Thorn Metal Halide Kolorarc MBI/BD 400W

The first British metal halide lamp was the Mazda Metalarc 400W from British Lighting Industries, a Thorn-AEI joint company. It was unusually based on the American sodium-thorium-scandium chemistry, in view of the fact that Thorn had a strong technical agreement with Sylvania. In 1968 it was replaced by this Kolorarc model, following Sylvania's lead of eliminating the thorium to attain improved life.

Sodium-Scandium lamps have the advantage of higher luminous flux than competing tri-band and rare earth chemistries, but were not developed elsewhere in Europe because of the challenges associated with running them on simple choke-type ballasts. However Thorn took the lead in solving many of these problems. The principal difficulty is that scandium iodide reacts with hot quartz to form stable scandium oxide + silicon iodide. The latter reacts with the tungsten electrodes, in which the silicon dissolves, releasing elemental iodine. Iodine is strongly electronegative, and the gradual increase in its concentration limits lamp life by its effect on increasing the reignition peaks of the arc voltage.

Thorn's first lamps required a special autotransformer ballast to deliver a high open circuit voltage, but this requirement was lifted in 1972 by the development of exceptionally clean lampmaking processes. These included polishing of arc tubes with ammonium bifluoride and hydrofluoric acid, and the world's first use of anhydrous metal halide pellets - a process developed jointly by Thorn and APL Materials. Thorn was also unusual for sandblasting its arc tubes to improve colour homogeneity in precision optical systems. This made lamps such as this Base Down model the favoured light source in demanding applications such as Lighthouses.
Manufacturer: Thorn Lighting Ltd Lamp No. 91-7142
Lamp Power: 375 Watts
Lamp Current: 3.5 Amps
Lamp Voltage: 120 Volts 3.5-5.0kV ignition
Cap Type: E40s/45 Ni Plated Brass + Vitrite
Bulb Type: ED-120 ED-37 in eighths/inch
Bulb Finish: Clear Borosilicate glass
Electrodes: Backwound Tungsten Thoriated Tungsten
Arc Length:
Atmosphere: Inner: Hg,(Na,Sc)Ix | Ar Outer: Nitrogen
Luminous Flux: 29,000 lm (@ 100 hrs) 24,000 lm (@ 2000h)
Luminous Efficacy: 77.3 lm/W (@ 100 hrs)
Colour Temperature & CRI: CCT: 4100K CRI: Ra 65
Chromaticity Co-ordinates: CCx: 0.380 CCy: 0.385
Rated Lifetime: 7,5000 hours 70% flux & 80% survival
Warm-up & Re-strike Time: 2 minutes 8.5 minutes
Burning Position: Base Down to 15° above horizontal
Overall Length: 292 mm 7 3/8 inches
Light Centre Length:
Factory: Leicester England
Date of Manufacture: January 1979 Date Code: 61
Original Value:
References: 1) Thorn Lighting Comprehensive Catalogue, UK, 1979
2) Thorn Lighting Comprehensive Catalogue, UK, 1971-72
3) Thorn Lighting Comprehensive Catalogue, UK, 1972-73
4) Thorn Lighting Comprehensive Price List, UK, 19th April 1972
5) Thorn Lighting Discharge Lamps Brochure, April 1980
6) Thorn Lighting Product Data Sheet KolorArc 250/400/1000W Lamps 4:91:6, December 1987