GB1594976A - High pressure electric discharge lamps - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO HIGH PRESSURE ELECTRIC DISCHARGE LAMPS (71) We, THE GENERAL ELECTRIC COMPANY LIMITED, of 1 Stanhope Gate, London W1A lEH, a British Company, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement::- This invention relates to high pressure electric discharge lamps of the type (hereinafter referred to as the type specified) including a tubular discharge envelope formed of fused silica, and closed at each end by a molybdenum foil pinch seal, a pair of electrodes sealed into the respective pinches so as to be located in the opposing end regions of the discharge envelope, and, contained within the discharge envelope, a filling consisting of mercury, one or more metal halides which are vaporisable at the operating temperature of the lamp, and a small amount of rare gas for facilitating starting of the discharge. The term "high pressure", as used herein with reference to the lamps concerned, is to be understood to mean that the total vapour pressure developed within the discharge envelope in normal operation of the lamp is at least one atmosphere.

A molybdenum foil pinch seal is a well known form of end closure for the fused silica discharge envelopes of high pressure discharge lamps, consisting of a strip of molybdenum foil to the respective ends of a strip of molybdenum foil to the respective ends of which are attached an electrode support member and a conducting lead, and over which an end portion of the silica envelope is pinched so that the molybdenum foil is completely embedded in the silica, the electrode extends into the discharge envelope, and the lead extends to the exterior of the pinch, the molybdenum foil thus forming part of the lead assembly for conducting electric current to the electrode whilst being effectively sealed into the pinch.If desired, more than one molybdenum foil may be incorporated in a single pinch, for supporting and providing conducting leads to more than one electrode, as for example in a lamp of the form described in the specification of copending Patent Application No. 31668/76, (Serial No 1579624) which has two pairs of electrodes. Accordingly, the phrase "molybdenum foil pinch seal", as used herein, is to be understood to mean a pinch seal incorporating one or more molybdenum foils.

We have found that the operating life of a lamp of the type specified is liable to be restricted as a result of premature failure of the molybdenum foil seals. During normal operation of such a lamp, some condensation of the metal halides of the filling occurs in the relatively cool end regions of the discharge envelope, adjacent to the pinches: failure of the seals appears to be due to penetration of condensed halides into the pinches along the electrode support members, to the molybdenum foils, with resulting chemical attack of the halides on the inner ends of the foils, which causes cracking of the inner ends of the pinches. (The "inner end" of a foil or pinch is to be understood to be that end thereof which is adjacent to the interior of the discharge envelope.) It is an object of the present invention to provide an improvement in lamps of the type specified, whereby premature failure of the seals, caused by attack on the molybdenum foils by condensed halides, can be reduced or prevented.

According to the invention, in a high pressure electric discharge lamp of the type specified, the inner end (as hereinbefore defined) of each of the molybdenum foils incorporated in the said pinch seals is tapered so that the edge of the said end is thinner than the central region of the foil.

The tapered inner end edge of the molybdenum foil is of the form which is well known in the art of forming glass-to-metal seals, and which is generally known as a "feather-edge" or "feathered edge". Accordingly these terms will be used hereinafter to refer to the said inner end edge of the foil, the process of forming such edge being referred to as "feathering" or "feather-edging".

In the conventional procedure for producing molybdenum foils for use in the seals of lamps of the type specified, long strips of molybdenum foil of appropriate width are cut into short lengths, as required for the seal foils: the longitudinal edges of these long strips are usually feathered, but the central region of the foil is considerably thicker than the feathered edges, so that in the short lengths cut from these strips the transverse edges, which constitute the inner and outer ends of the foils when incorporated in the lamp pinches, are tapered from the relatively thick centre to the thin feathered lateral edges, that is to say the inner and outer ends of the foils are of substantially elliptical cross-section.A thicker outer end of a foil in a pinch is not particularly disadvantageous, but the elliptical face of the inner end of the foil is vulnerable to attack by condensed metal halides, since a true hermetic seal is not formed adjacent to the cut end surface of the foil. By forming the foils with feather-edged inner ends, in accordance with the invention, tensile stresses in the pinches resulting from foil ends of elliptical cross-section are eliminated and the area of the inner end surface, of each foil, which is exposed to condensed halides penetrating into the pinch is much reduced, hence the rate of erosion of the inner ends of the foils is correspondingly reduced, with the result that the life of the seals, and therefore of the lamp, is prolonged.

