1981 Osram Vialox NAV-T 150W

The lamp shown here is a first-generation Osram 150 W NAV-T introduced in the mid-1970s, the last Vialox model and the smallest one featuring an internal construction characteristic of an early West-German design. Most representative is the lamp's hand-assembled frame with a sliding spring support at its extremity so as to properly center the arctube and accommodate its thermal expansion. The latter feature prevents stresses in the burner end seals, which is critical to avoid early failures. A pair of aluminium-zirconium St101 getter bands are welded on the sliding spring support so as to maintain vacuum in the bulb, needed for the proper thermal insulation of the burner. Those getters are cut sideways so as to prevent their overheating. Another detail which reveals the level of attention to the lamp design is the side frame wire which is twisted to limit shadowing when the lamp is used in luminaires with light-projecting optics.



The NAV-T 150W is built around a five-piece arc tube with thermocompressive niobium tube end seals, a burner construction invented in 1965 by T. Tol and B. de Vrijer at Philips, in the Netherlands. This was a significant departure and a major improvement over the original Lucalox design of GE (USA), which was characterized by niobium cup end seals and an external sodium-mercury amalgam reservoir. Bringing that amalgam inside the burner volume solved the restriction on the operating position of early American sodium lamps. This also ensured tighter and more stable electrical characteristics, a key requirement for a long lamp service life on European series-choke ballasting circuits. Moreover, Philips's thermocompressive end seal significantly improved the lamp's reliability by reducing the risk of seal failure from sodium corrosion. This risk is further mitigated by the addition os a ceramic ferrule sealed at each extremity of the burner, which increases the path length for seal cracks to cause leakage.

Typical of early high-pressure sodium lamps, the burner in the present 150 W Vialox is filled with xenon at low pressure (30–50 mbar) and the arctube vessel is made of a relatively opaque coarse-grained polycrystalline alumina ceramic with a rough surface. As a result, the sodium discharge is characterized by high thermal losses while the burner has high light scattering losses, which all combine yield an initial light output of 14,000 lm only, corresponding to a lumen efficacy of 93 lm/W. Nevertheless, this lamp and its ballast could still replace 250 W high-pressure mercury lamp systems (13 klm initial at lamp level) while providing massive energy saving.

The Vialox lamp design was eventually improved before the mid 1980s. Although the St101 getter effectively prevents the buildup of hydrogen, a light gas which diffuses through the niobium feedthroughs of the burner and makes the metal brittle, it was not entirely satisfactory for all other impurities. Consequently, a more effective Zr-Al pellet and barium mirror getter combination was implemented. The burner, while retaining its 5-piece design, was chemically polished so as to improve its optical transmittance, which raised the light output by 500 lm to 14,500 lm in the second-generation of the lamp introduced in 1984.