Patent Application
20110304267
This application claims the priority of German patent application no. 10 2010 028 472.6 filed May 3, 2010, the entire content of which is hereby incorporated by reference.
The invention is related to a noble gas short-arc discharge lamps comprising a discharge vessel and electrodes arranged therein.
The term noble gas short-arc discharge lamp here designates short-arc discharge lamps comprising a discharge vessel composed of quartz glass which is filled exclusively with a noble gas or noble gas mixture. Lamps of this type are usually operated with DC current or pulsed DC current. Said lamps are suitable for diverse fields of use, in particular also for cinema projection, profile spotlights and searchlights and also microscopy and endoscopy.
As a measure for increasing the luminance of a noble gas short-arc discharge lamp (AC, DC, pulsed operation) it is known in accordance with the prior art to increase for example the pressure in the discharge vessel. In this case, however, the starting behavior of the discharge lamp is altered negatively since the required starting voltage (cold start and hot restart) increases.
The patent specification EP 1 217 644 B1 discloses in FIG. 1 a generic noble gas short-arc discharge lamp with a xenon filling for cinema projection.
One object of the present invention is to provide a noble gas short-arc discharge lamp having improved cold starting behavior with the luminance remaining the same or with a luminance that is increased with the cold starting behavior remaining the same.
The noble gas short-arc discharge lamp according to an embodiment of the invention has a discharge vessel and electrodes arranged therein, between which an arc is formed during the operation of the lamp. According to the invention, the discharge vessel has at least in sections a coating for at least partly reflecting the electromagnetic radiation, in particular the infrared (IR) thermal radiation, emitted during operation by the lamp components situated in the discharge vessel and also by the excited filling gas. As a result of the IR radiation back reflection brought about thereby, the wall of the discharge vessel, the filling gas and also those lamp components of the discharge vessel on which the reflected radiation impinges are additionally heated during the operation of the lamp. This leads to an increase in the operating pressure of the filling gas and also to a lengthening of the electrode rods on account of the greater thermal expansion and, consequently, to a shortening of the electrode spacing. Without the IR reflective coating, by contrast, the IR radiation would largely be emitted through the discharge vessel into the surroundings.
These relationships can advantageously be utilized for the following two aims. In this case, the starting point of the following considerations is a conventional reference lamp without an IR reflective coating. If, then, an otherwise structurally identical lamp is provided with the IR reflective coating according to the invention, during operation with the same power consumption, the filling pressure is higher than in the case of the reference lamp, as a result of which the discharge arc contracts to a greater extent. As a result of this and as a result of the shortening of the electrode spacing during operation, a higher luminance is obtained in comparison with the reference lamp.
Alternatively, the increase in the operating pressure of the lamp which arises on account of the heating effect explained above can be utilized to reduce the cold filling pressure of the lamp in such a way that the regular operating pressure of the lamp is established, which would also have been established in the case of normal filling without a heat reflective coating but with the same power consumption. The lower cold filling pressure affords the advantage that the required starting voltage (cold start and hot restart) of the discharge lamp is lower than in the case of the reference lamp. At the same time, however, on account of the IR reflective coating, the filling pressure during operation and hence the luminance is the same as in the case of the reference lamp.
Particularly good reflection properties in the infrared spectral range and thus optimized thermal insulation of the noble gas short-arc discharge lamp according to the invention can be achieved if the reflective coating is composed of a layer system comprising a plurality of layers.
Preferably, a layer of the layer system composed of a material having a low refractive index and a layer composed of a material having a high refractive index are respectively alternated.
The layer composed of the material having a low refractive index can comprise a material which consists of an oxide or a nitride or an oxynitride composed of one of the metals Si, Zr, Al, Sn, ZN and of mixtures thereof (for example SiO2, ZrO2, Al2O3). One preferred material for the layer composed of a material having a low refractive index is SiO2.
