Thorn First 70W SON-E High Pressure Sodium

The first 70W HPS lamp was introduced by GE in 1975, having a low arc voltage to suit that country's mains supply. The lamp on this page represents the first variant adapted for use on higher European voltages, and was introduced in 1976 by Thorn Lighting of the UK. Just like with its American counterpart, there were a number of technical challenges to be overcome before a low wattage European HPS lamp could be perfected - notably in maintaining a high luminous efficacy and long life in such small lamps.

A major problem was that as the arc tube size was reduced, the blackening that occurs during life around the electrodes covered a rather large portion of the arc tube surface. That led to the combined problems of rapid lumen depreciation, and increasing arc tube temperature that also raised amalgam temperature, lamp voltage, and consequently a short life. To ensure a reasonable life in this earliest model Thorn had to use a large bore arc tube of 5.3mm, but this resulted in a low wall loading and disappointing efficacy. Niobium heat reflectors also had to be fitted around the tube ends to achieve sufficiently high sodium pressure.

This original model was intended to replace low wattage mercury fluorescent lamps, and as such it was offered only in an ovoid bulb. The majority of lamps were made with an internally-frosted pearl bulb, but it was occasionally also produced in this clear ellipsoidal variant for floodlighting applications. It was manufactured until 1984 when Thorn once again led the way in improving the efficacy and life of low wattage HPS lamps with a new arc tube design that overcame the blackening issues, allowing the switch to a narrow bore with considerably better performance.
Manufacturer: Thorn Lighting Ltd.
Lamp Power: 70 Watts
Lamp Current: 0.93 Amps
Lamp Voltage: 95 Volts
Cap Type: E27s/30 Ni plated brass + vitrite
Bulb Type: E-70 E-22 in eighths/inch
Bulb Finish: Clear Soda-lime glass
Electrodes: Backwound Tungsten BCT Emitter
Arc Length:
Atmosphere: Na20:Hg80 | Xe 25 torr Outer: Hard Vacuum
Luminous Flux: 5,800 lm (@ 100h) 5,300 lm (@ 2000h)
Luminous Efficacy: 82.9 lm/W (@ 100h)
Colour Temperature & CRI: CCT: 2000K CRI: Ra 35
Chromaticity Co-ordinates: CCx: 0.542 CCy: 0.415
Lifetime: 10,000 hours 90% flux & 85% survival
Warm-up & Re-strike Time: 5 minutes <1 minute
Burning Position: Universal
Overall Length: 154 mm 6 inches
Light Centre Length: 100 mm 4 inches
Factory: Leicester England
Date of Manufacture: 1976
Original Value:
References: 1) Thorn Lighting Comprehensive Catalogue, UK, 1977.
2) Thorn Lighting Datasheet 4:96.1 SON Lamps, January 1982.
3) Thorn Lighting Brochure, Discharge Lamps, Publication 612, April 1979.
4) Thorn Lighting Brochure, Discharge Lamps, Publication 63:9, April 1980.
5) A Decade of SON, D. Peach, Thorn Lighting Journal No 22, pp.3-5.
6) Thorn Lighting Company History, The 60s & 70s, Thorn Lighting News Staff Newspaper, 1987 September/October p.6.
7) Recent Advances in the Design of High Pressure Sodium Lamps, P.L.Denbigh & P.D.Molesdale, National Lighting Conference 1984 (UK), pp.201-207.