GB2058448A - Metal Halide Arc Discharge Lamp Having Improved Colour Uniformity - Google Patents

Abstract

A metal halide arc discharge lamp comprises an arc tube having press seals (2) at its ends. An electrode at each end is supported on an electrode rod (4) which extends into the press seal and is connected to a thin metal ribbon (5) embedded in the press seal (2) and providing an electrically conductive connection to an external lead in wire. There is a protrusion (8) in the press seal directly above the end (7) of the electrode rod in order to prevent thermal cycling cracks in the press seal during lamp life. However, the protrusion does not extend to the end chamber (9) of the arc tube, in order to prevent formation of a crevice around the electrode rod during press sealing. <IMAGE>

Description

SPECIFICATION Metal Halide Arc Discharge Lamp Having Improved Colour Uniformity This invention is concerned with metal halide arc discharge lamps. Examples of such lamps are shown in U.S. Patents Nos. 3,761,758 and 3,886,309. Such lamps comprise a tubular arc tube having electrodes and press seals at its ends.

The arc tube is made of high silica glass, for example, quartz or fused silica, and contains mercury, a starting gas and one or more metal halides.

This invention is particularly concerned with the arc tube press seals of metal halide arc discharge lamps. In such press seals, a thin metal ribbon, usually molybdenum, is connected between an electrode rod and an external lead-in wire, the thin ribbon enabling provision of the necessary hermetic seal in the press seal.

Such press seals are generally flat throughout most of their area except for protrusions and/or recessed provided to aid in securing metal straps clamped onto the press seals and except for ridges directly over the embedded electrode rods.

The former type of protrusions and/or recesses are generally not located over any of the embedded metallic conductors. The latter type of ridges generally extend from the arc tube end chamber to beyond the junction of the electrode rod with the metal ribbon, the purpose of the ridge being to prevent cracking of the seal during lamp life.

Other types of quartz press sealed lamps, for example, tungsten-halogen filament-containing lamps, can also have protrusions in their press seals, the purpose being to prevent movement of the metallic conductors during pressing or to accommodate excess quartz. Still other types of press sealed lamps are made of soft or hard glass and do not require thin metal ribbons in the press seal to obtain a satisfactory hermetic seal. The press seals thereof can have protrusions or recesses in the press seals, the reasons being to provide for an excess or deficiency of softened glass during pressing, or to provide means for mounting the lamp, or to strengthen the points where wires exit the press seal.However, the thermal cycling to which metal halide arc discharge lamps are subjected is more severe than that of the other types of lamps, because of the higher operating temperature of the press seal. As a result, metal halide arc discharge lamps generally require a ridge over the embedded electrode rod in order to prevent cracking.

We have found that in press sealed lamps a crevice often exists within the press seal that extends from the end chamber down along the electrode rod a short distance, for example, 70 mils (1.78 mm). In most types of lamps such a crevice presents no problems. But in metal halide arc discharge lamps, the metal halides are only partially vapourized during lamp operation, and the vapour pressure thereof is determined by the temperature of the coldest site within the arc tube during operation. We have found that the temperature in the crevice is significantly lower than that in the end chamber site, the latter temperature having generally been accepted as the cold spot temperature for the lamp. In a typical case for a 400 watt lamp, the crevice temperature was 6800C and the end chamber temperature was 7800C.We have further found that it is the crevice temperature which determines the vapour pressure of the metal halide additives and that the presence and extent of the crevices critically affect lumen maintenance performance and lamp colour. Furthermore, the need to provide relief in the form of a ridge in the press seal directly over the electrode rod in order to prevent cracking aggravates the crevice problem because the softened quartz is being squeezed less at the point where the electrode rod exits the end chamber. For example, in a 1 75 watt lamp, the thickness of the press seal was 81 mils (2.06 mm), but at the ridge it was 96 mils (2.44 mm). In a 400 watt lamp, the thickness of the press seal was 116 mils (2.95 mm), but at the ridge it was 141 mils (3.58 mm).Thus the thickness of the press seal at the ridge is 1 5 or 25 mils (.4 mm or .6 mm) more than the rest of the press seal, which results in less squeezing thereat.

