Importance

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Abstract

A driver (10) produces a current to generate traveling waves of voltage for low levels of illumination and an arc voltage for high levels of illumination through a gas discharge lamp (50). At the low illumination levels the traveling waves of voltage are produced in a manner so as to increase the current in the lamp at a controlled rate so that the increase in current can be stopped by an optical or ionization feedback loop when the lamp reaches the glow discharge region, after the Townsend discharge region and before the arc discharge region. Without careful control of the rate of the current increase, the desired current level can easily be overshot or undershot. Also, the feedback is critical given the varying nature of the impedance of gas discharge lamps. The process is repeated at selected intervals to produce a desired average level of illumination. The nature of the traveling waves assists the direction of ion acceleration toward the walls of the lamp, allowing the lamp to be brought to the glow discharge region without damage to the cathode filaments. The cathode filaments are further preserved by the fact that driving the lamp to the glow discharge region for brief time periods rather than to the arc discharge region does not require multiple transitions through the highest voltage regions that precede the arc discharge region. A current feedback loop is used to make the system self-resonating and to increase the frequency of operation when the lamp smoothly transitions to the high illumination arc discharge mode of operation. The method and apparatus of the present invention have been shown to operate cold cathode, hot cathode, serpentine lamp and flat lamp technologies effectively. Dimming ratios have been observed above 20,000:1 for serpentine lamps and above 50,000:1 for flat lamps, with these ratios being effectively doubled when viewed from behind an AM LCD.

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First Claim