scrap rate through better fixation of the getter on the filament, and which reduces unsightly blackening of the lamp envelope. 
     SUMMARY OF THE INVENTION 
     According to the invention, the above objects are accomplished in that the getter comprises a metal strip having a first portion extending across the filament with a closed, circumferential bounding edge which defines a closed aperture through which the filament extends and retaining means integral with the strip and biased against the filament for retaining the bounding edge of the aperture between a pair of filament turns. The integral retaining means may be a tail portion of the strip having an end resiliently biased between an adjacent pair of filament turns. 
     Such a getter strip may easily be assembled on the filament by simply feeding the filament through the aperture in the strip. Because the closed bounding edge of the aperture completely encloses the filament, the getter strip cannot fall completely off the filament, in contrast to the disc-shaped getters of the prior art. The integral retaining means ensures in a simple fashion that the getter does not skip axially along the filament. 
     According to another embodiment of the invention, the strip includes first and second opposing portions each extending across said filament. Each of these portions includes a respective closed aperture through which the filament extends and whose bounding edges engage between a respective pair of coil turns. The retaining means may be comprised by the opposing portions being resiliently joined for biasing the bounding edge of each aperture, or strip face adjacent thereto, generally axially against a respective coil turn. The retaining means may additionally, or alternatively, include a tail portion extending from at least one of the opposing portions which is bent against the filament for biasing the bounding edge of its respective aperture generally transverse to the filament between adjacent turns. 
     An additional advantage of the lamp according to the invention is that the edges of the getter need not engage the inner wall of the lamp envelope to be secured in the lamp as with the disc shaped-getters of the above-mentioned U.S. Pat. No. 3,313,264. By selecting the width of the strip such that it is spaced from the inner wall of the lamp envelope, blackening of the lamp envelope along these edges is avoided upon flashing of the filament. The lamp then has a higher quality appearance. 
     These and other advantages and features of the invention will become apparent from the following drawings, detailed description, and claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an elevation of a tubular infrared heat lamp; 
     FIG. 2 is an enlarged perspective view of a getter strip according to a first embodiment; 
     FIG. 3 is an enlarged elevational side view, partly in cross-section showing the getter strip of FIG. 2 assembled on the filament; 
     FIG. 4 is an enlarged perspective view of a getter strip according to another embodiment; 
     FIG. 5 is an enlarged elevational side view, partly in cross-section, showing the getter strip of FIG. 4 assembled on the filament; and 
     FIG. 6 is an elevational side view, partly in cross-section, showing tails of each of the opposing portions bent against the filament according to a third embodiment of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The incandescent radiant heat lamp shown in FIG. 1 includes an elongate tubular lamp envelope 1 having opposing seals 2, 3 at each end thereof sealing the lamp envelope in a gas-tight manner. A coiled tungsten filament 4 extends longitudinally within the lamp envelope between seals 2, 3 and includes successive filament turns. Means for connecting the filament to a source of electric potential outside of the lamp envelope include conventional conductive feed-throughs 6 connected to each end of the filament and extending through the seals 2, 3 in a gas-tight manner. Conventional spiral-wound tungsten supports 5 support the filament on the axis of the lamp envelope. Insulative ceramic bases 7, 8 are provided on each of the seals 2, 3o The lamp includes a fill of argon at a pressure of about one atmosphere. To clean up impurities within the lamp envelope, a getter 10 comprised of a metal strip is secured to the filament 4. 
     FIG. 2 illustrates a getter according to a first embodiment of the invention. The getter strip includes a first portion 11 with a closed, circumferential bounding edge 12 which defines a closed aperture 13 and integral retaining means in the form of a tail portion 14. The tail portion forms a &#34;V&#34; shape with the first portion 11 about a bend 15. 
