||Following AEI's 1959 invention of the linear sodium concept, the next major change arrived with the 1966 introduction of discharge tubes having the shape of a 4-leaf clover. In parallel a semiconductor IR coating of tin oxide was applied to improve thermal insulation. This allowed the 20,000lm output of the original 200W lamp to be boosted 25% to 25,000lm. A spinoff advantage is a lower striking voltage thanks to the unobscured line-of-sight between electrodes, which enabled the use of a low pressure Ne-Ar-Xe gas filling to bring a further efficacy increase without loss of life.
This so-called HO or High Output model was introduced around 1971 and marks yet another advance in efficacy. The ratio of surface area to cross-sectional area of the discharge tube has been increased by switching to a 5-leaf shape, which allows still greater light extraction. In parallel the IR coating is the superior indium-tin oxide composition, whose better reflectivity allows a wider discharge tube - the decrease in current density bringing the output to 27,500lm. Rather inconveniently the larger inner required the outer jacket to be increased to the non-standard T12½ diameter.
The electrode design was also shifted at this time to employ the more efficient and robust beehive SOX design. Since these heavier cathodes require some 6 Amps to be brought to incandescence, greater than the short-circuit current available from the SLI 200W ballast, the new HO lamp was no longer compatible with the original 200W circuits having a thermal starter-switch. It could only be installed in new luminaires having leakage reactance autotransformer ballasts. Because of this, the regular 200W lamp had to be continued in production alongside the new HO model.