Double Envelope Halogen with Backbone - K/19

This lamp is representative of Thorn's second generation halogen retrofits for GLS lamps. It superseded the earlier version which employed a moulded ceramic component to which the capsule, outer bulb and screw cap were all cemented. That original design was rather expensive, due in part to the cost of the ceramic part, as well as the manual labour and curing time required to cement the different components together. Following the development of the Philips Halogenà lamp which featured a low-cost full glass construction, Thorn needed to revise its own design to remain competitive, and the result was this lamp.

The capsule relies on one of Thorn's major innovations, the so-called Backbone support, which permitted the extension of mains voltage halogen technology to lower wattages and allowed universal burning. Previously, linear halogen lamps could not be made in such low wattages owing to the inability of the filament to support the weight of the traditional molybdenum spiral supports, and they had to be burned horizontally. The quartz spine is a most elegant solution, but it was undercut in cost by the Philips technique of dimpling the quartz to fix its wire supports in place.

The capsule is mounted on a short stem from a T8 fluorescent lamp, and sealed into a thin-wall BT-shaped outer bulb to provide a new appearance for this high-performing lamp. Both clear and frosted versions were made. The capsule's gasfill pressure is slightly reduced vs the earlier version to permit use in open luminaires. This design was only manufactured for about two years until Thorn was taken over by GE, which had access to a cheaper design from Tungsram of Hungary.
Manufacturer: Thorn EMI
Lamp Power: 100 Watts  
Lamp Voltage: 240-250 Volts  
Lamp Current: 0.41 Amperes  
Cap Type: E27/27 Ni plated brass + vitrite
Bulb Type: BT-45 BT-14 in eighths/inch
Bulb Finish: Clear Soda-Lime
Filament Type: CC-8 Axial Coiled Coil
Atmosphere: Bromine+Iodine in Kr Nitrogen
Luminous Flux: 1,350 lm  
Luminous Efficacy: 13,5 lm/W  
Peak Intensity: N/A  
Beam Distribution: N/A  
Colour Temperature & CRI: CCT: 2850K CRI: Ra 100
Chromaticity Co-ordinates: CCx: 0.444 CCy: 0.406
Rated Lifetime: 4000 hours to 50% survival
Burning Position: Universal  
Max. Overall Length: 110 mm 4 3/8"
Light Centre Length:
Factory: Enfield, London United Kingdom
Date of Manufacture: c. 1989
Original Value: GB £10.98 (1988)
 
References: 1) Thorn Lighting Comprehensive Catalogue, UK, 1987/88
2) Thorn UK Price List, 7th November 1988