Electric discharge lamp

Abstract

Electric discharge lamps of extended life can be produced or cheaper materials used in their manufacture by providing on the internal surface of the envelope and on the exposed surfaces of internal components a protective coating of a metal phosphate or arsenate glass. External surfaces can also be protected against corrosion in air or in the atmosphere within an outer jacket, where this is employed. The coating may be applied in a liquid medium which, preferably after draining, is warmed to evaporate the medium and thereafter baked to form the vitreous coating.

What we claim is: 
     
       1. An electrical discharge lamp comprising: an arc tube; components including a pair of spaced electrodes in said arc tube and respective conductive leads or supports for said electrodes; a gaseous fill in said tube; and a transparent, defect-free, solution-deposited protective coating of a vitreous compound selected from metal phosphates and arsenates .Iadd.which is inert to the gaseous fill in the tube .Iaddend.on at least the .[.internal surface.]. .Iadd.surfaces internally .Iaddend.of the arc tube .[.and th exposed surfaces of said leads or supports.]. which .[.tend to react with.]. .Iadd.are reactive to .Iaddend.said .Iadd.gaseous .Iaddend.fill .[.during operation of the lamp.].. 
     
     
       2. A lamp according to claim 1 further comprising metallic end caps on said tube, the internal surface of said end caps being also provided with said coating. 
     
     
       3. A lamp according to claim 1 further comprising a coating of a metal phosphate or arsenate glass on the external surfaces of said arc tube and components of said lamp. 
     
     
       4. A lamp according to claim 3 further comprising a light-transmitting outer jacket enclosing said arc tube and components. 
     
     
       5. A lamp according to claim 1 wherein said coating comprises a phosphate or arsenate glass of at least one of the metals aluminium, iron, chromium, titanium, vanadium and tin. 
     
     
       6. A lamp according to claim 1 wherein said coating is composed of an aluminium phosphate glass. 
     
     
       7. A lamp according to claim 5 wherein the atomic ratio of metal to phosphorus or arsenic in the glass composition is from 1:0.1 to 1:2.9. 
     
     
       8. A lamp according to claim 1 wherein said coating is the deposited and baked residue of a solvent-soluble complex phosphate or arsenate. 
     
     
       9. A lamp according to claim 1 wherein said coating comprises the deposited and baked residue of a compound of said metal and an acid moiety selected from oxyacids of phosphorous or arsenic.