1986 Tesla SHC 250W (Deluxe)

Tesla of Czechoslovakia developed its first color-improved high-pressure sodium lamps in the mid-1980s. To this end, the Czechs followed a design strategy first devised by Westinghouse (USA) in the late 1970s. The light color of sodium discharge is improved by widening both its quasi-molecular spectral emission and the optical self-absorption which arises from the colder vapor that surrounds the hot plasma channel. This is primarily achieved by increasing the sodium vapor pressure via a higher sodium fraction in the Na-Hg amalgam contained in the burner, whose cold-spot temperature is increased with metal foil heat shields located at the ceramic vessel’s extremities. The arc tube is also widened to promote optical self absorption, while the electrode gap length is shortened so as to keep a 100 V arc voltage, compatible with the series-choke ballasts used in Europe on 220 V mains. This also results in an increased arc temperature which enhances the spectral emission in blue from the mercury-sodium plasma.

Those design changes resulted in a significantly improved light color, with a color temperature raised by around 200 K to ~2200 K and a color tendering index increased by 40‒45 points to ~65 Ra8. The drawback, however, is a shortened service life due to the lamp’s higher operating temperature, combined with a lower lumen efficacy, which is reduced to about 100 lm/W as a result of increased optical losses in the burner and a less efficacious spectral output (more energy radiated at the extremities of the visible spectrum, away for the eye’s peak sensitivity). Nevertheless, the light color was sufficiently improved to take the efficient sodium lamp out of its purely utilitarian lighting applications.

This development was a success because of the burner’s highly resilient monolithic end seal, a design invented by Philips in 1967 and adopted by Tesla five year later. In 1986 the Czechs eventually produced small batches of their Deluxe SHC lamps using the standard 250 W bulb and frame. Even the stamp printed on the lamp is a standard one, with no reference to the improved light color. Interestingly, and despite successful pre-production runs with low scrap rates, Tesla never offered this SHC variant for sale, perhaps because it was deemed that the demand for such a lamp would be too limited in the Eastern Bloc, the company’s main market. Ironically, the following year (1987) Philips released its first SON Comfort lamp built with a similar burner design but with a high xenon fill pressure (a different development, also related to Westinghouse’s design) and which went on to become a successful product.