Methal halide lamp comprising a shaped ceramic discharge vessel

The invention provides a metal halide lamp having a ceramic discharge vessel, wherein the discharge vessel has a spheroid-like shape with a length L1 and a largest outer diameter d2, the discharge vessel further having curved extremities and openings at the curved extremities which have a curvature r3. The discharge vessel has an aspect ratio L1/d2 of 1.1?≦L1/d2?≦2.2 and a shape parameter r3/d2 of 0.7≦r3/d2≦1.1. This lamp has the advantage that it can be operated at a relatively high power. Furthermore, the lamp has a relatively high efficacy. Moreover, the lamp can be operated horizontally and vertically, i.e. it can be qualified for universal burning.

FIELD OF THE INVENTION

The present invention relates to a metal halide lamp comprising a ceramic discharge vessel, particularly a shaped ceramic discharge vessel.

BACKGROUND OF THE INVENTION

Metal halide lamps are known in the art and are described in, for instance, EP 0215524 and WO 2006/046175. Such lamps operate at high pressures and have burners or ceramic discharge vessels comprising ionizable gas fillings of, for instance, NaI (sodium iodide), TlI (thallium iodide), CaI2(calcium iodide) and/or REIn. REInrefers to rare-earth iodides. Characteristic rare-earth iodides for metal halide lamps are CeI3, PrI3, NdI3, DyI3, and LuI3.

Most present-day discharge vessels for metal halide lamps have a spherical shape, as described in, for instance, DE 20205707, a cylindrical shape, as described in, for instance, EP 0215524 or WO 2006/046175, or an extended spherical shape as described in, for instance, EP 0841687 (U.S. Pat. No. 5,936,351). The latter document describes a ceramic discharge vessel for a high-pressure discharge lamp constituted by a cylindrical central part and two hemispherical end pieces, wherein the length of the central part is smaller than or equal to the radius of the end pieces. In this way, the isothermy of the discharge vessel is improved.

SUMMARY OF THE INVENTION

These prior-art metal halide lamps or ceramic discharge metal halide lamps (CDM lamps) have one or more of the drawbacks that their lumen maintenance is less than would be desired. Another drawback may be that the combination of a high color rendering, indicated by means of the commonly used general color-rendering index Ra, also known as CRI, with values of about 90 or more, and a high efficacy, such as about 110 lm/W or more, does not seem to be easily possible. Color rendering for nine standard colors, particularly important for the red part of the color spectrum and indicated by R9, is generally very poor at very low values, which can even be negative. Particularly when they are operated at a relatively high power of about 150 W or more, such prior-art lamps sometimes have the further drawback that they are not qualified for universal burning, i.e. burning in a universal position, and can therefore be operated, for instance, only in a vertical arrangement of the burner (discharge vessel) in order to prevent cracks in the burner or its protruding end plugs, which may result in explosion of the burner.

It is an object of the invention to provide an alternative metal halide lamp which preferably further obviates one or more of the drawbacks described above.

To this end, the invention provides a metal halide lamp comprising a ceramic discharge vessel, wherein the discharge vessel has a wall enclosing a discharge space with an ionizable filling, the discharge space further enclosing electrodes having electrode tips arranged opposite each other and arranged to define a discharge arc between the electrode tips during operation of the lamp, the discharge vessel having a spheroid-like shape with a major axis and a length L1(outer length), a largest inner diameter d1and a largest outer diameter d2and further having curved extremities with a curvature with radius r3, wherein an aspect ratio L1/d2is 1.1≦L1/d2≦2.2 and a first shape parameter r3/d2is 0.7≦r3/d2≦1.1.

This lamp has the advantage that it can be operated at a relatively high power, e.g. at more than about 150 W. Furthermore, the lamp has a relatively high efficacy; efficacies of over 115 lm/W are possible at these high power values. Moreover, the lamp can be operated horizontally and vertically, i.e. it can be qualified for universal burning. It also appears that the lamp is less apt to forming cracks in the discharge vessel during its lifetime as compared with state-of-the-art lamps. For instance, when a lamp having a shape parameter of 0.5 is used (which is outside the claimed range), cracks are often observed in the wall of the discharge vessel at high power values. Likewise, discharge vessels of lamps having a large shape parameter often show cracks. However, the discharge vessel of the lamp according to the invention has a shape that provides stability while allowing a high power during operation of the lamp, as well as a high efficacy and universal burning.

