GB2064215A - Improvements in lamp filament supports - Google Patents

Abstract

An incandescent filament, especially the fragile filament of a low- power linear tungsten-halogen lamp, is supported by a refractory rod 22, e.g. of quartz, extending alongside or parallel to the filament 15 and connected thereto by support wires 23, one end of each of which passes through a respective laser-drilled hole in the rod. The rod may be supported by wires 20 welded to refractory metal elements 17, between which the filament is also mounted, thus providing a convenient assembly for insertion into the tube 10 during manufacture of the lamp. <IMAGE>

Description

SPECIFICATION Improvements in lamp filament supports The present invention relates to support structures for filaments in incandescent lamps and other electrical devices, and more particularly to supports for the filaments in linear halogen cycle lamps.

Elongated filaments, whether coiled or otherwise, require support at points along their length. In so-called linear tungsten-halogen lamps this support has been provided by support wires of refractory metal, usually tungsten, having the form of spirals fitting loosely in the lamp tube and terminating in a coil which engages the filament coil. These are, however, unsatisfactory with finer filaments, such as are required for low power linear tungsten-halogen lamps, for example below 300 W, because the fineness of the filament makes it difficult to assemble without damage or distortion, and provides insufficient restraint against movement of the support wires.

British Patent Specification 1,035,454 describes an incandescent filament lamp of strip form in which the filament is supported by a glass rod constituting a spine, and filament support wires which at one end are anchored in the rod and at the other are 'pigtailed' about the filament.

Where the high operating temperature of the device requires the use of a refractory insulating support rod, for example of quartz, as in the case with a tungsten-halogen lamp, however, this technique is not applicable. We have found great difficulty in inserting tungsten wire into fused areas in a quartz rod because the great heat required causes embrittlement of the wire.

We have found, in accordance with this invention, that satisfactory filament support can be provided by a refractory insulating rod, such as quartz, extending alongside the filament and perforated at intervals, and a series of refractory support wires, one end of each of which passes through a respective perforation and is retained thereby, while the other end engages the filament.

We have further found that the very small diameter holes required in a quartz rod which is itself of relatively small diameter can be formed by the operation of a laser, preferably in the infra-red.

A suitable laser for this purpose is a carbon dioxide laser, which operates in the region of 10 microns.

At this wavelength, practically ali the energy is absorbed by the quartz to give intense localized heating, whereby the desired perforation is easily achieved.

desired perforation is easily achieved.

The support wires can be retained in the perforations in a number of ways. For example, the end of the wire may be bent back into hooked form to engage the outer surface of the rod alongside the perforation. This structure does, however, allow a degree of twisting movement of the wire which may be undesirable in devices where accurate positioning of the filament is essential.

Alternatively, the wire may be fixed in the perforation by fused material, for example by localized heating of the lateral surface of the rod accompanied by inwardly directed pressure, or by fusion of a protruding portion of the wire at the end of the perforation. The heating required by both these techniques can be provided by laser. A further alternative is to apply a cement or fusible grit to the end of the perforation over the end of the wire and to form this into a bead securing the wire.

The end of the wire engaging the filament should do so in a manner that will not damage the filament and will preferably facilitate assembly. It is at present preferred that the wire should terminate in a helical portion or coil of several turns loosely embracing the filament or filament coil. If the turns of the support coil are closely spaced, the turns of the coil cannot enter or mesh with those of the filament, and the possibility of damage to a fine filament coil, which exists with the open loops or 'pig-tails' of previous support structures, is avoided.

The invention also provides a filament assembly comprising a filament connected at its ends to respective refractory metal elements, a refractory sinsulating support rod also connected at its ends to the said elements and perforated at intervals along its length, and a plurality of filament support wires one end of each of which passes through a respective perforation in the rod and is retained thereby and the other end of which engages the filament. This assembly constitutes a unit which can be conveniently fabricated, and the filament tensioned, before being introduced into the envelope of the lamp or other device.

The invention will be further described in greater detail, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a side elevation view of a linear tungsten-halogen lamp incorporating the invention; Figure 2 is a detailed view on a larger scale of a portion of the support rod and a support wire in Figure 1 showing one technique of securing them together; Figure 3 is a cross-section along the line 2-2 in Figure 2; Figures 4 and 5 are views, corresponding to those of Figures 2 and 3, of another technique of securing the support wire; and Figures 6 and 7 are corresponding views of still another technique of securing a support wire.

The lamp shown in Figure 1 comprises a tubular envelope 10, for example of quartz, which in the finished lamp contains a gas fill 11 of inert gas and halogen vapour. The envelope is sealed at each end by a press seal region 12 and exhausted through a tubulation 1 3 which is thereafter sealed off and caps 14 are fitted and the generally "linear" coiled tungsten filament 1 5 is connected by leads 16, screwed or otherwise fitting into the ends of the coil, and respective foil and elements 17 (usually of molydenum) in the seal, to contacts 18 in the caps.

