A METHOD FOR PROLONGING THE LIFE OF A GAS DISCHARGE LAMP

Abstract: A method for increasing the useful life of a gas discharge lamp having an envelope (1) comprising quartz or a high silica content glass and a neon gas fill, includes placing a heat sink (3) in thermal contact with the envelope (1) in use, thereby reducing diffusion of the neon gas through the envelope. Additionally, an optional flexible or resilient thermally conductive layer (9) may be placed between the heat sink (3) and the envelope (1) such that it is in thermal contact with both the heat sink and the envelope, thereby improving the uniformity of heat flow.

Description: A METHOD FOR PROLONGING THE LIFE OF A GAS DISCHARGE LAMP This invention relates to a method of prolonging the useful life of a gas discharge lamp having an envelope comprising quartz or a high silica content glass and a neon gas fill. It also relates to an apparatus for performing this method. It also relates to an improved illumination system. Gas discharge lamps having a neon fill have been known for over a century, and have been used since the 1930's to providing red lights for advertising signs. More recently, neon filled gas discharge lamps have been used as vehicle hazard beacons and signals, as described for example in EP-A-0673 183. This prior art patent application discloses that neon lamps have envelopes which normally comprise lead glass or lime glass or alumina silicate glasses, and teaches that envelopes made from quartz have a serious disadvantage. This is because quartz has a structure which allows neon to diffuse through it. As the neon diffuses out of the lamp there will be a pressure change, and concomitant changes in the electrical and optical properties of the lamp. These changes will happen faster in lamps which have a high power, because the diffusion rate of neon through quartz increases with temperature. According to a first aspect of the invention, there is provided a method of prolonging the life of a gas discharge lamp as claimed in claims 1 - 4. According to a second aspect of the invention, there is provided an apparatus as claimed in claims 5 - 11. According to a third aspect of the invention there is provided an illumination system as claimed in claims 12 - 16 Embodiments of the invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawings, in which Figure 1 shows a cross-section of an illumination system in accordance with the present invention, and Figure 2 shows a) plan view and b) a cross section of a preferred attachment means. Illumination systems having a heat sink in thermal contact with the lamp envelope are known, and one such device is described in GB-B-2073483. Such systems have in the past been developed to maintain an optimal mercury partial pressure in mercury vapour discharge lamps. Thus the heat sink in the present invention is provided for a completely different purpose, and provides a different technical effect to that disclosed in the prior art devices. Figure 1 shows a gas discharge lamp having an envelope (1) comprising a material consisting of at least 93% silica (in the present example fused quartz, although other materials such as Vycor glass available from Corning Inc. may be used as an alternative), the envelope being 1 mm in thickness and being filled with neon (2), a heat sink (3) in thermal contact with part (4) of the envelope, and attachment means (7, 8) for releasably securing the heat sink to the envelope. In the present example, the envelope is constructed to provide one or more protrusions (6) which co-operate with the attachment means in use. In the present example, the attachment means used comprises one or more resilient thermally conductive members (7, 8), which in use are placed in engagement with, the one or more protrusions. In the present example the one or more protrusions comprise a lip (6) which forms a part of the lamp envelope and which is located adjacent a substantially flat portion of the envelope, which portion is in thermal contact with the heat sink. As an alternative, the lip may be replaced by a skirt or flange or lugs which may co-operate with an attachment means of choice, such as for example one or more loops of metal or plastics, or spring clips. The protrusions provided on the envelope may be hollow or solid. One way of forming such protrusions is by a process called buttoning, which is well know to those skilled in the art of glass blowing. Hollow protrusions (as shown in Figure 1) are preferred as this arrangement provides good thermal transport from the discharge to the attachment means. The attachment means are preferably in contact with the heat sink. The gas discharge lamp shown in Figure 1 is provided with an external electrode (not shown) in the form of a coil which is provided with r.f. power to induce the glow discharge within the lamp envelope in use. This type of lamp is commonly called an electrodeless lamp, although electrodes for other purposes may nonetheless be present within the lamp envelope. The electrical properties of the material from which the envelope is made is especially important for such lamps. The material should have as low a dielectric constant as possible and as low a loss tangent as possible. This is to minimise energy absorption by the lamp envelope and to maximise the brightness of the lamp for a given drive circuit arrangement. Suitable drive circuit arrangements are described in our co- pending patent applications number GB 9702555.5 and PCT7GB97/00099 which are incorporated herein by reference. If such an electrodeless lamp is used in the present invention, it is advantageous for the attachment means to consist of elongate members which extend substantially radially from the glow discharge, thereby minimising the circulation of induced currents in the attachment means. Such an attachment means is shown in plan view in Figure 2, which shows elongate members in the form of fingers (10) which extend along radii from the centre of what would be the position of the discharge in use. In this Figure the lamp and the heat sink are not shown to improve clarity. The attachment means shown in Figure 2 may be connected to the heat sink by any suitable means, such as for example screws, or C - clamps, or a thermally conductive adhesive material. The illumination system of Figure 1 also shows an optional flexible thermally conductive layer (9), which is arranged between the heat sink (3) and the envelope (1) such that it is in thermal contact with both the heat sink and the envelope. In the present example the flexible thermally conductive layer is also resilient, and comprises a layer of Silpad or Gap Pad material. This material is obtainable from The Bergquist Company in Minneapolis in the USA, and comprises a silicone polymer which is loaded with thermally conductive alumina and fibreglass. This layer should be as thin as possible to provide good thermal conductance whilst still being able to compensate for variations in the gap between the imperfectly flat envelope/heat sink surfaces. Layers in the thickness range 0.5 to 1.5 mm are preferred. The illumination systems of the present invention are particularly advantageous for use on vehicles, for example as rear /brake lights or hazard beacons, as the construction is mechanically very robust and provides excellent thermal management which is required to maximise lamp efficiency and safety in flammable ambients. In order to minimise neon loss in use, the envelope temperature is preferably maintained below 250°C. At this temperature it is estimated that less than 10% of the neon fill will be lost from the envelope after 12,000 hours use. Although the provision of a thermally conductive flexible and/or resilient layer between the lamp envelope and a heat sink has been described above in relation to neon filled quartz envelopes, this aspect of the invention may be used advantageously in other systems, for example systems employing light transmissive envelopes made from lead glass or lime glass or alumina silicate glasses, and /or systems employing other gas fills such as for example argon or mixtures containing sodium, mercury etc.. Although the above embodiments have included an r.f. powered electrodeless lamp, the invention may be put into practice using electroded lamps and/or d.c. powered lamps as alternatives. The lamp envelope in the above embodiments preferably comprises a light transmissive glassy material having a silica content of greater than 93% by weight, very preferably greater than 95%, such as for example fused quartz or Vycor.

