| 摘要 |
In a preferred embodiment, a lighting device may include a metal halide lamp and a polarity inversion step-up circuit which generates a DC voltage Va of minus value. An inverter circuit inverts the output of the polarity inversion step-up circuit to create AC voltage and supplies it to a discharge lamp. Consequently, the discharge lamp has applied a square wave AC voltage with voltage between zero and peak being nearly |Va|. Potential of a lighting tool to enclose the discharge lamp is set to ground potential. Since the discharge lamp is lit always at a mean lighting potential which is less than the potential of the lighting tool, the Na loss phenomenon is suppressed. That is, life of the discharge lamp can be lengthened. Other embodiments also operate to keep the mean lighting potential of the discharge lamp less than the potential of the lighting tool.
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