GB2147735A

GB2147735A - Fluorescent lamps

Info

Publication number: GB2147735A
Application number: GB08423967A
Authority: GB
Inventors: Ashutosh Roy
Original assignee: Thorn EMI PLC
Current assignee: Thorn EMI PLC
Priority date: 1983-10-08
Filing date: 1984-09-21
Publication date: 1985-05-15
Also published as: DE8427957U1; GB8326980D0; GB2147735B; GB8423967D0

Abstract

In a low pressure discharge lamp, in which each electrode assembly at opposite ends of the discharge tube comprises lead wires 15,16 supporting a coil electrode 25, and each coil electrode is surrounded by a metal shield 26 which is supported independently of the coil electrode and its supports, end darkening is reduced by causing one tail (29) of the electrode coil (25) to contact the electrode shield (26). This may be effected by making the tall (29) longer than the other tail or by displacing shield (26) by bending its support (27) or displacing coil (25) by bending its leads (15, 16). <IMAGE>

Description

SPECIFICATION Improvements in method of and means for reducing end darkening in fluorescent lamps This invention relates generally to low pressure gas or metal vapour discharge lamps, more particularly, to fluorescent lamps and concerns a method of and means for protecting the lamp against darkening of the discharge tube ends and achieving a reduction in temperature of the electrode supports at the tube ends.

In operation of the discharge lamp, the electrodes and their supporting members are subject to ion bombardment during negative half cycles.

The bombardment of the electrode which usually comprises a coiled coil or triple coiled coil of tungsten wire covered in electron emitter material, such as, for example, a mixture of barium, strontium and calcium oxides, results in barium vapour being given off, which in turn results in blackening of the discharge tube ends. End blackening due to this phenomenon has been greatly reduced by the provision of a protective metal shield surrounding the cathode and its two supports. This shield is supported independently of the cathode support structure by a support wire embedded in the glass flare making up part of the discharge tube end. Such a floating shield will not prevent electrons from impinging on respective support wires during the positive half cycle unlike the case of wire probes partially coated with boron nitride and attached to support wires as disclosed in US patent 4013914.

It has been observed, however, that end blackening can still occur and it is believed this is due to overheating of the electrode support wires resulting in vapourisation of the metal of the support wires which is detrimental to the life of the lamp and contributes to the said end blackening problem. An object of this invention is to alleviate to some extent the above mentioned problem of end blackening.

According to the present invention there is provided a method of assembling a low pressure mercury vapour discharge lamp, said lamp comprising a discharge tube having tube ends and electrodes disposed at each tube end to sustain a discharge therebetween, each said electrode comprising a glass flare, lead wires sealed in the glass flare and supporting at the ends thereof and therebetween an electrode coil, a metal shield surrounding each said electrode coil, the shield being carried by a support member independent of said electrode coil, wherein said method of assembly includes the step of varying the distance between said shield and one end of the coil such that said shield and said one coil end are brought into contact.

According to a further aspect of the invention there is provided a low pressure discharge lamp comprising a discharge tube having tube ends and an electrode assembly disposed at each tube end to sustain a discharge therebetween, each said electrode assembly comprising a glass flare, lead wires sealed in the glass flare and supporting at the ends thereof and therebetween an elongate electrode coil, said electrode assembly further comprising a metal shield surrounding each said electrode coil and being supported independently of said coil and its supports wherein one end of each said electrode coil is in contact with the metal shield.

In a preferred embodiment of the invention contact of the electrode coil with the shield is ensured by making one end of the coil longer than the other.

The invention will now be described by way of example only and with reference to the accompanying drawings wherein; Figure 1 is a view generally of a fluorescent lamp embodying the invention.

Figure 2 is a perspective view, much enlarged, of an electrode assembly used in the lamp of Figure 1, with the metal shield partly cut away to expose the electrode coil.

Figure 3 is a diagrammatic representation of an electrode assembly according to another aspect of the invention.

Figure 4 is a graph of Temperature v Lamp Current in lamps using and not using the invention.

Figure 5 is a diagrammatic representation to indicate the extent of end darkening in lamps using and not using the invention.

In Figure 1, reference numeral 10 denotes generally a fluorescent lamp having a discharge tube 11 coated with a suitable phosphor and containing mercury in the usual manner. The discharge tube 11 is hermetically sealed at each end to a glass flare 12 which has an opening 13 leading to exhaust tube 14 as best seen in Figure 2. Lead wires 15, 16 are embedded and sealed within the pressed stem 17 of the glass flare 12 and extend outside the discharge tube 11 to make electrical contact with contact pins 18, 19 respectively extending from cover 20 sealed to the end of the discharge tube 11. As best seen in Figure 2 lead wires 15, 16 are bent over at these ends 21, 22 to clamp the tail members 23, 24 of electrode coil 25, which is covered with an electron emissive material such as a barium, strontium, and calcium carbonates which decomposes into oxide during processing.

The electrode coil 25 can be a coiled coil or triple coil of tungsten wire as well known in the art. A rectangular cathode shield 26 of 0.2mm thick mild steel strip surrounds the electrode 25 and is carried by a single support 27 of 0.6mm diameter nickel plated wire embedded in the pressed stem 17. As can best be seen from Figures 1 and 2 the electrode coil 25 and cathode shield arein generally parallel alignment and both are substantially transverse to the main tube axis 28 (Figure 1).

