WO2007026288A2

WO2007026288A2 - High-pressure gas discharge lamp

Info

Publication number: WO2007026288A2
Application number: PCT/IB2006/052938
Authority: WO
Inventors: Michael Haacke; Norbert Lesch
Original assignee: Philips Intellectual Property & Standards Gmbh; Koninklijke Philips Electronics N. V.
Priority date: 2005-09-02
Filing date: 2006-08-24
Publication date: 2007-03-08
Also published as: WO2007026288A3

Abstract

Various embodiments of high-pressure gas discharge lamps are described whose electromagnetic compatibility (EMC) is substantially improved, particularly during the starting or switching-on phase of the lamp, and which are thus particularly suitable for use in an environment that is sensitive in the way it reacts to electromagnetic interference, such as in vehicles for example. This is achieved in essence by reducing the work function for electrons of the electrode material and/or by increasing the mean temperature of at least one of the electrodes and/or by increasing the homogeneity of the surface of at least one of the electrodes.

Description

HIGH-PRESSURE GAS DISCHARGE LAMP

The invention relates to a high-pressure gas discharge lamp particularly for use in vehicles or in other environments that may be particularly sensitive in the way in which they react to electromagnetic interference (EMI).

Because of their high efficiency and their good radiating properties, high- pressure gas discharge lamps have become widely used. However, it has been found that when these lamps are used in an environment in which there are other electrical or electronic components, such as in vehicles for example, the risk of these components being electromagnetically affected by, or being subject to electromagnetic interference from, the discharge lamps can no longer be ignored. It has also been found, particularly with mercury-free gas discharge lamps, that the risk of such interference is especially high during the starting or switching-on phase. The interference becomes apparent essentially in the form of a noise signal that is superimposed on the lamp voltage or lamp current and that is also emitted and that may considerably detract from the electromagnetic compatibility (EMC) of the lamp. This may cause serious problems particularly when the lamps are used in vehicles, due to the many other electronic components that are present in them.

A general object of the invention is therefore to make it possible for a high- pressure gas discharge lamp to operate even in an environment in which there are components sensitive to electromagnetic interference by making the risk of such interference substantially lower, particularly during the starting or switching-on phase of the lamp.

A further object of the invention is to provide a mercury-free high-pressure gas discharge lamp intended particularly for use in vehicles, whose electromagnetic compatibility is substantially better particularly during the starting or switching-on phase. As claimed in claim 1, the object is achieved with a high-pressure gas discharge lamp having a means of improving electromagnetic compatibility (EMC) with other electrical or electronic components, particularly during the starting or switching-on phase, by increasing the mean temperature of at least one of the electrodes. As claimed in claim 3, the object is also achieved with a high-pressure gas discharge lamp having a means of improving electromagnetic compatibility (EMC) with other electrical or electronic components, particularly during the starting or switching-on phase, by reducing the work function for electrons of the electrode material.

Finally, as claimed in claim 6, the object is achieved with a high-pressure gas discharge lamp having a means of improving electromagnetic compatibility (EMC) with other electrical or electronic components, particularly during the starting or switching-on phase, by increasing the homogeneity of the surface of at least one of the electrodes.

The dependent claims relate to advantageous embodiments of the invention.

Particular advantages of the embodiment claimed in claim 7 are, for example, the fact that the lamp has an increased life because the risk of chemical reactions, which may result in corrosion of the electrode feedthroughs inside the pinches and may thus speed up the occurrence of cracks in the pinches, is reduced as a result of the abandonment of the use of thorium and thorium compounds (and particularly thorium oxides). What is more, the environmental compatibility of a lamp of this kind is, of course, also considerably better than that of lamps containing thorium.

These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.

In the drawings:

Fig. 1 is a diagrammatic graph of the temperature of the electrodes of a high-pressure gas discharge lamp during the starting or switching-on phase and;

Fig. 2 is a diagrammatic longitudinal section through a lamp of this kind.

Fig. 1 is a diagrammatic graph showing the curve followed by the temperature of the electrodes of a discharge lamp immediately after it is switched on, with time t in seconds being plotted along the horizontal axis and the temperature T of the electrode tips in degrees Celsius being plotted along the vertical axis. This curve is of the normal form in which there are three temperature zones. Consequently, after approximately 5 to 15 s from the striking of the lamp, the temperature declines relatively steeply from a first, higher value until, after a period of between approximately 30 s and approximately 3 minutes it reaches a second, lower value that indicates the settled or operating state. This decline in temperature is preset chiefly by the dynamic behavior of commercially available ballasts.

It has been found that the electromagnetic compatibility of the lamp in the transitional zone between the higher and lower temperatures, i.e. in the zone where the electrode temperature declines, is relatively poor. This is due principally to the fact that the ends of the arc at the electrodes are relatively unstable during this transitional phase. One of the reasons for this is, in turn, the fact that the work function for electrons at the electrodes is still relatively high in this transitional phase while at the same time the mean temperature of the electrodes is falling.

