EP0703600B1 - High pressure discharge lamp - Google Patents

Description

The invention relates to a high-pressure discharge lamp according to the Preamble of Claim 1. In particular, these are high-pressure mercury lamps low power (up to 250 W), e.g. for use in fluorescence microscopy, but also around high pressure xenon lamps similar services. Basically, the application is for higher performances however not excluded.

A high-pressure discharge lamp is known from EP-A 299 230, in which the uneven arc is reduced in that the cathode at least in Area of their tapered tip coated with a carbide layer which either decreases continuously towards the tip or the discharge side Leaves third of the tip free. Any carburizing of the cylindrical electrode part with constant diameter was called classified as meaningless.

However, it has been shown that the arch stability achieved in this way meets the requirements not sufficient for special photometric applications and in addition, the production of these electrodes is very time consuming.

The object of the present invention is to improve the arc stability in generic lamps and at the same time to increase the maintenance of the luminance.
This object is achieved by the characterizing features of claim 1. Preferred designs can be found in the dependent claims.

Surprisingly, it has been shown that the arch stability can not only be maintained but can even increase if the carburization in the tapering region in front of the electrode tip is reduced under suitable framework conditions. This behavior is achieved when the tapered area of the electrode is completely free of carbon, while a large part of the cylindrical base body of the electrode is covered by a carbide layer. The easiest way to produce such an electrode is to carburize a cylindrical rod, as described in EP-A 299 230, and only then to produce the tapered tip by etching or grinding and / or polishing.
The unsteadiness of the bow can be kept below 10% during the entire service life.

The carburization particularly advantageously interacts with a special material composition and structure of the cathode. In particular when using thin electrodes with a maximum of 5 mm diameter (values below 2 mm, particularly preferably below 1 mm are preferred), an electrode material has proven to be favorable which, in addition to tungsten, contains a maximum of 0.6% by weight ThO ₂ (preferably 0.2 up to 0.45%). Additional doping with 50-100 ppm potassium, up to 20 ppm aluminum and up to 10 ppm silicon (in each case based on weight) is particularly preferred. This material is subjected to a special manufacturing process in order to form a distinctive long crystal structure and to achieve a finely divided thorium oxide distribution.

Using a wet chemical process similar to that described in U.S. Patent No. 5,284,614 described, it is now possible to add very little thorium oxide (previously around 3%) use. This is already added to the tungsten powder. The endowment favors the formation of the desired long crystal structure, its Structure similar to that described in DE-AS 1 088 155. To be particularly advantageous has proven that the usual deformation work, consisting of Rolling, hammering and drawing (see e.g. DE-OS 40 02 974 and others Citations), is modified by restricting the usual hammering process is (advantageously it is completely dispensed with) and the drawing process instead is increasingly used to make the long crystal structure particularly strong shape, stabilize and train in a defined manner. The each electrode diameter used is advantageous with the degree of Restriction of the hammering process coordinated. With particularly small ones Electrode diameters can U. be hammered entirely. After the rod is carburized, a tapered area becomes created by the tip to a truncated cone or the like. etched or sanded and then polished. The basic body is advantageous at least more than 30%, preferably more than 50%, of its total length, starting from the approach of the tapered area, covered with carbide. The truncated cone preferably has a maximum height of 5 mm; the optimal height depends on the opening angle and the cathode diameter. With large diameters (e.g. 4 mm) a large opening angle (e.g. 60 °) advantageous, corresponding to a truncated cone height of approx. 4 mm. There is also a narrow area at the end facing away from the discharge of the base body is freed from the carbon layer for better contact.

Through a suitable choice of the electrode geometry, the system is inherent Luminance decrease greatly reduced, which is due to the Electrode erosion. By using the o.e. carburized cathode namely, the electron exit at the tip of the electrode facilitates, so that the required current density even at a lower operating temperature is reached, which in turn reduces the electrode erosion. While so far, with a lamp life of 200 h, the average Electrode erosion increased the electrode distance by approx. 100% (from typically 0.6 to 1.3 mm), the burn-off in the invention Lamp only 30 to 50%. Another consequence is that the Firing voltage increase during the service life is also small. He can are now limited to a maximum of 50% of the previously usual value.

