Abstract:
A high pressure gas discharge lamp includes a ceramic discharge vessel that has a container wall enclosing a discharge space having a filling. First and second electrodes are mutually oppositely arranged in the discharge space and are mounted on first and second feedthroughs, respectively, which extend in a gas-tightly sealed manner through the container wall. The high pressure gas discharge lamp further includes a UV-enhancer that has a wall portion and a chamber. The chamber is enclosed by the wall portion of the UV-enhancer and an end part of the container wall.

Description:
CROSS-REFERENCE TO PRIOR APPLICATIONS 
     This application is the U.S. National Phase application under 35 U.S.C. §371 of International Application No. PCT/IB2013/056253, filed on Jul. 30, 2013, which claims the benefit of U.S. Provisional Patent Application No. 61/679,112, filed on Aug. 3, 2012. These applications are hereby incorporated by reference herein. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates to a high pressure gas discharge lamp comprising a ceramic discharge vessel having a container wall enclosing a discharge space having a filling, further comprising a first and a second electrode, mutually oppositely arranged in the discharge space and defining a length axis of the discharge vessel, still further comprising a first and a second feedthrough, both extending in a gas-tightly sealed manner through the container wall and on which feedthroughs a respective electrode is mounted, and further comprising a UV-enhancer. 
     The invention further relates to a method of manufacture of said electric lamp. 
     A lamp of the type mentioned in the opening paragraph is known from U.S. Pat. No. 5,811,933. The known lamp is a high-pressure discharge lamp, more in particular a metal halide lamp. Such a lamp is suitable for various applications such as general interior lighting, general exterior lighting, video illumination, etc. The discharge vessel of the known lamp is made of ceramic material and is often obtained, via an extrusion process, in a tubular shape and subsequently provided with end plugs/end parts. Alternatively a slip-casting process or an injection molding process is used to manufacture the discharge vessel with end parts. Ceramic material in the present description and claims is understood to be a densely sintered polycrystalline metal oxide such as, for example, Al 2 O 3  or YAG, and densely sintered polycrystalline metal nitride such as, for example, AlN. 
     A known problem of this type of lamp is the comparatively wide spread in ignition time. This points to a shortage of free electrons during lamp ignition. The addition of a small quantity of  85 Kr in the discharge vessel can supplement such a shortage. A disadvantage of this, however, is that  85 Kr is radioactive. Efforts have been made to avoid this through the use of a UV-enhancer, which is a small UV discharge tube positioned adjacent the discharge vessel and acting as a UV source. The UV-enhancer in the known lamp is formed by a UV-transmitting ceramic tube positioned parallel to and at a distance from the discharge vessel. Upon breakdown, the UV-enhancer will generate said UV-radiation. The influence of this UV-radiation leads to the production of free electrons in the discharge vessel, which in turn strongly promote lamp ignition. Disadvantages of the known lamp are the relatively complicated construction, and the relatively cumbersome manufacture process which, in addition, is relatively expensive. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a lamp in which at least one of the abovementioned disadvantages is counteracted. According to the invention, a lamp of the kind as described in the opening paragraph has a UV-enhancer with a wall portion and a chamber, said chamber being enclosed by the wall portion of the UV-enhancer and an end part of the container wall. It is thus attained that the UV-enhancer is directly adjacent to the discharge vessel, whereby its functioning as a fast and reliable ignition aid is improved. Furthermore, a relatively compact, readily manufacturable lamp is obtained in which the number of components is reduced as, contrary to the known lamp, no complex, separate mounting construction for the UV-enhancer is required and the container wall of the discharge vessel simultaneously functions as a wall of the UV-enhancer. The UV-enhancer has a wall which is made from densely sintered polycrystalline Al 2 O 3 . The fact that this is widely used as a wall material for high-pressure discharge lamps has the major practical advantage that an existing technology for ceramic discharge vessels can be utilized. A very high degree of miniaturization is possible here. Although it was found that a combination of a rare gas and Hg is suitable as a filling, the UV-enhancer preferably has a rare gas filling. A suitable rare gas is inter alia Ne. Ar was found to be particularly suitable as a filling. A pressure (filling pressure) is preferably chosen for the filling which accompanies a minimum breakdown voltage. This filling pressure may be readily ascertained experimentally. A fair approximation can be realized by means of the Paschen curve. A mixture of rare gases in the form of a Penning mixture is also suitable. 
