CP/32 Twin Filament Quartz Halogen for TV Studio

Just two years after Osram-GEC launched the world's first twin-filament incandescent lamps for television studios, its competitor, Thorn Lighting, announced on 3rd March 1968 the first halogen versions. Thanks to changing from the GEC's large hard glass bulbs to a compact quartz envelope, the Thorn lamps doubled the lifetime to 200 hours. This was made possible due to the greater mechanical strength of the quartz bulb, which can be filled with higher gas pressure so as to retard the rate of filament evaporation. Additionally it featured a small increase in luminous flux, which was maintained during life thanks to the halogen cycle.

A year later GEC caught up with its own improved lamp, opting for a glass halogen version instead of quartz. The use of glass limited the gas pressure needed for longer life, but did offer improved lumen maintenance as well as a lamp that was much cheaper than Thorn's quartz designs, and as a result it was still able to compete well.

This quartz lamp is of particularly unusual design. Due to the difficulty of forming a pinch-seal with four conductors, there are two separate seal tubes, each pinch-sealed around two conductors. These are fused to a flat quartz plate, whose outer rim has been fused to a length of large diameter quartz tube which forms the lamp envelope. Four small tubes of quartz attached to the dome of the bulb provide support for the filament mount assembly. The cap has been made from sheet metal crimped around a ceramic insert that carries the four contact pins. It has been clamped to the seal tubes with an intermediate cushion of asbestos fibre. The bulb is filled with bromine so as to avoid a shift in colour due to the purple vapour of the usual iodine fills of the time.
Manufacturer: Thorn Lighting
Lamp Power: 2500 + 2500 Watts
Lamp Voltage: 240 Volts
Lamp Current: 10.42 + 10.42 Amperes
Cap Type: GX38q Steel Pins
Bulb Type: TA-70 TA-22 in eighths/inch
Bulb Finish: Clear Quartz
Filament Type: 2x C-13 Parallel Flat Grid
Atmosphere: Nitrogen + CH2Br2
Luminous Flux: 55k / 55k / 117k lumens
Luminous Efficacy: 22.0 / 22.0 / 23.4 lm/W
Beam Intensity: N/A
Beam Distribution: N/A
Colour Temperature & CRI: CCT: 3200K CRI: Ra 100
Chromaticity Co-ordinates: CCx: CCy:
Rated Lifetime: 250 hours
Burning Position: Vertical base down ± 45°
Overall Length: 220 mm 8 5/8 inches
Light Centre Length: 143 mm 5 5/8 inches
Factory: Leicester England
Date of Manufacture: November 1973 Date Code: 95
Original / Present Value: GB £52.04 (1972) GB £613.20 (2014)
References: 1) Thorn Lighting Comprehensive Catalogue, UK, 1973/74, p.9:13
2) Thorn Lighting Comprehensive Price List, UK, 19th April 1972
3) New Halogen TV Studio Lamps, Light & Lighting Journal (UK) June 1968 p.182
4) An Improved Colour TV Lamp, Light & Lighting Journal (UK) December 1969 p.346
5) Private Communication, Graham Skeldon, Osram Wembley, 1994
6) Private Communication, Roger Hume, GE Lighting Leicester, 1998