This overall improved operating behavior leads to an extension of the lamp life by 50% from 200 to 300 hours. The invention is applicable to high-pressure mercury lamps. Typical doses are 10 to 80 mg / cm ³ , an electrode spacing of 0.5 to 4 mm and burning voltages up to 50 V.

The invention can be used with particular advantage in the case of low-power mercury lamps (typical values are 50-200 W). The above measures create the prerequisite for optimizing the radiation intensity in the wavelength range relevant for the respective application. This happens primarily through a higher mercury dose. The application of this measure known per se has hitherto failed at these low powers because it has caused lamp failures. Due to the improved electrode, however, high doses between 70 and 130 mg / cm ^{3 of} mercury are now particularly advantageous without impairing the service life. In particular, the short-wave radiation intensity (in particular the range between 400 and 500 nm) can now be increased in a targeted manner (20-40%) without loss of other wavelength ranges which are also used.
Finally, the invention makes it possible for the first time to produce high-pressure mercury lamps in conjunction with reflectors as an extremely small structural unit. These are used, for example, in endoscopy.

Another area of application is high pressure xenon lamps small power up to 250 W.

Fig. 1

eine Quecksilberhochdrucklampe

Fig. 2

eine Kathode für die Lampe gemäß Fig. 1

Fig. 3

einen Vergleich der Bogenunruhe mit früheren Lampen

Fig. 4

den Elektrodenabstand einer Lampe gemäß Fig. 1 als Funktion der Brenndauer

Fig. 5

einen Vergleich des Spektrums erfindungsgemäßer und früher verwendeter Lampen

Fig. 6

eine Reflektorlampe

Fig. 7

eine Xenonhochdrucklampe

The invention is explained using several exemplary embodiments. It shows

Fig. 1

a high pressure mercury lamp

Fig. 2

a cathode for the lamp of FIG. 1st

Fig. 3

a comparison of the uneven arch with previous lamps

Fig. 4

1 as a function of the burning time

Fig. 5

a comparison of the spectrum of lamps according to the invention and previously used lamps

Fig. 6

a reflector lamp

Fig. 7

a high pressure xenon lamp

1 shows a DC-powered 100 W high-pressure mercury lamp 1. It is suitable for fluorescence microscopy and endoscopy as well as for light guide applications, Schlieren photography and the reproduction of holograms. The elliptical discharge vessel 2 made of quartz glass is filled with mercury in an amount of 18 mg. The volume is 0.2 cm ³ . The total length of the vessel 2 is 73 mm. In the discharge vessel 2, the anode 3 and the cathode 4 are arranged axially at a distance of 0.6 mm from one another. Each electrode has a cylindrical shaft 5.

The electrical supply takes place in a known manner via molybdenum foils 6, which are connected to the metal sleeve bases (not shown) via pins are. The molybdenum foils 6 are vacuum-tight in the two ends 7 of the discharge vessel 2 melted down. Instead of melting with molybdenum foils another technique, e.g. Rod melting or cup melting, be used.

The anode 3 is made as a solid cylinder block from hammered tungsten and has a wide, slightly bevelled end face.
The comparatively small cathode 4, onto which a coil is pushed, is shown enlarged (but not to scale) in FIG. 2. In order to ensure high arc stability, the cylindrical base body 8 of the cathode 4 (diameter approx. 0.6 mm, length 16 mm) tapers in the manner of a cone 9, the tip 10 of which is truncated. The stump that forms the base for the arch has a diameter of 0.1 mm. The cone forms an opening angle α of 15 ° and has an overall length of approximately 1.7 mm. The cone 9 is free of carbide. The cylindrical base body 8 is surrounded over its entire length by a layer 11 of tungsten carbide, with the exception of the end region 12 of 4.5 mm in length facing away from the discharge.

The cylindrical base body can also be only partially covered by carbide his. For example, the cylindrical body is at least 50% its total length, starting from the base of the truncated cone, carburized.

The cathode is preferably made of tungsten with a small amount of other substances (in addition to 0.4% by weight thorium dioxide 75 ppm potassium, 10 ppm Aluminum and 5 ppm silicon) is doped. The carbide layer has one 5 µm thick. In general, the layer thickness can be between 1 and 15 µm, it is preferably between 3 and 8 pm. The tapered area can be divided into several sections instead of a cone or a truncated cone, e.g. Truncated cones with different opening angles become.

