FIELD OF THE INVENTION

The present invention relates to a high intensity discharge lamp with UV enhancer. BACKGROUND OF THE INVENTION

UV enhancers for metal halide lamps are known in the art. US6392343, for instance, describes a metal halide lamp having an outer bulb containing besides a discharge vessel a UV enhancer (UVE). The UVE has a ceramic wall and is provided with a pair of internal electrodes. At least one of the electrodes may have an electrode winding with emitter.

US5942840 describes a high-pressure discharge lamp having a discharge vessel, an outer bulb enclosing said discharge vessel and defining an intervening space there between, a UV-enhancer positioned in the space between the outer bulb and the discharge vessel, the UV-enhancer provided with a wall fabricated of a ceramic material and an internal electrode, wherein an end portion of the UV-enhancer is closed with a compressive seal.

SUMMARY OF THE INVENTION

A known problem of many types of HID lamps is the comparatively wide spread in ignition time. This points to a shortage of free electrons during lamp ignition. In order ensure the lamp ignition reliability of a HID (high intensity discharge) lamp the radioactive isotope of krypton, ⁸⁵ Kr, is often used instead of a UV-enhancer. The addition of a small quantity of ⁸⁵ Kr in the discharge vessel can supplement such a shortage. This appears to be an easy solution, but a disadvantage of this, however, is that ⁸⁵ Kr is radioactive.

Alternative UV-enhancers, without ⁸⁵ Kr, have been developped and are available , but they are often costly, and also complicate the lamp making process.

Hence, it is an aspect of the invention to provide an alternative HID lamp, which is preferably easily ignited, especially also in the dark, without the need of added ⁸⁵ KR, or other radioactive materials that might be used to improve ignition, and which alternative HID lamp preferably further at least partly obviates one or more of above- described drawbacks.

A surprisingly relative simple way to generate UV light and so free electrons in a HID lamp surprisingly appeared to be a field emission diode structure and is suggested here. Each lamp has an anode and cathode. During the ignition step one or more pulses of > 2 kV may be used. When the anode is at the right distance from the cathode an electrical field strength up to for instance 2-3ν/μιη may be generated. By applying a field emitter material to one of the electrodes and a UV luminescent material to the other electrode, UV light might be generated, because electrons may move from the anode to cathode, when an electrical field strength of such as >0.5ν/μιη is generated, and hit the luminescent material. This

luminescent material, upon excitation, emits UV light which enhances start up of the HID lamp by producing free electrons inside the discharge vessel. When the lamp has started up, the voltage drops significantly between the anode and the cathode, so,then there will be no UV emission from the UV luminescent material. However, this is also not necessary anymore, since the lamp has already successfully been ignited.

The luminescent material can for instance be applied as paste. The field emitter material can consist of very sharp metal tips but can also be a applied as an emitter paste. In an embodiment this emitter paste may contain (very small) conductive carbon nano tubes (CNT) with threshold values of about 0.5 V/μιη or larger. This value indicates that from electrical field strengths of about 0.5 V/μιη and higher the field emitter material may start to emit electrons.

Hence, in a first aspect the invention provides a high intensity discharge lamp (HID lamp) comprising an outer bulb containing a first current conductor, a second current conductor, and a discharge vessel enclosing a discharge space which accommodates two electrodes, which are in electrical contact with the current conductors via current lead- through conductors, respectively, and which (discharge vessel) may especially contain a salt filling (as may be in the case of metal halide lamps and ceramic metal halide lamps), wherein the first and the second current conductors further comprise a field emitter material (herein also indicated as "field emitter") and a UV emitter material (herein also indicated as

"luminscent material"), respectively, wherein the UV emitter material is excitable by the field emitter material.

The phrase "wherein the first and the second current conductors further comprise a field emitter material and a UV emitter material, respectively", indicate that the first current conductor may further comprise the field emitter material and the second current conductor may further comprise the UV emitter material. The terms "first" and "second" are only used to distinguish. Hence, this phrase may equally indicate that the second current conductor may further comprise the field emitter material and the first current conductor may further comprise the UV emitter material. The phrase "wherein the UV emitter material is excitable by the field emitter material" implies that the field emitter material when providing electrons (due to the field) to the UV emitter material, excites the UV emitter material, upon which the UV emitter material emits UV radiation.

