||Since the first high pressure sodium lamp, a 400W product, was put on the market in the mid 1960s, consumers expressed a keen interest in the development of lower wattage versions. Every few years a new smaller lamp was placed on the market, but it was not until 1984 that a 70W product could be reliably produced. There were a number of very severe challenges to be overcome in the design of low wattage lamps, notably in finding methods of maintaining a high luminous efficacy and long life in these smaller sizes. Their arc tubes tended to blacken much more rapidly.
The lamp on this page is the first 70W SON lamp to have been manufactured by Thorn, which was introduced as something of a rush-job in 1983. To ensure a reasonable life, the arc tube bore had to be large (5.3mm) to reduce the rate of blackening, and the reduced wall loading meant that only a low efficacy was possible, also with poor colour rendering. Niobium heat reflectors had to be welded to the tube ends to provide sufficiently high sodium pressure.
A more efficient lamp could be made once the reason for the premature blackening had been discovered and halted, and Paul Denbigh's work at Thorn was largely responsible for this discovery. He identified that the blackening occurred as a result of rectification which places a high loading on the electrodes during warm-up, this occurring as a result of amalgam being in contact with the electrode shank. The problem was eliminated in later lamps by introducing a stepped end plug with a small alumina sleeve to keep the amalgam away from the electrodes. Then a 4mm narrow bore tube could be adopted, and lamp life and efficacy raised tremendously.