     An embodiment of the high pressure gas discharge lamp is characterized in that one of the feedthroughs forms an internal electrode of the UV-enhancer, said internal electrode extending through the chamber and extending in a gas-tightly sealed manner through said wall portion. Thus, compared to the known lamp, a further reduction in the number of components is attained, as the separate feedthrough and the internal electrode in the known lamp are combined into one part. 
     An embodiment of the high pressure gas discharge lamp is characterized in that the UV-enhancer is tubular and concentric with the longitudinal axis extending through the UV-enhancer. This has the advantage that, compared to the known lamp, the manufacture of a rotationally symmetric discharge vessel with UV-enhancer and hence a simpler manufacture process of the lamp is enabled. One way to further simplify the manufacture process is to separate the feedthrough that is (to be) located at the side of the UV-enhancer into a first part sealed in the end part and a second part sealed in the wall portion, the first part in electrically conductive contact with the second part once the lamp is fully assembled. The separation into two parts enables separate, simple, dedicated processing of the sealing parts without the necessity to take precautions to counteract mutual detrimental effects on the sealing quality of the respective dedicated sealing processes. In particular the dedicated process for the manufacture of the discharge vessel comprises shrink-sinter sealing of the discharge vessel under vacuum or Hydrogen atmosphere to at least 98% density to obtain a gastight, relatively clear discharge vessel. The dedicated process for the UV-enhancer comprises a pre-sinter shrink-sealing process to about 60% density under an inert (non-corrosive/oxidative) atmosphere, the wall-portion of the UV-enhancer still having a porous structure, which pores are still accessible for ambient gases. Moreover, said separation into two parts enables an embodiment of the high pressure gas discharge lamp which is characterized in that said electrically conductive contact between the first part and the second part is free from a weld. Such an electrically conductive contact without a weld is obtainable as follows: 
     the discharge vessel, which has a relatively clear container wall which is fully sintered to at least 98% density, and the UV-enhancer, of which the wall portion is pre-sintered to only about 60% density are assembled together, taking care that the first part of the feedthrough in the discharge vessel and the second part of the feedthrough in the UV-enhancer are such that said first and second part abut or substantially abut each other; subsequently, the combination of fully sintered discharge vessel and pre-sintered UV-enhancer undergo a final sinter step under, for example, an Ar or N2-atmosphere, to fully, i.e. to 98% density, shrink-sinter the UV-enhancer wall-portion onto the container wall. 
     Said final shrink-sinter step involves four phenomena, i.e.: 
     the UV-enhancer shrinks in the radial direction, thereby clamping itself in a gastight manner onto the container wall, thus realizing the gastight chamber of the UV-enhancer; 
     the UV-enhancer shrinks in the axial direction, thereby forcing the first part and the second part of the feedthrough towards each other, thus establishing the electrically conductive contact without a weld; 
     the chamber of the UV-enhancer is automatically filled with the gas used during the final sinter step, i.e. with Ar or N2; a fill pressure of about 0.15 bar will be obtained inside the chamber when during the final sinter step a gas pressure of about 1 bar is applied; 
     due to the Ar or N2 gas atmosphere the wall portion of the UV-enhancer becomes opaque as a result of inclusions of the ambient gas during the final sinter step, which gas is trapped inside the wall portion once the final sinter step is completed. Said opaque wall portion reflects the generated UV-radiation towards and into the container wall and into the discharge space during operation of the lamp, thus inducing a fast and reliable ignition of the lamp. 
     An embodiment of the high pressure gas discharge lamp is characterized in that the first part is made of iridium and the second part is made of niobium. Iridium is directly sealable, i.e. without the need/use of a sealing glass/frit, into the wall of the container, thus providing a seal which is comparatively very resistant to the salt filling inside the discharge vessel. Niobium is well-known to be readily sealable in the wall portion of the UV-enhancer because its coefficient of thermal expansion excellently matches that of alumina. 
     An embodiment of the high pressure gas discharge lamp is characterized in that the second part comprises niobium wire having a thin-diameter part and a thick-diameter part, for example 250 μm, the niobium wire being sealed in the wall portion with its thick-diameter part. This embodiment is in particular suitable for lamps having a relatively high nominal power, for example lamps having a nominal power of at least 400 W. 
