Article - February 27, 2016 By LarsonElectronics.com

HMI Lighting on Set and Why it Needs to Change

Hydrargyrum medium-arc iodide (HMI) lamps are the current standard for lights in the film industry. They are very powerful luminaries that can easily illuminate dark sets, and are capable of outshining traditional fixtures, such as fluorescent lamps and tungsten lights. Unfortunately, HMIs are also staggeringly expensive and fragile, making them a liability during filming.

Below covers an overview of HMI lighting and its role on movie sets.

HMI Lamps 101

HMI fixtures operate on metal-halide gas discharge medium arc-length technology. When in use, the bulb generates an arc, while activating mercury vapor and metal halide components inside the light. During this process, the unit relies on an electrical ballast to initiate the activation of compounds. The ballast also prevents flickering by regulating the arc and limiting the current coming into the lamp. HMIs are subject to warm up periods ranging between 5-10 minutes before reaching full light output. This can become an issue on movie sets, often causing delays while filming, when the crew needs to move the lamps around the location.

When it comes to the type of light that HMIs emit, the fixtures are capable of dishing out brilliant, white beams with a color temperature of 6,000K. At this rate, the luminaries can match conditions produced by natural sunlight. Moreover, the units can reach between 85-108 lumens per watt, and can project beams up to four times brighter than incandescent fixtures. HMI bulbs are dimmable down to 50 percent. However, dimming can sometimes affect the color temperature rating of the lamp, causing it to increase and turn blue.

During the lifecycle of HMIs, the bulbs take on different characteristics. A new bulb can reach color temperature ratings up to 15,000K during the first few hours of use. Over time, it will normalize to roughly 5,600K-6,000K. The color temperature rating of the lights decrease at a steady rate of one Kelvin per hour burnt. HMIs should never be used past their intended lifespan (some switch out the lamps after half of the luminary’s rated lifecycle), because they are known to explode violently when stressed.

Pitfalls of HMI

HMIs come with numerous disadvantages for film production crews. First, the lamps are extremely fragile. Accidentally dropping the fixture could result in an explosion, which could spread shards of hot quartz glass around the set. Direct contact with the bulb should also be avoided, as skin oil could create weak points on the surface. Some operators have reported cracking on the front lens of the light from exposure to rapidly escalating temperatures. When striking hot, the units can reach peak voltages around 70,000 volts. The first five minutes of striking is the most dangerous part of operation, because during this stage the lamp is highly prone to exploding. It is also important to consider that the units generate UV light during operation. Unwanted exposure without a UV safety glass at close ranges may result in retinal damage and skin burns.

As mentioned earlier, HMIs are very expensive. As a result, only production houses with large budgets can afford to purchase the lamps. Other groups with limited budgets are either forced to rent the lights, or utilize other types of luminaries on the set. The problem is, the light that HMIs produce is incomparable with other lighting technologies, including LEDs. With the film industry moving towards fast-paced recording, via smaller cameras, robust recording software, tiny audio devices, and mobile power distribution systems on remote locations, a new lighting standard must also be adopted to allow the sector to fully move forward- from a technological perspective. HMIs utilize outdate technology to produce light; and working around the inconsistencies and pitfalls of the luminary is holding back the industry, typically causing delays during production and maintenance.

HMI Alternatives

Tungsten fixtures may not seem like a viable alternative for HMIs, but they can be tweaked to produce similar lighting conditions on the set. The units are known for producing an orange hue with a color temperature rating of 3,200K. Like HMIs, they require a ballast during operation, feature dimming controls and get very hot during long periods of use. To stimulate daylight conditions, some operators install a blue gel or lens on the fixture. Tungsten lamps are cheaper than HMIs, but individuals will need to invest in other accessories to mimic the effects of HMIs on the set.

LEDs can closely match the color temperature ratings of HMIs, at 6,000K. The luminaries offer a sturdy, solid-state design and do not generate burning hot temperatures during use (they also don’t explode). Furthermore, they are dimmable, and are capable of producing other colors using RGB diodes. Currently, LEDs are popular around small production sets and filming projects with tight budgets. Because the lights offer directional lighting, operators must use different light manipulation methods to get the most out of the units. For now, LEDs still can’t match light output levels produced by HMIs. However, out of all the lighting technologies available today, LEDs are the most promising and may one day replace HMIs on movie sets.