1984 Philips SOXD-E 8W

Following its successful development of efficient narrow-bore TLD and T4 fluorescent lamps during the 1970s, Philips tried to apply a similar design approach to SOX lamps. The goal of this research work done during the 1980s was to increase the efficiency of low-wattage compact sodium lamps while enabling a compatible operation on standard PL lamp systems.

The key design strategy consisted in reducing the discharge tube diameter so as to increase charge losses by ambipolar diffusion of electrons and ions towards the wall. In order to maintain the ionization balance of the plasma, its electric field increases to raise the ionization rate to that of charge loss, thereby resulting in a higher lamp voltage and in a lower discharge current. The latter change is crucial as it decreases the electrode losses while increasing the electrical-to-light energy conversion efficiency of the low-pressure sodium plasma, helped by the more energetic electrons in the discharge. Moreover, the lamp efficacy was increased further thanks to the enhanced ionic pumping of sodium in the narrow discharge tube, which reduces optical losses by resonant self-absorption of Na's D lines in the burner volume.

In order to obtain a lamp electrical impedance suitable for an operation with a given PL-S ballast, both the discharge length and the neon-argon gas fill pressure were adjusted. The latter was optimized considering the lamp efficacy and the risk of neon-sodium segregation by cataphoresis. While the neon-argon fraction was finely tuned to facilitate ionization via the Penning reaction between the two noble gases, the loss of free charges in the plasma as a result of the burner's high surface-to-volume ratio still causes the lamp's ignition voltage to lie well above the standard European mains voltage of 220–250 V rms. As a result, a parallel-connected soft ignitor (< 1 kV) in the form of a glow-switch starter is required for a reliable ignition of the sodium lamp.

This design approach not only enables an operation on existing CFL-ni circuits, it also results in a significant reduction in lamp size. During the 1980s Philips developed a whole range of experimental SOXD lamps (“D” for “dun”, or “thin” in Dutch) from 8 to 30 W for testing and evaluation purposes. In this process, the company also substituted the BY22d end cap for a standard E27 one in the smaller models (only the 30 W type retained the bayonet cap). The platform was a technical success since efficacies in excess of 100 lm/W were achieved in very compact lamp formats, but lifetime issues were of concern, especially because of the rapid degradation of the narrow-bore discharge tube in the electrode areas and as a result of sodium electrolysis.

The model shown here is the smallest SOXD lamp developed at Philips’s Central Lighting Laboratory in Eindhoven, the Netherlands. With a power consumption of only 8 W and an efficacy of 100 lm/W, this source provides 33 % more light than the PL-S 9W. Its 800 lm output is ideally suited for addressing lighting needs normally catered for by GLS lamps in the 60–75 W range. The total SOXD system power consumption lies around 12 W and enables a 80–85 % energy saving compared to the incandescent lamp it replaces.

The SOXD-E 8W is built with a 30 mm-diameter glass tube jacket which, like generic SOX lamps, is internally coated with a layer of tin-doped indium oxide that reflects infrared radiation back towards the burner. Because of its extremely low power dissipation, the lamp requires the best thermal insulation possible so its burner can reach an optimal operating temperature of 260 °C, which is achieved thanks to a high-conductivity ITO coating as used in SOX-E lamps, and by maintaining the highest level of vacuum in the jacket. To that end the lamp is provided with a barium flash getter, located near the end cap, and with a platinum catalyst pellet, placed near the electrodes and whose role is to crack hydrocarbon contaminants. The structure holding the discharge tube is relatively simple and consists of a pierced metal disk at the burner's U-bend end, while the other extremity of the burner is held by its lead wires.

Although these compact sodium lamps were ideally suited for outdoor lighting applications due to their higher efficacies and the lack of blue light compared to CFLs, SOXDs were never developed into commercial products. Lifetime issues certainly could have been solved, but Philips chose not to release the compact sodium lamps possibly because of the lack of significant perceived advantages (from a consumer's point of view) over CFLs and certainly to avoid competing with compact fluorescent lamp systems that were released and promoted at the time for the similar applications. Besides, there was still the 18 W SOX, released in 1977, for security lighting that required more light and a lower power consumption than could be delivered by twin-lamp PL-S luminaires.