Such a lamp appears to be easiliy ignited, even in the dark, while addition of radio active isotopes, such as ⁸⁵ Kr or the addition of Th is not necessary. Hence, an embodiment of such a lamp can be essentially free of added ⁸⁵ Kr. Such a lamp can also be essentially free of added Th (thorium) and other optionally added radioactive elements or radioactive isotopes. However, the exclusion of any radioactive isotope and/or radioactive element is not compulsory. So, it is still possible to have a radioactive elements or radioactive isotopes supplied in a lamp of the inventionin addition to the advantageous presence of field emitter material and UV emitter material.

Such HID lamp may contain substantially no radioactive isotope of krypton while being able to be ignited without substantial problems and in the absence of (additional) UV enhancers. Further, such HID lamp may be relatively easily be made, without too much impact on the production process and/or without bulky add-ons, like additional light sources, etc. Hence, an environmental more friendly lamp may be provided, in an even easier production way, with a reliable UV enhancer. Here, the UV enhancer is the arrangement of field emitter material to the first current conductor (including a branch thereof, see also below) and the UV emitter material to the second current conductor (including a branch thereof, see also below).

The term "essentially free from added ⁸⁵ Kr" herein indicates that the discharge vessel is substantially free of ⁸⁵ Kr enriched Kr gas. This may in an embodiment be obtained by not using Kr gas, but this may in an embodiment also be obtained by using Kr gas, but not enriching such gas with the radioactive isotope ⁸⁵ Kr. Hence, essentiall free from added ⁸⁵ Kr may in an embodiment indicate that krypton is present, with all its naturally occuring isotopes with their natural occurrence. Likewise, this applies to other radioactive elements that are sometimes used to improve ignition, such as Th enriched tungstun as electrodes. For thorium applies that almost all isotopes are radioactive. Hence, the lamp may in an embodiment comprise electrodes that are not enriched with Th. The high intensity discharge lamp may for instance be a mercury vapour lamp, a metal halid lamp, a quartz metal halide lamp (QMH), a ceramic metal halide lamp

(MH/CDM), a sodium vapour halide lamp, a xenon short-arc halide lamp or an ultra-high performance lamp (UHP). In an embodiment, the high intensity discharge lamp is a metal halide lamp. Especially, the discharge vessel is a ceramic discharge vessel, i.e. the HID lamp is a CDM lamp.

Metal halide lamps are known in the art and are described, for example, in EP0215524, WO2006/046175, and WO05088675. Such lamps operate at a high pressure and comprise ionizable gas fillings of, for example, Nal (sodium iodide), Til (thallium iodide), Cal ₂ (calcium iodide), and/or REI _n . REI _n refers to rare-earth iodides. Characteristic rare earth iodides for metal halide lamps are Cel ₃ , Prl ₃ , Ndl ₃ , Dyl ₃ , and Lul ₃ . An important class of metal halide lamps includes ceramic discharge metal halide lamps (CDM-lamps), which are described in those documents. In the art, the term "salt filling" is sometimes also indicated as "ionizable gas filling" or "ionizable salt filling".

In an embodiment, the field emitter material may comprises carbon nano- tubes. The relative large aspect ratio of the tubes as well as the conductivity of the tubes may allow them to function as Spindt tip or corona tip. As will be clear to the person skilled in the art, also metal tips (Spindt tips) may be applied. However, carbon nano tubes appear to be very suitable and relatively easily applied to the electrodes (or branch thereof, see also below).

Carbon nano tubes may for instance be obtained by a carbon CVD (chemical vapour deposition) process. For instance, part of the electrode may be pre-treated to provide seeds, such as Fe seeds, Ni seeds, etc. (via solved salts like iron nitrate or nickel nitrate, etc.) or thin film deposition, such as of appr 1-20 nm iron and/or nicker metal film. After the pretreatment, carbon CVD may be applied. In this way, carbon nano tubes are formed on the electrode part. Alternatively, a carbon nano tube containing paste may be applied to part of the electrode. Especially the latter option is a relative easy option for a standard production process of the HID lamp.