We have found that it is the end of the electrode rod within the press seal that is the major stress point at which thermal cycling cracks occur. More particularly, it is the point on the electrode rod that is opposite the weld point of the rod to the thin metal ribbon. Consequently, we have eliminated the ridge found in the prior art and we provide only a raised dimple in the press seal at said stress point. This solves the thermal cycle cracking problem while permitting the softened quartz at the edge of the end chamber to be sufficiently squeezed at press sealing to eliminate the previously mentioned crevices.

The invention is illustrated by way of example in the accompanying drawings, in which: Fig. 1 is an elevation of an arc tube for a metal halide arc discharge lamp in accordance with this invention, and Fig. 2 is a section along the line 2-2 of the press seal of the arc tube of Fig. 1.

As shown in the drawing, a quartz arc tube 1 for a metal halide arc discharge lamp in accordance with this invention has press seals 2 at each end thereof. The arc tube 1 contains a filling of mercury, an inert gas and one or more metal halides. A coiled tungsten electrode 3 is supported on an electrode rod 4 at each end of the arc tube 1. Embedded in each press seal 2 is the usual thin molybdenum ribbon 5. The rod 4 is welded to one end of the ribbon 5, and an externally protruding lead-in wire 6 is welded to the other end of the ribbon 5. The major stress point in the press seal 2, as previously discussed, is at point 7, which is at the end of the rod 4 opposite the welded joint of the ribbon 5 to the rod 4. The quartz directly over the point 7 is thickened by means of an elevated dimple 8 in the press seal 2.The elevated dimple 8 does not extend as far as the end chamber 9 in order that crevices may be prevented at pressing, as previously discussed.

In a specific example, 250 watt lamps were made having the elevated dimple 8 in the press seal 2, as described. The electrode 4 was made of 23 mil (.58 mm) tungsten wire and was welded to the molybdenum ribbon 5 by means of the usual platinum welding strip. The lead-in wire 6 was made of 30 mil (.76 mm) molybdenum wire and was similarly welded to the molybdenum ribbon 5. The arc tube 1 was made from 1 5x 17 mm quartz tubing. The press seal 2 was 96 mils (2.4 mm) thick. The elevated dimple 8 protruded about 25 mils (.6 mm) above the surface of the press seal 2 and was about 94 mils (2.38 mm) from the end chamber 9. The elevated dimple 8 was formed by means of a depression in one of the two jaws used to form press seal 2, the depression being somewhat oval shaped, measuring 35 mils (.89 mm) deep by 1/8" (3.18 mm) wide by 3/16" (4.76 mm) long. The starting electrode 10 was a 1 5 mil (.38 mm) tungsten rod and was small enough in diameter that thermal cycling cracks were not a problem therefrom. The arc tube 1 contained a filling of argon, mercury, scandium iodide and sodium iodide.

The 250 watt lamps made in accordance with this invention were compared with prior art 250 watt lamps which had a ridge on the press seal over the electrode rod extending from the end chamber. Elimination of the crevices as per this invention resulted in more uniformity of colour between the lamps. For a statistically significant sampling, the colour from lamps as per this invention fell within a five step MacAdam ellipse, while the prior art lamps required an eight step MacAdam ellipse to contain their colour points.

Claims (4)

Claims

1. A metal halide arc discharge lamp comprising: a high silica glass arc tube containing a filling of mercury, starting gas and one or more metal halides; an electrode disposed in an end chamber at each end of the arc tube; a press seal sealing said end chamber at said end of the arc tube; an electrode rod supporting said electrode and extending into said press seal where it is joined to a thin metal ribbon embedded within said press seal; and an elevated dimple on the surface of the press seal directly above the joint of the electrode rod to the thin metal ribbon, the elevated dimple increasing the thickness of glass above said joint in order to eliminate thermal cycling cracks thereat, said dimple being spaced from said end chamber in order to permit the glass between the dimple and the end chamber to be sufficiently squeezed during press sealing to eliminate a crevice around the electrode rod.

2. A lamp according to Claim 1, wherein said dimple is located on the surface of the press seal that is opposite said joint.

3. A metal halide arc discharge lamp substantially as described herein with reference to the accompanying drawings.

4. The features as herein described, or their equivalents, in any novel selection.