     An embodiment of the high pressure gas discharge lamp is characterized in that the container wall has an outer container surface and the wall portion has an outer wall surface and in that an active antenna extends over said outer container surface and said outer wall surface. In this embodiment the (electrode of the) UV-enhancer and the active antenna are mutually arranged such that a capacitive coupling between the UV-enhancer and the active antenna is achieved, thus further enabling a fast and reliable ignition of the lamp. A convenient method of providing such an active antenna on the container wall and the wall portion makes use of a printing process of, for example, a Tungsten-containing electrically conductive paste. To simplify the printing process, it is favorable when an embodiment of the high pressure gas discharge lamp is characterized in that the outer container surface is flush with the outer wall surface of the UV-enhancer. 
     The invention further relates to a method of manufacturing a high pressure gas discharge lamp, comprising the steps of: 
     manufacturing a sealed discharge vessel by providing its discharge space with a filling and first and second electrodes mounted on a respective first feedthrough part and sealing said first feedthrough parts gas-tightly in a container wall of the discharge vessel; 
     shrink-sintering a ceramic wall portion of a concave UV-enhancer portion to about 60% density, either simultaneously with sealing a second part of one of the feedthroughs in a ceramic wall of a concave UV-enhancer portion so as to extend therethrough, or prior to a separate sealing step of the second part of the feedthrough in the wall portion of the UV-enhancer portion, using a sealing glass; 
     assembling the discharge vessel and the UV-enhancer portion such that they both abut against the first feedthrough and the second feedthrough part and subsequently shrink-sintering (at about 1600° C., i.e. 1500-1700° C., and about 1 bar Argon, i.e. 0.4-2 bar Argon) under a chosen gas atmosphere the UV-enhancer part to a density of about 98% and onto the discharge vessel to form the closed wall of the gas-filled UV-enhancer and to establish a fixed electrically conductive contact of the first feedthrough with the second feedthrough part. 
     This method has the advantage of a relatively simple manufacture process. The separation into two parts enables separate, simple, dedicated processing of the sealing parts without the necessity to take precautions to counteract mutual detrimental effects on the sealing quality of the respective dedicated sealing processes. In particular the dedicated process for the manufacture of the discharge vessel comprises shrink-sinter sealing of the discharge vessel under vacuum or Hydrogen atmosphere to a density at which the sintered container wall material is no longer porous or permeable to gas, for example in that it has at least 90% or 98% density, to obtain a gastight, relatively clear discharge vessel. The dedicated process for the UV-enhancer comprises a pre-sinter shrink sealing process to about 60% density, i.e. 50%-65% density, under an inert (non-corrosive/oxidative) atmosphere, the wall-portion of the UV-enhancer still having a porous structure, which pores are still accessible for ambient gases. Subsequently, the combination of fully sintered discharge vessel and pre-sintered UV-enhancer undergo a final sinter step under a desired gas atmosphere, for example, an Ar or N2-atmosphere, to fully, i.e. to at least 90% or at least 98% density, shrink-sinter the UV-enhancer wall portion onto the container wall. Moreover, as already said hereinabove, said separation into two parts enables an embodiment of the high pressure gas discharge lamp which is characterized in that said electrically conductive contact between the first part and the second part is free from a weld. 
