GB2162683A

GB2162683A - High pressure sodium vapour lamp

Info

Publication number: GB2162683A
Application number: GB08516698A
Authority: GB
Inventors: Jozsef Laz
Original assignee: Tungsram Rt
Current assignee: Tungsram Rt
Priority date: 1984-07-30
Filing date: 1985-07-02
Publication date: 1986-02-05
Also published as: NL8501637A; JPS6182657A; DE3526848A1; GB8516698D0; HUT39030A

Abstract

A high pressure sodium vapour lamp contains a ceramic alumina discharge tube (1) placed into an external light-passing glass bulb (2). The discharge tube (1) contains a respective electrode (15,16) located at its opposite ends, a gas filling, additive(s), a lamp base (5), fixed to the external glass envelope (2) as well as electric lead wires (3,4) connecting the electrodes (15,16) with the lamp base (5). Conductive surfaces (7a, 7b) including light-permeable parts are wrapped around the external surface of the discharge tube (1) and is maintained at a higher positive potential than the potential - averaged over a period of time - prevailing at any point of the discharge space within the tube (1). In one embodiment, there are conductive surfaces (7a,7b) around the relatively lower-temperature parts of the tube (1) near the electrodes (15,16) and their structure is made denser than that of a further conductive surface (8) around the relatively higher-temperature middle section of the tube (1) in order to suppress the migration of sodium through the wall of the tube (1). <IMAGE>

Description

SPECIFICATION High-pressure sodium vapour lamp The invention relates to a high-pressure sodium vapour lamp comprising a discharge tube made of ceramic material based on alumina, said tube being surrounded by an external transparent glass envelope, included within the discharge tube an electrode at each of its ends, a gas filling at least one additive, a lamp base fixed to the external glass envelope as well as conductive electric lead wires for connecting electrodes to the lamp base, further a conductive surface including translucent parts around the external surface of the discharge tube, where the electric potential of the conductive surface is at higher positive level than the potential - averaged over a period of time - in any of the discharge space within the discharge tube.

The wall temperature of the discharge tube of high-pressure sodium vapour lamps is very high, lying in the range of 1000 to 15000 "K. At such temperatures, the sodium ions tend to escape from the discharge tube across the tube wall made of a ceramic material based on alumina, at an increased rate, by way of diffusion and electrolysis taking place in the electric field. The rate of migration through the wall is influenced by several factors. The increasing wall temperature and/or increasing quantity of contaminants (e.g. calcium) results in decreasing ability of the wall to suppress the migration of sodium ions. According to the theoretical investigations known so far, the process of sodium migration across the tube wall is enhanced by increasing temperature.

For cold starting of high-pressure sodium vapour lamps a starting pulse in the range of several thousand volts is required. To reduce this voltage a trigger electrode can be formed from a metal wire wrapped around the discharge the latter. Also this auxiliary electrode may lead to sodium migration.

In US patent 4 179 640 granted to D.A. Larson it has been stated that the auxiliary electrode has to be separated from the electrodes of the lamp by a bimetallic switch, a high-resistance resistor or a capacitor, to prevent the trigger electrode from increasing sodium migration to an undesirable extent.

In a paper published in the Journal of the IES, Vol. 8, No. 5, April 1979, pp. 166 to 179, "Electrolysis of sodium through alumina arc tubes" the author, E.F. Wyner states that also in high-pressure sodium vapour lamps an electric field may spontaneously develop, by which electrolytic migration through the wall of alumina discharge tubes is brought about. According to Wyner's experiments, sodium escaping from the discharge tube and on the surface of the external envelope of the lamp, has been found to migrate partly back into the discharge space under the effect of the positive potential of the metal screen surrounding discharge tube, making thereby the black spots impeding the passage of luminous flux disappear.

In his experiments, Wyner surrounded the middle section of the discharge tube with a grid from metal screen connected to a d.c. voltage, which obviously influenced the luminous flux in a considerable way. No metal screen was applied to the ends of the arc tube over a 2 cm length at each end. However, just these sections lying adjacent the tube ends are most severely exposed to the attack of sodium aluminate, consequently -according to the Applicant's experience - although the mobility of the sodium ions in the tube wall increases at higher temperature, less sodium is dissolved at the hotter middle section of the alumina discharge tube than at its colder sections adjacent to the electrodes, since the sodium aluminate forming in the course of operating the lamp is unstable and easily decomposes at higher temperature, whereby the sodium present in the discharge tube dissolves in higher concentrations in the wall at the colder ends of the discharge tube, which causes a substantial loss of sodium. Further, crystal structure of the alumina wall is transformed by the sodium aluminate present in high concentration and produces beta-phase aluminium oxide which is more readily permeable by sodium, hence the resistance of the colder ends of the discharge tube to the diffusion of sodium diminishes considerably. The ends of the discharge tube blacken at a high rate, reducing the luminous flex and the sodium content of the discharge space, in addition to making the tube ends weaker and brittle.

