Thorn SON XL-T with Sintered Moly Antenna
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.
Manufacturer: GE-Thorn Lamps Ltd.
Lamp Power: 392 Watts
Lamp Current: 4.6 Amps
Lamp Voltage: 100 Volts 3.5-4.5kV x 2μs/cycle ignition
Cap Type: E40s/45 Ni plated brass + vitrite
Bulb Type: T-46 T-14½ in eighths/inch
Bulb Finish: Clear Borosilicate glass
Electrodes: Backwound Tungsten BCT Emitter
Arc Length: 96.0 mm 3.778 inches
Atmosphere: Na25:Hg75 | Xe 220 torr Outer: Hard Vacuum
Luminous Flux: 56,500 lm (@ 100h) 55,000 lm (@ 2000h)
Luminous Efficacy: 144.1 lm/W (@ 100h)
Colour Temperature & CRI: CCT: 2000K CRI: Ra 25
Chromaticity Co-ordinates: CCx: 0.530 CCy: 0.430
Rated Lifetime: 28,500 hours 50% survival, 78% flux
Warm-up & Re-strike Time: 2.5 minutes <1 minute
Burning Position: Universal
Overall Length: 285 mm 111/4 inches
Light Centre Length: 173 mm 67/8 inches
Factory: Leicester England
Date of Manufacture: September 1991 Date Code 21
Original Value: GB £68.07 (1992)
References: 1) Thorn SON XL Brochure Ref. 68499 1990.
2) Thorn Discharge Lamps Guide Ref. 44748 1989.
3) Thorn Lighting Product Data Sheet 400W SON-T & SONXL-T 96-2046C, 10-10-1989.
4) Thorn Lighting SON Manufacturing Specifications.
5) GE-Thorn UK Price List, March 1992.