The feather-edging of the inner ends of the foils may be effected by a conventional electrolytic etching procedure, using an alkaline electrolyte, for example caustic soda solution or Langmuir's solution.

In the construction of the lamp, the foils may be located either substantially in the centres of the pinches, that is to say with their longitudinal axes coincident with those of the pinches and of the discharge envelope, or with their longitudinal axes offset from the centres of the pinches, depending upon the desired dispositions of the electrodes within the discharge envelope. Thus, in a lamp having one pair of electrodes and designed to be operated with the longitudinal axis of the discharge envelope disposed vertically, or to produce a discharge of the wall stabilised form, the electrodes are required to be sealed into the centres of the respective pinches so as to extend substantially along the discharge envelope axis, and the molybdenum foils are therefore also located centrally in the pinches.Alternatively, in a lamp which is designed to be operated with the longitudinal axis of the discharge envelope disposed horizontally, and to produce a discharge of the constricted arc type, the electrodes are preferably sealed into the pinches at positions offset from the centres thereof, so that the discharge path between the electrode tips is substantially parallel to the discharge envelope axis: in such a case therefore, the foils are most conveniently offset from the centres of the pinches.A lamp of the latter type is mounted, for operation, with orientation such that the electrodes lie below the envelope axis: this arrangement prevents overheating of the upper part of the discharge envelope wall as a result of upward bowing of the arc, and reduces the temperature difference occurring between the upper and lower parts of the discharge envelope wall in operation of the lamp, thus enabling a relatively high vapour pressure to be maintained in the envelope, with resulting high luminous efficacy of the lamp. In a lamp having two pairs of electrodes the foils will, of course, have to be located in off-centre positions.

The invention will be further described with reference to the diagrammatic drawing accompanying the Provisional Specification, in which Figure 1 is a perspective view of a conventional molybdenum foil employed in lamps of the type specified, Figure 2 is a perspective view of a molybdenum foil of the form incorporated in a lamp in accordance with the invention, and Figure 3 shows, in elevation, the discharge envelope of a specific form of lamp in accordance with the invention, described by way of example.

Referring to Figures 1 and 2, each molybdenum foil 1 shown therein has feathered lateral edges 2 and 3; the conventional foil of Figure 1 has an elliptical end surface 4, 0.035 mm thick in the centre, as formed on cutting the foil from a longer strip, but in contrast to this the corresponding end of the foil of Figure 2 has a feathered edge 5, and is only 0.01 mm thick as measured at a distance of one millimetre from the edge, the feathering being effected by electrolytic etching of an end surface similar to the surface 4 shown in Figure 1.

The lamp of the example, shown in Figure 3, includes a tubular fused silica discharge envelope 6 containing a filling consisting of mercury, rare gas, and one or more metal halides which are vaporisable at the operating temperature of the lamp, for example a mixture of sodium iodide and scandium triiodide; the lamp is designed for operation with the longitudinal axis of the discharge envelope disposed horizontally. The discharge envelope is closed at each end by a pinch 7, into whch is sealed an assembly consisting of a molybdenum foil 1 with a feather-edged inner end 5 (as shown in Figure 2), a molybdenum lead wire 8, and an electrode located between the longitudinal axis of the envelope and the wall of the envelope: for supporting the electrodes in these positions, the foils 1 are offset from the centres of the pinches.Each electrode consists of a single coil 9 of tungsten wire with a linear extension of the wire at each end of the coil, the extension 10 at the outer end of the coil being attached to the inner end of the molybdenum foil and sealed into the pinch, and the extension 11 at the inner end of the coil extending into the envelope substantially parallel to the envelope axis; the interiors of the electrode coils are filled with activator material consisting of scandium oxide.The construction of the lamp is completed by mounting the discharge envelope in known manner within a tubular outer envelope of borosilicate glass fitted with a bipin cap at each end, each of the molybdenum lead wires 8 being connected to one of the pins which constitute the lamp terminals for connection to a source of electric current supply for operation of the lamp. (The outer envelope is of a well known form and is not shown in the drawing).