In this embodiment of the invention, the electrode coil 25 and electrode tail members 23, 24 are substantially symmetrically disposed between the ends 21, 22 of the support iegs formed by the lead wires 15, 16. However, one end 29, of one of the electrode tail members, for example, 23 is extended and made longer than the other so that it contacts the inside surface 30 of the heat shield 26.

A further embodiment of the invention is shown in Figure 3. In this embodiment an electrode coil 31 has tail members 32 of substantially equal length clamped within the ends 33, 34 of the lead wires 35, 36 respectively sealed within glass flare 37. The electrode coil 31 with its tail members 32 is disposed substantially symmetrically and transversely with respect to the main lamp longitudinal axis 38. In this embodiment the necessary contact is made by pushing the cathode shield 39 in the direction shown by arrow A until one of the tail ends 32 makes the necessary contact. In this case the distance moved by the cathode shield 39 is accommodated by a corresponding bending of the cathode shield support leg 40, set into the glass flare 37. The same effect can be achieved by moving either of the support wires 35 and 36.It has been found that maintaining contact between one of the coil ends and the heat shield, by varying the distance between an end of the electrode coil and the cathode shield by any of the above described methods, has resulted in a decrease in the temperature of the electrode support. From Figure 4 which is a graph of lamp current against temperature for tests carried out on a 8 ft; 100W, T12 fluorescent lamp with 0.8 x 18mm support wires made of nickel iron alloy, it can be seen the temperatures of the support members when no contact is made is 1130"C. When contact is made in accordance with the invention the support temperature drops to 975"C and 925"C respectively depending on the direction of flow of current indicated by the arrows.

Such a drop in temperature has extremely important advantages. For example a less expensive wire than nickel iron alloy could be used for the supports. There is also less of a problem in expansion mismatch between the glass of flare and the lead wires. By far the most important effect is the dramatic reduction in end darkening which takes place and the extent of which is indicated in Figure 5. This test which is for a lamp running for eight hours with a lamp current of 1.5 amps records end darkening in terms of a 360" angle representing complete darkening and 0 representing no darkening. It is known to those skilled in the art that as darkening spreads down the tube from the end, it also spreads in an enlarging patch around the tube eventually closing a loop right around the tube.For the purpose of the comparison of Figure 5, complete darkening has been arbitrary set at the point at which the loop around the tube is closed. In Figure 5, A is an example of a prior THORN EMI Lighting Limited lamp compared with another lamp, B, both lamps not embodying the present invention while C is THORN EMI Lighting Limited lamp incorporating the present invention. Lamps A and B exhibit substantial darkening, for example, 241 and 180 respectively while lamp C made in accordance with the invention has a darkening angle of only 1.8 .

As pointed out above the use of cathode shields generally is known, one such example being disclosed in US Patent 3,215,882 (Toomey). This patent discloses an electrode coil with the axis lying along the main axis of the tube and a cylindrical cathode shield surrounding the electrode coil. The cathode shield is connected in some undisclosed manner, perhaps by welding, along a substantial length of one of the lead wire supports while the connecting end of the other lead wires support is left unprotected by the shield. The axial position of the electrode coil is said to reduce the amount of heat radiation from the filament toward the end of the lamp, but by the same token the axial position of the coil means the cathode shield is much more exposed to electron bombardment and consequently could be expected to run hotter than a cathode shield placed transversely of the lamp axis.

From the above description it can be seen that by the simple expedient of making one end of the electrode coil contact the cathode shield the temperature of the coil support legs and end blackening can be reduced substantially. It has been found that mere contact is sufficient to practise the invention as opposed to joining of the members, for example, by welding. A welding operation, moreover, would mean a requirement for additional equipment in the production process thus adding to the overall cost of operation.

Claims

1. A method of assembling a low pressure mercury vapour discharge lamp, said lamp comprising a discharge tube having tube ends and electrodes disposed at each tube end to sustain a discharge therebetween, each said electrode comprising a glass flare, lead wires sealed in the glass flare and supporting at the ends thereof and therebetween an electrode coil, a metal shield surrounding each said electrode coil, the shield being carried by a support member independent of said electrode coil, wherein said method of assembly includes the step of varying the distance between said shield and one end of the coil such that said shield and said one coil end are brought into contact.

2. A method according to Claim 1 in which the distance is varied by displacing the coil relative to the shield.

3. A method according to Claim 1 in which the distance is varied by displacing the shield relative to the coil.

4. A method according to Claim 1 in which the coil has tail parts of unequal length, the longer of which is arranged to contact the shield.

5. A low pressure discharge lamp comprising a discharge tube having tube ends and an electrode assembly disposed at each tube end to sustain a discharge therebetween, each said electrode assembly comprising a glass flare, lead wires sealed in the glass flare and supporting at the ends thereof and therebetween an elongate electrode coil, said electrode assembly further comprising a metal shield surrounding each said electrode coil and being supported independently of said coil and its supports wherein one end of each said electrode coil is in contact with the metal shield.

6. A lamp according to Claim 5 in which the electrode coil includes tail parts of unequal length, the longer of which contacts the shield.

7. A lamp according to Claim 5 or Claim 6 in which the shield is situated asymmetrically about the coil to facilitate said contact.

8. A lamp according to Claim 7 in which the shield is situated asymmetrically relative to the discharge tube.

9. A low pressure discharge lamp substantially as herein described with reference tothe accompanying drawings.

10. A method of assembling a low pressure discharge lamp, the method being substantially as herein described with reference to the accompanying drawings.