||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 88 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.
So significant were these improvements that they allowed the 20,000lm output of the original 200W linear sodium lamp to be matched by this 140W model. These innovations earned Thorn Lighting the top place in the SLI lamps market and the reliability of the Linear design was second to none, with very few early failures. The 140W SLI/H holds the title of the first lamp to be used for motorway illumination, with 1200 having been installed in Thorn Alpha-5 lanterns on the M1 motorway between London and Luton in 1971.