||This powerful 280W lamp was unique to GEC's SOI range. It achieves its impressive output by making use of two 140-watt discharge tubes within the same outer envelope. There is insufficient room to accommodate the usual heat-insulating glass sleeves and this, combined with the self-absorption of some of the light from one discharge tube by the tube adjacent to it leads to a drop in efficacy vs the usual 140W model. However when operated at full power the penalty was less than expected, due to the fact that the two discharge tubes help to keep each other warm.
Introduced in 1959, the primary reason for this lamp was to offer high light output from existing 140W lanterns - it is much shorter than Philips' 5-foot long 300W SOI alternative. It also offered streetlighting authorities the opportunity to reduce electricity consumption as road traffic volumes eased late into the night, by simply switching off one discharge tube and therefore halving power consumption. Alternatively if both discharge tubes are connected in parallel, only one will ever strike up, and it was possible to double the lamp maintenance period by equipping each lantern with an auxiliary discharge tube - albeit with a drop in efficacy.
Perhaps because of the expense of this lamp, or the fact that there was insufficient room to incorporate any heat-reflecting glass sleeves to boost efficiency, or simply because of its complexity, fragility and considerable weight, it never achieved wide acceptance. Its requirement for two ballasts and duplicated wiring would also have added to the installation expense, and GEC decided to cease manufacture of this unusual product around 1969. This lamp is one of only four known examples still in existence.