GB2157883A

GB2157883A - Low-pressure mercury vapour discharge lamp

Info

Publication number: GB2157883A
Application number: GB08508197A
Authority: GB
Inventors: Cornelis Machiek Verheij; Jean Johan Heuvelmans
Original assignee: Philips Gloeilampenfabrieken NV
Current assignee: Koninklijke Philips NV
Priority date: 1984-04-02
Filing date: 1985-03-29
Publication date: 1985-10-30
Also published as: GB2157883B; FR2562324B1; GB8508197D0; BE902088A; NL8401030A; FR2562324A1; CA1239978A; DE3510156A1; JPS60225346A

Abstract

A low-pressure mercury vapour discharge lamp comprises a discharge vessel which is sealed in a vacuum-tight manner and has at least two parallel tube parts (1,2) arranged beside each other and connected to each other at or near their one ends (3). During operation a cool area (4,5) is present in the discharge vessel near the said connection (3). The discharge vessel contains an amalgam (13) for controlling the mercury vapour pressure in the discharge vessel during operation of the lamp in order to prevent condensation of mercury occuring at the cool area. <IMAGE>

Description

SPECIFICATION Low-pressure mercury vapour discharge lamp The invention relates to a low-pressure mercury vapour discharge lamp comprising a discharge vessel which is sealed in a vacuumtight manner and has at least two parallel tube parts arranged beside each other and connected to each other at or near their one ends, there being present in the discharge vessel during operation of the lamp a discharge path between electrodes provided at the other ends of the tube parts, discharge path which is curved at at least one area, the discharge vessel being shaped near the connection between the tube parts in a manner such that during operation of the lamp in situ a part of the inner wall of the discharge vessel has a comparatively low temperature.

Such a lamp is known from British Patent Specification 2050046.

The known lamp is comparatively compact and serves as an alternative to incandescent lamps for general illumination purposes.

In the case of an increase of the electrical energy supplied to such a lamp or if the lamp is operated in a space in which the temperature is higher than the normal room temperature (for example in a closed luminaire of small dimensions), the light output nevertheless remains comparatively high. The mercury vapour pressure in the discharge vessel is in fact stabilized at an optimum value (approximately 10-2 torr) due to the presence of the cool area in the discharge vessel near the end of a tube part.

However, it has been found that condensation of mercury is liable to occur at the cool area in the discharge vessel under normal operating conditions. This is especially disadvantageous if the lamp is arranged in a vertical operating position, in which the electrodes are situated at the lower end of the lamp.

They are then located substantially perpendicularly below the comparatively cool wall portion. There is then a risk that the mercury drips downwards along the wall of the discharge vessel, as a result of which a luminescent layer present on the inner wall of the discharge vessel is attacked. This is not only detrimental to the light output, but moreover the appearance of the lamp is spoilt. Furthermore there is also a risk that the liquid mercury falls downwards in the form of drops and impinges on the filament spiral of an electrode. This adversely affects the operation of the lamp. Moreover, with the occurence of this phenomenon, fluctuations in the light output are obtained and the light intensity distribution over the discharge vessel is inhomogeneous.

The invention has for its object to provide a lamp having a high light output, in which, however, the aforementioned disadvantageous phenomena are avoided.

According to the invention, a low-pressure mercury vapour discharge lamp of the kind mentioned in the opening paragraph is for this purpose characterized in that the discharge vessel contains an amalgam for controlling the mercury vapour pressure in the discharge vessel during operation of the lamp.

In the lamp according to the invention, the aforementioned undersirable phenomena do not occur. The mercury vapour pressure is controlled in the discharge vessel under normal operating conditions by the amalgam, as a result of which no condensation of mercury takes place at the cool area in the discharge vessel. The mercury vapour pressure during operation of the lamp is moreover stabilized at a value of approximately 10-2 torr over a wide temperature range. The light output of the lamp then remains as high as possible even if the lamp is operated in a compratively warm area, such as in a luminaire.

The amalgam is preferably situated at an area behind one of the electrodes. A suitable area is, for example, the base of the so-called mount. Such a mount is provided not only with an electrode, but, for example, also with wires for this electrode as well as with an exhaust tube located behind the electrode which is used for pumping the discharge vessel.

In a preferred embodiment of the lamp according to the invention, the amalgam is present in the exhaust tube. It has been found that such an area is favourable for a satisfactory vapour pressure stabilization.

The amalgam present in the discharge vessel of the lamp according to the invention consist, for example, of an alloy of indium and mercury or of an alloy of indium, bismuth and mercury (see, for example, British Patent Specification 1503636). Favourable results were obtained with an amalgam consisting of bismuth, lead, tin and mercury (see, for example, British Patent Specification 1572657).