     The getter is secured on the filament before the envelope is sealed by passing the filament through the aperture 13 with the first portion 11 substantially transverse to the axis of the filament so that the tail portion 14 clears the filament turns. The bounding edge 12 is then inserted between a desired pair of filament turns and the tail portion 14 is bent resiliently towards the first portion and its end inserted between an adjacent pair of turns. The length of the tail portion and the spring force of the strip are selected such that the face 12a of the strip adjacent the bounding edge 12, or the bounding edge 12 itself, is biased against the respective filament turn while the end of tail portion 14 is biased against its respective turn (FIG. 3). This secures the getter 10 to the filament. 
     In the second embodiment shown in FIG. 4, the getter 20 is a V-shaped part having first and second opposing portions 26, 27, with respective closed apertures 21, 23 equally spaced about a bend line 25 in the middle of the strip. The apertures are defined by respective closed bounding edges 22, 24. The strip is easily stamped from strip stock. 
     The getter strip is assembled onto the filament simply by inserting the filament coil through both apertures 22 of the two opposing portions 26, 27 of the getter strip 20 (FIG. 5). The size of the apertures 21, 23 and the angle α between the opposing portions are selected such that the filament turns pass through the apertures when the ends of the opposing portions 26, 27 are biased towards each other a predetermined amount. After locating the strip on the filament with the bounding edges between respective pairs of coil turns, release of the biasing pressure causes the opposing portions 26, 27 to move outwardly and axially bias the bounding edges 22, 24, or the strip face adjacent thereto, against a respective coil turn with spring force. With a tantalum strip having a thickness of about 0.1 mm, there is sufficient force to reliably retain the strip. 
     With some combinations of filament diameter and filament wire diameter the filament coils may be so elastic that it is not possible to obtain a sufficient axial biasing force between the two opposing portions 26, 27 and the coil turns to reliably secure the getter on the filament. This may generally occur with filaments having diameter of less than about 1 mm. For such filaments, the getter may be reliably secured by bending one or both tail portions 28, 29 transversely against the filament according to a third embodiment shown in FIG. 6. Prior to assembly on the filament, the tail portions are pre-bent at the location of the apertures, for example along the dashed lines 30 in FIG. 4. After locating the edges of apertures 21, 23 between respective pairs of turns, the tail portions are manually bent against the filament so that the portion of the bounding edge opposite the tail portion is firmly wedged between its pair of coil turns. The length of filament between the apertures is slightly bent as a result of the generally transverse biasing force exerted by the tail portions on the filament. During assembly, the opposing portions 26, 27 may also be biased towards each other as discussed with respect to FIG. 5 so that the faces of these portions are also biased generally axially against the filament turns. 
     With the getter strip according to FIG. 6, a labor savings of about 25% for the step of the securing the getter to the filament was achieved as compared to disc-shaped getters which were manually bent by peening. In addition to the labor savings, a far superior fixation of the getter to the filament is achieved. Furthermore, it has been found that fixation of the getter according to the invention on the filament does not form cold spots as with the disc-shaped getters formed by peening. 
     The strip is dimensioned such that its side edges 19 which extend longitudinally along the lamp envelope are sufficiently spaced from the inner wall of the lamp envelope so that no blackening of the inner wall occurs upon flashing of the filament. In the lamp shown in FIG. 1, the inside diameter of the lamp envelope was about 7.5 mm and the width of the getter strip 10 was 6 mm, leaving a clearance of about 0.75 mm between each side edge 19 and the lamp envelope. With this clearance, no blackening of the lamp envelope was observed upon flashing of the filament, in contrast to the distinct circles formed with the prior art discs. 
     Those of ordinary skill in the art will appreciate that various modifications may be made to the lamp base without departing from the scope of the appended claims. For example, instead of being bent along a bend line to obtain a &#34;V&#34; shape, the bend may be curved to obtain a &#34;U&#34; shape. Alternatively, the strip may be bent along two bend lines between the apertures, resulting in a square shape. Furthermore, the strip may be longer and include repetitions of the basic shape.