High-intensity discharge lamp with mercury arc — 5600K, matches daylight, extremely efficient. Standard for exteriors and large interior coverage without heat buildup.
The HMI lamp has dominated every set since the 1970s where true daylight needs to be simulated or massively amplified. The gas discharge between two electrodes in a quartz glass bulb produces a color temperature of exactly 5600K – this corresponds to midday sunlight and can be directly combined with daylight film stock or the appropriate WB setting. No color correction is needed if it's right.
What makes the HMI so valuable on set: efficiency and brightness without heat input like with tungsten. An 18K HMI delivers the same light intensity as a 10K Fresnel but consumes significantly less power and generates only a fraction of the heat. This is crucial for interior shots with actors – they don't sit in a sauna. Additionally: HMIs only ignite with a stabilized power supply and require a so-called ballast unit to regulate the voltage. That's the black or silver box next to the fixture, don't forget.
In practice, HMIs are used for large-scale exterior scenes – when sunlight is insufficient or falls undesirably. A 12K with a softbox fills an entire courtyard with diffuse light. Also for window backlighting or dramatic side lighting in studios: the controllability and neutral color temperature allow for color matching with real or artificial daylight. Flicker can be a problem with fast shutter speeds, which is why work is often done synchronized at 50Hz or 60Hz – some ballasts offer high-frequency operation to eliminate flicker.
Maintenance: HMI lamps last significantly shorter than regular light bulbs (200–2000 operating hours depending on type and age), and when changing them, you must never touch them with bare hands – skin oil destroys the quartz surface. Always use cotton gloves. Extinguished HMIs are hot and need time to cool down. And: Never switch them off under load, as this massively reduces their lifespan.
Alternative light sources like LEDs are increasingly replacing HMIs in modern productions – less weight, no flicker problems, better controllability. But the HMI remains the workhorse for raw power lighting outdoors and for those scenes where an incredible amount of light is simply needed.