GB2113001A - Heat shield for vapour discharge lamp - Google Patents

Abstract

A high pressure electric discharge lamp of the kind in which electric current is conveyed to the lamp electrodes by ribbon seals is provided with a heat shield in the form of a stack of plates (1,3) fitted round the lamp stem (11) intermediate the bulb (8) and at least part of the or each ribbon seal. Oxidation of the ribbon seals, which is liable to occur as a result of air leakage through the seal between the current supply pins and the end of the stem (11) remote from the bulb, is thereby largely prevented. <IMAGE>

Description

SPECIFICATION High pressure electric discharge lamps This invention relates to high pressure electric discharge lamps of the kind comprising an envelope of fused silica having a bulb portion containing a gas and/or vapour filling and a pair of electrodes between which an electric discharge passes in operation of the lamp, and in which electric current is conveyed to the electrodes from the exterior of the envelope via so-called ribbon seals.Such a seal comprises, essentially, a strip of a refractory metal foil, usually of molybdenum, to the respective ends of which are attached an electrode or a support therefore and a conducting lead, the foil and the adjacent ends of the electrode (or support) and the conducting lead being embedded in a silica steam projecting from the bulb portion of the envelope, with the electrode lying within the bulb portion and the lead extending to the exterior of the envelope. The strip of foil thus forms part of the lead assembly for conveying current to the electrode, whilst at the same time it readily adheres to the silica to provide an effective seal.

The invention is particulariy applicable to shortarc discharge lamps as are employed for film and television lighting, flood and stadia lighting and for photochemical reactions.

Lamps of the above type (i.e. those with fused silica bulbs and seals made from molybdenum and fused silica) are often run in air and it is well known that if the molybdenum foil adjacent to the conducting lead reaches a temperature of about 2500C or higher, oxidation of the foil takes place.

This oxidation often causes lamp failure by rupture of the molybdenum ribbon and/or the fused silica.

According to the invention a device for use with high pressure electric discharge lamps of the kind referred to comprises a heat shield arranged to fit around a lamp stem between the bulb and at least part of the seal, to protect the latter from high temperatures produced by the electric discharge in use of the lamp and a means of attaching said heat shield to the stem.

Preferably the means of attaching the heat shield comprises a tube of slightly greater diameter than the diameter of the lamp stem with the plane of the heat shield fixed perpendicular to axis of the tube. The shield should preferably be square and be fixed to an end of the tube, and the assembly being arranged to be fitted to the lamp with the shield at the end of the tube which is nearest to the lamp bulb.

Additional cooling of the heat shield could be achieved by having one or more cooling fins attached to the opposite end of the tube from the heat shield.

A device in accordance with the invention is suitable for lamps with either a single ended or double ended lamp construction and is particularly useful for high wattage lamps (500w and above).

The device maybe used on lamps employing conventional lamp caps which serve both for location of the lamps and for making an electrical connection to the lamp contacts for the supply of electric current thereto. However the preferred use is with a lamp where the functions of location and the supply of current are separated, the device in such a case serving to locate the lamp on which it is fitted, in addition to acting as a heat shield, and current being supplied to the lamp by use of e.g.

flying leads. This system has the advantage that any overheating due to contact resistance at the electrical connection is well away from the vulnerable end of the molybdenum seal.

A heat shield device in accordance with the present invention, will now be described, by way of example, with reference to the accompanying drawings of which: Figure 1 shows pictorially the device: Figure 2 shows a cross-sectional view of a double ended discharge lamp, and Figure 3 shows a cross-sectional view of a single ended discharge lamp.

Referring first to Figure 1, this shows the general construction of the device C. The device is formed from a ceramic material or an oxide resistant metal, e.g. stainless steel, aluminium or nickel plated brass, and comprises a flat square plate 1 with a central circular opening having a diameter slightly greater than the external diameter of the stem of the lamp to which it is to be fitted. The plate 1 is attached to one end of a tube 2, to the opposite end of which is fitted a similar plate 3. The bore of the tube 2 is of such a size that the plate/tube assembly can be fitted on to the lamp stem between the bulb and the end part of a molybdenum foil seal within the stem, the dimensions of the plate 1 being such that it protects that region of the seal against radiation from the discharge in use of the lamp.The outwardly facing surface of the plate 1 is brightly polished to aid heat reflection.

