Thorium-free discharge lamp with reduced halides and increased relative amount of Sc

A high-pressure gas discharge lamp has a sealed discharge vessel with an inner discharge space. Two electrodes project into the discharge space. The filling in the discharge space includes metal halides and a rare gas. The filling is free of Hg and Th and includes halides in an amount of 7.1-11.4 μg/mm3 of the volume of the discharge space. The halides includes at least NaI and ScI3, provided in such amounts that a ratio of the masses of NaI to ScI3 is 1.2-1.6.

FIELD OF THE INVENTION

The present invention relates to a high-pressure gas discharge lamp, in particular for use in automotive front lighting.

BACKGROUND OF THE INVENTION

A high-pressure gas discharge lamp comprises a sealed discharge vessel with an inner discharge space. Two electrodes project into the discharge space, arranged at a distance from each other, to ignite an arc therebetween. The discharge space has a filling comprising a rare gas and further ingredients such as metal halides. Discharge lamps are used in the automotive field due to their high efficiency and good radiating properties. While many discharge lamps used for automotive front lighting contain mercury, lately, mercury-free lamp designs have been proposed for environmental reasons.

However, besides mercury, also thorium is present in discharge lamps. On one hand, thorium may be present in the salts contained in the lamp filling, e.g. as thorium-iodide. On the other hand, thorium is commonly present as thorium oxide in tungsten electrodes.

EP-A-1349197 describes a mercury free metal halide lamp for use in an automotive headlight. In order to achieve an enhanced luminous efficiency, a low lamp voltage reduction, light with a chromaticity suitable for an automotive headlamp, and an increased, rapidly rising luminous flux, the amount of first halides containing a scandium halide (mass a) and a sodium halide (mass b) are chosen such that 0.25<a/(a+b)<0.8 and preferably 0.27<a/(a+b)<0.37. A second halide (mass c) is present for providing a lamp voltage in place of mercury in an amount such that 0.01<c/(a+b+c)<0.4, and preferably 0.22<c/(a+b+c)<0.33. The halides are present in the discharge vessel in an amount of 0.005-0.03, preferably 0.005-0.02 mg/mm3 of the inner volume. Additionally, Xenon gas is present in the discharge medium at 5-20 atmospheres cold pressure. Rod-shaped electrodes are provided with a shaft diameter of 0.3 mm or more which may be made of tungsten, doped tungsten, rhenium, a rhenium/tungsten alloy or the like. An outer envelope houses the discharge vessel, which may be hermetically sealed from the outside air or may have air or an inert gas at an atmospheric or reduced pressure sealed therein. In an example, tungsten electrodes of 0.35 mm diameter are provided in a discharge vessel of 34 mm3. The discharge medium contains 0.1 mg of ScI3, 0.2 mg of NaI and 0.1 mg of ZnI2 with Xe gas at 10 atm at 25° C. In a first comparative example with a higher amount of the second halide the amount of halides are 0.08 mg ScI3, 0.42 mg NaI and 0.30 mg ZnI2. In a second comparative example the amount of halides are 0.1 mg ScI3, 0.5 mg NaI and 0.2 mg ZnI2.

SUMMARY OF THE INVENTION

Lamps for use in the automotive field have to comply with certain requirements. Besides the run-up properties (amount of light delivered shortly after ignition of the lamp as well as possible electromagnetic emissions after ignition) and steady-state requirements such as a high luminous flux and specified color this concerns lifetime properties such as lumen maintenance (limited loss of light output in long-term use), limited lamp voltage increase and limited color shift.

It is an object of the present invention to provide a lamp with good environmental properties, yet suited for automotive use.

The inventors have recognized that simply removing thorium both from the lamp filling and the electrode material of known lamp designs will not lead to a lamp fulfilling automotive requirements. Thus, additionally to removing thorium from the lamp, the invention proposes special measures with regard to the filling within the discharge space which have surprisingly shown to lead to lamp designs with good lifetime behavior.

According to the invention, halides are provided within the filling in a tightly specified amount, which is significantly reduced with the regard to prior designs. The filling contains halides in an amount of 7.1-11.4 μg/mm3of the volume of the discharge space. The halides comprise at least NaI and ScI3, which are provided with a ratio of their masses 1.2-1.6.

