Last Updated 4 days ago
A ballast is the equipment that controls the electrical operating conditions of a discharge lamp. In film lighting, the word commonly refers to the separate unit used with an HMI lamphead. It helps the lighting system start and sustain the arc while regulating the lamp's operation. Fluorescent lamps also use ballasts, although their equipment and requirements differ.
The ballast is part of a matched lighting system, not a universal power adapter. Lamp type, wattage, lamphead, head cable and ballast compatibility all matter. A connector that appears to fit is not sufficient evidence that two pieces of equipment can be used together.
Why an HMI Needs a Ballast
An HMI produces light through an electrical discharge rather than a glowing tungsten filament. The lamp needs controlled operating current. Ignition also requires specialized circuitry, which may be located in the lamphead rather than entirely inside the ballast. Consequently, describing a ballast simply as a box that increases voltage misses its ongoing role after the lamp strikes.
A complete rental package normally includes the lamphead, an appropriate ballast and the specified head-to-ballast cable. Before a shoot, the electrical department checks the package as a system. Missing cables, incompatible connectors or an unsuitable ballast can make an otherwise functional lamphead unusable.
Advertisement
Magnetic and Electronic Ballasts
Older magnetic ballasts operate differently from modern electronic models. Their relationship to the mains supply makes camera timing an important consideration. Electronic ballasts can drive the lamp with controlled waveforms and offer operating modes intended to reduce visible flicker, audible noise or problems during high-speed photography.
Those modes involve trade-offs. ARRI's EB 1200/1800 High Speed manual, for example, distinguishes a low-noise setting from flicker-free and high-speed settings. Its low-noise mode carries camera-speed and shutter restrictions. A quieter lamp is therefore not automatically the right choice for a slow-motion shot.
A label such as “flicker free” should be read within the manufacturer's operating conditions. Camera frame rate, shutter duration, sensor readout, lamp condition and selected mode can affect the result. Test the actual combination rather than assuming one setting covers every camera speed.
Brightness, Color and Sound
Some electronic ballasts provide a limited dimming range. That range belongs to the particular lamp and ballast combination; an HMI is not normally dimmed to black like a tungsten practical. Observe the specified warm-up and dimming procedures. For a lower level, the crew may instead use suitable scrims, nets, bounce or a different lighting arrangement.
Dimming is not the only reason to change a ballast setting. A lamp or ballast may produce audible noise that matters during a quiet dialogue take. The gaffer and sound department need to coordinate any adjustment, because solving a sound problem can introduce an image problem.
Do not place an ordinary household dimmer ahead of a ballast unless the equipment manufacturer specifically permits that arrangement. A ballast's own intensity control and the AC supply feeding it serve different purposes.
Example on Set
A crew prepares an HMI for a daylight interior and later plans a high-frame-rate insert using the same light. The first setup looking clean does not clear the second automatically. The electrician confirms the lamphead and ballast pairing, selects an appropriate documented mode and records a camera test at the intended frame rate and shutter setting.
If flicker appears, the crew investigates the lighting system and camera settings before shooting the insert. Changing the shutter may alter exposure and motion rendering, so it should be a deliberate cinematography decision rather than an unnoticed technical workaround.
Handling and Safety
Ballasts and lampheads can involve hazardous voltages, heat and stored electrical energy. Keep ventilation clear, protect equipment according to its environmental rating and follow the manufacturer's shutdown and isolation procedures before changing connections. Qualified personnel should handle repairs and electrical faults. Repeatedly attempting to strike a failing lamp is not a substitute for diagnosis.
Record the ballast model and operating mode with the lighting notes when they are important to a setup. That small detail can save time when the scene returns several days later.
Advertisement