||Until 1977 the lowest power low pressure sodium lamp for general lighting was the 35-watt SOX. It was recognised that there would be an application for an even smaller lamp in outdoor security lighting, for which there was no alternative that combined low luminous flux with high efficacy. Philips therefore set about developing the Mini-SOX lamp, together with similarly miniaturised new control gear.
Scaling down LPS technology presented a major challenge owing to the fact that these lamps operate at optimum efficacy with a discharge tube temperature of 260°C, and at shorter lengths the temperature gradients at the electrode and U-bend ends make this difficult to attain over enough of the tube length. The thermal non-uniformity leads to sodium deficiency in the hotter parts of the lamp, where more of the rare gas filling is then ionised. This causes accelerated adsorption of the argon component onto the glass surface, and a consequent increase in lamp voltage, which necessitates large control gear in order to provide a high enough open circuit voltage for a long lamp life.
The problem was elegantly solved by introducing a small metal cap to provide additional thermal insulation over the relatively cold U-bend of the discharge tube. This performs the secondary function of supporting the discharge tube. Normally the cap itself would also lead to some light absorption, but when combined with a new dimpled outer bulb shape this effect was minimised. The new construction made the 18W SOX lamp technically viable, and it was launched in 1977 together with miniaturised control gear having a low open circuit voltage, achieving an impressive lamp efficacy of 100lm/W.