Dimmer rise time is the time required in each half cycle for the power devices in a phase-control dimmer to turn on (or turn off in a Dimmer, Reverse Phase-Control--in which case it's called fall time). Thyristor dimmers use a filter choke to create rise time. Higher rise times mean lower lamp filament noise, but at the cost of lower efficiency as larger chokes have higher voltage drop. Rise time should not be confused with Dimmer Response Time. Typical rise times for professional phase control dimmers are 350 microseconds, 500 microseconds; or 800 microsecond--for very demanding low-noise applications.