GB2248718A

GB2248718A - Metal halide lamp.

Info

Publication number: GB2248718A
Application number: GB9114768A
Authority: GB
Inventors: Dongha Kim
Original assignee: Samsung Electron Devices Co Ltd
Current assignee: Samsung SDI Co Ltd
Priority date: 1990-07-20
Filing date: 1991-07-09
Publication date: 1992-04-15
Also published as: DE4122694A1; JPH0498258U; HUT58428A; HU912313D0; GB9114768D0

Description

2 2 4,3 7 ', j, METAL HALIDE LAMP

FIELD OF THE INVENTION

The present invention relates to metal halide lamps and more particularly relates to metal halide lamps with an improved auxiliary electrode which improves the color quality of light from the lamp when the lamp is normally ignited.

BACKGROUND OF THE INVENTION

A metal halide lamp is a discharge lamp made by mixing, in a silica tube, inert gas or mercury with at least one element among the halides such as sodium, thallium, indium, dysprosium, and scandium. Metal halide lamps have the advantage of noticeably improving luminous efficiency and light color quality compared with a mercury arc lamp.

However, during operating of metal halide lamps, halogen dissociated from the metal halide reacts with an electron emission material under high temperature produced by the lamp.

Although some known material, such as barium and the like, have excellent electron emission quality, other materials such as thorium are often used because they have comparative stability X in halogen vapour, despite having low electron emission quality.

As a result, the start lighting or ignition voltage of metal halide lamps becomes higher by 100V than that of the former mercury arc lamps. Accordingly, the many-sided methods for lowering the start lighting voltage of a metal halide lamp such as a mercury arc lamp start to be studied. For example, Japanese Laid-Open Patent (Kokai) No. 51-57980 proposes the following method: disposing two main electrodes opposite to each other on both ends of a silica tube, installing an auxiliary electrode having at least one needle type projection proximate one of the main electrodes, starting glow discharge between the main electrode and the auxiliary electrode at the first stage of operation, causing electrolytic dissociation in the silica tube, and after starting, switching over arc discharge to the two main electrodes.

While a metal halide lamp formed by the above method has the advantage of lowering the ignition or start lighting voltage, the metal halide lamp has the fundamental problem that the 2 surroundings of the low position stem of the stems of the main electrodes has lower temperature than that of the opposite high position stem to form the coldest part and thus the metal halide material therein can not evaporate to remain, thereby not getting to arrive at the sufficient saturated state to cause bad light color quality.

Furthermore, the space between the main electrode and the auxiliary electrode becomes gradually wider due to enfacement of the electrodes by glow discharge, whereby the useful life of the metal halide lamp is greatly reduced.

SUMMARY OF THE INVENTION

One feature of the present invention is to provide a metal halide lamp having improved color quality at the normal operation by introducing heat produced by glow discharge between one main electrode and an improved auxiliary electrode to the low position stem to let metal halide material remaining thereabout rapidly evaporate.

Another feature of the present invention is to provide a 3 metal halide lamp, wherein the dischargeable time and useful life of the auxiliary electrode against the main electrode can be sharply extended.

To provide these and other features, the present invention provides the metal halide lamp comprising a convergent member formed in the cylinder type or in the truncated cone type which surrounds the stem of the low side main electrode to introduce heat produced by glow discharge to this stem, thereby letting metal halide material remaining therein sufficiently evaporate.

The convergent member is preferably made of refractory metal and formed in a truncated cone, wherein a small-diameter opening of the cone is arranged opposite to the other main electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the present invention will be apparent in the detailed description in connection with the accompanying drawings, in which:

FIG. 1 is a sectional side view of a metal halide lamp according to an embodiment of the present invention; and 4 FIG. 2 is an enlarged partial sectional side view of that embodiment, taken at a point forward of FIG.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates the structure of a metal halide lamp according to an embodiment of the present invention. Two main electrodes 4a, 4b are disposed on either end of a closed silica tube 2, and coils 6 comprising electron emission material 61 are wound on the ends of the main electrodes 4a, 4b. Electron emission material 61 such as sodium oxide or another rare earth oxide can be used in the present invention. The tube contains a gas of one of the metal halides, contained within side walls 24a, 24b and base 9 and end portions 22 of the tube, which define a gas-filled internal space 20.

