WO2004055862A2

WO2004055862A2 - Mercury-free high-pressure gas discharge lamp

Info

Publication number: WO2004055862A2
Application number: PCT/IB2003/005650
Authority: WO
Inventors: Pieter Postma; Manfred Peterek; Michael Haacke
Original assignee: Philips Intellectual Property & Standards Gmbh; Koninklijke Philips Electronics N.V.
Priority date: 2002-12-18
Filing date: 2003-12-03
Publication date: 2004-07-01
Also published as: AU2003283695A1; WO2004055862A3

Abstract

A high-pressure gas discharge lamp (HID [high intensity discharge]) lamp) is described which is free from mercury and which is suitable in particular for use in automobile technology. The metal halides conventionally used as voltage gradient formers in mercury-free discharge lamps have various disadvantages as their proportional quantity in the gas filling increases, which disadvantages manifest themselves in particular in the form of an intensified arc constriction and arc curvature, and possibly also of a reduced diffuseness of the discharge arc. All these disadvantages are at least substantially avoided in that the use metal halides as voltage gradient formers is dispensed with, and that instead xenon is introduced into the gas filling at an increased pressure which excites an arc discharge.

Description

Mercury-free high-pressure gas discharge lamp

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

Conventional high-pressure gas discharge lamps usually contain besides a starter gas also a discharge gas (for example a metal halide such as sodium iodide or scandium iodide), which is the actual light-emitting material (light generator), as well as on the other hand mercury, which serves primarily as a voltage gradient former and has the essential function of promoting the evaporation of the light-generating substances through a rise in temperature and pressure and increasing the luminous efficacy and burning voltage of the lamp.

Lamps of this kind have come into large-scale use on account of their good photometric properties and they are increasingly also used in automobile technology. An additional requirement in part is also in particular for this application that the lamps should not contain mercury for environmental reasons.

EP 1 063 681 discloses a discharge lamp in which the use of mercury as a buffer gas is dispensed with so as to avoid environmental pollution and to reduce the ultraviolet radiation component. It is furthermore described therein, however, that mercury- free lamps cannot be used in vehicles because they do not generate the required luminous flux quickly enough after switching-on. This problem is said to be solved in that the gas filling comprises a buffer gas serving as a starter gas with xenon at a pressure at room temperature of between 7 and 20 at, furthermore sodium iodide and scandium iodide, or compounds thereof, as well as a metal halide with a low melting point of at most 400 °C such as, for example, indium halide or gallium halide.

A further measure proposed therein is to reduce the heat capacity and the heat losses of the discharge space so as to achieve a faster pressure and temperature rise upon switching-on. For this purpose, certain ratios between the interior gas pressure at room temperature on the one hand and the volume of the discharge space as well as its maximum wall thickness in various cross-sectional planes in the discharge space on the other hand should be observed.

It is a disadvantage here, however, that a reduced distance between the vessel inner wall and the discharge arc, which is curved upwards owing to convection, causes the light-generating substances to react with the silicon oxide, in particular in the comparatively hot upper wall region of the discharge vessel, which could lead to an increased crystallization which reduces lamp life.

A general problem in connection with mercury-free lamps is, furthermore, that the metal halides (such as in particular zinc iodide) used as a replacement for mercury and serving as voltage gradient formers may have the result that the discharge arc is constricted and its curvature is increased. The diffuseness of the discharge arc is reduced at the same time. These effects manifest themselves the more strongly as the proportion of the metal halide serving as the voltage gradient former is higher.

This is particularly disadvantageous because the upper wall region of the discharge vessel is particularly strongly heated thereby, and the temperatures of the coldest spots of the discharge vessel are further reduced. As was noted above, this reduces lamp life on the one hand, while on the other hand the luminous efficacy of the lamp is reduced by the stronger tendency to condensation in the cold spots.

A further factor which also contributes to a decrease in luminous efficacy of the lamp is the fact that the higher temperature gradients accompanying the arc constriction cause the molecular radiation to decrease.

All these effects have the result that the mercury-free discharge lamps, in which a metal halide such as in particular zinc iodide is used as a voltage gradient former instead of mercury, attain or exceed the presently valid limit values of the specification for use in the automobile field in many cases, so that they are not suitable for this application.

It is accordingly an object of the invention to provide a mercury-free high- pressure gas discharge lamp which is suitable for use in the automobile field as regards its luminous efficacy and arc curvature.