If a lamp is designed simply starting from a known lamp design and eliminating ThO within the electrode material, the resulting Th-free lamps have been found to suffer from severe disadvantages, such as decreased lumen maintenance and flicker.

The specific choice of filling addresses the problems associated with eliminating thorium both from the filling and the electrode material of a discharge lamp. Within the electrode material, thorium (or, more specifically, ThO2) serves in its property as a solid state emitter to lower the work function of the electrode. Thus, in operation at electrically comparable parameters, an electrode without ThO will have a higher temperature. Electrode burn-back will increase. Due to the then reduced electrode length, more heat will be transferred to the discharge vessel material, e.g. quartz glass, leading to adverse lifetime behavior.

The inventors have recognized that the problems of Th-free lamps, specifically decreased lumen maintenance, are caused by different temperature behavior of the Th-free lamp as compared to prior designs. The inventors believe the observed low lumen maintenance to be caused by chemical reactions of the Sc component within the filling occurring at the elevated temperatures.

To counter this effect, it has surprisingly proven successful to limit the salt filling to a relatively small amount, but at the same time increase the relative amount of Sc within the filling. By providing an overall lower amount of halides, also the critical Sc amount is reduced. However, the amount of halides must not be reduced too much to avoid excessive color shift.

On the other hand, a reduced amount of halides alone would result in a limited luminous flux. Thus, the composition of the halides is changed to increase the proportion of Sc as a chief light-generating component. However, again the amount of Sc cannot be increased arbitrarily, because a too high amount of Sc would lead to light of a color no longer suited for automotive head lighting.

Thus, with the above measures, the amount of the critical component Sc has been on one hand reduced (by providing a reduced amount of halides), and on the other hand increased (by limiting the NaI/ScI3ratio). While it could be expected that these two opposite measures would neutralize, and that the resulting lamp would suffer from the known lumen maintenance problems, it has surprisingly be found that the proposed measures lead to a lamp with an excellent lumen maintenance, and still fulfill the remaining requirements for automotive head lighting.

According to a further development of the invention, the amount of halides within the filling is further specified to be 8.5-10.5 μg/mm3of the volume of the discharge space. Also, the NaI/ScI3mass ratio is most preferably 1.3-1.4. Within these intervals, the properties of the lamp with regard to lumen output and lumen maintenance have been found to be optimal.

According to a further development of the invention, the halides within the filling further comprise ZnI2, provided mainly for achieving a desired lamp voltage, which may initially be in the interval of 39-45 V. The amount of ZnI2may be chosen e.g. in the interval 0-20 wt-% of the halides, mostly depending on required electrical properties (voltage). Preferably it is proposed to be 4-12 wt-%, most preferably 6-10 wt-% of the halides. This has been found to limit the lamp voltage increase over the lifetime, especially if a rare gas is present at a cold pressure of no more than 17 bar. In this case it is possible that 60 V are only reached after more than 2000 hours.

As a further component, the halides preferably comprise InI, most preferably in an amount of 0.12-0.25 wt-% of the halides. Most preferably, the halides only consist of NaI, ScI3, ZnI2and InI.

In a preferred embodiment of the invention the volume of the discharge space is 10-45 mm3, especially preferred 19-25 mm3. The invention could be applied to many different types of lamps to provide Th-free designs with excellent lifetime behavior. Such lamps will typically have a quartz glass discharge vessel and may be designed for 20-45 W. Especially preferred are automotive lamps with e.g. 35±3 W, which further fulfill the R99 regulations.

A rare gas provided in the filling is preferably xenon, which may be present at a cold pressure of 11-17 bar. A pressure of 13-15 bar is especially preferred.

The electrodes are preferably made of tungsten. Generally, they could be of any shape, including stepped electrodes, or electrodes which—especially for large diameters—have a structure in the region embedded in the quartz material, such as coiled electrodes or laser treated electrodes. In a preferred embodiment, they are simply rod-shaped (i.e. pieces of drawn wire). A rod diameter of 280-320 μm is preferred, which is especially applicable for preferred wattages of 35±3 W. For strongly differing operating powers the diameter may need to be adjusted.