It has also been found that electromagnetic compatibility is appreciably better before and after the transitional zone mentioned. This is due principally to the fact that the ends of the arc at the electrodes are more stable, and the electromagnetic compatibility of the lamp is therefore substantially better, both at the first, higher temperature, at which the ends of the arc are substantially diffuse (the diffuse mode), and also at the second, lower temperature (i.e. in the settled state), at which an arc end substantially in the form of a spot forms at both electrodes (the spot mode).

Investigations have also shown that the electromagnetic compatibility of high- pressure gas discharge lamps during the starting or switching-on phase, and particular during the transitional phase between the diffuse mode and the spot mode, can be improved in at least one of three possible alternative ways, which ways will be elucidated below. In the first embodiment, a reduction in the diameter of the tips of the electrodes causes the electrode temperature to be higher in all three of the above-mentioned zones of temperature, which means that there is an overall upward shift of the curve shown in the graph in Fig. 1. The temperature is thus higher even during the transitional phase. As a result of this the electron emission is improved, thus enabling more stable arc ends and hence improved electromagnetic compatibility to be obtained.

The sizing of the diameter of the electrode tips, or in other words their reduction in size in comparison with known discharge lamps, is selected to be such that the desired reduction in the emission of electromagnetic interference signals is achieved. If however, to avoid any adverse effect there may be on lifespan or for other reasons, the diameter of the electrode tips is not to be reduced or if the said diameter may not be less than a certain value, and if for this reason it is not possible for a change to be made to the conditions governing temperature then, in a second embodiment, electromagnetic compatibility can be improved by reducing the work function for electrons at the electrodes.

This can be achieved by using a solid state emitter in the electrodes. What may be used as an emitter or emitters are for example ThO₂ and/or Y₂O₃ and/or HfC and/or Sc₂O₃ and/or Dy₂O₃ and/or La₂O₃ and/or CeO₃ and/or ZrO₂ and/or Pr₂O₃ and/or ZrC, with which material or materials the electrodes are doped. Doping of this kind results in a more severe, and in particular in a faster, reduction of the work function for electrons than is possible with the ThI₄ that is usually introduced into the salt filling of the discharge vessel for this purpose, which means that the ThI₄ is no longer required. The reason is the fact that the emitter is at once available at the surface of the electrodes, in contrast to ThI₄ that is added to the salt filling, which means that the emitter firstly has to vaporize, which takes a certain amount of time. However, during this time the electrode temperature may already have dropped appreciably due to the dynamic behavior of the ballast and the transitional phase may thus already have been initiated without there having been any lowering of the work function for the electrons. This results in electromagnetic compatibility being degraded in the way explained above. What is also achieved by the doping is that to a considerably greater degree the ends of the arc change from the diffuse mode to the spot mode simultaneously at both the electrodes, which means that in this way the transitional phase is, in addition, shortened by a corresponding amount and the spot mode is obtained at both electrodes after, for example, only approximately 10 to 15 seconds. In the third embodiment, electromagnetic compatibility is improved in an effective way by a particularly homogenous surface for the electrodes.

It has in fact been found that, when there is a particularly homogenous surface on which the end of the arc finds almost identical physical conditions everywhere on the electrode, arc instabilities such as generally occur when the electrode surface is not homogeneously covered, in particular with emitter material, can be substantially reduced or entirely avoided.

As has also been found, one major reason for non-homogeneous electrode surfaces is the presence of thorium or thorium compounds in the salt filling of the lamp, because these, when the lamp is switched on, change to the gaseous phase and, as was explained above in connection with the second embodiment, settle on the electrode surfaces only gradually, and result in the said non-homogeneities.

Thus, when electrodes that do not contain any thorium or thorium compounds are used, a particularly homogeneous surface can be obtained for the electrodes by virtue of the fact that no thorium or thorium compounds, and in particular no ThI₄, is or are introduced into the salt filling of the lamp.

The substantially more homogeneous conditioning of the electrode surfaces that is achieved in this way substantially improves the behavior of the lamp as far as EMC is concerned. The complete abandonment of thorium or thorium compounds (such as ThO₂ in particular) even in the electrodes gives the further advantage that the lamp has a high level of environmental compatibility.

The electrodes are preferably tungsten-doped electrodes that are doped with potassium and/or aluminum and/or silicon. As well as the good environmental compatibility that a lamp entirely free of thorium has, it also has the further advantage of a longer life, because chemical reactions with the thorium, which may result in corrosion of the electrode feedthroughs within the pinches and thus in speeding up of the occurrence of cracks in the pinches, are avoided. At the same time, there is also a saving of the money that, if thorium were used, would have to be spent on other measures for preventing corrosion of this kind and extending the life of the lamp.

Where required, the absence of thorium or thorium compounds in the lamp can be compensated for by a suitable thermal design for the electrodes and/or for the discharge vessel and/or by an adapted pinching or sealing process, to enable any specifications that may be relevant to be met in this way.

