Mercury arc lamps containing iodides of sodium, scandium and/or rare-earths develop voltage waveforms characterized by "spikes" of voltage appearing on the leading edge of each half-cycle of alternating current. This phenomenon is known in the lighting industry as "re-ignition voltage." Re-ignition voltage occurs as the alternating current passes through zero. The free iodine in the lamp envelope quenches the arc, which then partially extinguishes. In order to reestablish the arc with the current in the opposite direction, the voltage must be caused to rise, momentarily well above the sustaining voltage. The more free iodine in the lamp, the greater the re-ignition voltage.
In a typical metal halide lamp, the ratio of the re-ignition voltage to the RMS (or crest factor) voltage is about 1.2 to 1.5. In a mercury-free lamp, such as that described in copending United States patent applications entitled MERCURY-FREE METAL HALIDE ARC LAMP and CHEMICAL COMPOSITION FOR MERCURY-FREE METAL HALIDE LAMP, by P. B. Newell et al., application serial nos. 09/413,923 and 09/413,922, the re-ignition voltage increases with increasing envelope temperature (and vapor pressure of the salts) just as the performance measures of the lamp become attractive.
It has been discovered that when the voltage ratio (crest factor) exceeds 4, the re-ignition voltage often exceeds the open circuit voltage of the ballast, and the lamp extinguishes or goes out. Operation of the lamp at increased temperature becomes unstable. Small variations in power and temperature are observed to cause large variations in re-ignition voltage and oscillatory behavior.
The present invention features a ballast having a square-wave power supply for a mercury-free, metal halide arc lamp having a fused silica or ceramic envelope of, for example, polycrystalline alumina. The square-wave power supply reduces and, in some cases, eliminates the re-ignition voltage in the mercury-free arc lamp. The supply works by reversing the voltage polarity and reestablishing the arc current in the opposite direction before the free halogen (e.g., iodine) in the envelope can quench the arc. Switching times less than one microsecond accomplish this goal. Small inductances in series with the lamp from an igniter, for example, slow the switching time. The re-ignition voltages may then reappear.