The invention has for its object in particular to provide a mercury-free high- pressure gas discharge lamp with which a luminous discharge arc with a substantially lesser curvature and constriction than in known mercury-free lamps can be generated, while the diffuseness is at least not substantially reduced. Furthermore, a mercury-free high-pressure gas discharge lamp is to be provided which has a higher operating voltage and thus a lower lamp current than is usually achievable in mercury-free lamps.

Finally, a mercury-free high-pressure gas discharge lamp is to be provided with which a luminous efficacy and operating voltage can be achieved which correspond essentially to those of mercury-containing lamps, without the necessity of increasing the lamp power or changing the external dimensions of the outer envelope of the lamp.

This object is achieved according to claim 1 by means of a mercury-free high- pressure gas discharge lamp in which the gas filling is formed by a discharge gas (light- generating substance) as well as by a rare gas, wherein the pressure of the rare gas is at least so high that an arc discharge can be excited.

Particular advantages of this solution are that the volume of the discharge space can be reduced because of the lesser arc curvature, so as also to reduce the temperature gradients in the wall of the discharge vessel, without the risk of an excessive heating and crystallization of the upper wall regions. Furthermore, the luminous efficacy of the discharge lamp can be substantially increased, for a given lamp power, in comparison with lamps having metal halides serving as the voltage gradient formers. Finally, the diffuseness of the arc discharge, which may even be increased in certain cases, contributes to the creation of a mercury-free discharge lamp which is capable of complying with the specification valid at the present time in the field of automobile technology.

The dependent claims relate to advantageous further embodiments of the invention.

The embodiment of claim 2 relates to a rare gas that is to be preferably used. Claim 3 relates to light-generating substances to be preferably used. The embodiments of claims 4 to 6 render it possible to increase the operating voltage and luminous efficacy of the lamp further.

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

Fig. 1 diagrammatically shows a high-pressure gas discharge lamp in a longitudinal sectional view. Fig. 1 diagrammatically shows the construction of a high-pressure gas discharge lamp according to the invention. The lamp of Fig. 1 comprises a discharge vessel 1 of quartz glass which encloses a discharge space 2. The discharge space 2 is bounded inter alia by wall regions 10 which are in lowermost position when the lamp is in its operational position and by upper wall regions 11 opposite to the former regions.

The free first ends of electrodes 3 extend into the discharge space 2 from the mutually opposed ends thereof, which electrodes are made from a material with as high as possible a melting temperature such as, for example, tungsten. The second ends of the electrodes 3 are each fastened to an electrically conducting tape or foil 4, in particular a molybdenum foil, via which an electrical connection is achieved between connection terminals 5 of the discharge lamp and the electrodes 3.

To safeguard a vacuumtight entry of the electrodes 3 into the discharge space 2, the discharge vessel 1 merges into quartz glass portions (pinches or metal-quartz fused lead-throughs) 6 in the entry regions, into which the second ends of the electrodes 3, the electrically conducting foil 4, and portions of the connection terminals 5 have been embedded.

An arc discharge 12 (luminous arc) is excited between the tips of the electrodes 3 in the operational condition of the lamp. The discharge space 2 contains a gas filling which comprises one or several light-generating substances (discharge gas), which substances emit the luminous radiation owing to excitation and discharge.

The light-generating substances are chosen from the group of metal halides and are, for example, sodium iodide and/or scandium iodide. To replace a metal halide used as the voltage-gradient former in known mercury-free discharge lamps, according to the invention, exclusively a rare gas such as in particular xenon is introduced into the gas filling. The xenon pressure is chosen here such that an arc discharge 12 is generated in the operational condition. The pressure is chosen in particular such that a sufficiently high, i.e. desired operating voltage and luminous efficacy of the lamp are obtained, said lamp properties being better in proportion as the xenon pressure is higher.

It was found that the constriction of the arc discharge 12, which is connected with a high proportional quantity of a metal halide (such as, for example, zinc iodide) serving as a voltage gradient former, can be substantially reduced thereby. This again has the result that the temperature gradient in the arc discharge 12 is smaller and the molecular radiation is stronger, so that the lamp will have a higher luminous efficacy than in the case of metal halides being used as voltage gradient formers.

