Late-1970s Philips SON/C 50W Sapphire

Philips began experimenting with sapphire discharge tubes in the early 1970s in order to gain a direct optical access to the sodium discharge and to the electrodes of HPS lamps. This proved invaluable for the design and optimization of sodium lamps and special models with transparent burners were produced in the company’s lighting lab in Eindhoven well into the 2000s.

The SON/C 50W featured here was made in the second half of the 1970s, at a time when development efforts were focused on low-wattage sodium lamps. The sapphire tube, which constitutes the burner’s main body, was certainly supplied by Corning. Interestingly, each extremity of the discharge tube is closed by a feedthrough made of a PCA washer which holds a single niobium wire feedthrough, sealed with frit glass. This assembly is mounted inside a rather large elliptical jacket borrowed from 125 W mercury lamps, sealed with a short neck. A pair of barium mirror getters placed near the lamp base maintains a high vacuum in the bulb in order to ensure a proper thermal insulation of the burner.

The 50 W SON was Philips’s smallest conventional high-pressure sodium lamp, and the last model added to the family. When development on this particular lamp began just after the mid 1970s, a standard fill of low-pressure xenon (i.e., around 30 mbar) was used, which is the case of the present SON/C 50W. Philips eventually released this type of lamp in 1978 in the form of the SON 50W/I with a diffuse elliptical jacket and provided with an internal starter for a use on simple series-choke ballasts.

Because of the small physical size of the 50 W burner, the lamp’s lumen maintenance and longevity proved to be significantly poorer compared to higher wattage models. As a result, when Philips developed the clear tubular lamp version (without starter) in the early 1980s, the xenon fill pressure was increased just enough to reduce the rate of burner end blackening. The improved lamp was released around 1985 and was built with an antenna so as to help with the discharge ignition. Interestingly, the older low-pressure xenon design was kept for the self-starting 50 W lamp since the ignition voltage requirement had to be maintained as low as possible for this particular platform.