GB2103872A

GB2103872A - Electric discharge lamp seals

Info

Publication number: GB2103872A
Application number: GB08205149A
Authority: GB
Inventors: Angus Bernard Dixon
Original assignee: General Electric Co PLC
Current assignee: General Electric Co PLC
Priority date: 1981-07-22
Filing date: 1982-02-22
Publication date: 1983-02-23

Abstract

The silica envelope E of a high pressure electric discharge lamp is formed with an integral projection comprising a tubular outer seal member 9 surrounding and sealed to an inner seal member 10, and the lead assemblies 3, 4, 5 associated with both electrodes are sealed between the inner and outer seal members by collapsing the latter in a suitable non- oxidising atmosphere. As shown, the single ended metal halide discharge lamp has Mo foil lead- ins 4 brazed to a respective W electrode rod at their inner ends and to a lead wire 5 at their outer ends. Conveniently the pumping stem 7 is located diametrically opposite the ribbon seals. A number of specific configurations of seal construction, envelope shape and electrode arrangement are disclosed. <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 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 therefor and a conducting lead, the foil and the adjacent ends of the electrode (or support) and the conducting lead being embedded in the silica with the electrode lying within the envelope 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 particularly applicable to short-arc discharge lamps as are employed for film and television lighting, flood and stadia lighting and for photochemical reactions.

At present two basic constructions are used for short-arc high pressure electric discharge lamps having envelopes of fused silica. These are the double ended lamp, in which the foil seals associated with the two electrodes are located at opposite ends of the envelope, and the single ended lamp in which the foil seals are located at the same end of the envelope. In each case the seals are formed by pinching flat, on to the foils and the adjacent ends of the electrodes or supports and the associated leads, a tubular part of the envelope through which the lead assembly or assemblies extend.

An object of the invention is to provide an alternative form of sealing arrangement which has advantage over the present forms of lamps above described, as will be apparent from the following description.

According to the invention in a high pressure electric discharge lamp of the kind referred to the lamp envelope is formed with an integral projection comprising an inner seal member and a tubular outer seal member surrounding and sealed to the inner member, and the lead assemblies associated with both electrodes are sealed between the inner and outer members.

The inner and outer seal members are preferably of circular cross-section, but any convenient shape of cross-section may be used if desirable.

In forming such a seal the tubular outer member is conveniently collapsed on to the inner member in a suitably non-oxidising atmosphere whilst the lead assemblies are appropriately supported between them, preferably at diametrically opposite positions.

The foils employed are preferably of molybdenum as used in conventional ribbon seals.

A lamp in accordance with the invention, as it is single ended, avoids a number of disadvantages encountered with double ended high pressure electric discharge lamps, whilst also avoiding the disadvantages of the known pinched form of seal as used in existing forms of single ended lamps.

Thus as the lamp is single ended it requires only one cut-out in an associated reflector, thus increasing the forward light obtained from the reflector when compared with a double cut-out needed for a double ended lamp.

Furthermore the lamp may be located by gripping at one point only which eliminates stresses in the silica often encountered when a double ended lamp is gripped in two positions.

Moreover when the lead assemblies are sealed through the projection at diametrically opposite positions the tracking distance between the electrodes on the inside of the lamp envelope is approximately equal to half the external circumference of the inner seal mernber, so that the construction is more suitable for use with high voltage pulses required for starting and restarting than are existing single ended lamps with conventional pinch seals.

The seal of a lamp in accordance with the invention can readily be made a few millimetres long or as long as required to ensure that the end of the seal remote from the arc is cool enough to prevent seal failure by oxidation of the foil strips or associated lead conductors.

Moreover the bulb part of the envelope which contains the gas and/or vapour filling can be made substantially spherical which is the strongest shape to withstand the high pressures developed within it during operation. It also eliminates the potential weak spot which exists along the top of a flat pinch seal, and by arranging for the inside surface of the bulb to be symmetrical about the axis between the tips of the electrodes it allows the lamp to be rotated about the major axis of the arc without significantly modifying the cold spot temperature and therefore the colour or radiation from the lamp.

A lamp construction in accordance with the invention is suitable for a wide range of lamp sizes for a few watts to many kilowatts.

The filling can, for example, comprise mercury with an inert starting gas such as argon or a mixture of gases, or may comprise one or more metal halides and one or more inert starting gases with or without mercury.

A lamp in accordance with the invention has the further advantage that it can readily be mounted so that the arc is at right angles to the major axis of the reflector. This will allow an efficient reflector to be designed for use with the lamp, and at the same time the foils of the seal will lie in planes parallel to the main light beam, thus minimising the heat absorbed by the foil from the reflected beam.

