Electric discharge devices

Abstract

The inclusion of an alkali metal halide in the gas fill of a high pressure mercury and aluminium halide discharge lamp improves the color appearance of the lamp; notably by lowering the color temperature, while maintaining or even increasing the efficacy and stability of the discharge. Preferred contents are 10 to 30 mg cm -3 mercury, 5 to 100 μmol cm -3 aluminium halide and 0.5 to 150 μmol cm 3 alkali metal halide, with the molar ratio of alkali metal halide to aluminium halide in the range 0.1 to 1.5 and the total number of aluminium and alkali metal atoms preferably greater than the total number of halogen atoms.

What we claim is:  .[.1. A high-pressure electrical discharge lamp comprising: 
     
        and alkali metal halides are present during operation of the lamp..]. 3. A discharge lamp as claimed in claim .[.2.]. .Iadd.9.Iaddend., in which the total number of aluminium and alkali metal atoms is greater than the total 
     
     
        number of halogen atoms. 4. A discharge lamp according to claim .[.2.]. .Iadd.9 .Iaddend.wherein said gas fill comprises mercry in the range 10 to 30 mg cm -3 , aluminium chloride or iodide in the range 5 to 100 μmol cm -3  and sodium chloride or iodide in the range 0.5 to 150 
     
     
        μmol cm -3 . 5. A discharge lamp according to claim 4 in which the total number of aluminium and alkali metal atoms is greater than the total number of halogen atoms. .Iadd. 6. A high-pressure electrical discharge lamp comprising: a light-transmitting discharge vessel;   spaced electrodes in said vessel;   and a gas fill in said vessel comprising mercury vapour, rare gas, aluminum halide and at least one alkali metal halide, the molar ratio of the alkali metal halide to the aluminum halide varying from 0.1 to 1.5. .Iaddend..Iadd. 7. The lamp of claim 6 where the alkali metal halide is selected from the group of a sodium halide and lithium halide. .Iaddend..Iadd. 8. The lamp of claim 7 where the sodium halide is sodium iodide. .Iaddend..Iadd. 9. A high-pressure electrical discharge vessel comprising:   a light-transmitting discharge vessel having a wall power loaded at between 10 and 100 Wcm -2  during operation;   spaced electrodes in said vessel;   and a gas fill in said vessel comprising a rare gas, mercury in amount up to 30 mg cm -3 , aluminum halide and an alkali metal halide selected from the group of halides of lithium, sodium, potassium, rubidium, cesium and mixtures thereof, said alkali metal and aluminum halides being present in such quantities that the molar ratio of alkali metal halide to aluminum halide is from 0.1 to 1.5 and unevaporated aluminum and alkali metal halides are present during the operation of the lamp. .Iaddend..Iadd. 10. The lamp of claim 9 where the alkali metal halide is selected from the group of a sodium halide and a lithium halide. .Iaddend..Iadd. 11. The lamp of claim 10 where the sodium halide is sodium iodide. .Iaddend.