The material of the layer having a high refractive index comprises, for example, a material composed of an oxide or composed of a nitride or composed of an oxynitride composed of one of the metals Nb, Ti, Ta, Hf, and mixtures thereof (for example Nb2O5, TiO2 HfO2). It has proved to be advantageous to use Nb2O5 as a layer.
The layer system has at least 30 and at most 80 layers.
Since, during DC operation of the noble gas short-arc discharge lamp according to the invention, the anode, on account of its size, heats up to a particularly great extent and, accordingly, emits a particularly large amount of heat, it is preferred in accordance with a first exemplary embodiment if a region of the discharge vessel which is adjacent to the anode is coated.
In order to utilize as much as possible of the heat produced in the interior of the noble gas short-arc discharge lamp for increasing the pressure, it is preferred in accordance with another exemplary embodiment if the discharge vessel is substantially or even completely coated. As a result, during operation, the entire discharge vessel can be heated by the infrared-reflecting effect of the coating. However, in this case the layer system should preferably be designed such that it is sufficiently transparent to light.
In one exemplary embodiment, the discharge vessel is filled with pure xenon gas or a xenon-krypton gas mixture.
The invention will be explained in greater detail below on the basis of exemplary embodiments. In the figures:
In the region of the anode 2, the outer side of the discharge vessel 1 is provided with a coating 8. In this case, the coating 8 consists alternately of silicon oxide (SiO2) and of niobium oxide (Nb2O5) layers. These layers have refractive indices that deviate significantly from one another.
For operating the noble gas short-arc discharge lamp according to the invention, firstly an arc is struck by means of a comparatively high starting voltage between the two electrodes 2, 3, said arc extending through the xenon gas. The running voltage established after starting between the electrodes is significantly lower than the starting voltage. During operation, infrared thermal radiation arises in the discharge vessel 1 and, in particular, as a result of the hot anode 2, as a result of which the gas pressure increases in the entire discharge vessel 1. The thermal radiation partly impinges on the coating 8 and is substantially reflected back from the latter into the discharge vessel 1. In this case, the coating 8 is substantially transmissive to visible light. As a result of the reflection of the thermal radiation, the interior of the discharge vessel 1 is heated further, as a result of which the operating pressure is increased further. This leads to a constriction (concentration) of the arc and specifically of the point of maximum luminance directly in front of the cathode tip (hot spot), which consequently advantageously increases the luminance of the arc but also the hot spot luminance.
A curve 10 depicted comparatively thick represents the desired spectrum of the coating 8 (cf.
In the range of the wavelength from approximately 1050 to approximately 1200 nm, no measurement values are present on account of a measurement range jump of the spectrometer.
The diagram shows low reflection by the coating 8 in the range of visible light, while in the infrared range (up to 2100 nm) the majority of the thermal radiation is reflected back into the discharge vessel 1 by the coating 8.
The infrared-reflecting (IR) coating 8 according to the invention gives rise to an increased operating pressure in the discharge vessel 1 during operation, said pressure leading to a luminance increased by approximately 10% above that of comparable noble gas short-arc discharge lamps (in accordance with the prior art).
In an alternative variant of the above-explained exemplary embodiment of a 450 W xenon short-arc discharge lamp, the cold filling pressure of the discharge vessel is reduced by approximately 10% to approximately 9 bar. As a result, the required starting voltage (cold start and hot restart) of the discharge lamp is reduced by approximately 15%. The technological outlay in respect of apparatus (starting devices, line routing) is decreased as a result. During operation, on account of the IR reflective coating, the gas pressure increases to the operating pressure originally provided for this type of lamp. As a result the luminance obtained in the case of the conventional counterpart without an IR reflective coating is also achieved—despite a reduced cold filling pressure.
The scope of protection of the invention is not limited to the examples given hereinabove. The invention is embodied in each novel characteristic and each combination of characteristics, which includes every combination of any features which are stated in the claims, even if this feature or combination of features is not explicitly stated in the examples.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2010 028 472.6 | May 2010 | DE | national |