In an embodiment, the field emitter material comprises one or more of single wall carbon nano tubes, double wall carbon nano tubes and multi-wall carbon nanotubes. By choosing the type of carbon nano tubes, the threshold voltage may be chosen. Hence, dependend upon the type of HID lamp, the type and pressure of the filling of the outer bulb, the ignition voltage, etc., the type of carbon nano tube may be selected. For instance, the threshold for single wall carbon nano tubes may in about at least 0.5 V/μιη and for multi-wall carbon nano tubes about 1 V/μηι. The threshold for double wall carbon nano tubes is in between.

The precise threshold further depends upon the aspect ratio of the carbon nano tubes, which offers a further parameter to tune the threshold to the desired value. Above the indicated threshold, i.e. above the indicated field strengths, the carbon nano-tube based field emitter starts to emit electrons, which can be absorbed by the UV emitter material.

As will be clear to a person skilled in the art, the field emitter material may also comprise a plurality of different field emitter materials

In an embododiment, the UV emitter material comprises a material suitable to emit at a wavelenght shorter than 320 nm upon excitation by the field emitter material. Most electrode work functions have a threshold of 320 nm (i.e. 3.88 eV). For instance, tungsten (W), has a workfunction in the range of 4.32-5.22 eV. Hence, for a tungsten electrode (in the discharge vessel), a wavelength of 285 nm and smaller is desriable. Hence, in a specific embodiment, the UV emitter material comprises a material suitable to emit at a wavelenght shorter than 285 nm upon excitation by the field emitter material. Other electrode materials, like WRe (tungsten rhenium), Ta (tantallum), Ta-carbide, hafnium carbide, tungstun carbide, etc. may then also be succesfully be illuminated with the UV photons and thereby generate electrons starting/enhancing the discharge within the discharge vessel.

In an embodiment, the UV emitter material comprises a luminescent material selected from the group consisting of (a) LnP0 ₄ :Pr ³⁺ , wherein Ln comprises one or more of La, Lu, Y and Gd, (b) LnP0 ₄ :Bi ³⁺ , wherein Ln comprises one or more of Lu and Y, and (c) LnP0 ₄ :Ce ³⁺ ,wherein Ln comprises one or more of La and Gd. Such materials may emit in the UV, especailly at wavelengths lower than 320 nm and are excitable by field emitters. Other luminscent materials emitting in the UV may also be applied. For instance, luminscent materials known from UV water treatment. As will be clear to a person skilled in the art, the luminscent material may also comprise a plurality of different luminscent materials.

The field emitter material and UV emitter material have a shortest distance to each other. This distance is non-zero, otherwise the field could not be generated. Especially the high intensity discharge lamp is configured to generate an initial potential difference between the current conductors during a start up process of the high intensity discharge lamp, wherein the shortest distance is chosen to have an electric field strength generated between the field emitter material and a UV emitter material during the start up process of at least 0.5 V/μιη. The higher the predetermined initial potential for the ignition stage, the larger the distance may be. The initial potential difference may for instance be in the range of 1.5-30 kV. One or more ignition pulse may be used to generate the discharge in the discharge vessel. Electronics in the lamp will control the ignition pulse(s). As indicated above, the desired electrical field strength may also depend upon the type of field emitter material. The person skilled in the art may chose those parameters that may give the best ignition results (in the sense of quick and reliable start).

The luminescent material and field emitter material may be arranged in different type of arrangements within the outer bulb (and outside the discharge vessel). The respective current conductors can be used, and the materials may be applied to these respective electrodes. However, it might also be beneficial to use one or more branches of such electrodes. Without too much change of the production process, in such way the distance bewteen the materials can more easily be tuned.

In an embodiment, one or more of the field emitter material and a UV emitter material are arranged on a first branch of the first current conductor and a second branch of the second current conductor, respectively. Hence, in embodiments the field emitter material may be arranged on a first branch of the first current conductor, or the luminescent material may be arranged on a second branch to the second current conductor, or the field emitter material may be arranged on the first branch of the first current conductor and the

luminescent material may be arranged on the second branch to the second current conductor. As indicated herein, the terms "first" and "second", etc. are only use for distinguishing substantially identical items.

For good results, preferably the UV emitter material is arranged downstream of the field emitter material, and relative to the field emitter material, upstream of the discharge vessel. Hence, in an embodiment the UV emitter material is arranged between the a field emitter material and the discharge vessel. In this way, most photons of the UV emitter material may upon excitation by the electrons travel in the direction of the discharge vessel, especially in the direction of one or both electrodes.