Further developments of the invention relate to an outer envelope provided around the discharge vessel. The outer envelope is preferably sealed and filled with a gas to provide well-defined thermal properties. Most preferably, the outer envelope is filled with a gas having a thermal conductivity of 60-90 mW/m*K at 800° C. This property may be achieved, as will become apparent in connection with the preferred embodiment, by choosing an appropriate combination of a gas filling material and pressure.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1shows a side view of an embodiment10of a discharge lamp. The lamp comprises a socket12with two electrical contacts14which are internally connected to a burner16.

The burner16is comprised of an outer bulb18of quartz glass surrounding a discharge vessel20. The discharge vessel20is also made of quartz glass and defines an inner discharge space22with projecting electrodes24. The glass material from the discharge vessel further extends in longitudinal direction of the lamp10to seal the electrical connections to the electrodes24which comprise a flat molybdenum foil26.

The outer bulb18is arranged around the discharge vessel20at a distance, thus defining an outer bulb space28. The outer bulb space28is sealed.

The discharge vessel20has an outer wall arranged around the discharge space22. The discharge space22is of ellipsoid shape. Also, the outer shape of the outer wall is ellipsoid.

The discharge vessel20is characterized by an electrode distance d, and a volume V of the discharge space.

The lamp10is operated, as conventional for a discharge lamp, by igniting an arc discharge between the electrodes24. Light generation is greatly influenced by the filling comprised within the discharge space22, which is free of mercury and includes metal halides as well as a rare gas.

The discharge vessel20of the lamp10has a wall thickness of 1.85 mm. The discharge space22has a length (rim distance) of 8 mm and a central inner diameter of 2.4 mm. The volume of the discharge space22is 21 mm3.

The electrodes24are rod-shaped electrodes of pure tungsten material, and are free of Th. Alternatively, the material may be tungsten with dopants such as Al, Si and/or K. The rod electrodes have a diameter of 300 μm. The electrode distance is 3.7 mm (x-ray), such that the length of each of the electrodes24projecting into the discharge space22is around 2.15 mm.

The outer bulb18serves to control heat transfer during operation of the discharge vessel20to the outside. The outer bulb18is sealed and filled with a filling gas. The filling is chosen to achieve a heat conductivity (to be measured at 800° C.) within a range of 60 to 90 mW/m*K, preferably 68-76 mW/m*K. This heat conductivity may be achieved e.g. by a oxygen (68 mW/n*K) or air (76 mW/m*K) filling in the outer bulb, which both already have a suitable conductivity in the specified range, or alternatively by a filling combining gas of higher conductivity (such as, e.g. neon at 110 mW/m*K) with gas of lower conductivity such as, e.g. argon at 45 mW/m*K. Generally a pressure of the gas filling in the outer bulb18is preferably in the range of 10 mbar-2 bar, most preferably 30-200 mbar.

The outer bulb is essentially cylindrical, with a distance between the inner walls of the outer bulb18and the outer surface of the discharge vessel20of around 0.3 mm measured in a transversal plane arranged centrally between the electrodes24.

In the following, examples of lamp fillings will be given.

FIRST, PREFERRED EXAMPLE

In the first example, the filling within the discharge vessel22of the lamp described above provided as follows:

SECOND EXAMPLE

In the second example, the filling within the discharge vessel22of the lamp described above is provided as follows:

amount of halides220μgamount of halides per mm310.48μg/mm3of the discharge space 22mass ratio NaI/ScI31.39ZnI2proportion10wt-%composition of halides115 mg NaI, 82.6 mg ScI3,0.4 mg InI, 22 mg ZnI2rare gas fillingxenon at 15.1 bar (cold pressure)
Lamp Properties

FIG. 2shows the lumen maintenance for the above first example. As shown, the lumen maintenance is significantly above specifications by automotive manufactures.

Also, as shown inFIG. 3, the increase in lamp voltage over lifetime is limited, such that a limit of 60 V is not exceeded within a lifetime of 2000 hours. The color shift X (FIG. 4a) and Y (FIG. 4b) for the example is also well-confined within acceptable bounds.

Thus, lamp designs have been shown for environmentally compatible Th-free and Hg-free lamps, which have a good lumen maintenance and fulfill all further requirements for automotive front lighting.