SLI/H Linear Low Pressure Sodium

The Linear Sodium concept was first conceived by BTH Mazda in the mid 1950's. It was an idea that came into existence following some co-operative work on fluorescent lamps of non-circular cross-section that took place between GE of America and BTH, its allied British operation. GE later launched its distinctive "Power-Groove" fluorescent tubes in the USA while BTH adapted the technology for the sodium lamp.

In all sodium lamps, it is unfortunate that the wavelength of yellow light created by the discharge is heavily absorbed by the hot sodium vapour itself. If a hot sodium lamp is switched off and it is viewed under the light of another sodium lamp, its vapour-filled discharge tube looks distinctly opaque. Therefore, tubes of narrow diameter would seem to be favourable to make it easier for sodium light to escape. However as always in lamp technology, another mechanism works against this - for the efficacy of the sodium discharge is always greatest at the lowest current density, and this of course calls for a tube of large diameter. In all standard lamps, the tube diameter has to be chosen as the best compromise between low current density and light absorption.

What makes the linear sodium concept special is that the discharge tube has a non-circular cross section. Therefore the current density can be made lower while the light can escape more easily because from any given point where it is created, it has to travel a shorter distance through the opaque sodium vapour than it would have to in a circular tube. The greater efficacy allowed discharge tube length to be reduced for each wattage and the awkward U-shape was no longer required (U-bending a sodium discharge tube is a perennial nuisance and source of many problems in lamp production). The crescent-shaped "Power-Groove" section was the initial design, but Thorn Lighting, the successor of BTH Mazda, subsequently adopted a more advanced clover-leaf design in the late 1970's. Various lamps of differing section and thermal insulation method are illustrated here.



SL/H Dimmable lamp with heated jacket
c. 1960



SLI/H with triple heat reflecting glass sleeves



SLI/H with striped outer coating of pure gold
c. 1961-64



SLI/H with tin film and crescent shape arc tube



SLI/H with indium film & 4-leaf clover arc tube



SLI-HO High Output with indium & 5-leaf arc tube