High-pressure discharge lamp with getter

Abstract: The present invention relates to high-pressure discharge lamp having a discharge tube (5), an envelope (1) and a current lead (4), wherein a getter ring (6) is arranged in a vacuum space between the envelope (1) and the discharge tube (5) for producing a getter film in order to maintain the vacuum in said space, wherein the surface of the getter material (7) in the getter ring (6) is arranged opposite to one side of the pinched stem (2), said surface forming an angle with the geometrical axis of the lamp of up to 90°, preferably between 35-45°.

Description: The present invention relates to a high-pressure discharge lamp with a getter for increasing service life, having a discharge tube, an envelope and a current lead, wherein a getter film is arranged in a vacuum space between the envelope and the discharge tube, for maintaining the vacuum in said space. Earlier, the space between the discharge tube and the envelope of discharge lamps was filled with inert gas, as shown for instance in the EP-B-0 165 587. The role of the inert gas was to eliminate oxidation of the metal elements in the discharge tube. A further advantage of applying inert gas was that it increased the breakdown voltage in the envelope and due to this no breakdown occured between metal elements of the discharge tube. On the other hand, however, the heat loss due to the convection flow of the inert gas decreased the efficiency of the whole lamp. That was the reason that later vacuum has been produced between the envelope and the discharge tube of discharge lamps and the remaining gases were bound by a getter film in the vacuum space after fusing the end of the pumping out tube of the lamp. In this way, the pressure in the vacuum space could be held below 105 mbar (see Debreczeni et al: Fényforrások, Budapest, 1985, page 153). This pressure could be held during the whole thousand or some ten thousand hour service life of the lamp. The getter in the envelope has to bind the gases remaining after pumping out the air as well as gases released and escaped from the discharge tube during operation. During operation of the discharge lamps hydrogen and oxygen are mainly releasing and therefore it is reasonable to apply getter material which is capable of binding both gases. The barium, a getter film applied in the electronics, is highly active, binds the gases quickly and keeps them permanently bound in rather big quantities. In the case of high-pressure discharge lamps, however, the barium-film is warmed by the heat radiated by the discharge tube and, in this way, the hydrogen binding activity is not effective enough (SABS gettercatalogue, 1982, page 28). Therefore, combined (evaporated and metal) getters are generally applied, which are able to bind the remaining gases as well as those ones which release during operation. According to the US-B-3 626 229, zirconium, titanium or zirconium-titanium alloys are used for binding hydrogen and maintaining a pressure of below 0,01 bar (10⁻⁷ atmosphere). According to US-A-519 864 a Ba0₂ getter is used for binding hydrogen. The most often applied getter material for maintaining the required pressure is, however, a barium-film evaporated to the inner surface of the neck part of the envelope. The pure barium itself is generally produced by heating a BaAl₄ intermetallic alloy up to a temperature in the range of 900-1000 C° within the envelope. The releasing barium can quickly and completely be evaporated at the above temperature range by e.g. induction heating. The DE-B-3 346 130 discloses a discharge lamp wherein such a getter film is produced in the vacuum space between the envelope and discharge tube at the lower part of the inner surface of the envelope. The value of the pressure in the envelope of the high-pressure discharge lamps has a considerable influence on the concentration of the contaminating materials in the discharge space as the electric strength of the envelope and in this way, on the ignition characteristics and service life of the lamp, because the gases in the vacuum space penetrate into the discharge tube by diffusion and so they contaminate the discharge space as well. On the other hand, the metal parts in the envelope can easily bind oxygen and, accordingly, a quick oxidation occurs if oxygen is present in the vacuum space. If hydrogen can diffuse into the discharge space, it would influence the ignition characteristics and other operation parameters of the lamp. The object of the present invention is therefore to solve the above problems and to provide a getter system which can bind both oxygen and hydrogen releasing during operation of the lamp without applying a further