An aim of the invention is to provide a method by which the migration of sodium through the alumina discharge tubes of sodium vapour lamps can be reduced.

To achieve this aim, according to the invention, around the relatively lower-temperature parts at each end of the discharge tube and adjacent to the electrodes, conductive surfaces of positive potential and containing light-passing parts are formed, or around the discharge tube a continuous conductive surface of positive potential and containing light-passing parts is placed, this latter continuous conductive surface, showing maximum-density (i.e.

maximum capability of effectively covering the tube surface) above the colder parts of the discharge tube. Correspondingly, the subject of the invention is a high-pressure sodium vapour lamp containing an aluminous ceramic discharge tube placed into an external transparent envelope, the discharge tube having at each of its opposite ends, an electrode, a gas filling and at least one additive, a lamp base connected to the external glass envelope as well as electric lead wires connecting the electrodes with the lamp base, further a conductive surface containing light-passing parts around the external surface of the discharge tube, where the electric potential of the conductive surface is at higher positive potential than the potential averaged over a period of time -at any point of the discharge space within said discharge tube, so that around the parts of the discharge tube of relatively lower service temperature, near the electrodes the conductive surface is made denser than the conductive surface around the middle section of the discharge tube of relatively higher service temperature, or so that at the external surface of the dis charge tube, around the parts of relatively lower service temperature, near each electrode, a separate conductive surface is formed.

By means of the arrangement according to the invention the migration of sodium from the discharge tube can be reduced considerably, thereby extending the service life of the sodium vapour lamp, improving its quality, and fully or partially eliminating the need for reducing the luminous flux over the highest light-density middle section of the discharge tube, since the middle tube section is fully left uncovered or is only covered partially with the conductive surfaces providing partial screening. At the same time, the arrangement complying with the invention also facilitates the starting of the arc tube.

It may seem astonishing that long-life discharge tubes are to be protected just at their coldest parts, although Wyner's experiments have shown that the ability of the wall to resist to the harmful migration of sodium reduces with increasing wall temperatures. These experiments, however, lasted a few hours, so that the processes of the formation of sodium aluminate and beta-alumina and that of the intrusion of sodium into the wall, -the two effects mutually intensifying each other -could not become complete.Further, it is sufficient to expose the discharge tube only at its ends to the effect of an electric field of high intensity since the sodium escaping across the middle section of the arc through thermal diffusion may be driven back into the discharge tube at the ends of the latter -in full accordance also with Wyner's experiments -by applying to these regions a strong electric field.

According to the invention, the partly light-passing conductive surface may be advantageously a grid-form metal screen or some other setup made of electrically conductive material containing interstices, or for this purpose, a fully or partly transparent oxide layer, e.g. zinc oxide layer may be used.

The d.c. power supply for maintaining at required level the potential of the conductive layer surface is preferably mounted into the lamp base, and it is fed from the internal electric wiring supplying the sodium vapour lamp itself.

In the following, the invention is described in detail by way of preferred embodiments shown with reference to the drawings wherein: Figure 7 shows a high-pressure sodium vapour lamp according to the invention provided with a conductive surface of varying density, Figure 2 is a cross-section of a high-pressure sodium vapour lamp according to the invention, provided with a conductive surface at each end of the arc tube and Figure 3 is an arrangement of a direct current supply applied in a high-pressure sodium vapour lamp according to the invention for providing the potential of the conductive surface.

In the arrangement shown in Figure 1, a discharge tube 1 is located in an external glass envelope 2. Electrodes 15 and 16, serving as support rods at the same time, are connected to the terminals of a lamp base 5 by means of wires 3,4. The discharge tube 1 is surrounded, in the vicinity of its ends, by conductive surfaces 7a and 7b made of pieces of a closer-meshed metal grid and by a conductive surface 8 in the middle. At the colder sections of the ends of the discharge tube 1 the vapour pressure of sodium under discharge is higher than the pressure of sodium vapour forming above the sodium aluminate present there.

These endangered sections of the discharge tube 1 are the longer, the higher the sodium content in the sodium amalgam. With usual discharge tube parameters, the length of sections attacked by the sodium aluminate is about 1 to 2 centimetres. The sections should be covered up with pieces of a closer-meshed wire grid. The middle position of the discharge tube 1 is covered with conductive surface 8 made of a coarser-meshed wire gauze.

So, the luminous flux suffers but a minimum loss along the middle section of the discharge tube 1.

The conductive surface 8 shown in Figure 1 is especially a single metal conductor connecting electrically the conductive surfaces 7a and 7b, further by virtue of its positioning -it facilitates the starting of the high-pressure sodium vapour lamp from cold condition.

