Electromagnetic ballast for HMI lamps operating at 50/60 Hz, producing 100/120 Hz light flicker; requires precise shutter angles to minimize visible strobing.
Technical Details
Magnetic ballasts operate at mains frequencies of 50 Hz (Europe) or 60 Hz (USA), producing 100 Hz or 120 Hz light flicker, respectively. Typical power classes range from 200W to 18,000W, with 1.2kW, 2.5kW, and 6kW representing standard configurations. Efficiency is between 85-90%, with power loss dissipated as heat. Professional film ballasts (Electronic Ballasts) from manufacturers like ARRI or K5600 feature flicker-free modes with frequencies starting at 300 Hz and enable hot-strike ignitions without cool-down periods.
History & Development
Magnetic ballasts emerged in the 1930s, coinciding with the development of discharge lamp technology. The first film production using HMI lighting was "The Thomas Crown Affair" in 1968, exclusively using magnetic 50Hz ballasts at the time. In 1985, ARRI introduced the first high-frequency electronic ballasts (Electronic Ballasts), which eliminated problematic flicker. Modern ARRI True Blue or K5600 Joker systems now operate at a switching frequency of 1000 Hz.
Practical Application in Film
In "Blade Runner 2049" (2017), cinematographer Roger Deakins deliberately utilized 50Hz HMI flicker for dystopian neon atmospheres, while high-frequency ballasts were employed for natural daylight. Magnetic ballasts require precise 172.8° shutter angles for flicker avoidance when shooting at 24fps, whereas the standard 180° shutter functions at 25fps. The sound level of 40+ dB necessitates separate audio recording for dialogue scenes, which is why electronic ballasts with a noise level of <25 dB have become prevalent.
Comparison & Alternatives
Electronic ballasts (Electronic/Digital Ballasts) offer stepless dimming from 10-100%, hot-strike ignition, and flicker-free operation, but cost 3-5 times more than magnetic variants. LED systems like ARRI SkyPanel eliminate the need for ballasts entirely with integrated power supplies. Magnetic ballasts remain relevant for budget productions and rental houses due to their robustness and lower acquisition costs of €800-€2000 (vs. €3000-€8000 for electronic).