The terms "upstream" and "downstream" relate to an arrangement of items or features relative to the propagation of the light (here in fact electrons) from a light generating means (here the especially the field emitter material), wherein relative to a first position within a beam of light/electrons from the light generating means, a second position in the beam of light/electrons closer to the light generating means is "upstream", and a third position within the beam of light/electrons further away from the light generating means is "downstream". In an embodiment, the UV emitter material is comprised in a pressed body, such as a pressed sintered body. A pressed body may be contained in a electrically conductive holder or may be arranged to, such as attached to, an electrical conductive plate, in electrical contact with the respective current conductor.

Further, one or more of the UV emitter material and/or field emitter material may be applied as a coating to the respective current conductors or to electrically conductive branches thereof, but may also be contained in electrically conductive holders, in electrical connection with the respective current conductors.

Non-pressed material may be applied to such electrically conductive holder or electrically conductive plate, for instance by coating the material (UV emitter material or field emitter material, respectively) to (in) such holder or plate or branch. For instance, carbon nano-tubes may be grown to (in) such holder or plate or branch. For example, the material may be applied to the holder or plate before assembling the lamp. After arranging the field emitter materail or UV emitter material to the holder or plate, the holder or plate may be brought into connection with a predetermined connector, for instance by welding or soldering. An advantage of carbon nano tubes, but possibly also for other type of field emitters, may be that during assembling the lamp, the carbon nano tubes in the container are better protected and may less easily be damaged.

In a further embodiment, the UV emitter material is applied as coating to a conductive plate. For instance, such conductive plate may comprise a UV transmissive plate, like a UV transmissive glass plate, to which a conductive layer is applied, preferably a transmissive conductive layer, such as an indium tin oxide layer (ITO). The UV emitter can be applied to the conductive layer. Though transmissiveness of the plate and the conductive layer is not necessary, it may be advantageous in view of minimizing light loss.

The lower the pressure in the outer bulb, the easier electrons may travel from the field emitter material to the UV emitter material. Hence, the lower the pressure, the larger the distance may be. Therefore, also pressure is a paremeter that can be varied to obtain the desired ignition parameters. Larger pressures than 1 arm may be less desired in view of the excitability of the UV emitter material. In an embodiment, the pressure is 10 mbar or smaller, such as 1 mbar or smaller.

Hence, the invention provides a high intensity discharge lamp, wherein it is no longer necessary to have added, for example[to the volume contained by the outer bulb, radioactive isotopes (such as ⁸⁵ Kr or thorium). The volume contained by the outer bulb also includes the (volume of the) discharge vessel. The volume of the discharge vessel includes the discharge space. Especially the discharge vessel can be kept free of added radioactive isotopes. In a specific embodiment, the invention provides a high intensity discharge lamp, wherein the volume contained by the outer bulb, especially the (volume of the) discharge vessel, is free of added ⁸⁵ Kr. Further, in a specific embodiment, the volume contained by the outer bulb is free of thorium. Herein, the phrase "free of added radioactive isotopes", or "free of added ⁸⁵ Kr", or "free of thorium" especially refers to essentially free of the indicated item(s).

Use of the adverb "substantially" as used in this description and claims, such as in "substantially all emission" or in "substantially consists", will be understood by the person skilled in the art. "Substantially" may also include embodiments mentioning the adverbs "entirely", "completely", and the adjective or pronoun "all", etc. Hence,

"substantially" may also be removed in embodiments. Where applicable, "substantially" may also relate to 90% or more, such as 95% or more, particularly 99% or more, more particularly 99.5%) or more, including 100%). The verb "comprise" also includes embodiments in which it means "consists o '.

The lamps mentioned hereinbefore are described, inter alia, in their state of operation. As will be clear to the person skilled in the art, the invention is not limited to methods of operation or lamps in operation.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb "comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. In the device claim enumerating several means, several of these means may be embodied by one and the same item of hardware.

