Electronic ballast for HMI lamps that generates ignition voltage up to 65,000 volts and operates at 20–50 kHz high frequency to eliminate flicker.
Technical Details
Magnetic ballasts operate at 50/60 Hz mains frequency and produce a 100/120 Hz flicker, which leads to banding with cameras using electronic shutters. Electronic ballasts (EVG) operate lamps with high frequencies between 20-50 kHz and eliminate flicker almost completely. Modern flicker-free ballasts achieve frequencies of 1,000 Hz and allow shooting with shutter speeds up to 1/2000 second without visible artifacts. The ignition voltage for HMI lamps is 20,000-65,000 volts, after which the ballast regulates down to an operating voltage of 90-575 volts.
History & Development
OSRAM developed the first film-compatible HMI ballasts in 1969 together with the German company Geyer. Arri launched the first series-production flicker-free ballasts for 2.5/4 kW HMI lights in 1975. Electronic Theatre Controls (ETC) introduced digital ballasts with DMX control in 1991. From 2005 onwards, switch-mode power supplies became established, reducing weight by up to 60% and increasing efficiency to over 95%.
Practical Use in Film
For "Blade Runner 2049" (2017), DoP Roger Deakins exclusively used flicker-free ballasts to achieve extreme slow-motion sequences without banding. In "The Revenant" (2015), portable 1.2 kW Arri Electronic Ballasts enabled lighting in remote Canadian forests without generators. Standard workflows use 2.5/4 kW ballasts for daylight simulation and 6/12 kW units for large area lights. The disadvantage: ballasts generate their own noise of 35-45 dB, which can be disruptive during sound recording.
Comparison & Alternatives
In contrast to transformers for tungsten lamps, ballasts work with complex electronics for arc stabilization. LED lights require drivers instead of ballasts, consuming 80% less power for the same light output. Plasma lights like the Hive Plasma use 2.4 GHz microwave technology and require specialized high-frequency generators. Modern remote phosphor LED systems (Creamsource, Litepanels) are increasingly replacing HMI setups under 4 kW.