1984 Philips SOXD-E 18W

The lamp shown here is the largest variant of the ultra-compact SOX fitted with an E27 end cap, developed by Leo Sprengers at Philips from the late 1970s to the mid-1980s (see there for a more detailed introduction). Built with a T12 outer glass jacket, the SOXD-E 18W features a more evolved design involving a thin burner supported by reflective metal discs at both extremities, forming an IR-reflecting cavity with the ITO-coated bulb which contains the entire discharge tube, thereby resulting in its optimum thermal insulation. Heat losses are further limited by a vacuum environment which is maintained by two getters: a barium mirror near the end cap and a platinum catalyst pellet on the other end. The latter cracks hydrocarbon impurities. To ensure a proper and stable sodium vapor distribution between the electrodes during operation, the burner is provided with two sodium retention chambers and a narrow U bend that ensures a sufficiently high operating temperature there. Stability is further secured thanks to the electrically floating disks and ITO coating which limits the rate of electrolytic sodium loss from the burner.

This optimized lamp design is characterized by a 120 lm/W efficacy, which corresponds to a 20 % gain over the standard SOX 18W. This is realized for one part from a more efficient sodium discharge operating at a lower current (0.25 vs. 0.35 A), and for another part from reduced electrode losses (also due to the lower drive current). These changes arise primarily as a result of a higher discharge electric field that is caused by the narrower burner tube diameter, which confers the lamp a higher operating voltage (75 vs 57 V).

Like other lamps in the SOXD series, this variant is compatible with the standard PL lamp series choke and starter circuit. The sodium lamp was even made with similar dimensions as those of the 18 W PL-L so as to fit standard compact fluorescent luminaires too, while delivering 80 % more light for the same system power consumption of ~24 W. The SOXD-E 18W thus targeted safety lighting applications catered for by 150 W GLS lamps while enabling an energy saving of nearly 85 %. However, like the rest of the compact sodium lamp series, this model was never released on the market.