WO2003094198A1

WO2003094198A1 - High-pressure gas discharge lamp

Info

Publication number: WO2003094198A1
Application number: PCT/IB2003/001609
Authority: WO
Inventors: Ralf Muckel; Michael Haacke
Original assignee: Philips Intellectual Property & Standards Gmbh; Koninklijke Philips Electronics N.V.
Priority date: 2002-05-02
Filing date: 2003-04-28
Publication date: 2003-11-13
Also published as: AU2003222381A1; TW200404327A

Abstract

A high-pressure gas discharge lamp, i.e. a HID (high intensity discharge) lamp, is described, which lamp is in particular free from mercury and suitable for use in automobile technology. The lamp is characterized in particular in that an increase in the temperature of the coldest (lowermost) bottom surface (10) is achieved by means of an eccentric arrangement of the discharge space (2) relative to the pinches (5), while the temperature of the hottest (upper) wall regions (13) is not increased, or may even be reduced. An essential advantage of this lamp is that its outer shape and the dimensions of the individual components, as well as the groove diameter of the lamp itself, need not be changed, which renders possible a very economical manufacture.

Description

High-pressure gas discharge lamp

The invention relates to a high-pressure gas discharge lamp, i.e. a HID (high intensity discharge) lamp, in particular one which is free from mercury and suitable for use in automobile technology.

Conventional high-pressure gas discharge lamps contain on the one hand a discharge gas (in general a metal halide such as sodium iodide or scandium iodide), which forms the light-emitting material or light generator proper, as well as mercury, which serves primarily as a voltage gradient generator and essentially has the function of increasing the luminous efficacy and burning voltage of the lamp.

Lamps of this kind have found wide acceptance because of their good properties, and they are increasingly used also in automobile technology. It is also partly required in particular for this application, however, that the lamps should contain no mercury for environmental reasons.

A general problem with mercury-free lamps, however, is that a given lamp power in continuous operation leads to a lower burning voltage and thus to a higher lamp current and a lower luminous efficacy.

It is accordingly an object of the invention to provide a high-pressure gas discharge lamp, which is capable with a mercury-free gas filling to achieve a luminous efficacy which corresponds substantially to that of lamps that do contain mercury.

Furthermore, a high-pressure gas discharge lamp is to be provided which has a higher burning voltage in combination with a mercury-free gas filling than can be normally achieved with mercury-free lamps.

In particular, a high-pressure gas discharge lamp is to be provided with which at least one of the objects mentioned above (higher luminous efficacy and higher burning voltage) can be achieved without the necessity of increasing lamp power or substantially changing the shape and dimensions of the pinches and of the discharge vessel of the lamp.

A further object is to provide a high-pressure gas discharge lamp which can be operated with a mercury-free gas filling and which has a lumen maintenance usual for automobile applications, i.e. in which the luminous decrement during lamp life shows a gradient similar to that of lamps containing mercury.

Finally, a high-pressure gas discharge lamp is to be created which is particularly suitable for use in automobile technology. The object is achieved in accordance with claim 1 by means of a high-pressure gas discharge lamp with a discharge vessel which encloses a discharge space having a bottom surface which is in lowermost position in a horizontal operational position of the lamp and which has a first region, on which first region light-generating substances collect in the switched-off state of the lamp, as well as pinches with electrodes which are embedded at least substantially symmetrically in the pinches in the direction which is vertical in the operational position and whose free ends extend into the discharge space, wherein the pinches are offset eccentrically in the direction of the bottom surface with respect to the discharge space such that the light-generating substances enter the gaseous state at least substantially owing to heating after switching-on of the lamp. A particular advantage of this solution is that the temperature of the coldest

(lowest) regions of the discharge vessel is increased by the eccentric arrangement of discharge space and pinches without a rise in the maximum temperature of the upper wall regions in the operational position occurring. The maximum temperature may even be reduced in the case of a suitable eccentricity, so that the temperature drop and thus the maximum thermal load (in particular in the form of thermal stresses and a risk of crystallization of the glass wall of the discharge vessel) in the lamp are substantially reduced.

