Early-1980s Philips SOXS 18W

In an effort to reduce the size of low-pressure sodium lamps, engineers and scientist at Philips designed and tested different compact burner configurations during the 1980s. In the lamp shown here, the discharge tube is shaped into a tight spiral placed coaxially inside a shortened SOX 18W jacket. Interestingly, the structure holding the burner into place is quite elaborate and consists of a glass tree in the middle, a highly polished aluminum disc at the cap end of the lamp, and and a glass plate coated with an infrared-reflecting dielectric mirror at the other end. These two discs ensure an optimal thermal insulation of the burner while enabling the light emitted inside its coil structure to escape the lamp. In order to reduce the risk of sodium electrolysis, the aluminum disc has been left electrically floating while the lead wire connecting the electrode opposite to the cap is passed through the central tube of the glass tree support structure.

This kind of compact sodium lamp was not new as GEC of England had already developed a similar design in the late 1950s. Philips revisited this concept with an optimized thermal management and an improved discharge operation so as to maximize the lamp's lumen efficacy. To that end the operating voltage was increased to 87 V (64 % more than that of the standard 18 W SOX), which lowered the discharge current to 235 mA (vs. 350 mA), thereby reducing electrode and plasma losses. This is achieved mostly via a narrower (8 vs. 12 mm) and longer discharge tube (30 vs. 12 cm electrode gap). Despite the narrower discharge, the magnitude of the electric field in the plasma is around 2.7 V/cm, which is significantly lower than the 4.0 V/cm found in standard 18 W SOX lamps. Had the neon buffer fill pressure been kept constant, the electric field would have been higher, so is it clear that the fill pressure has been reduced, certainly to increase the electron energy in and to lower the heat production from the discharge. This is consistent with the fact that the smaller overall surface-to-volume ratio of the burner results in less heat being needed to bring it to its optimum operating temperature of 260 °C.

Despite its significantly longer discharge, the overall axial length of the helical burner is 6 cm only, which is more than twice shorter than that of the standard U-shaped burner used in standard 18 W SOX lamps. This thus enabled a reduction of nearly 25 % of the overall length of the lamp (165 vs. 216 mm). Because of the lack of a cold U-bend, the risk of sodium migration is also much more limited here, so there is no need for sodium reservoirs (i.e., Philips's trademark dimples) to distribute the metal vapor homogeneously. Although the SOXS 18 W is electrically compatible with existing 18 W control gears, the lamp cannot start by itself because of its narrower and longer burner which results in an increased ignition voltage. An additional ignitor is thus required to ensure a reliable starting.

While a helical configuration of the burner proved very effective to reduce the size of SOX lamps, this particular geometry also causes a significant absorption loss of the resonant radiation emitted by the sodium vapor. This intrinsic limitation offsets any gain in efficiency achieved by the improved thermal insulation of the burner and the reduced current operation of its discharge. Overall, the concept was thus a failure and the SOXS 18W was never developed into a commercial product as the standard SOX lamps with the simpler U-shaped burner kept the high ground in terms of efficiency. It is interesting to note that the concept was revived nearly three decades later in China by Shandong Blessed Huihuang PV Energy, who developed a series of low-pressure sodium lamps with a spiral discharge tube for solar-powered lighting systems. However, none of the lamps were ever released commercially due to the rise (at that time) of more efficient LEDs in the general lighting market.