The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

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

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which:

Fig. 1 schematically shows an embodiment of a lamp according to the invention in a side elevation;

Fig. 2 schematically shows an embodiment of the discharge vessel of the lamp of Fig. 1 in more detail;

Fig. 3 schematically shows an embodiment having an alternatively shaped discharge vessel;

Figs. 4 schematically shows a specific embodiment of the arrangement of field emitter and UV emitter material;

Fig. 5a-5c schematically shows some embodiments of the arrangement of field emitter and UV emitter material.

The drawings are not necessarily on scale.

DESCRIPTION OF EMBODIMENTS

Figures 1-3 describe a specific embodiment of high intensity discharge lamps, for the sake of understanding without yet incorporating the presence of the field emitter and UV emitter material. Figures 4 and 5a-5c schematically depict for high intensity discharge lamps in general, for instance for the embodiments schematically depicted in figures 1-3, embodiments of arrangements of field emitter and UV emitter material.

As mentioned above, in an embodiment the lamp of the invention may comprise a ceramic discharge vessel. This particularly means that the walls of the ceramic discharge vessel preferably comprise a translucent crystalline metal oxide such as

monocrystalline sapphire and densely sintered polycrystalline alumina (also known as PCA), YAG (yttrium aluminum garnet) and YOX (yttrium aluminum oxide), or translucent metal nitrides such as A1N. The vessel wall may consist of one or more (sintered) parts, as known in the art (see also below).

An embodiment of the lamp of the invention will now further be described with reference to Figs. 1-3, but as indicated above, for the sake of understanding yet without the field emitter and UV emitter material. Arrangements with field emitter and UV emitter material are shown in the figures 4 and 5a-5c. However, the lamp of the invention is not confined to the embodiments described below and/or schematically shown in Figs. 1-3. Lamp 1 may be a high- intensity discharge lamp. Figs. 1-3 schematically show discharge vessels 3. Current lead-through conductors 20, 21 are sealed with two respective seals 10 (sealing frits, as known in the art). However, the invention is not limited to such embodiments. Lamps wherein one or both of the current lead-through conductors 20, 21 are, for example, directly sintered into the discharge vessel 3 may also be considered.

Specific embodiments are herein described in more detail, with both current lead-through conductors 20, 21 being secured in discharge vessel 3 by means of seals 10 (see also Figs. 1-3). Two electrodes 4, 5, for example, tungsten electrodes, with tips 4b, 5b are arranged at a mutual distance EA in the discharge space 11 so as to define a discharge path between them. The cylindrical discharge vessel 3 has an internal diameter D at least over the distance EA. Each electrode 4, 5 extends inside the discharge vessel 3 over a length forming a tip-to-bottom distance between the wall 31 of the vessel (i.e. reference signs 33a, 33b (see also below) and the electrode tips 4b, 5b. The discharge vessel 3 may be closed at either side by means of end wall portions 32a, 32b forming end faces 33a, 33b of the discharge space. Each end wall portion 32a, 32b may have an opening in which a respective ceramic projecting plug 34, 35 fits in a gastight manner by means of a sintered joint S. The discharge vessel 3 is closed by means of these ceramic projecting plugs 34, 35, each of which encloses a current lead-through conductor 20, 21 (generally including respective components 40, 41; 50,51, which are explained in more detail below) to the electrodes 4, 5 positioned in the discharge vessel 3 with a narrow intervening space and is connected to this conductor in a gastight manner by means of a melting-ceramic joint 10 (further indicated as seal 10) at an end remote from the discharge space 11. Here, the wall 30 of the ceramic discharge vessel comprises wall 31, ceramic projecting plugs 34, 35, and end wall portions 32a,32b.

The discharge vessel 3 is surrounded by an outer bulb 100 which is provided with a lamp cap 2 at one end. A discharge will extend between the electrodes 4 and 5 when the lamp 1 is operating. The electrode 4 is connected via a current conductor 8 to a first electric contact forming part of the lamp cap 2. The electrode 5 is connected via a current conductor 9 to a second electric contact forming part of the lamp cap 2.