For the manufacture of the lamp described above with reference to Figure 3, the electrode coils are first filled with an aqueous paste of scandium oxide powder, and the electrodes are heated to a temperature of 3200"C in a flowing argon atmosphere, by means of an eddy current heating coil coupling into a tungsten cylinder surrounding the electrode, to cause the scandium oxide to melt so that, on cooling, it forms a solid mass adhering to the coil. The lead wires 8 and electrode extensions 10 are welded to the respective molybdenum foils in the positions shown in the drawing, and the discharge envelope is formed in conventional manner by pinching the ends of a suitable length of fused silica tubing over the foilelectrode-lead assemblies.The envelope is baked at 1000 C for 7.5 hours in vacuum, and the desired filling is introduced into the envelope via side tubes, which are then sealed off to leave the pips 12; it is necessary to ensure that no moisture or elemental iodine is introduced into the discharge envelope during or subsequently to the baking of the envelope or during the introduction of the filling. Finally the discharge envelope is mounted in the outer envelope and the latter is sealed and capped, in known manner.

For operation, the lamp is mounted in a pair of lampholders and is disposed horizontally and so orientated that the inner extensions 11 of the electrodes lie substantially vertically below the longitudinal axis of the discharge envelope.

In a specific example of a lamp of the form described above with reference to Figure 3 of the drawing, the discharge envelope 6 has an internal diameter of 12mm, the molybdenum foils 1 are 5 mm wide, and the inner extensions 11 of the electrodes are positioned 3 to 4 mm from the interior surface of the adjacent part of the discharge envelope wall, and thus 2 to 3 mm below the envelope axis when the lamp is in the correct horizontal operating position. In each electrode, the coil 9 consists of three turns of 0.5 mm diameter tungsten wire, the internal diameter of the coil being 0.6 mm, and the inner extension 11 is 2 mm long; the horizontal length of the discharge path between the tips of said extensions is 21 mm.The discharge envelope filling consists of 21 mg of mercury, 20 mg of sodium iodide, 10 mg of scandium tri-iodide, and argon at a room temperature pressure of 35 torr. In normal operation this lamp dissipates 175 watts at a tube voltage of 120 volts, has an initial luminous efficacy exceeding 100 lumens per watt, and shows 80% maintenance after 1000 hours operation. The useful operating life of the lamp is about 6000 hours. By comparison, lamps which are identical in all respects except that the molybdenum foils are of the conventional form, that is to say with relatively thick inner ends, have been known to fail after operating for only 2000 hours.

WHAT WE CLAIM IS: 1. A high pressure electric discharge lamp of the type specified, wherein the inner end (as hereinbefore defined) of each of the molybdenum foils incorporated in the said pinch seals is tapered so that the edge of said end is thinner than the central region of the foil.

2. A lamp according to Claim 1, wherein one or more molybdenum foils of the form shown in, and hereinbefore described with reference to, Figure 2 of the drawing accompanying the Provisional Specification is or are sealed into each of said pinches.