     Said final shrink sinter step involves three or four phenomena, i.e.: 
     the UV-enhancer shrinks in the radial direction, thereby clamping itself in a gastight manner onto the container wall, thus realizing the gastight chamber of the UV-enhancer; 
     the UV-enhancer shrinks in the axial direction, thereby forcing the first part and the second part of the feedthrough towards each other, thus establishing the electrically conductive contact without a weld; 
     the chamber of the UV-enhancer is automatically filled with the desired gas used during the final sinter step; a fill pressure of about 0.15 bar will be obtained inside the chamber when during the final sinter step a gas pressure of about 1 bar is applied; 
     in the case that use is made of an Ar and/or N2 gas atmosphere during the final sinter step, due to the Ar or N2 gas atmosphere, the wall portion of the UV-enhancer becomes opaque as a result of inclusions of the ambient gas during the final sinter step, which gas is trapped inside the wall portion once the final sinter step is completed. Said opaque wall portion reflects the generated UV-radiation towards and into the container wall and into the discharge space during operation of the lamp, thus inducing a fast and reliable ignition of the lamp. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and further aspects of the lamp according to the invention will be explained in more detail with reference to a drawing (not true to scale), in which: 
         FIG. 1  is a side elevation of a first embodiment of a lamp according to the invention; 
         FIGS. 2 a - c    show various stages in the manufacture process of the lamp; 
         FIG. 3  shows a second embodiment of the lamp according to the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  shows a high-pressure metal halide lamp  1  comprising a discharge vessel  3  having a container wall  2  enclosing a discharge space  4  filled with a filling  6 , which discharge vessel  3  is surrounded, with an interspace  5 , by an outer envelope  7 , which supports a lamp cap  9 . The discharge vessel  3  is made of densely sintered polycrystalline aluminum oxide and has a first lamp electrode  11  and a second lamp electrode  13 , which electrodes are connected to respective contacts  15  and  17  on the lamp cap  4  by means of a respective first  19  and second feedthrough  21 , extending over a longitudinal axis A of the discharge vessel  3 . The lamp  1  is provided with an UV enhancer  23  having a wall portion  24 , said UV-enhancer  23  is situated at an end part  26  of the discharge vessel  3 . The UV enhancer  23  has the first feedthrough  19  as an internal enhancer electrode  20 . The UV enhancer  23  has a capacitive coupling with an antenna  25  extending over an outer container surface  27  of the container wall  2  and over an outer wall surface  28  of the wall portion  24 . The UV-enhancer  23  contains Ar with a filling pressure of 170 mbar in a chamber  29  which is formed by the container wall  2  and the wall portion  24 . Preferably, the filling pressure lies between  50  mbar and 300 mbar. At pressure values of less than 50 mbar, the UV output of the enhancer appears to become smaller; at pressure values of more than 300 mbar, the ignition voltage of the enhancer may assume too high values. A combination of mercury and an inert (rare) gas, for example N2, Ne, Ar, Xe, or Kr, is also possible as a filling for the UV enhancer. However, an inert (rare) gas or a mixture of inert (rare) gases is preferred, because this precludes the use of the heavy metal mercury. 
     A number of lamps having a construction as shown in  FIG. 1  were subjected to an ignition test. The lamps are 39 W CDM lamps, make Philips, connected to a supply voltage source of 220 V, 50 Hz via a stabilizer ballast provided with an igniter circuit. These lamps have ceramic discharge vessels with fillings comprising metal halide. The ceramic material of the discharge vessel reaches a temperature between 800° C. and 1000° C. during lamp operation. The UV-enhancer in these lamps is situated as shown in  FIG. 1 . The lamp electrode is therefore directly irradiated by the UV radiation generated in the UV-enhancer. The ignition circuit comprises a starter, type SN57 (Philips); ignition pulses having a maximum value of about 2 kV and a pulse width of about 8·s are supplied. Prior to the ignition test, the lamps were operated for 10 to 15 minutes and subsequently switched off and maintained in a dark room for at least 1.5 hours. All lamps ignited after an ignition time that was well within the requirement of 30 s. It clearly appeared that there was only a very small ignition delay at relatively low ignition voltage pulses (2 kV). Furthermore, the spread of this ignition delay appeared to be very small. 
       FIGS. 2 a - c    show a UV-enhancer  23  and a discharge vessel  3 , which are similar to the ones of  FIG. 1 , in a cross-sectional view and in greater detail in various stages of the manufacture thereof. In  FIG. 2 a   , a first stage of the manufacture is shown in which the discharge vessel  3  and UV-enhancer  23  are in an unassembled configuration. The discharge vessel  3  has a tungsten electrode  11  mounted on an Iridium wire  31  which is fully, gas-tightly embedded in an aperture  33  in an end part  26  of the ceramic container wall  2  via a shrink-sealing process. The end part is made of ceramic material, for example densely sintered polycrystalline aluminum oxide sintered to about 98% density, and it has an outer diameter  41  which essentially will not change during a subsequently applied sinter step. The UV-enhancer  23  has a cylindrical, cup/concave-shaped wall portion  24  through which a Niobium wire  35  extends, the wall portion  24  is made of ceramic material, for example sintered polycrystalline aluminum oxide sintered to about 60% density. The cylindrical, cup-shaped wall portion  24  has an open side  37  and an internal diameter  39  which is just somewhat larger than the outer diameter  41  of the end part  26  of the container wall  2  so that it snugly fits over said end part  26 . 