An auxiliary electrode 8 extends through the end portion 9 parallel to one main electrode 4a, and includes a contact 15 suspended from or attached to the bottom surface 17 of convergent member 10 surrounding the main electrode 4a, by means of soldering.

The convergent member 10 is substantially formed as a hollow tube or cylinder or in a hollow truncated cone (f rustoconical) shape as shown in FIG.1. The member may be made of refractory metal such as tungsten alloy or by the deposition of refractory metal on ceramic. Further, the convergent member 10 can be formed as a truncated cone coil of refractory metal.

When the convergent member 10 is formed as the truncated hollow cone or coil, the small diameter opening lla of the convergent member 10 is preferably disposed proximate electron emission material 61 of the low side main electrode 4a. The large diameter opening llb faces the base 9 of the silica tube 2.

The interval or space between electron emission material 61 of the low side main electrode 4a and the edge of the convergent member 10 disposed thereabout is within the range of 1 - 2 mm, whereby satisfactory glow discharge can be done. In order to draw out the main electrodes 4a, 4b and the auxiliary electrode 8, molybdenum links 12 are provided, embedded in a base portion 9 and an end portion 22 of the tube, and an electrically connected 6 with the corresponding leads 14, 16 of one main electrode 4a and the auxiliary electrode 8.

A voltage is applied across one low side main electrode 4a and the auxiliary electrode 8 of the metal halide lamp formed by the above method, causing a glow discharge to start between electron emission material 61 of the main electrode 4a and the nearest edge 13 of the small diameter portion 11 of the convergent member 10. At that time, metal halide material H gets to gather around the stem of low side electrode 4a of the coldest part in the tube.

FIG. 2 illustrates an operational state where glow discharge occurs between electron emission material 61 and the edges of the small diameter portion of the convergent member 10.

Heat produced by glow discharge is introduced to the stem of low side electrode 4a by the convergent member 10 to let metal halide material R remaining thereabout evaporate. The convergent member 10 such as a truncated cone introducing heat produced by glow discharge to the stem of low side electrode 4a has the 7 advantage of spreading heat more widely than the other. This operation after glow discharge lets metal halide material H gathering around low side main electrode rapidly evaporate, so that when the normal operation by arc discharge between the main electrode 4a, 4b starts, saturated metal halide material H in the tube make original color quality.

In addition, the convergent member 10 extending to the auxiliary electrode 8 surrounds low side one of the main electrodes 4a, 4b so that the effacement of the auxiliary electrode 8 by glow discharge begins at a certain part proximate the low side main electrode 4a from the edge of the convergent member 10. After effacing this part, another part thereabout gets to be put proximate low side main electrode 4a and thus the change of discharge point can be naturally performed.

Accordingly, the natural change of discharge point according to the electrode effacement makes possible longer useful operation life of the metal halide lamp according to the present invention than the former metal halide lamp comprising a needle typ E3 electrode as the auxiliary electrode.

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Claims

1. A metal halide lamp comprising:

a closed silica tube; a rare gas in the tube; a first main electrode in the tube attached at a first end of the tube; a second main electrode attached at a second end of the tube opposite the first main electrode; a convergent member surrounding the first main electrode axially of the first main electrode;and an auxiliary electrode on the convergent member to induce an arc discharge between the first main electrode and the second main electrode, the auxiliary electrode being attached to a lower end of said convergent member to converge heat produced by the discharge and to spread the heat to the second main electrode.

2. The metal halide lamp as claimed in claim 1, wherein the convergent member is formed as a cylinder.

3. The metal halide lamp as claimed in claim 1, wherein the ]L 0 convergent member is formed as a truncated cone.

4. The metal halide lamp as claimed in claim 3, wherein the convergent member is formed as a truncated conical coil.

5. A metal halide lamp substantially as herein described with reference to the accompanying drawings.

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