Monday, October 8, 2007
DPP Solutions: HMI
In the realm of continuous lighting, these lamps are king, and for good reason
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.
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