In a preferred embodiment, the electrode tips are arranged at a distance L3of each other, and a second space parameter, L3/L1, is in the range of 0.4≦L3/L1≦0.7. Within this range, stable discharge vessel (operation) is found, whereas the formation of cracks increases outside this range.

In a specific embodiment, the discharge vessel further comprises protruding end plugs which surround at least part of the electrodes.

In a preferred embodiment, the ionizable filling comprises NaI, TlI, CaI2and X-iodide, wherein X is one or more elements selected from the group comprising rare-earth metals, scandium and yttrium. Particularly lamps having such fillings according to the invention show good optical properties and maintenance. In yet another preferred embodiment, the filling of the discharge space also comprises one or more halides selected from Mn and In, which is especially useful for obtaining lamps with a high correlated color temperature (CCT). Hence, in an embodiment, the ionizable filling further comprises one or more halides selected from the group consisting of Mn and In, especially Mn and/or In iodides.

DESCRIPTION OF EMBODIMENTS

General Description of the Lamp

Metal halide lamps or ceramic discharge metal (CDM) halide lamps are generally known. An embodiment of such a metal halide lamp is schematically depicted inFIG. 1. In general, metal halide lamps, here denoted by reference numeral25, comprise a discharge vessel1surrounded with clearance by an outer envelope105and having a ceramic wall or vessel wall30(with an internal surface12and an external surface13, seeFIG. 2) which encloses a discharge space22having a filling comprising an inert gas, such as xenon (Xe) or argon (Ar), and an ionizable salt, and with two electrodes4and5arranged in said discharge space22. The discharge vessel1is surrounded by an outer bulb or an outer envelope105which is provided with a lamp cap2at one end. The outer envelope105may be vacuum or filled with an inert gas such as nitrogen. In operation, a discharge extends between the electrodes4and5. The electrode4is connected to a first electric contact forming part of the lamp cap2via a current lead-through conductor8. The electrode5is connected to a second electric contact forming part of the lamp cap2via a current lead-through conductor9.

In the schematicFIGS. 1 to 4, the discharge vessel1further comprises protruding end plugs34,35, each at one side and each arranged to enclose at least part of the electrodes4,5, respectively. However, the invention is also applicable to discharge vessels1which do not comprise such protruding end plugs34,35(see also below).

In this description and claims, the ceramic wall30is understood to mean both a wall of metal oxide such as, for instance, sapphire or densely sintered polycrystalline Al2O3and metal nitride, for instance, AlN. According to the state of the art, these ceramics are well suited to form translucent discharge vessel walls30.

FIG. 2shows a preferred embodiment of the lamp in more detail. A shaped discharge vessel1is schematically depicted. The lamp shown is not drawn to true scale.FIG. 2shows that the electrodes have electrode tips4b,5bhaving a mutual interspacing so as to define a discharge path between them during operation of the lamp. In the embodiment, each electrode4,5is supported by a current lead-through conductor20,21entering the discharge vessel1. The current lead-through conductors20,21preferably consist of a first part made of a halide-resistant material such as, for instance, a Mo—Al2O3cermet, and a second part made of, for instance, niobium. Niobium is chosen because this material has a coefficient of thermal expansion corresponding to that of the discharge vessel1and prevents leakage from the lamp25. Other possible constructions are known, for instance, from EP0587238 (herein incorporated by reference, wherein a Mo coil-to-rod configuration is described). The current lead-through conductors may be sealed into the protruding end plugs34,35with seals10.

