1. Field of the Invention
The invention relates to a discharge lamp with high radiance (HID lamp), such as a high pressure mercury discharge lamp, a metal halide lamp or the like, which is used for example as a light source for a projector.
2. Description of Related Art
In a light source device for an optical device such as a liquid crystal projector, a DLP projector or the like, a discharge lamp with high radiance, such as a high pressure mercury discharge lamp, a metal halide lamp, or the like is used. In these discharge lamps it is generally necessary when starting to apply a high voltage between the main discharge electrodes or between the electrode for the main discharge and the inside of the discharge vessel, to produce an insulation breakdown in the discharge medium within the discharge vessel, and to induce a glow discharge or an arc discharge, the electrons of the plasma which has been produced thereby acting as the triggering substance.
The voltage which is necessary for the insulation breakdown during starting is generally roughly a few kilovolts when the discharge lamp is in the temperature state of roughly room temperature. The voltage which is necessary for the insulation breakdown during starting however changes depending on the running time after turning off following completion of prior operation, i.e. depending on the temperature of the discharge space. It can be imagined that the reason for formation of one such change is the following:
According to the drop in the temperature of the discharge space after it is turned off, part of the vaporized discharge medium, such as mercury, halogen or the like, condenses. This gradually changes the composition of the gas portion of the discharge space. For this or similar reasons the voltage which is necessary for the insulation breakdown changes.
The voltage which is necessary for the insulation breakdown is very low for example in the case of a discharge lamp with a discharge medium which comprises halogens such as bromine and mercury and the like, and a rare gas such as argon or the like, and which contains for example greater than or equal to 0.15 mg mercury per cubic millimeter volume of the discharge space, because there is residual plasma immediately after turning off the discharge lamp. Afterwards it does rise quickly until however shortly afterwards it begins to drop again (under the condition of natural cooling without compressed air cooling of the discharge lamp) until it reaches the minimum value after roughly 30 seconds. The insulation breakdown voltage repeatedly rises and falls however afterwards in a complicated manner during the interval during which the temperature of the discharge space finally drops to roughly 100° C., i.e. for the duration of a few minutes after turning off.
In order to enable hot restart at a time as early as possible after turning off, the absolute value of the high voltage which can be applied during starting can easily be made high. But in this case the possibility of occurrence of the following disadvantages increases:                The high voltage which is to be applied causes the dangerous phenomenon of unintentional insulation breakdown, i.e. an insulation breakdown of the coating of an insulated cable, and a creep discharge on the connector, on the terminal or the like.        The noise when the high voltage is applied causes a malfunction of the electronic circuit of the device of the main projector part.        
Therefore, conventional methods for enabling the starting of the discharge lamp without overly increasing the absolute value of the high voltage which is to be applied during starting, i.e. concepts for improving the starting property, have been developed. For example, U.S. Pat. No. 4,328,446, Japanese patent disclosure document HEI 2-61957, Japanese patent disclosure document HEI 2-61958, and the like have proposed a technique for improving the starting property of a discharge lamp, in which by placing a close-by conductor in the form of a wire or the like which is connected to the electrode of the main discharge, a high voltage is applied outside the main discharge vessel not only between the main discharge electrodes, but also between the inside of the main discharge vessel and the electrode for the main discharge.
Generally this is often called the outside trigger method which is used mainly for blinking lamps, in which without applying a high voltage between the main discharge electrodes a high voltage is applied to the conductor adjacent to the discharge vessel and in which thus a discharge is begun. In Japanese patent disclosure document HEI 5-54983 and Japanese utility model application SHO 37-8045, application of the outside trigger method for a discharge lamp with high radiance is proposed. The purpose here is, however, not to improve the starting property.
Since only action by a close-by conductor is of course not sufficient for improving the starting property, a proposal was made for reducing the absolute value of the high voltage which is to be applied during starting, in which light with a short wavelength such as UV radiation or the like is emitted and in which ionization of the discharge medium is promoted.
As the first conventional example, for example, U.S. Pat. No. 4,987,344 proposed a discharge lamp in which there is an auxiliary UV light source with an auxiliary discharge vessel which has a pair of inner electrodes. In U.S. Pat. No. 4,721,888 a discharge lamp is schematically shown in which in the vicinity of its hermetically sealed area there is an auxiliary UV light source with a pair of inner electrodes.
