||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. The lamp became known as the SON-L and it employed a new kind of contact at each end.
Several years later, Thorn Lighting unveiled a similar product - differing only in its length and the type of end contact. Thorn's SON-TD was made to fit directly into luminaires originally designed to accept linear tungsten halogen lamps, and consequently it was rather more successful, but still a highly specialised and low-volume product.
The lamp shown here is instead designed as an alternative to compact double-ended metal halide lamps which became very popular during the early 1990's. Such metal halide lamps were offered in two or even three different 'white' colour temperatures, but a low colour temperature lamp could not be made. Consequently Osram decided to offer the 'Plus' type high pressure sodium arc tube inside the same outer jacket. Since both the SON and metal halide arc tubes operate at the same lamp current, this lamp could be retrofitted directly into existing metal halide luminaires to give the characteristic golden light of high pressure sodium. As an alternative it was aimed at exterior applications of double ended metal halide, where luminous efficacy could be increased considerably but at the expense of colour rendering.