General Description of the Ionizable Filling

The ionizable filling generally comprises a salt (including a mixture of salts). The ionizable filling used in the invention preferably comprises one or more components selected from the group comprising iodides of Li, Na, K, Rb, Cs, Mg, Ca, Sr, Ba, Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, In, Tl, Sn, Mn, and Zn, particularly one or more components selected from the group comprising LiI, NaI, KI, RbI, CsI, MgI2, CaI2, SrI2, BaI2, ScI3, YI3, LaI3, CeI3, PrI3, NdI3, SmI2, EUI2, GdI3, TbI3, DyI3, HoI3, ErI3, TmI3, YbI2, LuI3, Inl, TlI, SnI2, MnI2, and ZnI2. Furthermore, the discharge space22generally contains Hg (mercury) and a starter gas such as Ar (argon) or Xe (xenon), as known in the art. In a preferred embodiment of a lamp according to the invention, the discharge vessel1further contains mercury (Hg). In an alternative embodiment, the discharge vessel1is free from mercury, i.e. the filling quantities do not take the quantity of mercury that is present into account. Mercury is dosed to the discharge vessel1in quantities known to the person skilled in the art.

The ionizable filling preferably comprises NaI, TlI, CaI2, and X-iodide, wherein X is one or more elements selected from the group comprising rare-earth metals, yttrium and scandium. X can thus be formed by a single element or by a mixture of two or more elements. For the sake of simplicity, the terms “rare earth” and “X” include Sc and Y.

In a preferred embodiment of the lamp25according to the invention, X is the total quantity of rare earth, and the molar percentage ratio X-iodide/(NaI+TlI+CaI2+X-iodide (+optionally MnI2and/or InI)) is above 0% up to maximally 10%, particularly in the range of 0.5 to 7%, more particularly in the range of 1 to 6%. At a too low quantity of X, experiments have proved that the electrodes may reach too high temperature values to operate satisfactorily. At quantities of X above the indicated maximum, it becomes more difficult to maintain a W-halide cycle in the discharge vessel1during lamp operation.

With X being the total quantity of rare earth (including Sc and Y), the molar percentage ratio CaI2/(NaI+TlL+CaI2+X-iodide (+optionally MnI2and/or InI)) is preferably in the range of 10 to 95%. In another preferred embodiment of a lamp according to the invention, the quantity of NaI, TlL, CaI2and X-iodide (+optionally MnI2and/or InI) is in the range of 0.001 to 0.5 g/cm3, particularly in the range of 0.005 to 0.3 g/cm3. The volume of the discharge vessel particularly ranges between 1.0 and 10.0 cm3, depending on the lamp power. Characteristic quantities of ionizable gas fillings are salt doses of about 5 to 50 mg.

To have a lamp which emits light at a color temperature (CCT) above 3500 K during its stable nominal operation, the filling of a preferred embodiment of the lamp according to the invention also comprises one or more halides selected from Mn and In. With the addition of a halide of Mn and/or In, the color point of the light emitted by the lamp can be adjusted primarily along the x-axis of the CIE color triangle having x,y-coordinates.

Varying the quantity of Tl halide in the filling has a major impact on the adjustment of the color point along the y-axis. In this respect, stable nominal operation is understood to mean that the lamp25is operated at a power and voltage for which it is designed. The designed power of the lamp25is referred to as the nominal or rated power. Wall load as herein defined is the lamp power divided by the surface of the external wall13excluding the optional protruding end plugs34,35. Characteristic wall loads of the wall of the discharge vessel on the surface13of the lamp25of the invention are in the range of about 18 to 30 W/cm2, particularly in the range of about 20 to 28 W/cm2. Loads on the surface12of the internal wall are generally in the range of about 25 to 35 W/cm2.

Preferred fillings are described in WO2005/088675, which is herein incorporated by reference.

Shaped Discharge Vessel

The discharge vessel of the lamp25of the invention will now be described in detail. A preferred embodiment, including optional features such as the protruding end plugs34,35, is schematically depicted inFIG. 3(not drawn to scale).FIG. 3shows an embodiment of the discharge vessel1of a metal halide lamp25having a ceramic wall30which encloses a discharge space22containing an ionizable filling. Two, for instance, tungsten electrodes4,5with tips4b,5bat a mutual distance L3are located in the discharge vessel1. In this schematically depicted embodiment, the discharge vessel1is closed by means of ceramic protruding end plugs34,35and encloses current lead-through conductors20,21connected to electrodes4,5positioned in the discharge vessel1with a narrow intervening space, and is connected to these conductors20,21in a gastight manner by means of a melting-ceramic joint or sealing10at ends remote from the discharge space22(see also above). However, the invention is not limited to the embodiment depicted inFIG. 3, see, for instance, alsoFIG. 4. The description of the discharge vessel1below first concentrates on the general aspects of the shaped discharge vessel1of the lamp25of the invention, and then deals with some preferred embodiments.

