LED vs HMI ARRI Lighting, with Rubidium Wu

In this video, I take a look at the differences between the HMI and LED lights offered by ARRI. Each has advantages and disadvantages, as well as specific situations in which they work best.

Tungsten

While tungsten gives a beautiful light, its critical disadvantage has always been its color temperate – it just doesn't match daylight. Tungsten lamps also aren’t very efficient at converting electricity into light. They get very hot, which can be both dangerous and uncomfortable on set. In the 1960s, scientists tried to overcome these limitations by developing the Hydrargyrum medium-arc iodide lamp, or HMI for short (hydrargyrum is the old scientific name for mercury). HMI is much more efficient—four times moreso than tungsten—at turning electricity into light, so it needs less power for the equivalent output. HMIs run cooler and, most importantly, their color temperature matches daylight.

HMI

ARRI makes some awesome HMI lights, the most popular of which are their M-Series. These are PAR lights, or Parabolic Aluminized Reflectors, which means a bulb inside a semi-circular housing that directs the light forward to give a more total output. Unlike a Fresnel light that uses an adjustable magnifying lens to focus or diffuse the light, PAR lights focus and flood by moving the bulb in relation to the reflectors around them, so there is less light loss. You can go from 20° to 60° beam at the twist of a knob.

The ARRI M18 is the workhorse of the independent filmmaker. It plugs into the wall, requiring well under the typical 20 amps of current most household circuits are rated for, and generates a huge 6000 Kelvins of light: 1 million lux at 1m, though the safe work distance is 3m. This is the light to get if you need to double for the sun or light up a 10x10 scrim. You can bounce it off a 6x6 frame 20 feet from your subject and still have plenty of light to work with.

For its advantages, HMI still has some downsides. It takes time to get to full power and correct color – usually around five minutes depending on the life of the bulb. Because of the bulb's fragility when ignited, an HMI has to be turned off to be moved. This can result in long downtimes because you’ll need to turn off a light, move it, and then wait for it to power back up. Ten minutes doesn’t seem like long time, but on a film set it can be an eternity.

Despite being cooler than tungsten, larger HMIs still get pretty hot – too hot to touch in some cases. They also don’t dim as well, and if you want to adjust the level of light emitted, you’ll need to insert a scrim in front of the light, or add more diffusion. In short, HMIs need to be set up and moved by trained professionals. They aren’t a light that you can just put on a stand and start shooting with. The weight and level of power required to operate them can be dangerous if not managed properly.

LEDs

This is why over the last few years LEDs have come to the forefront. LEDs are so efficient at turning power into light that they don't get overly hot, which means they can be closer to talent and powered by house power and batteries. LEDs are also lighter, smaller and easier to get into overhead scenarios. Another bonus to LEDs is that they don’t have bulbs that can break or need to be replaced. They come up to full power immediately, and they can also be turned off and on again immediately – unlike HMIs that need time to power down before powering on again.

The most common LED fixture on film sets in the last 5 years has been the ARRI SkyPanel S60. You can practically put the SkyPanel on the talent's face and you aren’t going to burn them. The ARRI SkyPanels are RGB, which means they can be dialed to any color of the rainbow. They also have built-in effects like fire light, police car, and fireworks. And, they can be controlled remotely by ARRI's excellent app, Stellar. ARRI also makes an LED spotlight, the L7-C, which will give you a color controllable, hard-edge source. These aren’t the brightest lights, but if you need hard light in different colors, and close to talent, they are great.

Conclusion

You might be thinking, if LEDs are so good, why are HMIs still made and used? Well, they are the undisputed output kings. LEDs can’t match the brightness of HMIs, and for the foreseeable future, most DPs and Gaffers will continue to combine then. HMIs are great for recreating the sun, illuminating larger spaces, or where you need a hard, bright light source. They're also great to bounce or diffuse through large scrims. LEDs like the SkyPanel series, including the huge SkyPanel S360, give cool, color controllable, diffused light right where you want it. You can use LEDs where an HMI and diffusion aren’t practical, like rigged to a car, in a ceiling, off the grid with batteries, and any time you want a light close to talent.

So the answer is it just depends on your set's needs. I will say, if you’ve never shot with HMIs, I suggest you try them out to see their power in person – the output of these lights is breathtaking. A single M18 has more output at 10 feet than ten SkyPanel S60s, which is invaluable for daylight exteriors or anytime you are competing with the sun.