Metal halide lamp including a source of available oxygen

1. A lamp comprising:

a discharge vessel;

tungsten electrodes extending into the discharge vessel;

an ionizable fill sealed within the vessel, the fill comprising:

a buffer gas,

optionally mercury, and

a halide component comprising a rare earth halide selected from the group consisting of lanthanum halides, praseodymium halides, neodymium halides, samarium halides, cerium halides, and combinations thereof; and

wherein the fill is free of all rare earth halides other than halides of lanthanum, praseodymium, neodymium, samarium, and cerium;

a source of available oxygen in the discharge vessel, the rare earth being present in an amount such that, during lamp operation, in combination with the source of available oxygen, it maintains a difference in vapor phase solubility for tungsten species between a wall of the discharge vessel and at least a portion of at least one of the electrodes.

2. The lamp of claim 1 , wherein the fill includes metallic mercury.

3. The lamp of claim 1 , wherein the source of available oxygen, under the lamp operating conditions, decomposes to form available oxygen.

4. The lamp of claim 3 , wherein the source of available oxygen comprises a solid metal oxide.

5. The lamp of claim 1 , wherein the source of available oxygen comprises an oxide of tungsten, the oxide of tungsten comprising WO 3 , and wherein the tungsten oxide is scaled in the vessel in a sufficient amount to maintain a concentration of WO 2 X 2 vapor phase in the fill during lamp operation of at least 1×10 −9 μmol/cm 3 , where X is selected from Cl, Br, and I.

6. The lamp of claim 5 , wherein the tungsten oxide is present in the fill at a concentration, expressed in terms of O 2 content, of at least 0.1 micromoles/cm 3 .

7. The lamp of claim 5 , wherein the tungsten oxide is present in the fill at a concentration, expressed in terms of O 2 content, of from 0.2-3.0 micromoles/cm 3 .

8. The lamp of claim 1 , wherein the rare earth halide comprises cerium halide.

9. The lamp of claim 1 , wherein the rare earth halide is present in the fill at a total concentration of from 3 to 13 μmol/cm 3 .

10. The lamp of claim 1 , wherein the fill is free of halides of holmium, thulium, dysprosium, erbium, lutetium, yttrium, and ytterbium, terbium, scandium, and magnesium.

11. The lamp of claim 1 , wherein fill further includes at least one of the group consisting of an alkali metal halide, an alkaline earth metal halide other than Mg, and a halide of Tl or In.

12. The lamp of claim 1 , where during lamp operation, the fill includes WO 2 X 2 in vapor form, where X is selected from Cl, Br and I.

13. The lamp of claim 1 , where during lamp operation, the wall is at a temperature that is at least 200K lower than the portion of the electrode.

14. The lamp of claim 13 , wherein during lamp operation, the wall is at a temperature that is no more than 800K lower than the portion of the electrode.

15. The lamp of claim 1 , wherein in operation, the temperature adjacent at least the portion of one of the electrodes is higher than a temperature at which the solubility of tungsten in the vapor phase is at a minimum and a temperature at the wall of the discharge vessel is higher than the temperature at which the solubility of tungsten in the vapor phase is at the minimum.

16. A lamp comprising:

a discharge vessel;

tungsten electrodes extending into the discharge vessel;

an ionizable fill sealed within the vessel, the fill comprising:

a buffer gas,

optionally mercury,

a halide component comprising a rare earth halide selected from the group consisting of lanthanum halides, praseodymium halides, neodymium halides, samarium halides, cerium halides, and combinations thereof, and wherein the fill is free of all rare earth halides other than halides of lanthanum, praseodymium, neodymium, samarium, and cerium,

and at least one of the group consisting of: a) an alkali metal halide, b) an alkaline earth metal halide, other than magnesium, and c) a halide of an element selected from indium and thallium, the lamp fill being free of halides of holmium, thulium, dysprosium, erbium, lutetium, yttrium, and ytterbium, terbium, scandium, and magnesium, and

tungsten oxide sealed in the vessel in a sufficient amount to maintain a concentration of WO 2 X 2 in a vapor phase in the fill during lamp operation of at least 1×10 −9 μmol/cm 3 , where X is selected from Cl, Br, and I.

17. The lamp of claim 16 , wherein the tungsten oxide is WO 3 .

18. A method of forming a lamp comprising:

providing a discharge vessel;

providing tungsten electrodes which extend into the discharge vessel;

sealing an ionizable fill within the vessel, the fill comprising:

a buffer gas,

optionally free mercury,

a halide component comprising a rare earth halide selected from the group consisting of lanthanum halides, praseodymium halides, neodymium halides, samarium halides, cerium halides, and combinations thereof; and

providing a source of available oxygen in the discharge vessel, the rare earth halide being selected in type to form an unstable oxide and being present in an amount such that, during lamp operation, in combination with the source of available oxygen, the solubility of tungsten species in the fill is lower adjacent at least a portion of one of the electrodes than at a wall of the discharge vessel, such that tungsten from the electrode that would otherwise be deposited on the wall during lamp operation is transported back to the electrode.

19. A method of operating a lamp comprising:

providing a lamp comprising:

a discharge vessel;

tungsten electrodes extending into the discharge vessel;

an ionizable fill sealed within the vessel, the fill comprising:

a buffer gas,

optionally mercury, and

a halide component comprising a rare earth halide selected from the group consisting of lanthanum halides, praseodymium halides, neodymium halides, samarium halides, cerium halides, and combinations thereof; and

wherein the fill is free of all rare earth halides other than halides of lanthanum, praseodymium, neodymium, samarium, and cerium; and

a source of available oxygen in the discharge vessel comprising a tungsten oxide, the rare earth being present in an amount such that, during lamp operation, in combination with the source of available oxygen, it maintains a difference in vapor phase solubility for tungsten species between a wall of the discharge vessel and at least a portion of at least one of the electrodes; and

operating the lamp by supplying an alternating current to the lamp to generate a discharge in the lamp vessel, the available oxygen reacting with tungsten deposited on the wall of the vessel to generate a soluble tungsten species, the soluble tungsten species being deposited on the electrodes.

20. The method of claim 19 , wherein, during lamp operation, the temperature adjacent at least the portion of one of the electrodes is higher than a temperature at which the solubility of tungsten in the vapor phase is at a minimum and a temperature at the wall of the discharge vessel is higher than the temperature at which the solubility of tungsten in the vapor phase is at the minimum.

21. The method of claim 18 , wherein, during lamp operation, the temperature adjacent at least the portion of one of the electrodes is higher than a temperature at which the solubility of tungsten in the vapor phase is at a minimum and a temperature at the wall of the discharge vessel is higher than the temperature at which the solubility of tungsten in the vapor phase is at the minimum.