The discharge vessel1has a spheroid-like shape with a major axis60and an outer length L1, a largest inner diameter d1and a largest outer diameter d2. Furthermore, the discharge vessel1has curved extremities114,115and openings54,55at (or in) the curved extremities114,115. These openings54,55are arranged to surround the electrodes4,5. The curved extremities114,115have a curvature with radius r3. The shaped discharge vessel1of the lamp of the invention is defined by an aspect ratio AR=L1/d2and a first shape parameter SP=r3/d2.

Spheroids are known in the art and are obtained by rotating an ellipse about one of its major axes. The discharge vessel1of the invention has a spheroid-like shape, more particularly a prolate spheroid-like shape (i.e. a shape like a rugby ball). A prolate spheroid has a major axis, here denoted by reference numeral60, and a minor axis, here denoted by reference numeral61; the major axis60is larger than the minor axis61.

FIG. 4schematically depicts a plurality of possible discharge vessel constructions, both within and outside the aspect ratio and shape parameter values as described herein. The term “spheroid-like shape” is used because the discharge vessel1of the lamp25of the invention may have shapes close to spherical at low aspect ratios AR and small values of the first shape parameter SP. At intermediate aspect ratios and first shape parameter values, the discharge vessel1substantially has a spheroid shape. When the aspect ratio AR further increases, particularly to above about 1.5, the discharge vessel1can be characterized by a spheroid having a central cylindrical part. InFIG. 4, this is indicated by a cylindrical intermediate part116which may (substantially) be absent at low aspect ratios and low shape parameters but is particularly present at relatively high aspect ratios. Hence, the discharge vessel of the lamp of the invention has shapes varying from close to spherical shapes to cigar-like shapes. These shapes are herein indicated as “spheroid-like shapes”.

Since the discharge vessel1has a spheroid-like shape, this also implies that a discharge vessel1having a shape close to spherical has a radius r3that is substantially constant over the curved extremities114,115. However, when the discharge vessel1has a shape deviating from close to spherical and a shape that is more like a spheroid, the radius r3may vary over the curved extremities114,115in some embodiments. Radius r3may therefore also be indicated as mean radius r3. As will be clear to the person skilled in the art, the mean curvature1/r3can then be derived by integrating the local curvature along the contour of the curved part and dividing by the length of the contour along which the curvature is integrated.

The discharge vessel1of the lamp25of the invention is substantially symmetrical around major axis60. For the sake of clarity, a coordinate system is drawn inFIG. 3, wherein the major axis60extends along the y axis and the minor axis61extends along the z axis, perpendicular to the y axis. The discharge vessel1is essentially rotationally symmetric around major axis60. Furthermore, a longitudinal axis100through the discharge vessel1is drawn. Major axis60coincides with part of this longitudinal axis. The optional protruding end plugs34and35(see above and below) are also rotationally symmetric around the longitudinal axis100of the discharge vessel (and thus in fact also around major axis60).

The discharge vessel has a largest internal radius r1, i.e. the length of a perpendicular from major axis60to the internal surface12of vessel wall30, and a largest external radius r2, i.e. the length of a perpendicular from major axis60to the external surface13of vessel wall30. Hence, the discharge vessel1has a wall thickness w1which is equal to r2-r1. The thickness w1is preferably substantially equal throughout the wall30of the discharge vessel. The discharge vessel1preferably has a wall thickness w1in the range of 0.5 to 2 mm, more preferably from about 0.8 to 1.2 mm. As indicated inFIG. 3, the discharge vessel1also has a largest inner diameter d1, i.e. the largest diameter of the vessel from internal surface12to an opposite internal surface measured along a perpendicular to major axis60. This inner diameter d1is equal to the length of the minor axis61within the discharge vessel1. Furthermore, the discharge vessel1has a maximum outer diameter d2. The outer diameter d2is equal to the length of the minor axis61. As will be clear to the person skilled in the art, (d1+d2)/2=w1.

