DPP Solutions: HMI

Even though strobes are favored by many pros, there’s definitely a core group who prefers continuous lights and, in particular, HMIs. They have a phenomenal watt-to-luminance ratio, produce comparatively little heat, are flicker-free, and emit a quality of light that’s something to behold. The highlights and shadows they create and the way they bring out the detail in a model’s hair or the textures in clothing is unique. Everything seems to sparkle more and have more depth.

Yes, HMIs are expensive (really, really expensive). That’s why a majority of pros who use them simply rent as needed, and even that can run over $1,000 a day if you want a three-light setup with a 2.5K ARRI Fresnel and two 4Ks. But it’s money well spent (especially if you get to pass it along to the client).

What’s So Special About HMI?
At 5600 Kelvin, the bulbs are daylight-balanced, which is a big advantage if you’re shooting outside and want to match the ambient light or if you’re shooting indoors and want to replicate sunlight. You won’t run into white-balance issues as you would when using tungsten lights with a noticeably warmer color temperature of 3200K to 3400K, which can wreak havoc with skin tones.

Because of their origins in the film industry, HMIs are flicker-free, so you can use a greater range of aperture and shutter speeds and experiment with a little motion blur if you prefer. Flicker is caused by the alternating current powering a lamp, so if this wasn’t corrected by the HMI’s ballast, you’d only get half your frame at certain shutter speeds.

HMI bulbs are very efficient as well, producing four to five times the output per watt compared to standard filament bulbs, so a lot less power is needed and a lot less heat is generated. With a 2000-watt HMI, you’d need an 8000- to 10,000-watt bulb in quartz or tungsten to have the same output.

What makes this possible is how HMI light is actually made. As in arc welding, an electrical current is created between two electrodes, which gives the light some unique characteristics besides output-per-watt and color temperature. There’s a crispness of the arc source, which photographers often desire. It gives a sharp shadow because the light is being created from a very small point source.

John Gresch, vice president of ARRI’s lighting division, says, “In a filament source like tungsten—and let’s say it’s a large 5000-watt bulb—the size of that filament, which heats up and glows, is a much larger area compared to the HMI’s arc source. So if you were to measure it, instead of being this half-inch gap, you’d have a filament that’s 1½ or 2 inches. When this larger ‘blob’ of light hits the reflector, some of the light rays are going to hit the intended optical path and go where you want them to—properly out the front of the reflector. But some of the light isn’t, and you’ll have secondary rays of light going where you don’t want them to, hence your shadows aren’t as crisp.”

And this is really why HMI lamps are in a league of their own. It makes them special, and expensive, compared to other types of bulbs.

Fresnel & PAR
While HMI lights come in many different optical systems that range from ellipsoidal, follow spot, soft light, open face and floodlight, the clear favorites among connoisseurs of continuous lighting in the photographic world are Fresnel and PAR (parabolic aluminized reflector).

A Fresnel consists of a metal housing, a reflector, a lamp assembly and a glass Fresnel lens—named after the 19th-century French physicist Augustin-Jean Fresnel, who first developed them for use in lighthouses. By dividing a lens surface into a series of concentric rings, you can make a large aperture lens that’s thinner and lighter than a conventional lens and thus allows more light to pass through.

“The advantage of the Fresnel lens, with its circular glass rings,” adds Gresch, “is that you’re able to take the point source of the light, and with the use of a spherical reflector instead of a parabolic reflector, put a majority of that light to that lens. So you get a very smooth light, a very even distribution of those rays.”

You also have the ability to create a different relationship between the lamp and the lens by changing the distance between them. You can create either a spot or a flood focus in a matter of seconds. And this isn’t the case with some of the other optical systems.

Even though you can change lenses on a PAR HMI to achieve different levels of flood and spot light, there’s no way to quickly adjust the distance between the lamp and the lens as with the Fresnel. That’s why photographers who use HMI lighting will sometimes set their ambient or background light with a couple of PARs. This allows them to produce a big pool of light for establishing a mood. Then they can get really crisp highlights and shadows on their subject with a couple of Fresnels.

The Importance Of Establishing A Mood
Call it what you will—ambiance, atmosphere, état de rêve—it’s something you just don’t get with a strobe setup unless you’re using a combination of strobe and continuous. Even then, it’s still not the same as setting your lights to craft a sustained look.

“When people are creating their image,” says ARRI’s Gresch, “they often choose a particular lighting style because it does something for them emotionally in terms of a visual result. It creates a mood in the photographs and at the shoot itself, which can help stimulate their creativity. It’s kind of like a poet who goes to his or her favorite serene place to write. It’s different than if he or she is sitting in the subway and trying to write. And I think this is very much what we’re talking about when a photographer is crafting shots with the light—creating the way they’re actually going to be recorded. I think there’s something organic about that kind of creative process.”

Adds Gresch, “Now if you’re somewhat talented in the use of strobes, I’m not saying you can’t get your desired effect. But you’re extrapolating something that you’re not really seeing or experiencing at the moment. It’s kind of like calculating how much light you’ll need at the moment of impact in a crash test. There’s nothing really subjective about it. But if you’re creating a photographic image as an art form, you probably want to be availing light—using it to create the right mood before you snap the shutter.”

Admittedly, opinions are likely to vary on this. Beyond simply having enough light, there’s no right or wrong way to light a shoot. The important thing is to experiment periodically and try new approaches so you keep your photography fresh.

If you’ve never used HMI, give it a try. You just might find this light source provides some interesting possibilities on the other side of your camera’s lens.


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