GB2049271A - Discharge lamp - Google Patents

Abstract

The invention relates particularly to a high-pressure sodium vapour discharge lamp having a discharge vessel 1 in which xenon and an absorbent are present. The xenon is partly absorbed in the absorbent. When the temperature is raised xenon is released. Both for starting the lamp and for the operating condition a suitable xenon pressure can be realised. According to the invention the absorbent 11 is housed in a cup 10 located in a cavity 35 in the wall of the discharge vessel 1. The absorbent 11 preferably comprises carbon, but may be one or more of fine-granular oxides, carbides, borides and metals. As shown, the cup 10, e.g. of niobium, fits tightly in cavity 35 and has cuts at its open side so that strips 10a are formed which are bent inwards to form a connection point to which the electrode rod 5a is affixed. A specific example is given. The invention may also be used in low pressure lamps. <IMAGE>

Description

SPECIFICATION Discharge lamp The invention relates to a discharge lamp comprising a discharge vessel in which xenon and an absorbent which is in contact with the xenon are present, the xenon being absorbed at least partly in the absorbent and being released partly from the said absorbent when the temperature is raised.

Such a lamp is disclosed in British Patent Specification 669,033. The xenon pressure in the operating condition of the lamp described lies in such a range that a light source having a comparatively large light output can be realised with it. In the inoperative condition of the lamp the xenon pressure is reduced by means of the absorbent. In this known lamp the absorbent is provided in a holder around the electrode rod. A construction as used in this known lamp requires the use of a long electrode rod. This may be a disadvantage. It is the object of the invention to provide a construction in which the said disadvantage is removed or at least mitigated.

According to the invention, a lamp of the kind mentioned in the opening paragraph is characterized in that the absorbent is present in a cavity in the wall of the discharge vessel.

The lamp according to the invention has the advantage that the presence of the absorbent is independent of electrode rods, and is based on the recognition of the fact that the wall of the discharge vessel is a suitable place for storing the absorbent.

It is feasible that the cavity, in the form of a separate appendix of the discharge vessel, may be enclosed by the wall of the discharge vessel. However, a lamp in accordance with the invention preferably comprises at least one lead-through for a current supply member to an internal electrode and the cavity is present at the area of the lead-through.

The advantage of this lamp is that space is obtained for the location of the absorbent by means of a simple construction.

In a further preferred lamp in accordance with the invention the lead-through is a metal cup which is located in the cavity, which cup tightly enganges substantially throughout its length against the wall enclosing the cavity, said cup being gas-permeable on a side facing the electrode. The advantage of this lamp is that the absorbent is in a favourable place from a point of view of light-technology.

The wall of the discharge vessel consists, for example, of quartz glass or a ceramic material. In a further advantageous embodiment of a lamp in accordance with the invention the lamp of the discharge vessel mainly consists of densely sintered aluminium oxide. Such a lamp has for its advantage that the wall of the discharge vessel is a good heat conductor. As a result of this it is achieved that the absorbent rapidly assumes a sufficiently high temperature.

The absorbent may comprise one or more materials, for example, fine-granular oxides, carbides, borides and metals. In an improved embodiment of a lamp in accordance with the invention the absorbent is mainly carbon. This lamp thus has a material with good absorption properties, which is advantageous.

A lamp in accordance with the invention may be for example, a low-pressure discharge lamp or a high-pressure mercury vapour discharge lamp.

According to an improved preferred embodiment of a lamp in accordance with the invention, however, the lamp is a high-pressure sodium vapour discharge lamp. The advantage of this lamp is a high luminous flux and good ignition properties.

The invention will now be described in greater detail with reference to a drawing. In the drawing Figure 1 is a side elevation, partly broken away, of a lamp in accordance with the invention, and Figure 2 is a sectional view of a detail of a lead-through construction of the lamp shown in Figure 1.

Reference numeral 1 in Figure 1 denotes a discharge vessel the wall of which consists of density sintered aluminium oxide which is enclosed by an envelope 2 having a lamp cap 3. The discharge vessel 1 has two internal main electrodes 4 and 5 between which the discharge is maintained in the operating condition of the lamp. Main electrode 4 is connected to a metal strip 7 via a leadthrough 6. This strip 7 is connected to a pole wire 8 which is connected to a contact of the lamp cap 3. An extended portion 9 of the pole wire 8 serves to support and centre the discharge vessel 1 in the envelope 2. The main electrode 5 is connected to a strip-shaped conductor 13 by means of a leadthrough consisting of a tubular cup 10 and a rod 12.

