Solarstream Linear Sodium SON-L 250W

The double-ended high pressure sodium lamp was introduced by Osram-GEC in 1975. It marked an important development, being the first SON lamp having its outer jacket specifically engineered around the arc tube to deliver maximum system performance. Previously, the large outer jackets from mercury lamps had always been employed.

By switching the bulb material from glass to heat-resistant quartz, it could be brought closer to the arc without risk of melting. Additionally the outer is filled with high pressure argon, instead of the vacuum atmosphere of other sodium lamps. This permits a higher loading with increased output, the gas pressure restraining the rate of alumina evaporation and lumen depreciation. However it is imperative that luminaires be sealed, otherwise air draughts will cool the lamp, this cooling being conducted via the gas filling to the arc tube. Lamp voltage is about 20% lower in free air.

The SON-L range comprised 250, 310, 400 & 1000W types. The R12.5s cap was introduced with silver plated copper pins to ensure reliable electrical contact at the high current and temperature. Competitors were fast to offer their own variants - Thorn's SON-TD becoming the market leader as its length and contact type matched that of linear halogen lamps. An identical product was offered by Sylvania until quite recently. German Osram's lamps are characterised by their very short length and the Fc2 end contacts.

Linear SON has remained a small part of the market, owing to the high lamp cost, and because standard lamps are usually good enough. It is used only where optical precision is demanded. The SON-L range was discontinued in 1994.
Manufacturer: The General Electric Company of England PLC
Lamp Power: 250 Watts
Lamp Current: 3.0 Amps 4.1A Starting current
Lamp Voltage: 100 Volts (in luminaire) 80 Volts (in free air)
Cap Type: R12.5s Silver plated copper pin
Bulb Finish: Clear Quartz
Bulb Type: T-20 T-6 in eighths/inch
Overall Length: 273 mm 10¾ inches
Contact Length: 265 mm 10½ inches
Arc Length: 88 mm 3½ inches
Electrodes: Backwound Tungsten Barium Calcium Tungstate
Atmosphere: Na,Hg | Xe Outer: Argon high pressure
Luminous Flux: 29,000 lm @ 100 hours 28,500 lm @ 2,000 hours
Luminous Efficacy: 116.0 lm/W @ 100 hours 114 lm/W @ 2,000 hours
Colour Temperature & CRI: CCT: 2100K CRI: Ra 23
Chromaticity Co-ordinates: CCx: 0.526 CCy: 0.418
Burning Position: Horizontal ± 20°
Rated Lifetime: 10,000 hours 80% survival, 90% lumens
Warm-up & Re-strike Time: 3 mins to 80% light 30-60 seconds restrike
Factory: Shaw, Oldham England
Date of Manufacture: May 1992 Date Code Z5
Original & Present Value: £75.07 (1992) £104.07 (2005)
References: 1) Osram-GEC Lamp Catalogue, UK, 1975, p.33
2) Osram-GEC Solarcolour Brochure, 1983.
3) Osram UK Lighting Catalogue, 1992/93.
4) Osram Price List, January 1992.