||The improved reliability SOX-Plus lamp was launched by Philips in 1993, with its goal being to extend the group re-lamping interval for low pressure sodium installations from two to three years such that they could benefit from the same long-life as high pressure sodium systems. The technical improvement lies in the electrode assembly.
Traditional SOX lamps suffer a weakness at the glass-to-metal seals. These are normally made to unborated dumet wire sleeved in 2-ply soda-lime glass having an external borate layer. Since the borate does not extend over the end of the glass bead to the encapsulated leadwire, that weak spot is protected by embedding a small tube of magnesia into the glass to keep away sodium, but it is only partly successful. Cracking of the seals due to sodium attack affects a small percentage of lamps. Gas leakage between the core and sheath of dumet is another rare failure mechanism. In 1986 Philips developed an improved sodium resistant glass-to-metal seal, which was initially introduced only for its Belgian-made SOX-E range. It replaced the dumet seal with a single piece leadwire of 'Vacovit' iron-nickel-chromium alloy, sleeved with a single layer of pure borate glass. No ceramic bead is necessary. Elimination of premature seal failures leads to a small improvement in life.
Even though the SOX-Plus life claims showed a significant improvement, the actual gain was minimal as traditional lamps had been out-performing their life claim for several years. Rather than updating those claims to reflect reality, Philips cunningly developed the parallel range of SOX-Plus lamps, identifiable by their new red caps, and was able to increase profits by selling the new lamps at higher prices.