Electrode support tube for high pressure discharge lamp

Details Electrode support tube for high pressure discharge lamp Electrode support tube for high pressure discharge lamp

1999-04-12

2001-07-17

Patel, Ashok (Department: 2879)

Electric lamp and discharge devices

With gas or vapor

Having electrode lead-in or electrode support sealed to...

Reexamination Certificate

active

06262535

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to high pressure discharge lamps, and more particularly to short-arc lamps, especially mercury arc discharge lamps of high power. Additives of metal halides may be included within the discharge vessel of the lamps. The basic principle of the invention is also suitable for use in xenon short-arc discharge lamps.
BACKGROUND OF THE INVENTION
U.S. Pat. No. 5,140,222, Roznerski, discloses a high pressure discharge lamp of the type to which the present invention relates. This lamp has a fill which includes xenon. The lamp has a quartz glass bulb which defines a lamp axis. Two neck portions extend from the bulb in alignment with the axis. The lamp necks are provided with conically shaped support elements made of quartz glass, and with ceramic disks movably located in the lamp necks, and pressed by a spring to a constricted zone of the neck portions and the lamp.
Plug elements in the form of glass cylinders have been used in mercury arc high pressure discharge lamps for support of electrode connecting or holding rods. These glass cylinders are melt-connected to the necks extending from the bulb, and have a smooth generally cylindrical outer surface on which the melt connection is formed. The lamps described in the referenced U.S. Pat. No. 5,264,759, Lewandowski et al, U.S. Pat. No. 5,304,892, Lewandowski et al, and German Publication DE 196 18 967 A1 have such support tubes which, at least in the transition region to the bulb, have a constant wall thickness throughout their length. Thus, the outer diameter as well as the inner diameter of the support tubes are defined by the diameter of the foil melt connection in the more remote portion of the necks of the lamps. The support tubes usually were fitted directly at the beginning of the neck portion, and melt connected with the wall of the bulb in the region of the neck. This technology permitted only mercury fills up to a maximum of 20 mg/cm
3
and relative small bulb dimensions, to an overall length of about 80 mm. If higher pressure is used, the risk of bursting increases, since the stress accepting capability in the region of the transition of discharge space to the neck is exceeded.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a high pressure discharge lamp which can accept a higher fill pressure without bursting, and typically, to accept a value of operating pressure of from 30 to 70 bar.
Briefly, the support tube is shaped to have a conical outer surface which tapers in a narrowing direction towards the interior of the discharge space, that is, towards the respective electrode. The terms “inner end” and “outer end” will be used hereinafter with respect to the discharge space within the bulb of the lamp. Thus, the outer diameter at the inner end of the support tube will be smaller than the outer diameter of the support tube at the outer end thereof. The support tube is melt-connected to the respective neck of the lamp which, in the region of the support tube, is also slightly conical to closely fit the support tube.
The lamp in accordance with the present invention has a bulb of quartz glass with two generally cylindrical necks extending from the bulb. Two electrodes positioned diametrically opposite each other are located within the discharge space of the bulb. The electrodes are supported by electrode holding or support rods. At least one of the holding rods, and preferably, both holding rods, are surrounded by a respective support tube in the inner region of the neck; the support tube, as noted, has a conical outer surface, and is melt-connected to the neck. The outer diameter of the inner end of the support tube is smaller than the outer diameter of the more remote or outer end of the support tube.
The electrode holding rod can continue into the more remote portion of the neck, or it can terminate behind the support tube, for example at a molybdenum disk. The rods can be extended by an extension portion toward the outside. That, however, does not form part of the present invention and the further electrical connection can be in accordance with any well known arrangement, for example as described in the above-referred-to referenced U.S. patents.
The support technology in accordance with the present invention permits constructing higher powered lamps with a power of over 1 kW and a fill which operates under particularly high operating pressure, for example up to about 70 bar. Typically, a mercury fill with 20 to 100 mg/cm
3
can be used, rising even to a maximum of up to about 150 mg/cm
3
. The large bulbs have a length up to 120 mm, and over, and the diameters are typically of about 100 mm.
The danger of bursting of the bulb exists due to the high operating pressure and is counteracted by the practice of this invention. A careful analysis of bursts of bulbs has shown that there is a weak point at the transition between the discharge space of the bulb and the bulb neck. Shaping the support tube in conical form, preferably with essentially uniform wall thickness, surprisingly, substantially reduces the danger of bursting. Utilizing selected dimensioning, readily determinable by a few experiments, the bursting pressure can be increased up to about 300% over previously believed permissible pressures.
In a preferred embodiment of the invention, the ratio between the outer diameter of the remote or rearward end—with respect to the discharge space of the bulb—and the outer diameter of the inner end of the support tube is between about 1.1 and 2.5.
In accordance with an important aspect of the invention, the wall thickness of the support tube at its inner end is as small as possible, so that the transition of the wall thickness of the neck to the system neck-support tube is as continuous as possible. Good results have been obtained when the wall thickness of the support tube is not over the original wall thickness of the bulb at the transition region; preferably it is less and, most preferred, less than 50% of the original wall thickness of the bulb at the point where the support tube starts. With a 50% wall thickness of the support tube, the reinforced wall thickness will be at the most 1.5 the original wall thickness of the bulb after melt sealing the support tube to the neck and/or to the bulb, respectively. The transition zone of the neck, in the region of the inner end of the neck where it joins the bulb, is preferably free from the support tube. It is recommended that this transition zone at the most is twice the outer diameter of the support tube at its inner end. Absolute values, preferably, place this recess of the support tube in the transition zone, or the transition zone itself, between 3 and 25 mm.
The resistance against bursting is further improved and optimized by carefully selecting the cone angle of the bulb in the region of the transition zone to the conical outer surface at the inner end of the support tube. The angle &bgr; (
FIG. 3
) between the end facing surface of the support tube and the inner wall of the bulb, in the region of the transition zone, should be at the most 90°. Preferably at the most it is equal to an angle &agr;, in which the angle &agr; is the angle which corresponds to a tangent on the inner wall of the bulb to the end facing surface of the support tube. The most preferred relationship for the angle &bgr; is less than or equal to (&agr;−15°). The inner wall of the bulb, in the region of the transition zone, then typically extends inwardly over the support tube, so that the support tube is recessed from the bulbous region of the bulb.
The concept of a conical support tube does not require that the generatrix for the conical surrounding surface is a straight line; rather, the generatrix for the cone may be a curve, for example a bulged curve, so that the conical surface is somewhat bulged outwardly but, in general, still is generally conical. The generally conical support tube may, at its outer end, have a short cylindrical extension portion extending, preferably, at the most over 30% of the overall length.

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