Improvements in tungsten halogen incandescent lamps

Abstract

 Atungsten halogen incandescent lamp having a filament support structure made of molybdenum and a filament shield, also of molybdenum, to provide an assymetric passing beam in an automobile headlight. When operating with the halogen bromine small changes in the design or positioning of the shield can be critical and can cause darkening. This can be prevented by applying a small amount of the metal platinum to the back of the shield by plating, by means of a paste or in any convenient manner. In a specific example 13 microgrammes of platinum were used in a 55-60W H4 automobile headlight having a fill of bromine and krypton.

Description

[0001] This invention relates to incandescent lamps operating on the tungsten halogen transport cycle. More particularly, the invention relates to vehicle headlamp bulbs of the type known as H4. It should be understood, however, that the invention is generally applicable to incandescent lamps which incorporate a halogen whether or not the tungsten halogen cycle is fully operative and the term "tungsten halogen lamp" should be so construed. As is now well known the function of the tungsten halogen transport cycle is to help to keep the bulb wall clean. When a halogen is added to the gas filling, and provided certain temperature conditions are satisfied, a chemical transport cycle is established which prevents tungsten from being deposited on the inside surface of the bulb wall or lamp envelope. If the correct chemical balance is not achieved or the correct temperature conditions do not prevail blackening of the bulb wall can still occur.

[0002] The H4 lamp mentioned above is a twin filament bulb and is described in detail in the article "H4 Automobile Lamp Technology" published in Lighting Journal 20, published in the Spring of 1979 by the present applicants. It is evident from the article that the construction within the lamp envelope is extremely complex and includes a mount structure for the twin filaments and a filament shield.

[0003] In one example of an H4 lamp which uses Bromine as the only Halogen and in which the mount structure and filament shield are made of molybdenum, darkening of the bulb wall has been found to occur due to the presence of a brown-black deposit which on analysis shows the presence of molybdenum and bromine. It is believed that the same would apply if the halogen were predominantly but not solely Bromine. Since molybdenum is used in the mount structure, this is not surprising but what is surprising is the sensitivity of the transport cycle to temperature. Small changes in shield design and or the positioning of the shield which affect temperature can result in a darkening problem. It is desirable to produce a design of lamp which is less sensitive to changes in conditions within the lamp envelope.

[0004] According to the invention there is provided an incandescent lamp, said lamp having a filament mount structure made of molybdenum, a filament shield also made of molybdenum, a filament and the halogen bromine, all included within lamp envelope characterised in that the metal platinum is included within the lamp envelope to prevent blackening of the bulb wall during lamp operation.

[0005] In a particularly preferred embodiment of the invention the platinum is applied to the back of the molybdenum shield. Although the platinum can be applied in a variety of ways, for example, by plating or the application of a platinum paste, it has been found that it is not essential to cover the shield completely with the platinum.

[0006] In United Kingdom Patent 1 183 008 a tungsten halogen incandescent lamp is disclosed using either iodine or bromine as the halogen. In this patent a tab of platinum is used as an aid to effect a weld betweeen the foil and inner lead portions. This arrangement, however, occurs as part of the pinch seal and not in the lamp envelope. It is also known to use platinum as an aid to welding in a lamp using bromine as the halogen but where the filament mount structure is made of tungsten. In each case the platinum is used as part structural member whereas in the present invention the platinum is being used only to prevent darkening and need be present in very small quantities only. In United Kingdom Patent 807 137 a tungsten halogen incandescent lamp is disclosed in which it is suggested that platinum be used for the lead-in wires and other supports. Since platinum is an extremely expensive material this proposal is impractical and, in any case, this patent teaches that halogens other than iodine are unsatisfactory. Chlorine is said to cause disintegration of the filament supports and Bromine is said to be only slightly less destructive while Fluorine is not at all suitable.

[0007] One embodiment of the invention will now be described by way of example only and with reference to the accompanying drawing in which:

Figure 1 is an example of an H4 lamp embodying the invention.

[0008] In Figure 1, the reference numeral 10 depicts generally an H4 lamp. This comprises a quartz bulb or lamp envelope 11 set into a flat cap portion 12 which forms part of the barrel portion 13. Attached to the barrel portion 13 is the focussing flange 14-with lugs 15 serving to give the lamp the correct orientation with respect to an appropriate lamp holder on a vehicle. Attached to the barrel portion 13 are the external electrical connections 16.

[0009] Inside lamp envelope 11 the filament mount structure comprises filament support members 18, 19 and 20, in this .case made of molybdenum strip material, set into the pinch seal 21. Tungsten filaments 22 and 23 are attached to their respective supports by respective tungsten tails 24, 25 and 26, 27. Filament shield 28, also made of molybdenum is attached to filament support 20 and positioned behind filament 22. This is the passing beam filament and the shield is placed to produce an assymetric passing beam.

[0010] In accordance with the present invention a small amount of platinum is applied to the back 29 of the shield 28. As stated previously, this need only be a small amount which prevents darkening. Preferably but not necessarily, the platinum is applied to the position on the back of shield 28 specifically indicated by the lead line to numeral 29. In a typical 60-55W H4 automobile headlight bulb having a quartz bulb diameter of 13mm, and a fill comprising bromine and krypton, it was found 13 microgrammes of platinum prevented darkening. The method of application is not critical and, as stated previously this can be by plating or the application of a platinum paste. It is believed the invention is applicable to incandescent lamps containing halogen over a range of 2 to 10KW.

Claims

1. An incandescent lamp, said lamp having a filament mount structure (18, 20) made of molybdenum, a filament shield (28) also made of molybdenum, a filament (22) and the halogen bromine, all included within lamp envelope (11) characterised in that the metal platinum is included within the lamp envelope to prevent blackening of the bulb wall during lamp operation.

2. An incandescent lamp according to Claim 1 wherein the platinum is applied to the shield (28).

3. An incandescent lamp according to Claim 2 wherein the platinum is applied to the back (29) of the shield (28).

4. An incandescent lamp according to Claim 2 or Claim 3 wherein the platinum is applied as. a paste.

5. An incandescent lamp according to Claim 2 or Claim 3 wherein the platinum is applied by plating.

6. An incandescent lamp acording to any one of Claims 2 to 5 wherein the platinum does not extend to cover the whole of the shield surface.

7.. An incandescent lamp according to any preceding Claim wherein the amount of platinum is substantially 13 microgrammes.

8. An incandescent lamp according to any preceding claim operable on the tungsten halogen cycle.

9. A method of constructing an incandescent lamp, the lamp having a lamp envelope including the halogen bromine, .said method comprising the steps of:

providing a filament (22); providing filament suport structure (18, 20) made of molybdenum to which the filament is attached;

providing a shield (28) for the filament (22), the shield also being made of molybdenum, and adding the metal platinum to the shield (28).

10. A method according to Claim 9 wherein the platinum metal is added by plating.

11. A method according to Claim 9 wherein the metal platinum is added by applying a platinum paste.

12. A method according to any one of Claims 9 to 11 wherein the platinum metal is added to the back (29) of the shield (28).

Drawing