||This prototype lamp was manufactured around 1984 as part of the drive to make the high pressure sodium lamp available in smaller lumen packages. The technology was introduced in the form of the 400 watt version but on account of its high efficacy and moderate colour rendering properties it was recognised that there would be an indoor market for lower wattage lamps.
Thereafter, a new wattage joined the range every few years. The manufacture of low wattage SON lamps presents a number of very severe technical challenges, and indeed even when the first 70W lamp became available it was something of a rush-job which exhibited considerably lower luminous efficacy and lifetime than its higher wattage counterparts. It took a further two years to optimise the design of that lamp and learn how to make good performing light sources of such low wattage. The technological changes enabled a 50W lamp to also join the range.
But still there was a demand for an even smaller lamp, and Thorn addressed the situation with the development of this experimental 40-watt source. It employs a newly developed slim arctube and specially shaped end seals which keep the sodium amalgam away from the electrode shank. Amalgam in contact with the electrode was found to lead to rectification and rapid blackening, so the new ceramic end plugs were fabricated with a small sleeve to extend part way up the side of the electrode and prevent the amalgam coming into contact with it. The lamp pictured here was successful but it was found that a still lower rating of 35 watts became feasible, and that was the type to be marketed. Its market volume was so small though that only Thorn's competitors ever made it.