||Following AEI's 1959 invention of the linear sodium concept, the next major change arrived with the 1966 introduction of another novel form of discharge tube which pushed the luminous efficacy higher again. In parallel semiconductor IR coatings were applied to improve thermal insulation.
This lamp employs a discharge tube whose cross-section takes the shape of a 4-leaf clover. It offers an improvement over the original crescent shape by further increasing light extraction and decreasing current density. Maintenance of sodium position over life is ensured by 32 grooves known as 'greenhouses' running along the edge of each 'leaf'.
A spinoff advantage is a reduction in striking voltage thanks to the unobscured line-of-sight between the electrodes, enabling the use of a low pressure neon-argon-xenon gas filling to bring a further efficacy increase without loss of life. Following Thorn Lighting's takeover of AEI, the new lamps switched to Thorn's Braided cathodes, formed from seven tungsten wires woven into a hollow tube, which contain more emissive material and bring another life extension. The heat-reflective coating is of tin-doped indium oxide.
These improvements brought a modest gain of about 10% in efficacy and earned Thorn the top place in the SLI lamps market, but curiously the company was not successful in the 60W category. Possibly this is due to the fact that Thorn is not believed to have produced lanterns for the 60W lamp. The 60W was a development of its competitor Osram-GEC who did offer a number of lanterns, and even though Thorn had the better lamp, the replacements market continued to be dominated by the GEC SLI/H 60W alternative.