A particular advantage of the lesser curvature of the luminous arc is that also the temperature gradient across the wall of the discharge vessel 1 becomes smaller, because the upper wall regions 11 are not heated so strongly, and the lower wall regions 10 are not cooled down so strongly.

This situation provides the possibility of further increasing the operating voltage and luminous efficacy of the lamp in that in addition the volume of the discharge space 2 is reduced compared with known similar discharge lamps. Furthermore, the wall thickness may be increased so as to improve the thermal conductivity and to reduce the temperature gradient in the wall of the discharge vessel.

To increase the operating voltage and the luminous efficacy of the lamp further, the wall regions 10 of the discharge space 2 which are in the lowermost position in the operational condition of the lamp, and which are usually the coldest spots, may be raised and may be shaped into a contour, for example, such that they approach the arc discharge 12, so that their temperature is raised thereby.

A rise in the temperature of the lower wall regions 10 of the discharge vessel 1 may also be achieved in that the pinches 6 are shifted relative to the longitudinal axis of the discharge vessel 1 in the direction of the lower wall regions 10, and the electrodes 3 are positioned such that at least their tips come closer to these regions.

It was also found that a diffuseness of the arc discharge which is at least not smaller than with the use of zinc iodide can be achieved when metal halides are dispensed with as voltage gradient formers and a rare gas such as in particular xenon is used at an increased pressure. The diffuseness may indeed remain the same or may even be increased, in dependence on the xenon pressure.

To clarify the advantages and properties that can be achieved with the discharge lamps or lamp fillings according to the invention, two examples are given below for the sake of comparison. With a known gas filling of 200 μg Na/Sc (60/40% by weight) as the light generators and 50 μg ZnJ as the voltage gradient former, a xenon pressure of 10 bar in the cold state leads to a luminous flux Φ of 2950 lm, a curvature C of the discharge arc of 0.75 mm, and a diffuseness D of the discharge arc of 0.78 mm. When the quantity of zinc iodide is raised to 100 μg in this gas filling, an otherwise unchanged composition leads to a reduced luminous flux Φ of 2900 lm, an increased curvature C of 0.85 mm, and a slightly reduced diffuseness D of 0.77 mm.

A first gas filling according to the invention comprises only 200 μg Na/Sc (60/40% by weight) as the light generator and xenon at a xenon pressure of 10 bar in the cold state. This leads to a luminous flux Φ of 3000 lm, a curvature C of 0.61 mm, and a diffuseness D of 0.80 mm.

A second inventive gas filling again comprises 200 μg Na/Sc (60/40% by weight) as the light generator and xenon at an increased xenon pressure of 13 bar in the cold state. This gives an increased luminous flux Φ of 3150 lm, an only slightly increased curvature C of 0.64 mm, and a somewhat reduced diffuseness D of 0.77 mm.

It is apparent from these comparative examples that not only is the luminous flux higher in the two inventive lamp fillings, but that in each case the curvature of the discharge arc is substantially smaller and its diffuseness is approximately the same or even greater.

Claims

CLAIMS:

1. A mercury-free high-pressure gas discharge lamp in which the gas filling is formed by a discharge gas (light-generating substance) as well as by a rare gas, wherein the pressure of the rare gas is at least so high that an arc discharge (12) can be excited.

2. A high-pressure gas discharge lamp as claimed in claim 1 , wherein the rare gas is xenon.

3. A high-pressure gas discharge lamp as claimed in claim 1 , wherein the discharge gas is sodium iodide and/or scandium iodide.

4. A high-pressure gas discharge lamp as claimed in claim 1, wherein, for the purpose of raising the temperature of the wall regions (10) which are lowermost in the operational position, the volume of the discharge space (2) is reduced, and/or the wall thickness of the discharge vessel (1) is increased for the purpose of increasing the thermal conductivity.

5. A high-pressure gas discharge lamp as claimed in claim 1, wherein the wall regions (10) of the discharge space (2) which are lowermost in the operational condition have a contour such that they approach the arc discharge (12) for the purpose of increasing the temperature of said wall regions.

6. A high-pressure gas discharge lamp as claimed in claim 1 , wherein the electrodes (3) are positioned such that at least their tips approach the wall regions (10) of the discharge space (2) which are lowermost in the operational position for the purpose of increasing the temperature of said wall regions.

7. A lighting unit comprising a high- pressure gas discharge lamp as claimed in claim 1.