getter material. As it is known in the electronics, a barium-film is highly active, can bind and keep bound rather an high amount of contaminating gases. The only problem is to adsorb hydrogen if the temperature is rather high (SABS gettercatalogue, 1982, page 28). For binding oxygen, however, a higher temperature (300-400 C°) is advantageous. The present invention is based on the recognition that a getter film may be produced for binding both hydrogen and oxygen, if a getter ring and a getter film is arranged according to the invention. For adsorbing hydrogen, applying a high surface getter film on the lower part of the inner surface of the envelope is the best way, because here is the lowest temperature (120-250 C°) during operation. At the same time, for absorbing oxygen, a small surface thick layer of getter should be applied in a region, where the temperature is higher than 350-420 C°. According to the invention, a getter film may be produced by a single getter ring and the produced getter system is capable to bind effectively both hydrogen and oxygen. The above object is achieved by a high-pressure discharge lamp according to claim 1. It is pointed out that from DE-A-3 324 081 there is known a high pressure mercury vapour discharge lamp having a getter ring within the space between the discharge tube and the outer envelope. The getter ring is arranged such that it faces the wider side of the pinched lamp stem. According to the present invention, the angle between the surface of the getter material and the geometrical axis of the lamp is advantageously between 30 and 45°. In the lamp according to the invention, the barium releasing from the getter ring during induction heating impacts on the side surface of the stem and produces a rather thick layer in a high temperature region, which is just required for binding oxygen. Another, smaller part of the barium is deposited directly on the inner surface of the lower part of the envelope or is directed to the lower part of the envelope by the pinch part of the stem. Yet another part reaches a colder place by diffusion and therefore, a thin film is produced. These thin films in regions of low temperature are just required for adsorbing hydrogen. Further details and advantages of the present invention will be shown by an example with reference to the drawing. In the drawing: Figure 1 shows a high-pressure discharged lamp, partly in section. The discharge lamp shown in Figure 1 has an outer envelope 1, which contains a pinched stem 2 provided with a pumping out tubulation 3. There is a current lead 4 and a discharge tube 5 connected to the pinched stem 2. A getter ring 6 with getter material 7 is connected with the current lead 4 in a way that the surface of the getter material 7 in the getter ring 6 forms an angle with the geometrical axis of the lamp of about 30-45°. The getter material 7 may be for instance BaAl₄ and its surface is just opposite to the narrower side of the pinched part of the stem 2. If the getter material 7 is evaporated from the getter ring 6 by heating, 60 % of the barium is deposited to the surface of the pinched part of the stem 2. The other part of the barium produces a thin film on the lower part of the inner surface of the envelope 1. Accordingly, the getter film on the high temperature stem can bind oxygen and the getter film on the low temperature surface of the envelope can bind hydrogen from the vacuum space between the envelope and the discharge tube. It has been found that in lamps having an output of above 150 W it is advantageous, if the getter ring is opposite to the narrow side of the pinch.

Claims: High-pressure discharge lamp with a getter for increasing service life, comprising an inner discharge tube (5) arranged within an outer envelope (1), current leads (4), a pinched lamp stem (2) having two narrow sides and two wide sides, and a getter ring (6) having a surface with getter material (7) thereon and being located in a vacuum space between the envelope (1) and the discharge tube (5) adjacent to the pinched lamp stem (2) with said surface facing one of the narrow sides of the pinched lamp stem (2) at an angle of less than 90° with the longitudinal axis of the lamp for producing a getter film in order to maintain the vacuum in said space. High-pressure discharge lamp as claimed in claim 1, characterized in that the angle between the surface of the getter ring (6) with the getter material (7) thereon and the longitudinal axis of the lamp is between 35° - 45°.

Inventors: HORVATH GYOERGYI, UGROSDY LASZLO, HORVATH, GYOERGYI, UGROSDY, LASZLO, Horvath, Györgyi
Applicants: TUNGSRAM RESZVENYTARSASAG
Application year: 1992
Main classification: H01J61
IPC codes: H01J61