Demonstration Lamp with Half Coated Bulb

This lamp was prepared by Thorn Lighting in the early 1980s to illustrate the internal construction of the mercury fluorescent lamp. Ordinarily the outer bulb is completely covered with the opaque layer of europium activated yttrium vanadate, making it impossible to see inside.

This was one of the first Thorn lamps produced with the so-called integral frame assembly, copied from Philips products which were the first to use this method of construction. Previously, the arc tube had been welded into a metal support frame, which was mechanically clamped to the glass flare tube in the lamp neck. The process was labour intensive and required many components in its assembly. The Philips mounting method simply employs thicker wires emanating from the stem itself. The arc tube is spot-welded directly to these wires, thus eliminating the frame assembly.

Another feature of Thorn mercury lamps is their braided cathode construction. Mercury electrodes usually consist of a simple backwound coil of solid tungsten over a tungsten mandrel. Occasionally another coiled wire may be employed for the secondary coils. In the Thorn design, a hollow tube of eight finely braided tungsten wires was employed for the secondary electrode winding. This electrode holds a greater weight of emitter and guarantees enhanced lamp life.

Not so noticeable here is another advanced assembly method. Making one of the first uses of lasers in lamp assembly, the electrodes here have been welded to the molybdenum foils using a Neodymium:YAG laser beam, which provided much more reliable results than spot-welding methods.
Manufacturer: Thorn EMI
Lamp Power: 125 Watts
Lamp Current: 1.15 Amps
Lamp Voltage: 125 Volts
Cap: E27s/27 Ni plated brass
Bulb Finish: Yttrium Vanadate half-coated
Bulb Type: E-75 Soda-lime
Overall Length: 183 mm
Light Centre Length: 110 mm
Electrodes: Braided tungsten with double carbonate emitter
Inner / Outer Atmosphere: Inner: Ar | Hg Outer: Nitrogen
Luminous Flux: N/A
Luminous Efficacy: N/A
Colour Temperature & CRI: N/A
Chromaticity Co-ordinates: N/A
Burning Position: Universal
Rated Life: 8,000 hours to 70% survival
Warm Up / Re-strike Time: 4 minutes 5 minutes
Factory: Leicester England
Date of Manufacture: Circa 1986
Original / Present Value: No commercial value