The eccentricity has the result in particular that either mercury can be dispensed with without any substitute, or that a different, less environmentally unfriendly voltage gradient generator may be used instead of mercury, for example a suitable metal halide, such that in all cases the light-generating substances enter the gas phase at least to a substantial degree owing to the achieved higher temperature of the lower wall regions, i.e. in such a quantity that the luminous efficacy of the lamp and/or its burning voltage is increased thereby, i.e. reaches values which are comparable to those of lamps containing mercury. This may be additionally supported by the introduction of a rare gas (in particular xenon), by means of which the gas pressure in the discharge space is further increased.

Another advantage is that the symmetrical location of the electrodes in the pinches leaves the groove diameter of the lamp unchanged. The quality of the groove region as compared with known lamp shapes is not impaired, and a long lamp life is achieved. Since the pinches and the discharge vessel have substantially unchanged external shapes and dimensions, known production methods can be used for manufacturing the lamp.

A further advantage of this solution is that it can also be applied to discharge lamps with mercury in the gas fillings. The luminous efficacy of such lamps can be considerably increased thereby.

It should be noted here that EP 0 581 359 discloses a high-pressure gas discharge lamp in which the pinches are offset with respect to the axis of the discharge vessel in the direction of the lower wall thereof. The object of this is on the one hand to minimize the temperature difference between the upper and the lower wall of the discharge vessel and to raise the temperature of the coldest spot, and at the same time to shift the coldest spot from the regions of the seals below the electrodes to the region of the lower wall of the discharge vessel. On the other hand, the object is also that the pinches in the horizontal burning position of the lamp lie with their main surfaces (width dimension) in a horizontal plane, so that it is avoided that metal halides of the discharge gas condense in cracks of these pinches and thus adversely affect the photometric parameters. The discharge gas, however, comprises mercury, and it is not disclosed that the lamp can be operated with a mercury-free gas filling, so that the above requirements for use in automobile technology are not complied with. The dependent claims relate to advantageous further embodiments of the invention. The embodiment of claim 2 utilizes the fact that the light-generating substances have the characteristic that they migrate towards the coldest spots of their environment during switching-on of the lamp. The fact that the eccentricity is so dimensioned that said coldest spots are located not in the regions of the electrode entrances into the pinches, but, for example, in the first region mentioned above or at the side walls of the discharge vessel adjacent to said first region, prevents a migration of substantial quantities of the light-generating substances not evaporating upon switching-on of the lamp in the direction of the pinches and thus towards the entry locations of the electrodes, so that corrosion or other damage caused by these substances in the pinches is prevented.

The embodiment of claim 3 can be manufactured in a technically particularly simple manner and is accordingly preferred for reasons of economy. This is true also if the bottom surface is additionally provided with depressions and/or elevations.

Claims 5 and 6 relate to voltage gradient generators which are preferably used instead of mercury and by means of which a particularly good luminous efficacy can be achieved, while claim 7 provides an alternative possibility of achieving this object, describing in particular a higher luminous efficacy and burning voltage.

The embodiment of claim 8 achieves a particularly even temperature rise of the lower wall regions, while according to claim 9 the protection of the entry locations of the electrodes and the pinches situated behind them against the light-generating substances can be further improved.

Further details, features, and advantages of the invention will become apparent from the ensuing description of preferred embodiments, which is given with reference to the drawing, in which:

Fig. 1 is a diagrammatic side elevation of an embodiment of the invention.

Fig. 1 shows a high-pressure gas discharge lamp according to the invention in its horizontal operational position. The lamp comprises a discharge vessel 1 of quartz glass which encloses a discharge space 2 with a bottom surface 10 which is lowermost in the operational position and with an upper wall 13 opposite thereto, and which continues into the shape of pinches 5 at its mutually opposed ends. The discharge space 2 is filled with a gas, which is composed of a discharge gas (light generator) emitting light radiation through excitation or discharge and preferably a voltage gradient generator, which may both be chosen from the group of the metal halides.

