RLOD#33 (2020.06.15) Early-1990s Sylvania SHP 35/70W

The burners in dual-arc HID lamps are usually identical, but that’s not the case here. In the early 1990s Sylvania developed and investigated an ingenious step-dimmable lighting system that used sodium lamps built with two different arc tubes (35 and 70 W in the present case) in order to provide two different light levels while maintaining an optimum lumen efficacy. Differences in electrical characteristics enable an electronic driver to sense which burner is running, and the circuit could switch the lamp off and on so as to select the proper burner for a given set power level. To simplify things, the low-wattage burner (35 W here, the one with the longest heat shields) has a higher xenon fill pressure than the high-wattage one (70 W here) in order to ensure that the latter is always the first one to ignite when the lamp is cold.

This lamp-ballast system was aimed at streetlighting applications where significant energy saving can be realized by reducing power in the dead of night. Various arc tube combinations where investigated (e.g. 75/50 W) but this concept was never developed into a commercial product, certainly because of its complexity and higher cost compared to existing dimming systems based on commutable gears, which use standard lamps. Another issue is that of the more frequent switching needed to change power level, which is bound to reduce the service life due to electrode wear and end blackening, especially that of the higher-wattage burner.