Several different embodiments of the invention will now be described by way of example with reference to Figures 1 to 12 of the accompanying schematic drawings, which illustrate a number of different metal halide electric discharge lamps constructed in accordance with the invention.

In the drawings Figure 1 a represents an axial section through one of the lamps, and Figures 1 b and 1 c show an elevation and a plan view of this lamp; Figure 2 and 3 illustrate a manner of forming the lamp; Figure 4 shows the lamp of Figures 1 to 3 fitted with a base, and Figures 5 to 12 show the various alternative lamp constructions.

Referring first to Figures 1 a, 1 b and 1 c the lamp comprises an envelope E having a substantially spherical thick-walled bulb portion 1 of transparent fused silica, with a hollow cylindrical projection 2 extending from and integral with it. The lamp has a pair of refractory metal, e.g. tunsten, electrodes 3, in the form of short rods, each lying parallel to the axis of the projection 2 and sealed at their outer ends through the wall of the silica bulb and into the wall of the cylindrical projection 2 at diametrically opposite positions as shown.

The outer end of each electrode 3 has brazed to it a ribbon of etched molybdenum foil 4 extending lengthwise within the wall of the projection, and having brazed to its outer end one end of a molybdenum lead wire 5 which is also sealed into the wall of the projection 2 at that end, the opposite end of the lead wire protruding axially from the projection.

The envelope contains a quantity of mercury 8 and one or more metal halides, the filling being introduced through a pumping stem which is substantially sealed off as at 6.

The lamp construction described is suitable for lamps utilised for filming, one such lamp having a power dissipation of 1200 W with an arc length of 12 mm and a bulb volume of 5.5 ml.

In manufacturing such a lamp the bulb portion 1 with a pumping stem 7, and an outer hollow cylindrical extension projecting from opposite ends, has the electrodes 3 inserted into it through the extension 9 as shown in Figure 2. A fused silica tube 10, closed at one end, is mounted coaxially within the extension 9 with its closed end innermost, so that the outer ends of the two electrodes 3 with the molybdenum ribbons 4 and the adjacent ends of the respective lead wires 5 are located diametrically opposite each other between the tube 10 and the extension 9. The latter is then heated above its softening temperature to cause it in a non-oxidising atmosphere to collapse on to the ribbons 4 and the adjoining ends of the electrodes 3 and lead wires 5 to form ribbons seals as in Figure 3.The bulb is then evacuated through the pumping stem 7 and the appropriate filling introduced before being finally sealed off by collapsing the pumping stem close to the bulb as at 7a and removing the excess.

The outer surface 1 Oa of the closed inner end of the tube 10 is conveniently shaped to provide a substantially smooth continuation of the spherical inner surface of the bulb, although other shapes may be utilised if desired as will be described below.

Conveniently the lamp is fitted for use with a cap 11 of heat resistant insulating material, for example a suitable ceramic, in the form of a collar which fits coaxially around the projection 2 and is sealed to it by a suitable capping cement 12 as shown in Figure 4. Flying leads as at 13 are conveniently brazed or soldered to the lead wires 5. The lamp is arranged to be supported within an aperture in a base plate 14 and is held in position by a spring clip 1 5. The base plate 14 together with an outwardly directed flange 1 6 on the cap shields the seals at the end of the projection remote from the arc from the high temperatures generated in the arc.

As mentioned above the interior of the bulb need not be completely spherical, but may be modified, for example, to make an "isothermal bulb" in order to obtain a uniform inside temperature when the lamp is operated in one fixed operating position. This would allow higher vapour pressures of metal halides to be used, thus modifying the radiation or colour of the lamp.

Examples of different bulb shapes are illustrated in Figure 5, the bulbs of Figures 5a and 5b being modified by providing a different shaped end wall 1 Oa on the inner tube 10, and that of Figure 5c, which represents two sectional views of the same lamp at right angles to each other, by utilising a different bulb shape.

If desired a solid silica rod 1 7 as shown in Figure 6 may be utilised in place of the hollow inner tube 10. Also the pressures inside the lamp bulb may be modified by the provision of reflectors, which may be in the form of preformed cups 1 8a and 1 8b of a suitably reflective material, for example metal or ceramic as in Figure 7a, or a reflective coating 1 9a for example of zirconia or titania or other suitable material painted on to the bulb as shown in Figure 7b. A similar reflective coating 1 9b may be painted on to the interior of the inner sealing member at the end adjacent the lamp envelope.