Fig. 3 shows a comparison between the uneven arch of an inventive Lamp (Fig. 3a) and a previously used lamp (Fig. 3b). While the new version has an uneven arc of one percent When the burn time of 200 h is reached, the bow unrest in the old version is over an order of magnitude worse (Fig. 3b) and reaches values up to 100%.

4 shows the electrode spacing as a function of the burning time. To After 200 hours, the initial value of 0.6 mm only increased by 40% 0.85 mm. In contrast, in the old version the original electrode distance is increased from 0.5 mm to almost double (0.95 mm). Directly correlated with this is the mean increase in the While at the old version was more than 10 V, it is in the new invention Version limited to approx. 5 V (from 23 V to 28 V). This property is especially important because the ballast has a burning voltage above 30 V. can overuse.

Finally, FIG. 5 shows a comparison between the lamp spectrum of an old and a new version. The higher intensity of the new version is particularly pronounced in the short-wave spectral range and is still clearly visible up to 600 nm.
For example, the intensity in the new version is 10% higher in the 355 to 375 nm spectral band, 38% in the 450 to 500 nm band and 17% in the 535 to 555 nm range than in the old version.

6 also shows a structural unit of a high-pressure mercury lamp 1 a reflector 15 for use in endoscopy. The reflector lamp is characterized by a low overall height of only 83 mm and one Diameter of 67 mm. The lamp 1 sits axially in an elliptical Reflector 15, which is provided with a dichroic coating 16. The Reflector lamp mainly emits in the wavelength range 320 to 390 nm. It is used in particular for the curing of paints. The cathode 4 of the Lamp 1 is adjacent to the top of the reflector. A heat accumulation coating 18 covers approximately the lower third of the discharge vessel 2.

Fig. 7 shows a high-pressure xenon lamp with a power of 180 W. Sie has a cathode 21 with a diameter of 1.5, which is at the tip a truncated cone with a height of 3.5 mm, corresponding to one Opening angle of 26 °. The lamp 20 is axially in a reflector 22 placed, similar to that described in Fig. 6.

Claims (14)

1. High-pressure discharge lamp having a discharge vessel (2), a cathode (4) and anode (3) arranged axially therein, as well as an ionizable filling, the cathode having a cylindrical basic body (8) which tapers (9) to a tip (10), and it being the case that the cathode is partially coated with a carbide layer, characterized in that the entire tapering region (9) between the tip (10) and basic body (8) of the cathode is free from carbide, while the basic body is at least partially covered with carbide.
2. High-pressure discharge lamp according to Claim 1, characterized in that the lamp contains mercury in a quantity of 70 to 130 mg/cm³.
3. High-pressure discharge lamp according to Claim 2, characterized in that the electrode spacing is approximately 0.4 - 0.8 mm.
4. High-pressure discharge lamp according to Claim 2, characterized in that the operating voltage is approximately 20 to 29 V, in particular approximately 23 V.
5. High-pressure discharge lamp according to Claim 1, characterized in that it forms an assembly with a reflector.
6. High-pressure discharge lamp according to one of the preceding claims, characterized in that the lamp power is up to 250 W.
7. High-pressure discharge lamp according to Claim 1, characterized in that the layer thickness of the carbide layer is 1 - 15 µm, in particular 3 - 8 µm.
8. High-pressure discharge lamp according to Claim 1, characterized in that the length of the tapering region (9) is at most 5 mm.
9. High-pressure discharge lamp according to one of the preceding claims, characterized in that the cathode consists of tungsten with an admixture of up to 0.6% by weight of ThO₂, as well as, if appropriate, further additives, in particular potassium, aluminium and silicon, in smaller quantities.
10. High-pressure discharge lamp according to Claim 9, characterized in that the cathode has an elongated crystal structure.
11. High-pressure discharge lamp according to Claim 1, characterized in that the lamp contains xenon with a cold fill pressure of approximately 2 - 15 bars.
12. High-pressure discharge lamp according to Claim 8, characterized in that the length of the tapering region is greater than or equal to the diameter of the cathode.
13. High-pressure discharge lamp according to Claim 12, characterized in that the tapering region is a cone or conical frustum with a full opening angle of 10 to 30°.
14. high-pressure discharge lamp according to Claim 1, characterized in that the basic body is covered with carbide over a part of its overall length, in particular 30%, preferably 50%, starting from its step at the tapering region.

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