High Pressure Sodium Lamps:
Working With Exotic Materials
Development of this discharge lamp waited on the invention of a material which sodium would not corrode. A new type of alumina ceramic passed this test.
Although the material appears milky white, it is actually 95% transparent.
Sealing the ends of the tubes proved difficult since few materials would stick to the new ceramic, and it would not melt like glass. Niobium seals were the answer.
Since the invention of this lamp in the 1960s, many seal styles have been
developed as techniques for working these unusual materials have advanced. In
the 1970s a clear alumina material was invented by Corning Glass and used in
some lamps made by Westinghouse.