The part or region of the discharge vessel1with the largest diameter d2is indicated as intermediate region116. In fact, the discharge vessel1of the invention can be considered as two curved parts or curved extremities114,115between which an intermediate region116is found which may be, for instance, cylindrical. These regions or parts114,115and116are only indicated for the sake of simplicity.

The extremities114and115of the discharge vessel1are curved. Note that, in the Figures, protruding end plugs34and35are connected to these extremities. The protruding end plugs are optional and will be described below. These curved extremities have a certain curvature (or mean curvature) with radius r3(see above). Since the discharge vessel is rotationally symmetric around its major axis60and preferably also symmetric around its minor axis61, the curvature of these curved extremities114,115is the same at each side from an intersection (vertex) of major axis60and minor axis61. The vessel1is characterized by AR=L1/d2which is 1.1≦L1/d2≦2.2 and the first shape parameter SP=r3/d2which is 0.7≦r3/d2≦1.1.

The curved extremities114and115have openings54and55which are arranged to enclose or surround the electrodes4and5at least partially. Note that the electrodes4,5, or more precisely the current lead-through conductors20,21, may be directly sintered to the wall30of the discharge vessel, but may also be partially integrated into the protruding end plugs34,35. Furthermore, the current lead-through conductors20,21may also be directly sintered into the protruding end plugs34,35, respectively, or sealed into the protruding end plugs34,35with seals10. Anyhow, the current lead-through conductors20,21are arranged in discharge vessel1in a vacuumtight manner.

The electrodes4,5enter the discharge vessel1via openings54and55which surround at least part of the electrodes. The mutual distance between the openings54,55, or the distance from one side of the major axis60to the other side of the major axis60is indicated as length L1(or outer length L1) of the discharge vessel1. Hence, length L1is equal to the length of the major axis60and diameter d2is equal to the length of the minor axis61. The electrodes4,5have electrode tips4band5bwhich are arranged at a mutual distance L3. This distance is often also indicated as ED or EA. Note that the electrodes4,5are located in the discharge vessel1along major axis60.

The invention thus provides a metal halide lamp25comprising a ceramic discharge vessel1which has a wall30enclosing a discharge space22with an ionizable filling, the discharge space22further enclosing electrodes4,5having electrode tips4b,5barranged opposite each other and arranged to define a discharge arc between the electrode tips4b,5bduring operation of the lamp25, the discharge vessel1having a spheroid-like shape with a major axis60and a length L1, a largest inner diameter d1and a largest outer diameter d2and further having curved extremities114,115and openings54,55at the curved extremities114,115, which openings54,55are arranged to surround the electrodes4,5or the current lead-through conductors20,21, and the curved extremities114,115have a curvature r3, wherein the aspect ratio AR=L1/d2is 1.1≦L1/d2≦2.2 and the first shape parameter SP=r3/d2is 0.7≦r3/d2≦1.1.

As regards aspect ratio AR and first shape parameter SP, and particularly when using the preferred ionizable fillings as described above (i.e. NaI, T11, CaI2and X-iodide and optionally MnI2and/or InI), it appears that lamps25used under these shape conditions have excellent optical properties, maintenance, efficacy and universal burning.

At larger or smaller values of the first shape parameter SP and aspect ratio AR, cracks are often found, leading to failure of the lamp. A relatively low efficacy is found in some cases in which an aspect ratio AR close to about 1.0 is used. In other cases, in which a shape parameter SP of, for instance, 0.5 is used, cracks are often observed in the wall of the discharge vessel, particularly at high power values. The efficacy is reduced at lower values of L1/d2. The risk of failure increases at higher values of L1/d2. If the shape parameter r3/d2is too low or too high, the risk of failure will also increase. Hence, it appears that, particularly under the conditions of the discharge vessel1as defined above, the lamp25of the invention has the advantages of a high efficacy, good maintenance in a universal burning position and good optical properties (relatively high values for CRI (color rendering), R9and color temperature CCT) and a long lifetime. Efficacies of at least 110 lm/W during operation (stable operation at rated power) and even efficacies of at least 115 lm/W (stable operation at rated power) can be obtained for the lamp25of the invention.

