The invention relates to a metal halide lamp provided with a discharge vessel which encloses a discharge space and which is surrounded with intervening space by an outer bulb which is fitted with a lamp cap, in which intervening space a UV enhancer (UVE) with a ceramic wall is accommodated.
A lamp of the kind mentioned in the opening paragraph is known from U.S. Pat. No. 5,811,933. The known lamp, which has a discharge vessel with a ceramic wall, has a high ignition voltage. An installation suitable for operating the known lamp should accordingly be provided with an ignition circuit in addition to a ballast circuit. The UVE in the known lamp is provided with an internal electrode and is capacitively coupled to a current conductor to the discharge vessel. The UVE has a filling of only a rare gas. It is possible with such a UVE to ignite the lamp by means of an ignition pulse of 3 kV with a small ignition delay. A further favorable property of the UVE of the known lamp is its very compact construction. A disadvantage of the known UVE is that it ignites unreliably, or even not at all, at voltages well below 3 kV.
The term xe2x80x9cceramic wallxe2x80x9d in the present description and claims is understood to mean a wall of a metal oxide such as, for example, sapphire or densely sintered polycrystalline Al2O3 (referred to hereinafter as pca) as well as a wall made of metal nitride, for example AlN.
Besides lamps having a high ignition voltage and accordingly requiring a separate ignition circuit which supplies high ignition voltage pulses, there are also metal halide lamps which ignite reliably at a low ignition voltage of at most 600 V. In addition, there are metal halide lamps which are suitable for being ignited and operated directly on a ballast circuit, i.e. which do not require a separate ignition circuit. Said lamps having a low ignition voltage of up to 600 V are usually provided with radioactive ignition means for this purpose, for example in the shape of electrodes provided with a ThO2 pellet. A radioactive Penning mixture is also used as an ignition means in such lamps. The use of radioactive ignition means, however, is disadvantageous. Furthermore, these lamps may be provided with built-in starters, for example in the form of a glowswitch starter.
The invention has for its object to provide a lamp which ignites reliably at a low ignition voltage and in which the above disadvantages have been eliminated.
According to the invention, a lamp of the kind mentioned in the opening paragraph is characterized in that the UVE is provided with two internal electrodes between which a glow discharge can extend. It was surprisingly found that the use of a UVE provided with two internal electrodes substantially counteracts the occurrence of ignition delays at low ignition voltages and in the absence of radioactive ignition means. In a further advantageous embodiment of the lamp, one of the two internal electrodes is provided with an electrode winding. A further reduction in the ignition delay is achieved thereby. A further improvement can be achieved in that one of the internal electrodes is provided with an emitter. In a further preferred embodiment of the lamp, each of the electrodes of the UVE is provided with an electrode winding with emitter.
A further advantage of the UVE of the lamp according to the invention is that it is also suitable for use in lamps which have a high ignition voltage, for example the known lamp. This is also true in the case in which the UVE has an electrode winding and is provided with emitter. A universally applicable UVE is thus realized, which brings with it the possibility of a substantial simplification in lamp manufacture.
Although UVEs with glass or quartz walls are known per se, it is found that their use in a comparable metal halide lamp leads to unacceptable ignition delay times owing to impurities which are inevitably released from the wall. The ignition delays can only be reduced to practically useful values, it is found, through the use of additional ignition means. A further practical disadvantage of such UVEs is that their dimensions are considerably larger compared with the very compact construction which is possible for a UVE with a ceramic wall.