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The King of Daylight Brightness — An Introduction to HMIs

You’ll find HMIs doing the heavy lifting on almost every big Hollywood film set — lighting daytime exteriors, diffused windows, and turning night into day.

In the past decade, LEDs have become the technology of choice when it comes to lighting. As far as film and photography lighting go, they’re cheap, they’re light, they come in multiple colors, and most are cool enough to use close to talent without causing them to break a sweat.

LEDs have some key drawbacks, though. For all their brightness, they still can’t come close to matching the sun. This is important when you’re shooting outside and need to match existing light levels, or when you want to match the output in other scenarios, like using a window to illuminate a large room.

Tungsten lights are high output, but they don’t match the 5600 kelvin of sunlight, and any blue gels you place in front of the lamp further reduce the light output.

Enter the HMI — the undisputed king of daylight brightness.

The King of Daylight Brightness — An Introduction to HMIs - ARRI M18

The M18, a standard HMI in indie filmmaking.

The Aputure 300D M-II, an LED spotlight fixture, meters at 80,000 lux from three feet at full spot. That may seem like a lot of light, but the ARRI M18, the indie film standard HMI, is almost 1,000,000 lux zoomed in, at the same distance.

The extra power is indispensable when you need to diffuse light through a large fabric of skin, which are standard issue when lighting large sets. This is because cinematographers often need to light large areas with soft lights so actors are free to move around without radically changing their f-stop.

Most HMIs are typically named after their wattage, so an M18 is an 1800-watt light, just as a K5600 Joker 800 is an 800-watt light. That isn’t to say that every HMI with equivalent wattage shares the same output. Different brands use different technology to get more out of the same amount of power.

The King of Daylight Brightness — An Introduction to HMIs - Lighting a Scene

Diffusing an HMI.

If HMI’s are so much brighter than LEDs, why aren’t they used more? For their amazing output, HMIs have a few drawbacks that limit their appeal.

They use a lot of power to achieve their output. The M18 is the largest HMI that you can plug into a standard power socket — without blowing the breaker — and, as a result, it’s one of the more common lights on indie film sets, where the expense of renting and fueling a generator isn’t always feasible.

The King of Daylight Brightness — An Introduction to HMIs - Lighting the Scene

The brittle nature of an HMI bulb means you can’t move the light while it’s on.

They get hot, which makes them hard to handle. There’re also fragile and take around five minutes to get up to their correct temperature. Because the bulb is brittle, you can’t move the light while it’s on, so you have to turn the HMI off, wait five minutes for it to cool down, move it, then turn it back on and wait five minutes for correct brightness and temperature. This delay, every time you move a light, can kill the momentum on a film set, favoring a DP or gaffer who really knows what they’re doing.

The lights are also expensive. The M18 is $5000 for the lamp head, $7000 for the ballast, and $500 for the bulb, and you’ll need something like the Matthews Combo Stand (another $500) to mount it on. Because HMIs are such long-lasting fixtures, rental houses in big markets — such as L.A. and New York — are able to rent them for $250 a day or $500 a week, so they’re by no means out of reach.

Probably the biggest barrier for indie filmmakers is that HMIs require a qualified gaffer. They aren’t something you can rent and just mess around with, like you can with LEDs. They’re heavy, hot, high-power, and much more dangerous than most people are comfortable with.

For all their downsides, HMIs continue to dominate the industry. If you want to chose when and where the sun will shine, HMIs are the way to go.

Cover image via ARRI.

Looking for more on film and video lighting? Check out these articles.

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