||This lamp is representative of the North American style of energy-saving U-tube fluorescent lamp, made according to a design introduced in the 1970s. During the energy crisis of that decade, a broad variety of energy-saving lamps were developed. Due to the peculiarities of the type of control gear used with fluorescent lamps in North America, it was somewhat more difficult to achieve an energy-saving fluorescent lamp than in Europe.
The most notable change vs the original version of this lamp, the F4OT12 U-tube, is that the gas filling has been changed from the usual argon to krypton (or possibly a krypton-argon mixture). All other things being held constant, a krypton-filled lamp has a lower discharge voltage than the argon version. When operated on US-style rapid-start ballasts, whose output current is almost independent of lamp voltage, a reduction in voltage leads to a decrease in power dissipation and hence an energy-saving lamp.
However Krypton-filled lamps are considerably more difficult to start than their argon-filled counterparts. These difficulties were not encountered in Europe, where fluorescent lamps are more commonly operated on preheat circuits, which comprise a magnetic ballast and glowbottle starter, which delivers a powerful ignition voltage. The solution adopted for the American FB-series T12 krypton lamps was to apply a transparent electrically conductive coating of fluorine-doped tin oxide to the inner surface of the glass tube. This capacitively couples the two electrodes and results in a reduction of striking voltage. The presence of this coating can sometimes be observed due to the iridescent colours of surface reflections on the glass tube.