The light-generating substance is, for example, sodium iodide and/or scandium iodide, whereas the voltage gradient generator used may be, for example, zinc iodide and/or other substances instead of mercury.

Alternatively to or in addition to the voltage gradient generator, certain quantities of rare gases (for example xenon) may be introduced into the discharge space so as to increase the gas pressure, and thus the luminous efficacy and burning voltage of the lamp, yet further. Electrodes 3, which are made from a material of the highest possible melting temperature such as, for example, tungsten, extend into the discharge space 2 from the mutually opposed ends thereof, and an arc discharge (luminous arc) 6 is excited between the tips of said electrodes in the operational state of the lamp. The respective other ends of the electrodes 3 are each fastened to an electrically conducting tape or foil 4, in particular a molybdenum foil, by means of which an electrical connection is achieved between the electrodes 3 and contacts 15 of the discharge lamp. The electrically conducting foil 4 and the ends of the electrodes 3 fastened thereto are embedded in the respective pinches 5, preferably symmetrically, i.e. in the centers thereof both in plan view and in side elevation when the lamp is in its operational position. This has the advantage in particular that the groove diameter of the lamp is not changed, as is the case with electrodes which are eccentrically or obliquely fastened to the foil 4, and that the quality of the groove region and lamp life are not adversely affected in comparison with known lamps. The fact that the shape and dimensions of the pinches and of the discharge vessel themselves remain unchanged renders it possible to use known production methods also for the manufacture of the lamp.

Starting from the operational position of the lamp shown in Fig. 1, the pinches 5 and the discharge space 2 are mutually offset in vertical direction, i.e. eccentrically arranged, so that the electrode tips are closer to the lowermost bottom surface 10 than to the upper wall 13.

Various parameters and criteria are to be observed in the dimensioning of this eccentricity, as will be explained in more detail below. It was noted above that the gas filling of the high-pressure gas discharge lamp according to the invention preferably comprises one or several suitable metal halides as a voltage gradient generator instead of mercury. Since these substances have a comparatively low partial vapor pressure, however, it is necessary to change the temperature balance in the discharge vessel 1 if substantially the same luminous efficacy or substantially the same luminous flux is to be obtained as with the use of mercury, as well as a burning voltage which is as high as possible. The temperature of the light-generating substances that have accumulated in solid form on the bottom surface 10, lowermost in the operational position, in a first region 11 should in particular be increased during switching-on of the lamp to the point that these substances enter the gaseous state in a sufficient quantity for achieving a desired, i.e. as high as possible a luminous efficacy and burning voltage. It should be taken into account here that the bottom surface 10 has the lowest temperature in the operational position of the lamp.

It should be ensured here that a change in the temperature conditions will have the result that the light-generating substances accumulated in the first region 1 should not, or at least not in a substantial quantity, be able to reach the entry locations 7 of the electrodes and thus the pinches 5 upon switching-on of the lamp owing to the temperature rise then occurring and the migration of said substances occasioned thereby, because damage through corrosion or similar effects may be caused there in the course of time. It should furthermore be ensured that the initially arising molten light- generating substance does not cover the electrode tips or the arc discharge 6 when the lamp is switched on.

These conditions can be fulfilled by dimensioning the eccentric arrangement of the pinches 5 with respect to the discharge space 2, possibly with a corresponding adaptation of the shape and/or size of the bottom surface 10 of the discharge space 2. In the embodiment of the invention shown in Fig. 1, this eccentric arrangement is dimensioned such that the electrode tips are located comparatively close above the first region 11 of the bottom surface 10, but this distance is at least so great that the electrodes 3 and the arc discharge 6 arising between them including the diffuse region thereof are not hampered or interfered with, and that no damage to the discharge vessel 1

(devitrification, crystallization, cracks) shortening lamp life can occur owing to excessive heating.