3. A lamp according to Claim 1, substantially as shown in, and as hereinbefore described with reference to, Figure 3 of the drawing accompanying the Provisional Specification.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (3)

**WARNING** start of CLMS field may overlap end of DESC **. Figure 2), a molybdenum lead wire 8, and an electrode located between the longitudinal axis of the envelope and the wall of the envelope: for supporting the electrodes in these positions, the foils 1 are offset from the centres of the pinches. Each electrode consists of a single coil 9 of tungsten wire with a linear extension of the wire at each end of the coil, the extension 10 at the outer end of the coil being attached to the inner end of the molybdenum foil and sealed into the pinch, and the extension 11 at the inner end of the coil extending into the envelope substantially parallel to the envelope axis; the interiors of the electrode coils are filled with activator material consisting of scandium oxide.The construction of the lamp is completed by mounting the discharge envelope in known manner within a tubular outer envelope of borosilicate glass fitted with a bipin cap at each end, each of the molybdenum lead wires 8 being connected to one of the pins which constitute the lamp terminals for connection to a source of electric current supply for operation of the lamp. (The outer envelope is of a well known form and is not shown in the drawing). For the manufacture of the lamp described above with reference to Figure 3, the electrode coils are first filled with an aqueous paste of scandium oxide powder, and the electrodes are heated to a temperature of 3200"C in a flowing argon atmosphere, by means of an eddy current heating coil coupling into a tungsten cylinder surrounding the electrode, to cause the scandium oxide to melt so that, on cooling, it forms a solid mass adhering to the coil. The lead wires 8 and electrode extensions 10 are welded to the respective molybdenum foils in the positions shown in the drawing, and the discharge envelope is formed in conventional manner by pinching the ends of a suitable length of fused silica tubing over the foilelectrode-lead assemblies.The envelope is baked at 1000 C for 7.5 hours in vacuum, and the desired filling is introduced into the envelope via side tubes, which are then sealed off to leave the pips 12; it is necessary to ensure that no moisture or elemental iodine is introduced into the discharge envelope during or subsequently to the baking of the envelope or during the introduction of the filling. Finally the discharge envelope is mounted in the outer envelope and the latter is sealed and capped, in known manner. For operation, the lamp is mounted in a pair of lampholders and is disposed horizontally and so orientated that the inner extensions 11 of the electrodes lie substantially vertically below the longitudinal axis of the discharge envelope. In a specific example of a lamp of the form described above with reference to Figure 3 of the drawing, the discharge envelope 6 has an internal diameter of 12mm, the molybdenum foils 1 are 5 mm wide, and the inner extensions 11 of the electrodes are positioned 3 to 4 mm from the interior surface of the adjacent part of the discharge envelope wall, and thus 2 to 3 mm below the envelope axis when the lamp is in the correct horizontal operating position. In each electrode, the coil 9 consists of three turns of 0.5 mm diameter tungsten wire, the internal diameter of the coil being 0.6 mm, and the inner extension 11 is 2 mm long; the horizontal length of the discharge path between the tips of said extensions is 21 mm.The discharge envelope filling consists of 21 mg of mercury, 20 mg of sodium iodide, 10 mg of scandium tri-iodide, and argon at a room temperature pressure of 35 torr. In normal operation this lamp dissipates 175 watts at a tube voltage of 120 volts, has an initial luminous efficacy exceeding 100 lumens per watt, and shows 80% maintenance after 1000 hours operation. The useful operating life of the lamp is about 6000 hours. By comparison, lamps which are identical in all respects except that the molybdenum foils are of the conventional form, that is to say with relatively thick inner ends, have been known to fail after operating for only 2000 hours. WHAT WE CLAIM IS:

1. A high pressure electric discharge lamp of the type specified, wherein the inner end (as hereinbefore defined) of each of the molybdenum foils incorporated in the said pinch seals is tapered so that the edge of said end is thinner than the central region of the foil.

2. A lamp according to Claim 1, wherein one or more molybdenum foils of the form shown in, and hereinbefore described with reference to, Figure 2 of the drawing accompanying the Provisional Specification is or are sealed into each of said pinches.

3. A lamp according to Claim 1, substantially as shown in, and as hereinbefore described with reference to, Figure 3 of the drawing accompanying the Provisional Specification.