1. Field of the Invention
The present invention relates to a discharge lamp and an illuminating system containing such discharge lamp.
The invention relates to a discharge lamp, in particular also to a fluorescent lamp, in which all electrodes are arranged on the external wall of the discharge vessel. The external wall serves in this case, inter alia, as a dielectric layer which separates the electrodes from the discharge during operation of the lamp. This type of discharge is therefore also termed a bilaterally dielectrically impeded discharge.
The spectrum of the electromagnetic radiation emitted by such as lamp can, in this case, comprise both the visible region and the UV (ultraviolet)/VUV(vacuum ultraviolet) region and the IR (infrared) region. Furthermore, a fluorescent layer can also be provided for converting invisible radiation into visible radiation.
Furthermore, the invention relates to a discharge lamp having a tubular discharge vessel sealed at both ends. The cross section of the discharge vessel is preferably circular. However, even only approximately circular cross sections, for example regular polygons such as, for example, hexagons, etc are also suitable. The term "tubular" is not restricted here to straight tubular discharge vessels, but likewise comprises bent, for example angled, tubular discharge vessels. Since the discharge direction runs essentially perpendicular to the lamp longitudinal axis, the length of the lamp is also not limited in principle.
Such lamps are used, in particular, in equipment for office automation (OA), for example color copiers and color scanners, for signal lighting, for example as brake lights and direction indicator lights in automobiles, for auxiliary lighting, for example interior illumination of automobiles, and for background lighting of displays, for example liquid crystal displays, and so-called "edge-type backlights".
These technical fields require both particularly short start-up phases, and luminous fluxes which are as independent as possible of temperature. Consequently, these lamps contain no mercury. Rather, these lamps are usually filled with inert gas, preferably xenon, or inert gas mixtures.
The said applications require both a high luminous density and a luminous density which is uniform over the length of the lamp. To increase the luminous density, lamps for OA use are normally provided with an aperture along the longitudinal axis. Increasing the power injected into previous systems does not suffice to raise the luminous density further, since the loading of a lamp cannot be raised at will for lasting and reliable operation. A further difficulty is, that with the systems used so far in copiers and scanners, the efficiency of the discharge decreases with increasing injected power.
2. Background Information
U.S. Pat. No. 5,117,160 discloses an inert gas discharge lamp for OA equipment. There are two strip-shaped electrodes arranged along the lamp longitudinal axis on the outer surface of the wall of a tubular discharge vessel. The lamp is operated with AC voltage at a preferred frequency of between 20 kHz and 100 kHz. During operation, the 147 nm xenon line is excited. The efficiency of the useful radiation which can be achieved with the employed mode of operation, and consequently the resulting luminous density are relatively low.
It is known, furthermore, from U.S. Pat. No. 5,604,410 that the efficiency of dielectrically impeded discharges can be raised substantially compared with the dielectrically impeded discharges excited with AC voltage (see U.S. Pat. No. 5,117,160) with the aid of a pulsed operation (pulsed, dielectrically impeded discharge) adapted to the particular conditions (striking distance, electrode configuration, electrode geometry, filling gas and filling pressure).