The other end of said conductor 13 is connected to another contact in the lamp cap 3. The cup 10 fits tightly in the cavity 35 (Figure 2) in the vessel wall and is filled with carbon 11.

Near its end where the tubular cup 10 is present, the discharge vessel 1 is surrounded by a heat shield 25 extending the length of the cup. The heat shield preferably consists of tantalum.

The discharge vessel is provided with an external auxiliary electrode 20. Near the main electrode 4 said auxiliary electrode 20 is connected to the strip 7 by a capacitor 23. At the other end of the discharge vessel the auxiliary electrode 20 is connected to one end of an auxiliary member 21 in the form of a tension spring. The other end of the auxiliary member 21 is connected to the metal strip 13 by means of a conductive strip 22.

Reference numeral 1 in Figure 2 again denotes the discharge vessel of which is shown the part near the main electrode 5. The cup 10, which together with rod 12 constitutes the leadthrough to the electrode 5, consists of niobium and fits tightly in the cavity 35.

Before being provided in the discharge vessel the cup 10 is successively subjected to the following operations. First the absorbent 11 is placed in the cup. A number of sawcuts are then provided in the cup at its open side, which cuts extend in the longitudinal direction of the axis of the cup and the lengths of which are substantially half the diameter of the cup. The niobium strips 10a thus formed are then folded inwards and interconnected at their free ends to form a connection point. The main electrode 5 is connected to this connection point by means of an electrode rod 5a. Herewith it is achieved that the carbon can be reached by the xenon. It is also possible for this niobium cup to be covered by means of a layer consisting of a porous metal.

The lamp described relates to a high-pressure sodium vapour discharge lamp having xenon as a buffer gas. The pressure of the xenon at 300 Kis approximately 16 kPa. In the operating condition of the lamp in which the average temperature is approximately 2200 K, the xenon pressure is approximately 213 kPa. The niobium cup 10 which has a diameter of 4 mm and a volume of approximately 75 mm3 contains 45 mg of carbon. The lamp in question is suitable for being connected to a supply source of 220 V, 50 Hz via a stabilisation ballast (not shown) of approximately 0.11 H. In addition to the stabilisation ballast, a starter (not shown) is incorporated in the connection to the supply source, which starter may, for example, be of the type described in United Kingdon Patent No. 1,300,214.

The power consumed by the lamp is 400 W. The luminous flux is approximately 135 1 m/W. The ignition voltage presented to the discharge vessel is approximately 3 kV.

The lamp of the above described embodiment has a discharge vessel the wall of which consists of density sintered aluminium oxide. The length of the discharge vessel is approximately 110 mm and the inside diameter is approximately 7.5 mm. The distance between the two internal main electrodes of the discharge vessel is 82 mm, while the distance from a main electrode to the nearest end of the discharge vessel is approximately 11 mm. The discharge vessel has a filling which in addition to xenon comprises 25 mg of amalgam, containing 27% by weight of sodium and 73% by weight of mercury. The lamp combines good ignition properties and a high luminous flux with a favourable place for storing the absorbent.

Claims (7)

1. A discharge lamp comprising a discharge vessel in which xenon and an absorbent which is in contact with the xenon are present, the xenon being absorbed at least partly in the absorbent and being released partly from the said absorbent when the temperature is raised, characterized in that the absorbent is present in a cavity in the wall of the discharge vessel.

2. A discharge lamp as claimed in Claim 1, the discharge vessel of which comprises at least one leadthrough for a current supply member to an internal electrode, characterized in that the cavity is present at the areas of the leadthrough.

3. A discharge lamp as claimed in Claim 2, characterized in that the Ieadthrough consists of a metal cup which is located in the cavity and that the cup tightly engages substantially throughout its length against the wall enclosing the cavity, the said cup being gas-permeable on a side facing the electrode.

4. A discharge lamp as claimed in Claim 1, 2 or 3, characterized in that the wall of the discharge vessel comprises densely sintered aluminium oxide.

5. A discharge lamp as claimed in one or more of the preceding Claims, characterized in that the absorbent is mainly carbon.

6. A discharge lamp as claimed in any preceding Claim, characterized in that the lamp is a highpressure sodium vapour discharge lamp.

7. A discharge lamp substantially as herein described with reference to the accompanying drawing.