||Soon after the first halogen retrofits for the ordinary A-line lamps were first marketed in America, GTE Sylvania raised the bar with the introduction of its ultra high efficacy Capsylite MB-19 series in 1986. Offered in ratings of 42W, 52W and 72W and intended to replace 60W, 75W and 100W incandescent lamps, they not only delivered a significant energy saving but also raised the lifetime to an impressive 3500 hours - while competitive halogen equivalents typically achieved only 2000h life and had somewhat lower efficacy.
The Sylvania achievement was made possible thanks to its use of a small rectifier diode inside the lamp cap, which reduces the line voltage from 120V to about 85V by half-wave rectification with minimal electrical losses. The lamp therefore contains a halogen capsule designed for operation at 85V instead of 120V, and capitalises on the fact that lower voltage filaments can be driven at higher efficacy. For a given power, as voltage is reduced current must increase, and this necessitates the use of thicker tungsten wire. Thicker filaments take longer to evaporate to the point of failure, and part of this life increase can be offset by driving the filament harder. The resulting hotter operation leads to an increase in luminous flux and efficacy.
The concept can only be applied successfully on 60Hz mains supplies. At 50Hz the diode results in unacceptable levels of flicker. Moreover the diode was also found to interfere with operation on dimmer switches, and this made Sylvania's lamps so unpopular that the design had to be abandoned in favour of a return to less-efficient 120V capsules. For many years later in the Americas, halogen lamps were conspicuously marketed as "Diode-Free" designs.