As the second conventional example, U.S. Pat. Nos. 5,550,421; 5,811,933; 4,818,915 (corresponding to JP HEI 1-134848); and U.S. Pat. No. 5,990,599 (corresponding to JP 2001-512622 A) and the like have proposed a discharge lamp in which there is an auxiliary UV light source in which in an auxiliary discharge vessel which is provided with an inner electrode a high voltage is subject to electrostatic capacitance coupling.
As the third conventional example, U.S. Pat. No. 4,812,714 (corresponding to JP HEI 1-134849) have proposed a discharge lamp in which there is an auxiliary UV light source in which in an auxiliary discharge vessel without an inner electrode a high voltage is subjected to electrostatic capacitance coupling.
As the fourth conventional example, U.S. Pat. No. 5,323,091 and International Patent Application publication WO-A-00/77826 proposed arranging a bubble-like secondary discharge chamber such that it is in contact with the conductive foil of the hermetically sealed area of the discharge lamp and which then operated as an auxiliary UV light source.
As the fifth conventional example, U.S. Pat. No. 5,959,404 (corresponding to JP HEI 8-236080) proposed a discharge lamp in which an auxiliary UV light source is formed integrally with the outside of its hermetically sealed area.
As the sixth conventional example, U.S. Pat. No. 6,268,698 (corresponding to JP 2000-173549 A) proposed a discharge lamp in which an auxiliary UV light source is formed integrally with the end face of its hermetically sealed area and carries out the discharge in the open space.
As the seventh conventional example, International Patent Application publications WO-A-99/48133 and WO-A-01/59811 proposed a discharge lamp in which there is an auxiliary discharge vessel with or without an inner electrode adjacent to the main discharge space, in which a high voltage is subjected to electrostatic capacitance coupling, and which is thus operated as an auxiliary UV light source, and in which moreover using the conductivity which is induced by the discharge in the auxiliary discharge vessel a high electrical field is applied to the main discharge space.
However, none of these conventional examples were satisfactory. In the case of the first and the second conventional examples, for example, there are the disadvantages that a hermetically sealed arrangement for enclosing the inner electrodes from the outside is necessary since the auxiliary discharge vessel has inner electrodes and that the production of the hermetically sealed arrangement is time-consuming and costly.
In the case of the third conventional example, there is the disadvantage that the arrangement of the discharge lamp is complicated, that its installation takes a large amount of time, that the arrangement must be protected by an outside vessel and that as a result costs are high even if the production of the auxiliary discharge vessel in itself need not take much time since a hermetically sealed arrangement is not necessary because the auxiliary discharge vessel does not have an inner electrode.
In the case of the fourth example, an extra component is not necessary, therefore no additional material costs accrue. But since the bubble-like secondary discharge chamber must be installed in the discharge lamp itself, there are the disadvantages that production takes much time, that a highly developed production technology is necessary, because in addition a hermetically sealed area is needed, that the probability that the discharge lamp itself is free of faults is certainly reduced since more processing steps are needed and that as a result costs increase. Even in lamps which have been completed without faults, the arrangement of the hermetically sealed area itself, with a reliability which must be ensured to prevent breaking of the bulb, especially in lamps with greater than 150 atm, has the disadvantage that the reliability is certainly reduced more dramatically than without this arrangement of the hermetically sealed area.
In the case of the fifth and sixth conventional examples, the degree of technical difficulty may be lower than in the fourth conventional example. The aspects that the production takes much time and that the probability of a fault-free discharge lamp is reduced are however likewise present. There is, therefore, the disadvantage that costs increase. In the case of the sixth conventional example the discharge is carried out in an open space with a high pressure. Therefore, it is assumed that high precision in the dimensioning of the arrangement and the like is required to produce an effective discharge stably and moreover with high efficiency.
In the case of the seventh conventional example, using the conductivity which is induced by the discharge in the auxiliary discharge vessel, a high electrical field is applied to the main discharge space; compared to the arrangement with the close-by conductor in the form of a wire or the like this structure is therefore more indirect and less effective. Furthermore, there is the disadvantage that production of the discharge vessel of the auxiliary UV light source by a thin hollow tube which surrounds the main discharge space like a ring requires very highly developed technology which results in higher costs.
Besides these measures, there is a relatively old example that ionization of the discharge medium is promoted by a radioactive substance such as Krypton 85 or the like being added to the discharge space and that thus an insulation breakdown is simplified. With consideration of the increased interest in the environment in recent years, this solution is not a technology which can be easily used.