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Tuesday, June 7, 2011

Cool Lights

HMIs, LEDs and fluorescents are energy-efficient alternatives to hot lights for digital stills and HD video

This Article Features Photo Zoom

Broncolor Mobil A2L
The beauty of HMI is that it outputs daylight-balanced light (5600K) out of the box. Equally important, HMI has a very high CRI (Color Rendering Index) exceeding 90. (CRI of 90 and above is considered a "clean" light source and a value to look for in all the cool lights discussed here. Fixture manufacturers may not always publish CRI data, so you may have to fish for it.)

On the flip side, however, HMI emits excessive amounts of UV (relative to output), which limits how close the light can be positioned to people and animals. Proper use of these lights addresses this deficit with suitable filtration, usually in the form of safety glass or safety lenses.

Profoto ProDaylight Air with ProBallast 800
CDM lamps effectively simulate tungsten illumination in their raw state and currently peak at a much lower wattage than HMI. For instance, ARRI fixtures use 250W CDM bulbs. These lamps also have a much longer life, 4,000 hours, as compared to 300 to 500 hours for HMI (per data provided by ARRI, which may differ in fixtures from other sources). With any lamp technology, it's important to note that the fixture is a key mediating factor in lamp life, owing to how well heat is dissipated.

What's the downside to using HMI? John Gresch, VP Lighting Products, ARRI Inc., points out, "One of the disadvantages of HMI is that you cannot dim it past 50% or you're going to lose the [light-producing] arc inside the bulb. The light will just stop functioning. And when you do dim it down to 50%, color temperature increases—the light gets bluer."

Bowens Streamlite 530
Westcott 2-Pack Spiderlite TD6

LED And Fluorescent

The true "cool" lights, in practically every sense of the word, are LED and fluorescent. These light sources don't project heat toward the set. And, unlike HMI, UV doesn't appear to be a factor in LED or fluorescent lighting. The problem with LED illumination, however, is that these fixtures require numerous diodes to produce a usable quantity of light on set. The downside is that multiple point sources of light potentially lead to multiple shadows. In contrast, a comparable-output HMI casts a single shadow and does so more economically.

To get around that problem of multiple shadows, manufacturers may pack the LEDs more densely or add a mixing chamber or other device to unify the multiple lights into a single beam. Other solutions are to widen each LED's beam spread so that they overlap, or the light could be diffused, steps that would diminish the reach of the light. You also have to be careful not to point a raw pinpoint LED array in someone's face, as it could be blinding.


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