Claims: CLAIMS 1. A method for prolonging the useful life of a gas discharge lamp having an envelope comprising a material consisting of at least 93% silica, and a neon gas fill, the method including controlling the temperature of the envelope to be below the temperature at which significant diffusion of neon through the envelope occurs by placing a heat sink in thermal contact with the envelope in use. 2. A method as claimed in claim 1 including arranging a flexible thermally conductive layer between the heat sink and the envelope such that heat energy from the envelope passes through the layer to the heat sink in use. 3. A method as claimed in claim 1 or claim 2 in which the temperature of the envelope is maintained below 250°C in use. 4. A method as claimed in any preceding claim in which the envelope comprises fused quartz. 5. Apparatus for performing the method of claim 1 comprising a neon filled gas discharge lamp having an envelope comprising a material consisting of at least 93% silica, a heat sink in thermal contact with part of the envelope, and attachment means for releasably securing the heat sink to the envelope. 6. Apparatus as claimed in claim 5 in which the envelope is constructed to provide one or more protrusions which co-operate with the attachment means in use. 7. Apparatus as claimed in claim 6 in which the one or more protrusions are hollow and define one or more cavities which interconnect with the space defined by the rest of the lamp envelope. 8. Apparatus as claimed in claim 6 or claim 7 in which the one or more protrusions comprise a lip or skirt or flange located adjacent a substantially flat portion of the envelope, which portion is in thermal contact with the heat sink. 9. Apparatus as claimed in claims 5 - 8 in which a flexible thermally conductive layer is arranged between the heat sink and the envelope such that it is in thermal contact with both the heat sink and the envelope. 10. Apparatus as claimed in claim 9 in which the flexible thermally conductive layer is resilient. 11. Apparatus as claimed in claim 5 - 10 in which the attachment means comprises one or more resilient thermally conductive members. 12. An illumination system comprising a gas discharge lamp having an at least partly light transmissive envelope, a heat sink in thermal contact with part of the envelope, and attachment means for releasably securing the heat sink to the envelope, characterised in that the system further comprises a flexible thermally conductive layer which is provided between the heat sink and the envelope such that it is in thermal contact with both the heat sink and the envelope. 13. An illumination system as claimed in claim 12 in which the flexible thermally conductive layer is resilient. 14. An illumination system as claimed in claim 12 or claim 13 in which the envelope is constructed to provide one or more protrusions which cooperate with the attachment means in use. 15. An illumination system as claimed in claim 12 - 14 in which the one or more protrusions comprise a lip or skirt or flange which substantially surrounds a substantially flat portion of the envelope in thermal contact with the heat sink in use. 16. An illumination system as claimed in claim 12 - 15 in which the envelope comprises a material consisting of at least 93% silica.

Inventors: WRIGHT GRAHAM LESLIE, WRIGHT, GRAHAM, LESLIE
Applicants: CENTRAL RESEARCH LAB LTD, WRIGHT GRAHAM LESLIE, CENTRAL RESEARCH LABORATORIES LIMITED, WRIGHT, GRAHAM, LESLIE
Application year: 1998
Main classification: H01J61
IPC codes: H01J61