Halogen Theatre Spotlight T/15

Theatre lamps consist of a planar grid filament and prefocus cap, this arrangement delivering maximum beam uniformity and light extraction in fresenel-type lanterns - which also contain a spherical mirror to focus an image of the rear of the filament between the coils. Their colour temperature is lower than studio / photographic types since there is no requirement to match the colour balance of 3200K film stock - as such the Class T lamps have rather longer lifetimes.

The first theatre lamps were incandescent types, and during the 1970-1990s those were entirely superseded by halogen lamps offering 100% lumen maintenance and 2x-4x longer life. However they also cost far more, and in the theatre business it took several years before they could be made competitively enough to finally usurp the incandescents.

Halogen projection lamps can only be made with quartz of the finest chemical and optical purity. In the early days this was extremely expensive, especially in the large diameters needed for high wattage lamps. This lamp shows one of the most important steps in solving this problem, following the invention of Thorn Lighting's Alex Halberstadt. Thorn was first to achieve the large diameters by purchasing lower cost small-diameter quartz, and inflating this to a larger diameter only around the filament so as to prevent overheating at that critical location. The concept was patented and the resulting cost advantage made Thorn a global leader studio and theatre halogen business. The T/15 model shown here was introduced in 1972 to replace the earlier incandescent T/6 - a type which was operated base-up. This required an unusually long neck to prevent overheatng of the thermally-sensitive quartz-to-metal end seals.
Manufacturer: Thorn Electrical Industries
Lamp Power: 1000 Watts
Lamp Voltage: 220 Volts
Lamp Current: 4.55 Amperes
Cap Type: P28s Ceramic
Bulb Type: BT-35 BT-11 in eighths/inch
Bulb Finish: Clear Quartz
Filament Type: C-13 Monoplane Grid h=17.5mm w=18.0mm
Atmosphere: Nitrogen Bromophosphonitrile
Luminous Flux: 23,000 lumens
Luminous Efficacy: 23.0 lm/W
Luminous Intensity: N/A
Beam Distribution: N/A
Colour Temperature & CRI: CCT: 3050K Ra: 100
Chromaticity Co-ordinates: CCx: 0.434 CCy: 0.403
Lifetime: 750 hours
Burning Position: VBU ± 15°
Overall Length: 160 mm 65/16 Inches
Light Centre Length: 89.0 ± 0.5mm 31/2 Inches
Factory: Leicester United Kingdom
Date of Manufacture: 1974 November Date Code: 480 11
Original Value: GB £10.37 (1974)
References: 1) Thorn Lighting Comprehensive Catalogue, 1974/75 UK, p. 9:29
2) Thorn Lighting Comprehensive Catalogue, 1972/73 UK, p. 9:13
3) Thorn Lighting Comprehensive Catalogue, 1971/72 UK, p. 9:13
4) Thorn Lighting Comprehensive Price List, 1st October 1974, p.70
5) GB Patent 1,122,747 - Tungsten Halogen Lamps having Bulged Tubular Envelopes, A.L. Halberstadt & R.D. Swain, Thorn Lighting Ltd, Filed 1969
6) Biography of Alex Halberstadt - J.D. Hooker, www.lamptech.co.uk