Lamps25with a first shape parameter of 0.75≦r3/d2≦0.9 and/or an aspect ratio of 1.3≦L1/d2≦1.7 are particularly advantageous in terms of efficacy, color rendering and a long lifetime.

Lamps can be made with a nominal power of any suitable value ranging from about 20 W to about 1000 W or more. The lamp is preferably made with wattages of more than 100 W, preferably more than 150 W (even up to or more than 1000 W) that qualify for a universal burning position. Hence, the rated power of the lamp25may be larger than 100 W, preferably of the order of about 150 W or more, preferably in the range of 150 W to 1000 W, although larger power values are also possible. Characteristic wattages are, for instance, 150 W, 210 W, 315 W, 400 W, 600 W, and 1000 W.

Moreover, it appears that the ratio of the distance L3between the electrode tips4b,5band the length L1of the discharge vessel1is advantageously in the range of 0.4 to 0.7. In this way, the distance of the electrode (tips) to the wall30of the discharge vessel, i.e. particularly its internal surface12, is sufficient so that crack formation is prevented or reduced. Hence, the ratio L3/L1, indicated as second space parameter SPP, is preferably 0.4≦L3/L1≦0.7. If the second space parameter SPP=L3/L1is smaller than about 0.4, the lamp efficacy will become too low, and if the second space parameter is above 0.7, the electrode tips4b,5bmay come too close to the wall30, which leads to cracking of the discharge vessel1.

In a specific variant, which is preferably applied, the discharge vessel1further comprises protruding end plugs34,35, as schematically depicted inFIGS. 2 to 4. Together with the wall30of the discharge vessel, these protruding end plugs34,35may constitute one body. The protruding end plugs34,35are rotationally symmetric around a longitudinal axis100and are arranged to enclose the current lead-through conductors20and21, respectively. The conductors20,21, may be sealed into the protruding end plugs34,35by means of seal10or may directly be sealed into the plugs34,35without using a separate sealing material to form seal10. The protruding end plugs have an inner diameter d6, d7and an outer diameter d4,d5, respectively. Furthermore, the protruding end plugs34,35have a wall width w2which is preferably substantially equal to wall width w1of the wall30of the ceramic discharge vessel. The plugs34,35have a length L4,L5, respectively, which are preferably substantially equal. Hence, in one embodiment, the openings54,55at the curved extremities114,115may be arranged to surround the electrodes4,5(particularly when no protruding end plugs34,35are used) and, in another embodiment, they may be arranged to surround the current lead-through conductors20,21.

At the end of the extremities114,115, the wall30of discharge vessel1may have a further curvature which is different from the curvature with radius r3, in the direction of the protruding end plugs34,35. This curvature is indicated as radius r4. This curved part is generally only a minor part of the curved extremities114,115. The curvature radius r4is generally of the order of about 0.5 to 3.0 mm, preferably 1.0 to 2.0 mm.

The invention also relates to a metal halide lamp25to be used in a vehicle headlamp and to a headlamp comprising the lamp25according to the invention.

Examples

A large number of experimental lamps were made. Some examples and comparative examples with discharge vessels1described herein and fulfilling the criteria described above, as well as discharge vessels having aspect ratios and shape parameters outside these criteria were made and measured. An overview is given of the lamps that were made, with discharge vessel dimensions in Table 1, fillings according to Table 2 and results given in Table 3.

These data show that lamps25according to the invention with discharge vessels1as defined above, i.e. lamps1-7,11-12have excellent properties, whereas discharge vessels8,9and10not according to the invention show failures (cracks, etc.) or have a relatively low efficacy. Lamp10is similar to the lamp described in EP0841687 (SP about 0.5). All lamps according to the inventions have a R9of 60 or more.