Low-pressure mercury-vapor discharge lamp

Details Low-pressure mercury-vapor discharge lamp Low-pressure mercury-vapor discharge lamp

2000-11-20

2003-11-11

Kim, Robert H. (Department: 2882)

Electric lamp and discharge devices

With support and/or spacing structure for electrode and/or...

C313S492000, C313S613000, C313S352000

Reexamination Certificate

active

06646365

ABSTRACT:

The invention relates to a low-pressure mercury-vapor discharge lamp comprising a discharge vessel,
which discharge vessel encloses a discharge space containing a filling of mercury and an inert gas in a gastight manner,
electrodes being arranged in the discharge space for generating and maintaining a discharge in said discharge space,
and an electrode shield at least substantially surrounding at least one of the electrodes.
In mercury-vapor discharge lamps, mercury is the primary component for (efficiently) generating ultraviolet (UV) light. An inner surface of the discharge vessel may be provided with a luminescent layer containing a luminescent material (for example a fluorescent powder) for converting UV to other wavelengths, for example to UV-B and UV-A for tanning purposes (sunbed lamps) or to visible radiation. Such discharge lamps are therefore also referred to as fluorescent lamps.
A low-pressure mercury-vapor discharge lamp of the type mentioned in the opening paragraph is known from DE-A 1 060 991. In said known lamp, the electrode shield surrounding the electrode is made from thin sheet titanium. By using an electrode shield, which is also referred to as anode shield or cathode shield, blackening at an inner surface of the discharge vessel is counteracted. In this respect, titanium serves as the getter for chemically binding oxygen, nitrogen and/or carbon.
A drawback of the use of such an electrode shield is that the titanium in the electrode shield may amalgamate with the mercury present in the lamp and, thus, absorb mercury. As a result, the known lamp requires a relatively high dose of mercury to obtain a sufficiently long service life. Injudicious processing of the known lamp after its service life has ended adversely affects the environment.
It is an object of the invention to provide a low-pressure mercury-vapor discharge lamp of the type mentioned in the opening paragraph, which has a relatively low mercury consumption.
To achieve this, the low-pressure mercury-vapor discharge lamp in accordance with the invention is characterized in that, during nominal operation, the temperature of the electrode shield is above 450° C.
In the description and the claims of the current invention, the designation “nominal operation” is used to indicate operating conditions where the mercury vapor pressure is such that the radiant efficacy of the lamp is at least 80% of that during optimum operation, i.e. operating conditions where the mercury vapor pressure is optimal.
For the proper operation of low-pressure mercury-vapor discharge lamps, the electrodes of such discharge lamps include an (emitter) material having a low so-called work function (reduction of the work function voltage) for supplying electrons to the discharge (cathode function) and receiving electrons from the discharge (anode function). Known materials having a low work function are, for example, barium (Ba), strontium (Sr) and calcium (Ca). It has been observed that, during operation of low-pressure mercury-vapor discharge lamps, material (barium and strontium) of the electrode(s) is subject to evaporation. It has been found that, in general, the emitter material is deposited on the inner surface of the discharge vessel. It has further been found that Ba (and Sr) which is deposited elsewhere in the discharge vessel, no longer participates in the electron emission process. The deposited (emitter) material further forms mercury-containing amalgams on the inner surface, as a result of which the quantity of mercury available for the discharge decreases (gradually), which may adversely affect the service life of the lamp. In order to compensate for such a loss of mercury during the service life of the lamp, a relatively high dose of mercury in the lamp is necessary, which is undesirable from the point of view of environmental protection.
The provision of an electrode shield, which surrounds the electrode(s) and, during nominal operation, is at a temperature above 250° C., causes the reactivity of materials in the electrode shield relative to the mercury present in the discharge vessel, leading to the formation of amalgams (Hg—Ba, Hg—Sr), to be reduced.
It has further been found in experiments that emitter material which evaporates from the electrode reacts with the material of the electrode shield, thereby forming oxides (BaO or SrO). During (nominal) operation of the discharge lamp, mercury makes a bond with these oxides of evaporated emitter material. If reactive oxygen is present in the proximity of the electrode, then BaO, SrO and/or HgO and, possibly, SrHgO
2
and BaHgO
2
are formed. If, in addition, tungsten (originating from the electrode) is deposited (in the case of a cold start, tungsten is sputtered) also WO
X
and HgWO
X
are formed. Without being obliged to give any theoretical explanation, it seems that although BaO and SrO do not react with mercury under normal thermal conditions, the presence of the discharge in the discharge space plays a part in the formation of these compounds of mercury and the oxides of evaporated emitter material. At temperatures above 450° C. the mercury is released again, as a result of dissociation of said compounds of mercury and the oxides of evaporated emitter material, and the released mercury is available again for the discharge. Particularly HgO dissociates at a temperature of 450° C. or higher; the compounds SrHgO
2
and BaHgO
2
are slightly more stable. The inventors have recognized that by using an electrode shield having a temperature of 450° C. or higher, mercury is released from the compounds of mercury and oxides of emitter material. A particularly suitable temperature of the electrode shield is approximately 500° C., at which temperature also the dissociation of, in particular, SrHgO
2
and BaHgO
2
takes place relatively rapidly. It cannot be excluded, however, that the stainless steel also acts as a getter (corrosion) at the above-mentioned relatively high temperatures, leading to an additional reduction of the formation of HgO-type compounds.
The known lamp comprises an electrode shield of thin sheet titanium, which material relatively readily amalgamates with mercury. The mercury consumption of the discharge lamp is limited by substantially reducing the degree to which the material of the electrode shield, which surrounds the electrode(s), reacts with mercury and/or bonds with mercury.
In addition, the use of an electrically insulating material precludes the development of short circuits in the electrode wires and/or in a number of windings of the electrode(s). The known lamp has an electrode shield of an electroconductive material, which, in addition, relatively readily forms an amalgam with mercury. The mercury consumption of the discharge lamp is limited by substantially reducing the degree to which the material of the shield surrounding the electrode(s) reacts with mercury.
In order to obtain an electrode shield which can be heated to such high temperatures during nominal operation of the discharge lamp and, during operation, is capable of maintaining said high temperatures throughout the service life of the discharge lamp, the electrode shield is preferably manufactured from a metal or a metal alloy which can withstand temperatures of 450° C. or higher. An “electrode shield which can withstand high temperatures” is to be taken to mean in the description of the current invention, that, during the service life of the discharge lamp and at said temperatures, the material from which the electrode shield is manufactured does not show signs of degassing and/or evaporation, which adversely affect the operation of the discharge lamp, and that no appreciable changes in shape occur in the electrode shield at such high temperatures.
A preferred embodiment of the low-pressure mercury-vapor discharge lamp is characterized in accordance with the invention in that the electrode shield is made from stainless steel. Stainless steel is a material which is resistant to high temperatures. Stainless steel has a high corrosion resistance, a relatively low coefficient o

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