The connecting wires 3 and 4 of the lamp base 5 are linked up also with the input terminals of a direct current power supply 6, preferably mounted into the lamp base 5, from where the power supply is connected through a lead wire 9 passing through the external envelope 2 to the conductive surface 7a, i.e. to one of the coarser -meshed gauze wrappings.

In the arrangement shown in Figure 2 at the middle portion of the discharge tube 1 no coarsermeshed conductive layer 8 can be found. The conductive surfaces 7a and 7b made up of pieces of wire gauze and located at the two ends of the discharge tube 1 are connected by a piece of the lead wire 9. In this arrangement the light emission from the middle portion of the discharge tube 1 is impeded by the lead wire 9 to a lesser extent than by the conductive surface 8 shown in Figure 1. At the same time, a lower ignition voltage is sufficient with the arrangement of Figure 1 than with that of Figure 2. Reduction of sodium migration is ensured with both arrangements.

Naturally it is advisable to place also the ceramic sealing plugs of the discharge tube 1 under the effect of the electric field, since also these plugs are exposed to the damaging effect of sodium, just as the wall of the tube.

The conductive surface used corresponding to the invention is partly or fully brought into permanent contact with the external surface of the discharge tube 1. A leakage current of about 1 to 2 microamperes flows between the positive-potential conductive surface and the wall of the discharge tube 1. The electric energy required for maintaining the flow of this direct current is taken from connecting wires 3 and 4 of two electrodes 15 and 16 of the discharge tube 1 or from any other internal electric leads of the high-pressure sodium vapour lamp. There are alternating current voltages present on the internal electric loads of the sodium vapour lamp.From these alternating current voltages -advantageously by means of a semiconductor -type diode -a direct current voltage is produced by a bult-in electronic discharge power supply 6 and is passed to the conductive surface arranged around the discharge tube 1. Owing to the simple circuitry of the direct current power supply 6, it can made very small, so as to be accommodated within the lamp unit, advantageously in the lamp base 5.

In Figure 3 a possible circuit arrangement of the direct current power supply 6 is represented. Lead wires 3 and 4 shown in the Figure are at a potential equal to that of the electrodes of the discharge tube 1, they are connected to resistors 10 and 11.

The voltage present on the discharge tube 1 is attenuated in given ratio by a voltage divider composed of resistors 10 and 11, and the highfrequency voltage surges are filtered out by a capacitor 14.

This voltage is then rectified by a diode 12 and stored by a capacitor 13. The positive terminal of the capacitor 13 is connected to the conductive surface surrounding the discharge tube 1 through the lead 9. During the ignition of the sodium vapour lamp, a voltage of several thousand volts appears between the leads 3 and 4 that has to be withstood without damage by the resistor 10.

Since the load current of the direct current power supply 6 is small (1 to 2 microamps), the requirements to be fulfilled by the circuit components are minimal.

The presented examples refer to conductive surfaces made up of grid-form metal screens and expanded spirals, still other conductive surface structures can be conceived, either of metal, or of electrically conductive zinc oxide or of other metal oxides.

Claims

1. High-pressure sodium vapour lamp containing a ceramic alumina discharge tube placed into an external transparent glass envelope and within the discharge tube an electrode located at each of the two opposite ends of said tube, a gas filling, additive(s), a lamp base fixed to the external glass envelope, as well as electric lead wires connecting the electrodes with the lamp base, and conductive means including light- permeable parts, arranged around the external surface of said arc tube, said conductive means being maintained at a higher positive potential than the potential averaged over a period of time prevailing at any point of the discharge space within the discharge tube, wherein said conductive means includes conductive surfaces around the parts of the discharge tube of relatively lower service temperature and near the electrodes and a conductive surface around the middle section of the discharge tube of relatively higher service temperature, the former surfaces being more dense than the latter surface.

2. High-pressure sodium vapour lamp containing a ceramic alumina discharge tube placed into an external transparent glass envelope, within the discharge tube an electrode located at each of the two opposite ends of said tube, a gas filling, additive(s), a lamp base fixed to the external glass envelope, as well as electric lead wires connecting the electrodes with the lamp base, and conductive surface means, including light-passing parts, around the external surface of said discharge tube, where said conductive surface means is at a higher positive potential than the potential averaged over a period of time, prevailing at any point of the discharge space within the discharge tube.

3. A high-pressure sodium vapour lamp as claimed in claim 1 or 2, wherein the conductive surfaces contain light-permeable sections formed by pieces of wire gauze.

4. A high-pressure sodium vapour lamp as claimed in claim 1 or 2, wherein said conductive surfaces containing light- permeable sections, formed by layers of some translucent metai oxide, e.g. zinc oxide.

5. A high-pressure sodium vapour lamp as claimed in any preceding claim, including a direct current power supply for maintaining of potential level of the conductive surface, mounted in- the lamp base.

6. A high-pressure sodium vapour lamp substantially as herein described with reference to and as shown in Figures 1 and 3 or Figures 2 and 3 of the accompanying drawings.