Thorn MBT Sunlight Clear Mercury Blended

The lamp illustrated here was never formally offered for sale by Thorn Lighting, it is purely a laboratory sample made up for demonstration purposes.

Technologically it is no different than any standard mercury blended lamp, consisting of a quartz arc tube which is ballasted by an internal single coil tungsten filament. Designed for operation on the high UK mains voltage of 240/250 Volts ignition of the arc tube is therefore relatively straightforward and the third auxiliary electrode is sufficient.

The main electrodes are of the unique construction invented by Thorn and used in many of its discharge lamps, being of the braided tungsten construction. Seven fine thoriated tungsten wires are braided into a hollow tube, which is coiled around the tungsten shank of the electrode, and the hollow structure then impregnated with double carbonate + thoria emissive material (approx 3mg on each electrode). This structure allowed for a greater volume of emitter and reduced the rate of sputtering, allowing lamp performance to be increased. Electrode positioning is outstanding, resulting from a novel laser welding technique employed by Thorn.

The outer jacket is filled with argon:nitrogen and two getters are employed. Standard red phosphorus is applied to the ballast (removing moisture and oxygen) while a zirconium / aluminium getter pan removes hydrogen impurities. Physically the lamp construction is of the most recent design employed by Thorn Lighting, known as the Philips style construction as the inner assembly has been built up on a backbone made of the heavy-gauge inner lead wires, a practice first introduced by Philips.
Manufacturer: Thorn Lighting Ltd.
Lamp Power: 160 Watts
Lamp Current: 0.65 Amps 0.90 Amps starting current
Lamp Voltage: 240/250 Volts
Cap: E27s/27 nickel plated
Bulb Finish: Clear
Bulb Type: E-75 Soda-Lime
Overall Length: 170 ± 5 mm
Light Centre Length:
Electrodes: Braided  KN tungsten Double carbonate + thoria
Inner / Outer Atmosphere: Ar 30-35 torr + Hg 12mg Nitrogen 500 torr
Luminous Flux: 2,900 lm @ 100 hours 2,650 lm @ 2,000 hours
Luminous Efficacy: 18.1 lm/W @ 100 hours 16.6 lm/W @ 2,000 hours
Colour Temperature & CRI: CCT: 5000K CRI: Ra 40 (9% Red)
Chromaticity Co-ordinates: CCx: 0.349 CCy: 0.380
Burning Position: Universal
Rated Life: 6,000 hours (to 50% survival)
Warm Up / Re-strike Time: 3 minutes 5 minutes
Factory: Leicester England
Date of Manufacture: May 1987
Original / Present Value: Laboratory sample - no commercial value
References: 1) Thorn Lighting Lamp Specification 91-2075A, Issue 3, 29-August 1989.