High pressure sodium lamps, and particularly so-called "saturated" high pressure sodium lamps, are known in the art. Therein, an elongated art tube is positioned within an evacuated glass envelope and filled with large amounts of sodium and mercury. As is well known, sodium loss has long been a problem causing undesired increased voltage drop across the lamp and an accompanying reduction in the useful period of "life" of the discharge lamp. Thus, the large amount or "saturated" sodium content of the arc tube is an attempt to compensate for the uncontrolled loss of sodium during the operational use of the discharge lamp.
It has long been recognized that one of the principal causes for this undesired sodium loss is the presence of oxygen impurities in the gas fill of the arc tube. More specifically, it is known that the sodium fill gas tends to combine with oxygen and the aluminum oxide arc tube to provide sodium aluminate whereby undesired sodium loss is encountered.
One known attempt to reduce this undesired oxygen impurity level is set forth in U.S. Pat. No. 4,075,530 of Furukubo et al. Therein, a niobium exhaust pipe is coupled to an arc tube and a decomposable material, NaN3, is located within the exhaust pipe. The exhaust pipe is heated to decompose the NaN3 while the art tube is cooled to effect condensation. Thus, the material within this exhaust pipe is heated to effect decomposition, transferred to the arc tube wherein materials, such as sodium and mercury, are condensed and whereat undesired gases, such as nitrogen, are withdrawn.
Unfortunately, locating the decomposable materials in a container external to the arc tube necessitates the application of heat thereto in order to effect the desired decomposition. Thereafter, the decomposed materials must be transferred to the arc tube. Also, the arc tube must be cooled in order to effect the condensation of desired residual materials while permitting the exhaust of other undesired materials. Obviously, such a process is cumbersome of apparatus and unrealistically expensive of labor and materials.
USPTO Backgrounds
USPTO Backgrounds