||The Linear Sodium concept was first introduced in 1959 by Associated Electrical Industries (formerly the British Thomson-Houston company, a division of General Electric of USA). It was the first artificial light source to break the 100 lm/W barrier. The first model was rated 200 Watts, and it was joined in 1961 by this smaller 60W equivalent.
The range was characterised by a novel grooved discharge tube having an alternating crescent shaped cross-section, based on GE's PowerGroove Fluorescent products. Because hot sodium vapour absorbs its own radiation, narrow diameter tubes are desirable to allow escape of light created below the surface. However efficacy also rises as the current density in the discharge falls, thereby favouring a large diameter tube. In traditional sodium lamps, a compromise must be found to determine the optimum tube diameter.
The novel shape of AEI's linear sodium lamps achieved both efficient light extraction and low current density, and thereby delivered a notable increase in efficacy. Although other cross-sections were still more efficient, the crescent-shape was chosen because of its ease of manufacture. The tips of the crescents are made sufficiently sharp that they also serve as sodium reservoirs, holding the liquid metal in place by capillary action. The reason for the alternate side of grooving is to prevent the sodium running along the length of the tube, while also increasing discharge path length. Thermal insulation of the original 200W lamp was effected by enclosing the discharge tube in a heat-reflecting glass sleeve and a secondary outer envelope, but this smaller 60W model maintains the impressive 100lm/W efficacy by employing two glass sleeves within the outer jacket.