1. An amount of current still flowing even when a dimmer is set at zero.
2. To set a large wattage lamp at a level of 02-07 or so in a cue prior to when it is needed, so as to made a smoother fade up.
3. The act of gently warming lamp filaments over time prior to a show. Whether or not this adds to lamp life is hotly debated. See http://www.controlbooth.com/forums/lighting/6431-preheating-conventionals.html.
Note that some Sine Wave dimmers use preheat as a method of reducing cold-filament inrush current. This allows smaller, lower current power devices (IGBT's or MOSFET's) to achieve acceptable response time on a bump from off to full on. However, preheat means that the system draws current even when dimmers are set to zero. Preheat can also adversely affect the timing of a snap to off from full on. It is generally thought that preheat is not a good substitute for high power devices in Sine Wave dimmers that can handle cold-filament inrush currents to deliver fast response time.
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