For a fluorescent lamp to be dimmed over a full range without a reduction in lamp life, its electrode heater voltages must be maintained while the lamp arc current is reduced. As such, lamps operated in rapid start mode are the only fluorescent lamps suitable for wide-range dimming applications. The power required to keep electrode voltage constant over all dimming conditions means that dimming ballasts will be less efficient when operating lamps at dimmed levels.
Dimming ballasts are available in both magnetic and electronic versions, but there are distinct advantages to using electronic dimming ballasts. To dim lamps, magnetic dimming ballasts require control gear containing expensive high power switching devices that condition the input power delivered to the ballasts. This is economically viable only when controlling large numbers of ballasts on the same branch circuit. In addition, luminaires must be controlled in large zones that are determined by the layout of the electrical distribution system. Since the distribution system is fixed early in the design process, control systems using magnetic dimming ballasts are inflexible and are unable to accommodate changes in usage patterns.
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