Taming HMI Flicker: A High-Speed Headache

Dealing with flicker from HMIs on high-speed shots nearly killed a recent project, but we mitigated it by carefully adjusting shutter angles. We were shooting b-roll for a historical piece, capturing water droplets in slow motion with an AMIRA at 200fps, lit by an Arri M18 HMI through diffusion. Initially, we had severe banding due to the fixture's internal ballast frequency not syncing with the camera's frame rate. My immediate thought was to adjust the camera's FPS to find a flicker-free zone, but the client was adamant about 200fps for the desired slow-motion effect.

What worked was adjusting the shutter angle on the AMIRA. Instead of the typical 180 degrees, we found a clean image at 172.8 degrees. This small, precise adjustment to the shutter cycle effectively 'hid' the HMI's micro-flicker within the exposure window, eliminating the banding. Earlier, trying to use an old K5600 Joker 800 HMI with a square-wave ballast just made the flicker worse; its older tech couldn't keep up with the demands of such high frame rates, even at lower shutter angles. The key was the modern, high-frequency ballast in the M18, which gave us a fighting chance to find that sweet spot.

Does anyone have a workflow for quickly calculating flicker-free shutter angles and frame rates on set when using various HMI technologies, especially with modern cameras that offer such wide-ranging shutter options?