||Although the concept of linear HPS lamps existed since the late 1960s, it was not until 1974 Thorn Lighting launched the first product. The development makes perfect technical sense - the oversized outer bulbs inherited from mercury lamps were replaced by slender outer envelopes designed specifically around the HPS arc tube. By adopting the more refractory quartz glass, lamp diameter was reduced to just 21mm. This allowed the light source to be brought closer to the focus of compact and high efficiency optical systems, and better aligned, so as to achieve outstanding beam control and high efficiency in floodlighting applications.
Thorn's first lamp was the 400W SON-TD, developed with the same length and similar cap as the popular 1500W linear halogen. It was soon joined by this 250W variant, intended to replace 1000W halogen lamps. This is actually one of the first 250W lamps with the original spiral arc tube support construction, and strip-type hydrogen getter wrapped around one end of the arc tube.
Linear HPS lamps are unsuual in that their outer jackets are filled with a coolant gas instead of being evacuated. The reason is because the close-fitting outer reflects infrared radiation back to the arc tube, which would otherwise reduce life due to overheating of the seals, and accelerate sublimation of the ceramic arc tube. The conductive gas filling makes linear HPS lamps very sensitive to ambient temperature changes, and they can only be used in dedicated luminaires. Despite the technical advantages of this concept, the higher cost of double ended lamps and luminaires has meant that these lamps have been phased out for all but the most optically demanding applications.