The distance is chosen in particular in dependence on the curvature of the arc discharge 6 such that the temperature of the light-generating substances accumulated in the first region 11 is raised to the extent that said substances evaporate in a sufficient quantity after switching-on of the lamp and also remain in the gas state in a sufficient quantity for lamp operation so as to achieve a desired, i.e. the highest possible luminous efficacy and burning voltage of the lamp.

The temperature at the upper wall 13 is not increased, or is even lowered, owing to the now increased distance of the light arc 6 to this wall, so that the thermal stresses and the accompanying load on the discharge vessel 1 as well as the risk of crystallization of the glass walls of the discharge vessel 1 are considerably reduced, and a correspondingly longer lamp life is achieved. In particular, a lumen maintenance comparable to that of discharge lamps containing mercury can be obtained. Furthermore, said distance is preferably so small, on the other hand, that the coldest spot of the inner wall of the discharge vessel 1 is not shifted into the region of the entry locations 7 of the electrodes 3 into the pinches 5, but remains in the first region 11 or in the directly adjoining regions of the side walls of the discharge vessel 1. It is prevented thereby that substantial quantities of light-generating substances, which tend to migrate towards the coldest regions after switching-on of the lamp, enter the pinches 5 and cause damage through corrosion or similar effects there.

An optimum offset (eccentricity) of the discharge vessel 1 is accordingly achieved when on the one hand the light-generating substances accumulated in the first region 11 are heated so strongly that they evaporate in the sufficient quantity described above after switching-on of the lamp, while on the other hand the temperature in the regions of the entry locations 7 of the electrodes 3 is preferably higher (but at least not substantially lower) than in the first region 11 , so that the non-evaporated light-generating substances do not reach the entry locations 7, at least not in substantial quantities. In the practical realization of an embodiment, a distance between the electrode tips and the bottom surface 10 situated below them of, for example, approximately 0.7 mm was found to be particularly suitable in view of the requirements mentioned above.

The increase in temperature of only the bottom surface 10 also achieves that the temperature drop along the wall of the discharge vessel 1, in particular between the upper and lower sides thereof, is reduced, so that also the thermal stresses in the vessel are substantially smaller.

This construction renders it possible in particular to achieve a luminous efficacy of the lamp as could hitherto be achieved substantially only with gas fillings containing mercury. Furthermore, the spectral characteristics and the color point of the generated light correspond substantially to those of lamps containing mercury, which is of particular importance for the application in the field of automobile technology.

The burning voltage of the lamp is also raised thereby in comparison with known mercury-free lamps and can be further raised through an additional adaptation (increase) of the initial xenon pressure to the volume of the discharge vessel 1. In addition to the measure described above, a penetration of migrating light- generating substances into the entry locations 7 may also be prevented by a suitable mutual attunement of the quantity of light-generating substances accumulating in the first region 11 and the size of this region 11, and in particular in that the bottom surface 10 comprises at least one second region 12 surrounding the first region 11 for the purpose of accommodating the migrating light-generating substances.

As was noted above, the first region 11 is that region on which the substantial, major portion of the light-generating substances is deposited in the switched-off state of the lamp. In the elliptical discharge space 2 shown in Fig. 1, this region lies approximately below the center between the two electrode tips, and accordingly in the deepest location of the discharge space 2 in the operational position of the lamp.

The second region 12 may alternatively be shaped such that it rises in a direction towards the entry locations 7 of the electrodes 3, or forms an elevation or a depression dimensioned and shaped such that they can serve as a barrier to migrating light- generating substances. The discharge vessel 1 may thus also have an asymmetrical longitudinal section.

In the embodiment shown in Fig. 1, the regions 11, 12 merge into one another in that the bottom surface 10 has a cradle shape, i.e. substantially the shape of half an oval in longitudinal section. This embodiment is preferred because of its comparatively simple manufacture and the economic advantages connected therewith.