The examples of lamps so far illustrated incorporate seals with a single current carrying foil 4 associated with each electrode 3. This arrangement may not, however, be suitable for very high current lamps e.g. 30 amps or more, and multiple sealing foils and lead wires 5 may be used. Figure 8 shows one such lamp in which each electrode 3, only one of which is shown, is connected to a pair of foils 4a each connected to a respective lead 5a.

The shape of the seal itself may be modified by increasing the diameter of the sealing tubes 9, 10 at the end of the projection 2 remote from the bulb as shown in Figure 9. Such an arrangement increases the distance between the lead wires 5, allowing the use of very high voltages for hot restarting of small lamps, for example of 200 W rating or less; a lamp modified in this manner may be suitable, for example, for the attachment of a standard lamp cap (e.g. a bipost cap).

A slot 20 may be formed in the silica sealing tubes 9, 10 parallel to their axes as shown in Figure 10, and a separate insulator e.g. of mica (not shown), may be inserted in this slot to increase the tracking distance between the lead wires and thereby increase the voltage pulse height that can be used, e.g. for starting.

The electrodes 3 instead of being disposed parallel to each other may be modified by inclining the whole of the electrode to each other as in Figure 11, of bending the inner ends 3a of the electrodes towards each other as in Figure 12, to assist in the stabilisation of the arc on the tips of the electrodes.

Although electrodes 3 may, as in the examples illustrated consist of simple unactivated tungsten rods, activated rods with or without coils to hold a suitable emitter may alternatively be used.

Moreover in the lamps illustrated the sealing-off pip 6 is shown at the top of the buibs, i.e.

diametrically opposite the sealing projection 2, the pumping stem and consequently the pip 6 may be in any other convenient position on the bulb as may be desired.

Claims

1. A high pressure electric discharge lamp of the kind referred to hereinabove in which the lamp envelope is formed with an integral projection comprising an inner seal member and a tubular outer seal member surrounding and sealed to the inner member, and the lead assemblies associated with both electrodes are sealed between the inner and outer members.

2. An electric discharge lamp according to Claim 1 in which the inner seal member is in the form of a tube closed at the end adjacent the lamp envelope.

3. An electric discharge lamp according to Claim 1 or Claim 2 in which the lamp envelope is substantially spherical.

4. An electric discharge lamp according to Claim 1 or Claim 2 in which the surface of the inner seal member which forms part of the inner surface of the lamp envelope is substantially flat or convex.

5. An electric discharge lamp according to Claim 1 or Claim 2 in which the lamp envelope is elongated along the axis defined by the tips of the electrodes.

6. An electric discharge lamp according to any preceding Claim incorporating a reflective coating on the region of the lamp envelope adjacent the said projection.

7. An electric discharge lamp according to Claim 2 incorporating a reflective coating on the interior of the inner sealing member at the end adjacent the lamp envelope.

8. A light comprising an electric discharge lamp according to any of Claims 1 to 5 and a reflector in the form of a cup-shaped collar which fits coaxially round the said projection.

9. An electric discharge lamp according to any of Claims 1 to 7 in which each electrode lead assembly incorporates a plurality of currentcarrying foils adapted to be connected in parallel.

1 0. An electric discharge lamp according to any of Claim 1 to 7 in which the distance between the metal foils is greater than the distance between the tips of the electrodes.

11. A method of manufacturing an electric discharge lamp from a bulb portion provided with a tubular extension and a pumping stem, an inner sealing member adapted to fit loosely within the tubular extension and a pair of electrodes provided with current-conducting refractory metal foils, in which the inner sealing member is held within the tubular extension, the electrodes and foils are suspended in the gap between the tubular extension and the inner sealing member so that the electrodes extend into the bulb portion, and the tubular extension is heated in a non-oxidising atmosphere so as to collapse it onto the ribbons and the adjoining ends of the electrodes and form ribbon seals.

12. An electric discharge lamp substantially as described hereinabove with reference to Figure 1 of the accompanying drawings.

13. A method of manufacturing an electric discharge lamp substantially as described hereinabove with reference to Figures 2 and 3 of the accompanying drawings.

14. A lighting assembly substantially as described hereinabove with reference to Figure 4 of the accompanying drawings.

1 5. An electric discharge lamp substantially as described hereinabove with reference to Figures 5 or 6 of the accompanying drawings.

1 6. An electric discharge lamp substantially as described hereinabove with reference to Figure 7 of the accompanying drawings.

1 7. An electric discharge lamp substantially as described hereinabove with reference to Figures 8 to 12 of the accompanying drawings.