Thorn SON-TD Linear High Pressure Sodium
The concept of a linear double-ended version of the SON lamp was originally introduced by Osram-GEC in 1979, which offered its 250W, 310W and 400W arc tubes housed within a quartz outer jacket. It made perfect sense to enclose the long, slender SON arc tube in this kind of jacket, and for the first time allowed the arc tube to be precisely located very near to the focus of compact ellipsoidal reflectors.

Several years later, Thorn Lighting unveiled this similar product - differing only in its length and the type of end contact. Osram-GEC had chosen a new type of contact whereas Thorn instead employed a recessed contact of the type found on double ended linear halogen lamps.

The 400W version illustrated here was the first to be marketed, and was developed specifically for use in Thorn's series of outdoor floodlighting luminaires which had originally been developed for linear halogen and MBIL linear metal halide products. Shortly afterwards a 250W model appeared and relatively recently, in the early 1990's a 1000W version was offered. The lamp lengths correspond with halogen and MBIL types so that existing luminaires can be used and only the control gear needs to be modified.

Double-ended SON has remained a very small part of the overall market though, the principal reason being the high cost of the lamp and the fact that for nearly every application, the very tight optical control this lamp offers is either not required, or can be attained almost as well with a standard optical system and a single ended screwbase lamp. Consequently SON-TD only finds a home in precision exterior floodlighting of buildings etc.
Manufacturer: Thorn EMI Lighting Ltd.
Lamp Power: 400 Watts
Lamp Current: 4.4 Amps
Lamp Voltage: 105 Volts (in luminaire) 85 Volts (in free air)
Cap Type: RX7s-24 Silver plated contacts
Bulb Finish: Clear Quartz
Bulb Type: T-22 T-7 in eighths/inch
Overall Length: 254 mm 10 inches
Contact Length:
Arc Length:
Electrodes: Backwound Tungsten Barium Calcium Tungstate
Atmosphere: Na,Hg | Xe Outer: Argon high pressure
Luminous Flux: 48,000 lm @ 100 hours 46,000 lm @ 2,000 hours
Luminous Efficacy: 120.0 lm/W @ 100 hours 115 lm/W @ 2,000 hours
Colour Temperature & CRI: CCT: 2000K CRI: Ra 25
Chromaticity Co-ordinates: CCx: 0.530 CCy: 0.430
Burning Position: Horizontal ± 20°
Rated Lifetime: 18,000 hours to 50% survival
Warm-up & Re-strike Time: 5 minutes 30-60 seconds restrike
Factory: Leicester England
Date of Manufacture: April 1987 Date Code 64
Original & Present Value: £54.21 (1988) £97.37 (2005)
 
References: 1) Thorn Lighting UK Catalogue, 1987/88.
2) Thorn Lighting UK Price List, 1987.