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In 1984 a sigificant miniaturisation of compact fluorescent lamps ocurred, with the introduction of the Sylvania double-tube design in which a pair of U-tubes are mounted side-by-side. In 1985 both Sylvania and Osram improved this by fusing the two tubes together to create a single discharge path, raising efficacy by eliminating two of the original four electrodes. For glass-technical reasons it was not practical to apply three U-bends, and both companies adopted the Philips-style glass bridge to join the pair of U-tubes.
Philips began pilot production of a similar design in 1985, and in 1986 launched the PLC type featured here. The letter C means 'cluster' in English, and 'carré' or square in French. It was produced by joining all four tubes with glass bridges, and it is interesting to note from the X-ray image that one of the upper bridges is farther from the end than the other - this enabling the discharge path (and hence lamp power and output) to be increased. A second cold spot is created by the dead zone in the cap area, in case of cap-up operation.
The double-tube lamps immediately became popular due to the great reduction in length, and improved light distribution uniformity. It was this development that suddenly made CFL's much more viable light sources in the rapidly growing market for low energy downlight luminaires in the 1980s. Incidentally there is a small decrease in efficacy vs twin-tube types. This is owing to increased absorption losses of each tube by its neighbours - note the increased core brightness in the lighted photo. Additionally, they tend tend to run hotter with less optimum mercury vapour pressure. In the highest loaded PL*C 26W lamp, this required an amalgam dosing to achieve satisfactory efficacy. |