It has also been found, particularly with discharge lamps of the present kind that are subject to high thermal stresses, that the light flux falls off to an increased extent during their life and the lamp voltage rises to a corresponding degree. What is more, accelerated crystallization of the discharge vessel, reduced lumen maintenance and increasing arc instability may become apparent in such lamps, as a result of which there is a corresponding reduction in the life of the lamps.

It has been found that these problems can be substantially alleviated by reducing the absolute amount of salt mixture that is present in the discharge vessel. This reduction in the salt mixture preferably takes place only to a degree sufficient for there not to be any substantial adverse effect on the lighting and/or electrical characteristics of the lamp or for these characteristics not to drop below a desired lower limiting value. This may, furthermore, also be achieved by adjusting the xenon pressure or the salt composition in the discharge vessel.

The amount of salt mixture introduced into the lamp is preferably reduced to a value of between approximately 30% and approximately 60% of the amount that is normally used of between approximately 250 μg and approximately 350 μg, and preferably of approximately 300 μg. Starting from the preferred amount of 300 μg, this corresponds to a preferred reduction to between approximately 100 μg and approximately 200 μg in the case of a discharge vessel having a volume of between approximately 15 μl and approximately 30 μl, and preferably of 21 μl, the maximum inside diameter being between approximately 2 mm and approximately 3 mm, and preferably being 2.4 mm. The salt mixture comprises at least NaI and ScI₃ in this case and, as an option, may contain ZnI₂, InI and/or ThI₄ in addition. Fig. 2 is a schematic cross-section through a high-pressure gas discharge lamp of the present kind that is known per se and that has, in essence, a discharge vessel 1 having a discharge chamber 10, two pinches 2, 3 situated opposite one another, and electrodes 4, 5 that extend through the pinches 2, 3 and into the discharge vessel 1. When the lamp is in the operating state, an arc discharge 11 is excited between the tips of the electrodes 4, 5. In all the embodiments, the mean diameter of the electrode tip is preferably between approximately 200 μm and approximately 400 μm. This mean diameter is preferably taken over a length of the electrode tip of approximately 1 mm. What is basically true is that, as was described in connection with the first embodiment, electromagnetic compatibility is all the better the smaller this diameter is. However, at the same time allowance has to be made for the other known characteristics of the lamp that militate against an arbitrary reduction in diameter. A preferred mean diameter for the electrode tip is approximately 340 μm.

As has been described above, the electromagnetic compatibility of a discharge lamp, particularly during the starting or switching-on phase, is thus improved by reducing the work function for electrons of the electrode material, by increasing the homogeneity of the surface of the electrodes or by increasing the mean electrode temperature. These three measures may each be taken individually or in any desired combination with one another. The measures or measures selected and their combination will be governed essentially by the application for which the discharge lamp is intended and the desired level of electromagnetic compatibility.

Claims

1. A high-pressure gas discharge lamp having a means of improving electromagnetic compatibility (EMC) with other electrical or electronic components, particularly during the starting or switching-on phase, by increasing the mean temperature of at least one of the electrodes.

2. A high-pressure gas discharge lamp as claimed in claim 1, in which the means is formed by electrode tips of a diameter that is reduced to obtain the increase in temperature.

3. A high-pressure gas discharge lamp having a means of improving electromagnetic compatibility (EMC) with other electrical or electronic components, particularly during the starting or switching-on phase, by reducing the work function for electrons of the electrode material.

4. A high-pressure gas discharge lamp as claimed in claim 3, in which the means is formed by a solid state emitter with which at least one of the electrodes of the lamp is doped.

5. A high-pressure gas discharge lamp as claimed in claim 4, in which the solid state emitter is a ThO₂ and/or Y₂O₃ and/or HfC and/or Sc₂O₃ and/or Dy₂O₃ and /or La₂O₃ and/or CeO₃ and/or ZrO₂ and/or Pr₂O₃ and/or ZrC.

6. A high-pressure gas discharge lamp having a means of improving electromagnetic compatibility (EMC) with other electrical or electronic components, particularly during the starting or switching-on phase, by increasing the homogeneity of the surface of at least one of the electrodes.

7. A high-pressure gas discharge lamp as claimed in claim 6, in which the means is formed by electrodes that are at least substantially free of thorium or thorium compounds and by a salt filling for the discharge vessel that is free of thorium or thorium compounds.

8. A high-pressure gas discharge lamp as claimed in claim 7, in which the electrodes are composed of doped tungsten, the tungsten being doped with potassium and/or aluminum and/or silicon.

9. A high-pressure gas discharge lamp as claimed in claim 1 or 3 or 6, in which the lamp filling is at least substantially free of mercury.

10. A high-pressure gas discharge lamp as claimed in claim 1 or 3 or 6, in which the amount of salt mixture introduced into the discharge vessel is reduced to a value at which predetermined specified limiting values for the lighting and/or electrical characteristics of the lamp are still observed.

11. A high-pressure gas discharge lamp as claimed in claim 10, in which the amount of salt mixture introduced into the discharge vessel is reduced to between approximately 30% and approximately 60% of a known value, the salt mixture containing at least NaI and ScI₃.