Although the first six foot tube was introduced by GE of America in 1944 as part of its Slimline T8 series, it remained a rather uncommon size.
The first preheat cathode six foot tube was introduced by British Lighting Industries (a division of Thorn Lighting) in 1966 under the Atlas Super Six name. It had a T12 diameter and was rated 85 Watts. BLI introduced it as new standard with highly optimised performance of the total lighting system, and it was originally intended that this size should become the dominant tube for the British fluorescent market. Previously the highest runners in Britain had been the 5ft 80W tube, and after 1956 the 8ft 125W. Both were highly loaded tubes that offered impressively high light output for industrial and commercial applications, but that made them rather inefficient - their loading being similar to the American HO (high output) tubes introduced by GE in 1952. That situation was partially overcome in 1961 when BLI introduced the new standard of the 8ft 85W tube. That delivered an extremely impressive boost in lighting efficiency - but as noted by RRK such long tubes were considered unwieldy and difficult to handle.
The compromise of 1966 was to introduce the 6ft 85W tube as an intermediate model that satisfied the following criteria: - high light output suitable for industrial and commercial applications - high efficacy to attain lowest cost of operational ownership - the maximum length tube that could strike easily on the British 240V mains supply, without expensive or complex control gear such as autotransformer ballasts, or the capacitive ballast as used for the 8ft 125W tube
The 6ft 85W indeed quickly became one of the most popular sizes in Britain but curiously it did not enjoy the same success in other countries. As RRK also already pointed out, for low voltage countries in the Americas it would not be an advantage, because the high tube voltage would require a greater open circuit voltage of the autotransformer ballasts, resulting in greater size, weight, cost, and electrical losses of the ballasts. In other European countries it was also not adopted because their mains voltage of 220V was slightly too low to ensure reliable starting of the six foot tube, which was not a problem on the British 240V mains. As such, the six foot tube standard remained almost exclusively limited to the UK and other 240-250-260V countries around the world. Another restriction was that in continental Europe Osram and Philips had recently constructed new high speed tube production machinery - but not having foreseen the six foot development and also not having ever made the 8ft tubes in significant volumes, they built their lines to handle a maximum tube length of 5ft. It was therefore commercially very difficult for them to follow the Thorn lead in the new 6ft business.
Two years later in 1968, Thorn made another major development with the introduction of its "Superwhite" phosphor. That delivered an almost unprecedented boost of 6% in luminous efficacy, and allowed the 6ft tubes to rival even the 8ft 85W in total system efficacy. The same phosphor was not attractive to apply on 8ft tubes because those already delivered enough light, and the high cost of the new material would have made them too expensive on a longer tube. Also in 1968 Thorn developed a remarkably efficient new semi-resonant-starting ballast for the 6ft 85W tube, which eliminated flicker on startup and greatly extended lamp life. These two achievements further cemented the 6ft 85W as the leading British tube.
Following the global Energy Crisis of the early 1970s, in 1973-74 Thorn re-rated the 6ft T12 from 85W to a dual-rated 75/85W tube. New ballasts were introduced to run the tube at slightly lower current, which caused a drop of 10W in power consumption but due to the lower power loading, the decrease in light output was much less significant. Thereafter, most new 6ft installations used the newer 75W ballasts.
Finally, following Thorn's 1975 introduction of the first Krypton-filled T12 energy-saving tube in Europe, and especially after Philips' 1978 extension of the Krypton technology to the new T8 formats as T12 retrofits (first in 4ft 36W, then 2ft 18W and 5ft 58W), it was a logical step for Thorn to apply the same principle to its 6ft tubes. That resulted in the introduction of the 6ft T8 70W krypton lamp. Like its 6ft T12 predecessors, it remained almost exclusively used in countries having mains voltages of 240V or higher.
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