The discharge vessel of the lamp according to the invention may have different shapes.

The discharge vessel may comprise, for example, two tube parts which are connected to each other by a transverse connection, as described in the aforementioned British Patent Specification 2050046. However. it may alternatively comprise four such interconnected tube parts. Favourable results were also obtained with a lamp having a discharge vessel of the kind described in German Offenlegungsschrift 3112878. In this lamp, a cool wall portion is obtained by shaping a discharge tube which is in the form of a U at the area of the curved part in a manner such that a wall portion is obtained whose cross-section extends substantially at right angles to the direction of the axis.

An embodiment of the invention will now be described more fully with reference to the accompanying drawing. In the drawing, a low-pressure mercury vapour discharge lamp according to the invention is shown, partly in elevation and partly in sectional view.

The lamp comprises a discharge vessel which is sealed in a vacuum-tight manner (filled with mercury and a rare gas, such as argon) comprising two parallel tube parts 1 and 2, which are arranged beside each other and are connected to each other through a transverse connection 3. The inner wall of the tube parts and the transverse connection is coated with a luminscent layer. The transverse connection is provided, for example, by means of a method as described in GB-PS 2048562. This transverse connection is located at a certain distance from the one ends 4 and 5 of the tube parts 1 and 2, respectively. These ends are in the form of end faces arranged at right angles to the longitudinal axes of the tube parts. The electrodes 6 and 7 are arranged beside each other, each at the other end of a respective tube part.During operation of the lamp, a U-shaped discharge path is present in the tube parts 1 and 2 between these electrodes.

A lamp cap 8 is secured at the ends of the tube parts (1, 2), at which the electrodes are located, and this cap preferably consists of synthetic material. The lamp cap comprises a projecting wall portion 9, in which a starter is provided. Current connection pins 10 and 11 are arranged on either side of the projecting wall portion.

During operation of the lamp, portions of the inner wall of the discharge vessel located near the one ends of the tube parts (especially the wall portions 4 and 5) are comparatively cool. Without additional steps being taken, condensation of mercury can occur at these areas, which has the aforementioned disadvantageous consequences, especially if the lamp is arranged in vertical position.

In order to counteract these phenomena, an amalgam 1 3 for controlling the vapour pressure in the discharge vessel during operation of the lamp is present in the exhaust tube 1 2 behind the electrode 6 forming part of the mount associated with the electrode 6, the end of the exhaust tube 1 2 remote from the electrode 6 being sealed. Condensation of mercury at the aforementioned ends is then avoided. The alloy forming with mercury an amalgam preferably consists of the elements lead, bismuth and tin.

In a practical embodiment of the aforementioned lamp, the length of the tube parts is approximately 1 3 cm and the inner diameter is approximately 10 mm. The luminscent layer on the inner wall is a mixture of two phosphors, i.e. green luminescing terbium-activated cerium magnesium aluminate and red luminescing yttrium oxide activated by trivalent europium. 180 mg of an alloy with a ration in atoms of lead, bismuth and tin equal to 16:36.5:47.5 is present in the exhaust tube. Furthermore, approximately 6 mg of mercury is present. With a rare gas filling of argon (3 torr), the light output was approximately 600 lumen with a power supplied to the lamp of approximately 9 W.

Claims

1. A low-pressure mercury vapour discharge lamp comprising a discharge vessel which is sealed in a vacuum-tight manner and has at least two parallel tube parts, arranged beside each other and connected to each other at or near their one ends, there being present in the discharge vessel during operation of the lamp a discharge path between electrodes provided at the other ends of the tube parts, which discharge path is curved at at least one area, the discharge vessel being shaped near the connection between the tube parts in a manner such that during operation of the lamp in situ a part of the inner wall has a comparatively low temperature, characterized in that the discharge vessel contains an amalgam for controlling the mercury vapour pressure in the discharge vessel during operation of the lamp.

2. A low-pressure mercury vapour discharge lamp as claimed in Claim 1, characterized in that the amalgam is present at an area behind one of the electrodes.

3. A low-pressure mercury vapour discharge lamp as claimed in Claim 2, characterized in that the amalgam is present in an exhaust tube, provided behind said electrode.

4. A low-pressure mercury vapour discharge lamp as claimed in Claim 1, 2 or 3, characterized in that the amalgam consists of lead, tin, bismuth and mercury.

5. A low-pressure mercury vapour discharge lamp substantially as hereinbefore described with reference to the accompanying drawing.