The first and second regions 11, 12 are dimensioned in dependence on the quantity of light-generating substances accumulated in the switched-off state of the lamp and not evaporating in the operational state of the lamp so as to achieve the objects set out above. The temperature of the bottom surface and of the light-generating substances accumulated thereon can be further and more evenly increased in all embodiments by means of an additional coating which reflects incident infrared radiation and which is provided on the outer wall of the discharge vessel 1 in the region of the bottom surface 10, because the infrared radiation passes twice through these regions (once before and once after reflection). The coating may be formed substantially by zirconium oxide (ZrO ). Other materials, however, are also possible such as, for example, Nb₂O₅ and Ta₂O₅, which have an even better infrared-reflecting power than ZrO₂, but which are comparatively expensive. The use of SiO₂ in crystalline form, finally, is also conceivable.

Such a coating can be provided also on the outer wall of the discharge vessel 1 in those regions of all embodiments in which the entry locations 7 of the electrodes into the pinches 5 are located, so as to achieve alternatively to or in addition to the measures described above that the temperature in these regions is increased and accordingly as few as possible light-generating substances - or other deposited substances - migrate towards the entry locations 7 upon switching-on of the lamp. A luminous efficacy and/or burning voltage sufficient for certain applications may also be achieved in certain situations when mercury is dispensed with and no other voltage gradient generator is used instead, or when instead of the voltage gradient generator certain quantities of rare gases (for example xenon) are introduced into the discharge space for increasing the gas pressure. It should finally be pointed out that the principle of the invention may also be applied to advantage when a discharge gas comprising mercury is used and the disadvantages of mercury for the environment are accepted. In this case the temperature rise mentioned above may be used, for example, to increase the luminous efficacy or to reduce the input power of the lamp at a given luminous efficacy.

Claims

CLAIMS:

1. A high-pressure gas discharge lamp with a discharge vessel (1) which encloses a discharge space (2) having a bottom surface (10) which is in lowermost position in a horizontal operational position of the lamp and which has a first region (11), on which first region (11) light-generating substances collect in the switched-off state of the lamp, as well as pinches (5) with electrodes (3) which are embedded at least substantially symmetrically in the pinches (5) in the direction which is vertical in the operational position and whose free ends extend into the discharge space (2), wherein the pinches (5) are offset eccentrically in the direction of the bottom surface (10) with respect to the discharge space (2) such that the light-generating substances enter the gaseous state at least substantially owing to heating after switching-on of the lamp.

2. A high-pressure gas discharge lamp as claimed in claim 1, wherein the eccentricity is dimensioned such that the region of entrance (7) of each electrode (3) into the pinch (5) has a temperature in the operational state of the lamp which is higher than the lowest temperature at the bottom surface (10).

3. A high-pressure gas discharge lamp as claimed in claim 1, wherein the discharge space (2) in longitudinal section substantially has the shape of an oval, an ellipse, or a cylinder.

4. A high-pressure gas discharge lamp as claimed in claim 1 , comprising an at least substantially mercury-free gas filling.

5. A high-pressure gas discharge lamp as claimed in claim 4, comprising a voltage gradient generator in the form of one or several metal halides in its gas filling.

6. A high-pressure gas discharge lamp as claimed in claim 5, wherein the voltage gradient generator comprises zinc iodide.

7. A high-pressure gas discharge lamp as claimed in claim 1 , wherein the gas filling comprises additional quantities of rare gases, such as xenon, for increasing the gas pressure and raising the luminous efficacy of the lamp.

8. A high-pressure gas discharge lamp as claimed in claim 1, wherein the outer wall of the discharge vessel (1) situated opposite the lowermost, bottom surface (10) is provided with a coating comprising zirconium oxide (ZrO₂).

9. A high-pressure gas discharge lamp as claimed in claim 1, wherein the outer wall of the discharge vessel (1) is provided with a coating comprising zirconium oxide (ZrO ) in the region of each entry location (7) of the electrodes (3) into the respective pinches (5).

10. A lighting unit, in particular for motor vehicle headlights, with a high-pressure gas discharge lamp as claimed in any one of the preceding claims.