Each ceramic projecting plug 34, 35 narrowly encloses a current lead-through conductor 20, 21 of a relevant electrode 4, 5 having electrode rods 4a, 5a which are provided with tips 4b, 5b, respectively. Current lead-through conductors 20, 21 enter discharge vessel 3. In one embodiment, each current lead-through conductor 20, 21 may comprise a halide- resistant portion 41, 51, for example, in the form of a M0-AI ₂ O ₃ cermet, and a portion 40, 50 which is fastened to a respective end plug 34, 35 in a gastight manner by means of seals 10. Seals 10 extend through some distance, for example, approximately 1-5 mm, over the Mo cermets 41, 51 (during sealing, ceramic sealing material penetrates the free space within the respective end plugs 34, 35). Parts 41, 51 may be formed in an alternative manner instead of from a M0-AI ₂ O ₃ cermet. Other possible constructions are known, for example, from

EP0587238 (herein incorporated by reference, wherein a Mo coil-to-rod configuration is described). A particularly suitable construction was found to be a halide-resistant material. Parts 40, 50 are made from a metal whose coefficient of expansion corresponds very well to that of the end plugs 34, 35. Niobium (Nb) is chosen, for example, because this material has a coefficient of thermal expansion corresponding to that of the ceramic discharge vessel 3.

Fig. 3 shows another embodiment of the lamp according to the invention.

Lamp parts corresponding to those shown in Figs. 1 and 2 are denoted by the same reference numerals. The discharge vessel 3 has a shaped wall 30 enclosing the discharge space 11. The shaped wall 30 forms an ellipsoid in the embodiment shown here. Compared with the embodiment described above (see also Fig. 2), the wall 30 is a single entity, in fact comprising wall 31, respective end plugs 34, 35, and end wall portions 32a, 32b (shown as separate parts in Fig. 2). A specific embodiment of such a discharge vessel 3 is described in more detail in WO06/046175. Alternatively, other shapes, such as, for example, spheroid are equally possible.

Wall 30, which in the embodiment schematically shown in Fig. 2 may include ceramic projecting plugs 34, 35, end wall portions 32a, 32b, and wall 31, or wall 30 (as schematically shown in Fig. 3) is a ceramic wall here, which is to be understood to mean a wall of translucent crystalline metal oxide or translucent metal nitrides such as A1N (see also above). According to the state of the art, these ceramics are well suited to form translucent walls of the discharge vessel 3. Such translucent ceramic discharge vessels 3 are known; see, for example, EP215524, EP587238, WO05/088675, and WO06/046175. In a specific embodiment, the discharge vessel 3 comprises translucent sintered AI ₂ O ₃ , i.e. wall 30 comprises translucent sintered AI ₂ O ₃ . In the embodiment schematically shown in the Figures, wall 30 may also comprise sapphire.

The filling in the lamp 1 of the invention may comprise Ca^ Til and preferably Hg^. Furthermore, the discharge space 11 preferably contains Hg (mercury) and a starter gas such as Ar (argon) or Xe (xenon), as known in the art (but herein preferably not enriched with one or more radioactive isotopes).

Characteristic Hg quantities are between about 1 and 100 mg/ml Hg, particularly in the range of about 8-25 mg/ml Hg; characteristic pressures are in the range of about 2-50 bar. The quantity of mercury in the discharge vessel 3 is preferably chosen to provide a mercury gas at nominal use without condensation of mercury, i.e. the mercury vapor is unsaturated. In principle, the lamp of the invention may also be operated free of mercury, but Hg is present in the discharge vessel 3 in the preferred embodiments. During steady-state burning (herein also referred to as nominal operation), long-arc lamps generally have a pressure of a few bar, whereas short-arc lamps may have pressures of up to about 50 bar in the discharge vessel. Characteristic power values of the lamp are between about 10 and 1000 W, preferably in the range of about 20-600 W.

Nominal operation in this description is understood to mean operation at the maximum power and under conditions for which the lamp has been designed to be operated.

Characteristic volumes of the discharge vessel are in the range of about 0.03-3 ml.

The discharge vessel 3 is filled with the filling (i.e. starter gas, salt filling and Hg) by means of techniques known in the art. During (nominal) use, the salts dissociate into iodine and metal elements and ions.

The current conductors 8,9 are thus the electrical connections within the outer bulb 100 between the electrical contacts comprised by the lamp cap 2 and the current lead- through conductor 20,21, which on their turn are in electrical contact with the electrodes 4,5 within the discharge vessel 3. Herein, current conductors 8,9 refer to those electric connectors that are, at least partly not covered by isolating material (other than optionally the UV emitter material and field emitter material), and which connect within the bulb the exterior of the high intensity discharge lamp 1 (such as the cap 2) and the electrodes 4,5 within the discharge vessel 3.