The filament is supported, in accordance with the invention, by a structure which comprises at each end of the lamp a refractory wire 20 welded to the foil 1 7 and extending parallel to the lead 1 6.

An inner end portion 21 of each wire is bent at right angles and is attached to a refractory, insulating support rod 22, for example of quartz, which extends along the lamp generally parallel to the filament 15. The wire ends 21 may be attached to the rod by the same technique as the filament support wires 23 to be described hereinafter. Alternatively, the ends 21 may terminate in helical coils which are a push fit on the rod 22.

At intervals along the lamp, the support wires 23 interconnect the filament and the support rod.

One end 24 of each wire passes through a respective hole drilled in the rod 22 and the other terminates in a helical coil 25 of several turns which loosely embraces the filament coil.

The end foils 17, leads 16, rod support wires 20, rod 22 and filament support wires 23 provide a substantially rigid asssembly in which the filament 1 5 can be supported and desirably tensioned before the assembly is inserted in to the envelope 10 during fabrication of the lamp. The filament can be introduced by lowering it through the coils 25 at the ends of each wire 23.

One manner in which the support wires 23 may be attached to the rod 22 is shown in Figures 2 and 3. The end 24 of the wire is bent into a hooked shape, which is passed through the hole 26, previously drilled in the rod.

A further example of the fixing of a support wire in a quartz rod is illustrated by Figures 4 and 5.

After the wire 23 has been inserted in the perforation 26 in the rod 22, the lateral surface of the rod is locally heated, preferably by a carbon dioxide laser, and the heated portion pressed inwardly to bear on the wire. The depression so formed is shown schematically at 27.

Yet another example of a means of fixing a support wire in a perforation in the rod. In this example, the protruding end of the wire is fused into a bead 28, for example by means of the laser, which secures the wire to the rod.

In one example of the manufacture of a low power tungsten-halogen lamp incorporating a support according to the invention, a quartz rod of 1.5mm diameter was subjected to the action of a Ferranti MF 400 carbon dioxide laser producing about 400 W at 10.6 microns, focussed onto a spotless than 0.25mm in diameter, and if possible down to 0.1 Omm. A hole of approximately 0.2mm diameter was produced in less than one second.

The hole may be flared at one end. The operation is repeated at desired intervals along the rod, for example every 12mm, including holes for attachment of the rod support wires 20.

The preferred form of the end 25 of the support wires 23 is, as shown in the drawings, a coil of from one to three times of the wire, preferably having a pitch equal to the wire diameter. Because there are only one to three turns, in which the filament lies loosely, the cooling effect on the filament is considerably less than with the normal method of supporting, where there is a much greater mass of heavier, tungsten wire wound between the turns. This reduced cooling effect lowers the temperature gradient along the filament, so reducing migration of tungsten from the hot to cool areas due to the various transport cycles within the lamp, and improving lamp life.

Although the invention has been particularly described in relation to a low-power linear tungsten-halogen lamp, the same principles can be applied to provide filament supports in other types of lamp or device, for example studio or projection lamps.

Claims (13)

1. An electric lamp or other device having a filament supported by a refractory insulating rod extending alongside the filament and perforated at intervals along its length, and a plurality of filament support wires one end of each of which passes through a respective perforation in the rod and is retained thereby and the other end of each of which engages the filament.

2. A lamp or device according to claim 1, wherein the filament is generally linear and the support rod extends generally parallel to the line of the filament.

3. A lamp or device according to claim 2 wherein the rod is connected at its ends to respective elements of refractory metal and the filament is connected between the said elements.

4. A lamp or device according to claim 1, 2 or 3, wherein the perforations in the rod are laserdrilled.

5. A lamp or device according to any of claims 1 to 4, wherein the rod is of quartz.

6. A lamp or device according to claim 4 or 5, wherein the perforations are laser-drilled by a carbon dioxide laser.

7. A lamp or device according to any of claims 1 to 6, having a tungsten filament and gas fill comprising inert gas and halogen whereby a tungsten-halogen regeneration cycle is established during operation.

8. A lamp according to claim 7 in the form of a linear tungsten-halogen lamp or power less than 300 watts.

9. A filament assembly for an incandescent lamp or other electrical device comprising a filament connected at its ends to respective refractory metal elements, a refractory insulating support rod also connected at its ends to the said elements and perforated at intervals along its length, and a plurality of filament support wires one end of each of which passes through a respective perforation in the rod and is retained thereby and the other end of which engages the filament.

10. A filament assembly according to claim 9, wherein the said other end of each support wire is formed as a helical coil loosely embracing the filament.

11. A filament assembly according to claim 9 or 10, wherein the support rod is connected to the refractory metal elements by two rod support wires, each of which passes through a respective perforation near the end of the rod and is retained thereby.

12. A lamp having a filament support substantially as described with reference to Figures 1 to 3 or to Figures 4 and 5 or to Figures 6 and 7 of the accompanying drawings.

13. A filament assembly substantially as described with reference to Figures 1 to 3 or Figures 4 and 5 or to Figures 6 and 7 of the accompanying drawings.