||Thorn's SON-XL lamps represent the high lumen efficacy category - XL = Xtra Lumens. This is achieved through a significant increase in the xenon filling pressure, which lowers thermal losses from the arc. The higher pressure also preserves the electrodes for longer, slowing the rate of lumen depreciation. Indeed this 400W rating exhibits 95% lumen maintenance at the end of its 24,000 hour life.
Ignition of standard high pressure sodium lamps can easily be achieved with the pulse available from a standard ignitor. However the XL types with higher xenon pressure and thinner arc tubes, require a somewhat higher ignition pulse. To enable reliable starting on standard ignitors, an internal ignition aid is required.
Initially a plain molybdenum wire was mounted alongside the arc tube, connected to one electrode, and acting as a third external electrode. To prevent sodium ions being drawn out of the arc tube by this charged external conductor, it was standard practice to attach it to a pair of bi-metal strips, causing it to move away from the arc tube when hot. However the wire remained permanently connected and it still resulted in some sodium loss. Additionally the unreliability of the moving parts could result in premature failures.
In 1988 Thorn experimented with arctubes having integrated antennas. A stripe of molybdenum is sintered into the outer surface of the arc tube itself. Contact is made with a simple bi-metal strip welded to the frame. This proved a much more reliable method, both mechanically, as well as to fully isolate the strip. The design was taken further in 1997 when Philips eliminated the bi-metal with its PIA antenna.