     In  FIG. 2 b   , the discharge vessel  3  and UV-enhancer  23  are in assembled position, i.e. the wall portion  24  of the UV-enhancer  23  is shifted over the container wall  2  and the niobium wire  35  is inserted into the aperture  33  of the end part  26  of the container wall  2  such that it abuts against the embedded Iridium wire  31 . Subsequently, the assembled combination is sintered at about 1600° C. under 1 bar Argon atmosphere. The sintering causes, through shrink sealing from 60% density to about 98% density, the wall portion  24  to be gas-tightly sintered onto the end part  26  to form the UV-enhancer chamber  29 . Thus, the wall portion  24  of the UV-enhancer  23  together with a part of the container wall  2  forms a closed wall  30  of the UV-enhancer  23  which encloses said chamber  29 . Said chamber  29  is filled with the gas atmosphere applied during said sinter step, i.e. with Argon, its pressure being about 150 mbar at room temperature (•20° C.). Furthermore, due to the shrinking of the wall portion  24 , the niobium wire  35  is forced towards the Iridium wire  31  to establish a weldless, yet very good, reliable electrically conductive contact  43  therewith. Thus, a feedthrough is formed in which the iridium forms a first feedthrough part  18   a  and the niobium forms a second feedthrough part  18   b . As initially the wall portion  24  has only 60% density and is porous, i.e. ambient gas can penetrate into the bulk  45  of the material of the wall portion  24 , the pores of said wall portion  24  will be filled with ambient gas, i.e. with Argon. Subsequently, during the sinter step, said Argon gas is trapped inside the bulk  45  of the wall portion  24  due to shrinkage of the material of the wall portion  24 , resulting in the wall portion  24  becoming opaque and highly reflective. UV-radiation generated in the chamber  29  of the UV-enhancer  23  thus will be “guided” towards the transparent container wall  2  and towards the electrode  11  inside the discharge vessel  3 . The UV-enhancer electrode  20  is the feedthrough  19  extending through a first extremity  47  of the chamber  29 , at a side where the first lamp electrode  11  is mounted on the feedthrough, and said feedthrough extending through a second extremity  49  of the chamber  29  where the UV-enhancer  23  is sealed in a vacuum-tight manner by means of a sintered plug. Said plug forms the end part  26  of the container wall  2 , through which the feedthrough  19  extends and is connected to a current conductor (which in turn is connected to an electrical contact, see  FIG. 1 ). The UV-enhancer  23  has a length of 13 mm, an external diameter of 1.5 mm and an internal diameter of 0.675 mm. 
     In  FIG. 2 c   , the final stage of the manufacture of the discharge vessel  3 +UV-enhancer  23  is shown in that an antenna  25  is provided which extends on an outer surface  27  of the container wall  2  and on an outer surface  28  of the wall portion  24  of the UV-enhancer  23 . The antenna  25  may be formed from an electrically conductive material such as transparent conductive coatings such as, for example ITO (indium Tin Oxide), or from metal coatings, for example, tungsten that is deposited upon the outer surfaces  27 , 28  after the discharge vessel  3  and UV-enhancer  23  have been joined and sintered. 
       FIG. 3  shows the assembled UV enhancer  23  and the discharge vessel  3 , which are similar to the ones of  FIG. 1  and  FIG. 2 c   , in a cross-sectional view and in greater detail. The end part  26  of the container wall  2  of the discharge vessel  3  has a stepped profile  51  such that said profile and the wall portion  24  of the UV-enhancer  23  fit together in such a manner that the outer surfaces  27 , 28  of the container wall  2  and the wall portion  24  are flush after being shrink-sintered together. Thus, it is enabled to easily provide an antenna  25  on said outer surfaces  27 , 28 . Furthermore, the niobium wire  35  has a thin diameter part  34  and a thick diameter part  36 . The niobium wire  35  is sealed in the wall portion  24  with its thick diameter part  36 . Such a construction is especially suitable for electric lamps having a relatively high nominal power, for example a nominal power of 400 W or more.