Figure 4 schematically depicts part of an embodiment of a high intensity discharge lamp 1. Only the first current conductor 8, second current conductor 9, discharge vessel 3 and end plugs 34, 35 are indicated. The lamp parts shown may extend on the left side. Here, the first current conductor 8 has a first branch 258, on which the UV emitter material 159 has been applied (for instance by coating a luminescent material layer. This first branch 258 may simply be a (stiff) wire, for instance of the same material as the current conductor(s). The second current conductor 9 has no branch. Field emitter material 158 has been applied to the second current conductor 9. Upon starting the lamp, an electrical field is generated between the current conductors 8,9. This field stimulates the field emitter material 158 to emit electrons. Part of the total number of electrons impinges on the UV emitter material 159 and may consequently generate UV emission from the UV emitter material 159. Part of the total number of UV photons may enter the discharge vessel 3 and reach one or both electrodes 4,5. Then, electrons from the electrode(s) may be freed, which starts and/or enhances the discharge within the discharge vessel 3. The field emitter material 158 and UV emitter material 159 have a shortest distance 1 to each other. The high intensity discharge lamp, including electronics (not depicted), is configured to generate an initial potential difference between the current conductors 8,9 during a start up process of the high intensity discharge lamp 1. The shortest distance 1 between the field emitter material 158 and UV emitter material 159 (and other parameters) may be chosen to allow an electric field strength generated between the field emitter material 158 and UV emitter material 159 during the start up process of at least 0.5 V/μιη.

Figures 5a-5c schematically depict a non- limiting number of possible configurations.

Figure 5 a schematically depicts a simple variant, wherein the first and the second current conductors 8,9 comprise a field emitter material 158 and a UV emitter material 159, respectively. Please note that the terms "first" and "second" have no specific meaning, and thus the arrangement of field emitter material 158 and UV emitter material 159 may also be the other way around.

Figure 5b schematically depicts an embodiment where the first current conductor 8 comprises first branch 258 with field emitter material 158. Figure 5b is similar to figure 4. Figure 5 c schematically depicts an embodiment where the second current conductor 9 comprises second branch 259 with UV emitter material 159. Note that in an embodiment, not depicted, both current conductors 8,9 may comprise branches 258,258.

In figure 5c, the UV emitter material 159 is arranged between the a field emitter material 158 and the discharge vessel 3.

In figure 4, the field emitter material 158 and the UV emitter material 159 are shown as coating. In figures 5a-5c, those materials are very schematically depicted. For instance, they may be present as plate or as coating or as material, optionally pressed into a body, in a container. A pressed body may for instance be attached to an electrically conductive plate or a back side of such plate may be metalized.

As will be clear to a person skilled in the art, embodiments may be combined. EXAMPLES

Examples of lamps according to the invention and reference lamps

High intensity discharge lamps with CDM burners (discharge vessels) were made, with a quartz outer bulb. The pressure within the outer bulb was smaller than 1 mbar. The burner was a 70 W burner, free of ⁸⁵ Kr, and only containing argon, mercury, and metal (Na, Tl, Dy, Ho, Tm) halide salts. Lamp voltage was about 100 V and ignition pulse was about 5 kV. The field emitter material was double wall carbon nano tube, applied in a paste on one of the current conductors with a brush. Binder within the paste was burnt off in air. Likewise, the UV emitter material (YP0 ₄ :Bi) was applied to the other current conductor, and binder was burnt off in air too. A construction as shown in figure 4 was chosen. The distance between the field emitter material and UV emitter material was 3 mm. The lamp did ignite well, without the presence of radioactive krypton. 20 reference lamps without carbon nano tubes, UV emitter material and ⁸⁵ Kr were made and 20 with carbon nano tubes and UV emitter material, but also wihtout ⁸⁵ Kr, were made. The lamps were tried to be ignited in the dark. The reference lamps (without the ⁸⁵ Kr and without field emitter material and UV emitter material) did not ignite at all, whereas the lamps without the ⁸⁵ Kr but with field emitter material and UV emitter material, all did ignite in the dark.