Electric lamp and discharge devices – With gas or vapor – Having a particular total or partial pressure
Reexamination Certificate
2001-03-08
2003-10-07
Patel, Ashok (Department: 2879)
Electric lamp and discharge devices
With gas or vapor
Having a particular total or partial pressure
C313S492000, C313S613000, C313S642000
Reexamination Certificate
active
06630787
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a low-pressure mercury-vapor discharge lamp comprising a discharge vessel, which said 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 maintaining a discharge in said discharge space, and an electrode shield surrounding at least one of the electrodes.
2. Discussion of the Prior Art
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 coated 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 for general illumination purposes. Such discharge lamps are therefore also referred to as fluorescent lamps. Low-pressure mercury-vapor discharge lamps comprise a generally tubular discharge vessel which is circular in section, and which includes elongated as well as compact embodiments. In general, the tubular discharge vessel of so-called compact fluorescent lamps comprises a collection of comparatively short, straight parts having a comparatively small diameter, which straight parts or connected to one another by means of bridge parts or by means of, for example, arch-shaped parts. Compact fluorescent lamps are generally provided with a lamp cap (with integrated electronics).
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.
SUMMARY OF THE INVENTION
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 consumes comparatively little mercury.
In accordance with the invention, this object is achieved in that the electrode shield carries an electric current during operation, and the temperature of the electrode shield during nominal operation is above 250° C.
In the description and the claims of the current invention, the designation “nominal operation” is used to indicate operating conditions in which 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 in which 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). 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 compounds, for example 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 operation, carries an electric current 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.
A preferred embodiment of the low-pressure mercury-vapor discharge lamp is characterized in accordance with the invention in that the temperature of the electrode shield during nominal operation exceeds 450° C.
At temperatures above 450° C., the mercury abstracted from the discharge by amalgamation is released again. Particularly HgO and mercury-containing Ba and Sr compounds dissociate at a temperature of 450° C. or higher. By using an electrode shield which is heated to 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 the dissociation of said compounds takes place relatively rapidly.
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 operation, the electrode shield is customarily heated by the heat radiated by the electrode. It has been recognized by the inventors that it is advisable to heat up the electrode shield to the desired temperature. To achieve this, a preferred embodiment of the low-pressure mercury-vapor discharge lamp is characterized in that the electrode shield electrically contacts the electrode via a current conductor.
In general, the discharge vessel of the low-pressure mercury-vapor discharge lamp is provided with a first and a second current-supply conductor, which issue from the discharge vessel (
10
) to the exterior. Heating up of the electrode shield preferably takes place by the first current-supply conductor electrically contacting the electrode shield and the second current-supply conductor electrically contacting the electrode. By virtue thereof, the electric current that flows to the electrode via the current-supply conductors also flows through the electrode shield.
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.
In order to reach the desired temperature of the electrode shield during nominal operation of the discharge lamp, the current flowing through (a current-carrying portion of) the electrode shield, during operation, causes energy in the form of heat
Seinen Peter Arend
Van Der Pol Adrianus Johannes Hendricus Petrus
Koninklijke Philips Electronics , N.V.
Patel Ashok
Zimmerman Glenn
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