The plate 3 conveniently acts as a heat sink or cooling fin, and may be furnished in matt black to assist in the heat loss process.

In Figure 2 the device C is shown fitted to one stem 9 of a double ended lamp 6, the device being secured to the stem by the use of a layer of suitable high temperature cement 4 between the inner surface of the tube 2 and the stem.

The sides of the plate 1 are slightly longer than the diameter of the lamp bulb so that the plate completely shields the end of the molybdenum stem seal from direct radiation from the discharge.

In addition it enables the lamp to be placed on a flat surface without rolling.

The opposite stem 10 of the lamp is fitted with a similar heat shield device C' which is constructed in a similar manner to the device C, but, instead of being fixed to the stem 10, it is free to slide along the major axis of the stem.

This arrangement allows the lamp to be fixed in position within a surrounding luminaire (not shown) by the use of a support which grips or clamps the device C. This positions the lamp relative to reflectors or lenses used for optical control of the light output. Thus the function of locating the lamp is separated from that of supplying current thereto, and the latter can then be achieved by the use of flying leads, as at 5, so that any over-heating brought about by contact resistance at an electrical contact connection is well away from the vulnerable end of the molybdenum foil seal.

The slidable device C' acts as a heat shield and can be gripped by a second support within the luminaire. This system avoids the stresses in silica often encountered when a double-ended lamp is gripped firmly in two positions, and at the same time allows for considerable differences in thermal expansion between the fused silica of the lamp and the luminaire.

Of course, the device C' can also be fixed to the lamp in cases where the lamp is designed for use in a luminaire in which at least one of the supports allows for such differential movement.

The utilisation of square shield plates 1 having sides which are greater than the diameter of the lamp bulb enables the lamp to be placed on a flat surface without rolling and with the bulb, which may be hot after a period of use, held clear of the surface. However, it is not necessary for the plates to be square, and other shapes of plates could alternatively be employed.

In some case the plate 3 may be smaller than the plate 1 or dispensed with altogether, or alternatively one or more additional plates, with air gaps between them may be attached to the tube 2 adjacent the plate 3 to enhance the cooling effect.

The heat shield device C as above described may also be used on single ended lamps as shown at 8 in Figure 3, the device being similarly secured to the lamp stem 11 by a high temperature cement 7. The lamp illustrated is constructed in accordance with the invention forming the subject of copending patent application No. 8122619, although the device can also be employed on conventional single ended lamps.

A heat shield device in accordance with the invention may also be used on lamps having conventional lamp caps used for location of the lamps and for supplying current thereto, the device serving simply as a heat shield.

It will be understood that the invention includes within its scope high pressure electric discharge lamps employing one or more heat shield devices as above described.

Claims (11)

1. An arrangement comprising a high pressure electric discharge lamp of the kind referred to and a heat shield fitted around the lamp stem between the bulb and at least part of a said ribbon seal.

2. An arrangement according to Claim 1 wherein the heat shield comprises a stack of plates spaced apart and mounted on a supporting member transverse to the stem axis, said member being fitted round the lamp stem.

3. An arrangement according to Claim 1 or Claim 2 wherein the lamp is mechanically supported by one or more of said heat shields and is supplied with current via means electrically isolated from said one or more heat shields.

4. An arrangement according to Claim 2 wherein the lamp is rigidly connected to one said heat shield and slidingly located and/or supported by another said heat shield.

5. An arrangement according to any preceding Claim wherein the or each surface of a said heat shield exposed in use to direct radiation from the lamp is reflective.

6. An arrangement according to any preceding Claim wherein surfaces not exposed in use to direct radiation from the lamp are non-reflective.

7. An arrangement according to any preceding Claim wherein the least one said heat shield is constructed of a ceramic material.

8. An arrangement according to any preceding Claim wherein the lamp is connected to at least two said heat shields, the locations, dimensions and shapes of which are such that the lamp and heat shield assembly may be rested on a flat surface without any tendency to roll and with the lamp bulb supported clear of said surface.

9. An arrangement substantially as described hereinabove with reference to Figure 2 of the accompanying drawings.

10. An arrangement substantially as described hereinabove with reference to Figure 3 of the accompanying drawings.

11. A heat shield substantially as described hereinabove with reference to Figure 1 of the accompanying drawings.