Low-Pressure Mercury Vapor Discharge Lamp

The invention relates to a low-pressure mercury vapour discharge lamp having a closed discharge vessel which contains mercury and rare gas and within which electrodes are positioned between which the discharge is maintained during operation, each electrode being formed by a tungsten coil supported at either end by a respective current supply wire, which coil has central turns covered with at least one emitter and, on either side thereof, end turns which are free from emitter.

Low-pressure mercury vapour discharge lamps of the kind described are well-known. Such lamps include, for example, fluorescent lamps constructed as straight tubes as, for example, disclosed in U.S. Pat. No. 3,937,998, and compact fluorescent lamps as, for example, disclosed in U.S. Pat. Nos. 4,374,340 and 4,546,285.

The emitter on the central turns of the tungsten coil provides a reduction of the emission potential of electrons emitted by the tungsten coil during operation of the lamp. A mixture of barium oxide, strontium oxide, and calcium oxide, for example, is used as an emitter. The barium therein is the main active ingredient, while the strontium and the calcium reduce the vapour pressure of the barium. During the manufacture of the lamp, triple carbonate (Ba-Sr-Ca carbonate) is provided on the central turns and converted into Ba-Sr-Ca oxide by heating through passage of electric current through the tungsten coil, whereby CO.sub.2 is evolved. The end turns and the current supply wire ends are not coated with emitter because the temperature of the end turns and the current supply wire ends remains too low during passage of current for a good conversion of the triple carbonate into oxides to be realised. This would mean that later, in the finished lamp, CO.sub.2 would be evolved during operation, which is disastrous for lamp life.

Low-pressure mercury vapour discharge lamps can be subdivided into so-called hot starting and cold starting lamps. In hot starting lamps, the tungsten coil is preheated before ignition of the lamp in that an electric current is passed through it. In cold starting lamps, the discharge is initiated by a glow discharge at he area of a tungsten coil, for example, between one tungsten electrode and an ignition strip which is connected to the other tungsten electrode, or between the two electrodes.

A problem with cold starting low-pressure mercury vapour discharge lamps is the switching life of the lamps. It has been found that the lamps reach the end of their lives already after a comparatively low number of on/off switching operations.

The invention has for its object to provide an improved low-pressure mercury vapour discharge lamp which has a longer switching life because of a greater switching resistance.

To achieve the envisaged object, a low-pressure mercury vapour discharge lamp of the kind mentioned in the opening paragraph is characterized in that each of the current supply wire ends together with the adjoining end turns of the tungsten coil is surrounded by an electrically insulating sleeve.

The invention is based on the recognition that during starting of the lamp the transition from the glow discharge to the arc discharge at the tungsten coil takes place preferably at the area of the end turns of the coil not coated with emitter, near the ends of the current supply wires, with the result that a "hot spot" in the end turns will lead to the end of the life of the coil. By electrically insulating the end turns and the current supply wire ends from the discharge, the discharge is forced to strike at the emitter-coated central turns of the tungsten coil, and a much longer switching life is surprisingly found to be obtained.

The electrically insulating sleeve may be provided, for example, as a layer, for example, by means of the so-called CVD technology (chemical vapour deposition).

A favourable embodiment of a low-pressure mercury vapour discharge lamp according to the invention is characterized in that the electrically insulating sleeve consists of a glass tube which is closed at the side of the discharge.

Preferably, the glass tube is provided with a slot in axial direction through which the tungsten coil is passed. The slot renders it possible to slide the glass tube simply over the end turns and the adjoining current supply wire end.

A further favourable embodiment of a low-pressure mercury vapour discharge lamp according to the invention is characterized in that the glass tube is fixed around the current supply wire end through fusion by heating.

The invention will be explained with reference to a drawing.

In the drawing:

FIG. 1 shows a known low-pressure mercury vapour discharge lamp in longitudinal section;

FIG. 2 shows a mount with electrode construction for use in a low-pressure mercury vapour discharge lamp according to the invention.

The low-pressure mercury vapour discharge lamp of FIG. 1 has a closed glass discharge vessel 1 which contains mercury and a rare gas, for example argon, as a starting gas. Electrodes 2 and 3 (tungsten coils) are arranged inside the discharge vessel 1, between which electrodes the discharge is maintained during lamp operation. The discharge vessel 1 is provided on its inside with a luminescent layer 4 which comprises at least one luminescent material which emits visible radiation upon excitation by mainly 254 nm radiation from the mercury discharge.

In FIG. 2, the reference numeral 5 denotes a glass stemtube which is provided with an exhaust tube 6 and a pinch seal 7 in known manner. Two current supply wires 8 are sealed into the pinch seal 7 and support a tungsten coil electrode 9. The tungsten coil 9 has central turns 9a which are coated with an emitter, for example BaO in combination with SrO and CaO, and end turns 9b on either side thereof not coated with emitter. The number of non-coated end turns is determined in practice by margins in the emitter coating process and the degassing process. The safety margin used is a distance of 1 to 2 mm from the current supply wires to the emitter, which implies a varying number of non-coated end turns for the various electrodes. A glass tube 10 is fitted around each end of the two current supply wires 8 and around the adjoining end turns 9b, which tube is provided with a slot (not visible) in axial direction through which the tungsten coil 9 is passed. The glass tube 10 is closed at the top, i.e. at the discharge side in a lamp, so that the end of the current supply wire 8 and the end turns 9b are electrically insulated from the discharge. As a result, the discharge can only strike at the central turns 9a coated with emitter. It is achieved in this way that the switching life of the lamp is considerably increased.

The glass is softened in that the glass tube 10 is heated with a burner, and the tube fixed itself around the current supply wire 8, possibly after pinching.

During tests with compact fluorescent lamps of the PLC-E type (electronic lamp having four interconnected parallel discharge tubes in a square arrangement), in which the number of end turns not coated with emitter at either end of the tungsten coil was approximately 7, the end of the switching life of the lamp was achieved after approximately 8,000 on/off switching operations without a glass tube 10. When lamps of the same batch were provided with the glass tubes 10, the end of the switching life of the lamp was not reached until after approximately 11,000 on/off switching operations.