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:	400 Watts
Lamp Current:	4.3 Amps
Lamp Voltage:	105 Volts
Cap Type:	E40s/45	Ni plated brass + vitrite
Bulb Finish:	Clear	Hard glass
Bulb Type:	T-46	T-14½ in eighths/inch
Overall Length:	285 mm	11¼ inches
Light Centre Length:	173 mm	6¾ inches
Arc Length:
Electrodes:	Backwound Tungsten	Barium Calcium Tungstate
Atmosphere:	Na,Hg \| Xe 300 torr	Outer: Hard Vacuum
Luminous Flux:	56,500 lm	(@ 100 hrs)
Luminous Efficacy:	141.3 lm/W	(@ 100 hrs)
Colour Temperature & CRI:	CCT: 2000K	CRI: Ra 25
Chromaticity Co-ordinates:	CCx: 0.530	CCy: 0.430
Burning Position:	Universal
Rated Lifetime:	Not Published
Warm-up & Re-strike Time:	8 mins	5 minutes
Factory:	Leicester	England
Date of Manufacture:	September 1991	Date Code 21
Original & Present Value:	£68.07 (1992)	£97.90 (2005)


References:	1) Thorn SON XL Brochure Ref. 68499 1990.
	2) Thorn Discharge Lamps Guide Ref. 44748 1989.