Patent Application
20060152160
The invention relates to a high-pressure mercury vapor discharge lamp comprising an envelope made from high-temperature-resistant material, which contains two electrodes made from tungsten and a filling in a discharge space, which filling essentially consists of mercury, rare gas, and a halogen that is free in the operating condition.
High-pressure mercury vapor discharge lamps with a cyclical process of the halogen for avoiding wall blackening are known from DE 38 13 421 A1 and are used as light sources in video and data projectors. Long burning periods can be reached only if the lamps are not subject to blackening. This can be achieved by adding the halogen to the filling, which can prevent precipitation of evaporating tungsten from the electrodes on the envelope wall. The available halogen reserve, however, is lost in the course of the burning period due to the reactions with the envelope and electrode material and this collapses the halogen cycle. A filling of the lamp with a larger quantity of halogen leads to a high halogen concentration in the gas phase at the beginning of the burning time and consequently to more electrode corrosion and shorter burning period.
It is accordingly an object of the invention to ensure a balanced halogen concentration in the gas phase over a long period.
This object is achieved according to the features of claim 1. According to the invention, the envelope comprises besides the discharge space a second space, which is connected to the former. During the first burning period the mercury evaporates and collects in the second space. If the position of the space is selected suitably, part of the mercury filling will condense within the second space, which is also called hollow space, and will form liquid mercury, which does not evaporate again during the operation. A portion of the filled halogen quantity is soluble as mercury halide in this liquid mercury.
Although mercury halide does not dissolve in the mercury at room temperature, it has surprisingly shown a dissolving phase at temperatures above 200° C. Such a dissolving phase of mercury halide may be used as a storage reservoir or buffer for the halogen concentration in the gas phase in a burning lamp. In this case a dissociation pressure above this solution determines the halogen vapor pressure in the gas phase. As a result the lamp contains a halogen buffer, i.e. a liquid or solid halogen reservoir, which can provide the halogen quantity necessary for a cyclical process in case of loss of halogen from the gas phase.
Advantageously, the second space has a volume that is between 0.5% and 40%, preferably between 1% and 10% of the volume of the liquid mercury filling. The major portion of the mercury thus remains in the discharge space and cannot condense in the second space, so that the operating pressure of the lamp is maintained. Hence the volume of the reservoir is chosen to be so small that only a small portion of the entire mercury filling can condense there.
Simply put, the second space is arranged within the electrode lead-through; as a result its temperature is lower than the coldest spot of the wall of the discharge space while the lamp is on.
Advantageously, the second space is arranged at an inner end of an electrode rod or laterally of the rod. On account of the distance from the discharge space, the temperature of the reservoir is chosen such that enough mercury halide can dissolve and the dissociation pressure adjusts itself above the solution phase in a range leading to an optimum halogen transport cycle. The discharge space and the hollow space are connected to each other by capillaries or slots in order that a pressure and concentration balance can be set between both spaces. Generally, capillaries, cracks or slots arise as a result of the production process in the vicinity of the electrode rod and can be used for connecting. The lamp envelope has one or more second hollow spaces connected to the internal volume of the discharge space, also called interior space of the envelope, which has a lower temperature than the coldest spot on the inside wall of the discharge space during the operation in order that part of the mercury filling can condense there.
In lamps which are used for video and data projectors, the mercury filling is measured advantageously such that a mercury quantity of more than 0.15 mg/mm2 remains in the internal volume during operation. The mercury vapor pressure in these lamps must be very high during operation if a favorable emission pressure is to be reached, which can be achieved only if the envelopes are very compact. The lamps contain mercury fillings of more than 0.15 mg/mm2.
Simply put, the used halogen bromine is in a filling quantity between 10−6 and 10−1 mole per mm3, preferably between 10−5 and 10−2 μmole per mm3 of the internal volumes.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.
In the drawings:
DE 3813421 A1 describes mercury maximum-pressure lamps with a concentration of free bromine in the gas phase of between 10−4 and 10−6 μmole/mm3. This range ensures an optimum halogen transport cycle. This corresponds to a necessary dissociation pressure of HgBr2 of between approx. 0.4 and 40 mbar in the burning lamp. It is advantageous for stabilizing the halogen concentration in the gas phase to operate the lamp near the lower threshold for keeping electrode corrosion as small as possible.
Lamps 1, 21 as described in DE 3813421 A1 were built for a trial series and were used as light sources for video and data projectors for displaying video images, with a reservoir 15, 16 or 35 being provided at one end 17, 18 or laterally of an electrode pin 10, 11, 30. The filling comprised argon as a starting gas, mercury in a quantity of 0.25 mg/mm3 internal volume, and bromine in a quantity of about 1.5×10−4 μmole/mrnm3. The size of the reservoir 15, 16 or 35 was selected such that that less than 10% of the filled mercury could be accommodated there. At a reservoir temperature of approx. 1000K, the dissociation pressure was approx. 4 mbar, whereas approx 50 mbar would be expected at complete vaporization.
The lamps 1, 21 clearly showed lower tungsten transport rates and better long-time stability than corresponding reference lamps without reservoir. No appreciable decline in the bromine quantity in the gas phase was observed during a 2000 h burning period.
| Number | Date | Country | Kind |
|---|---|---|---|
| 03100560.6 | Mar 2003 | EP | regional |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/IB04/00572 | 2/27/2004 | WO | 9/1/2005 |
