Arc lamps can be categorized as direct current arcs, such as carbon arcs, and alternating current arcs, such as HMI globes. The carbon arc works on the principle of a gaseous arc that is formed between two carbon rods connected to a DC supply, such as a generator or a powerful battery.
The HMI (Hydrargyum Medium Arc-Length Iodide) globes represent the most advanced concept in modern lighting technology. HMI fixtures are a highly efficient source of daylight-balanced light of approximately 5,600ºK – 6,000 ºK. Comparing to a tungsten bulb, the HMI lamp has no filament; basically it is a mercury arc that’s created between tungsten electrodes enclosed in a glass envelope, with metal halide additives.
All HMI globes produce light that has harmful amounts of UV radiation. Skin and retinal burns can result from direct exposure to the light. For this reason, a special housing and protective lens must be used. UV rays are reduced to a safe level when the light passes through the glass lens or when the light is indirect, bouncing off the inside walls of the fixture, for example. HMIs always have a safety switch that shuts off the globe if the lens door is opened or if the lens breaks.
An HMI puts out about four times as much light as a tungsten light of the same wattage (85-108 lumens per watt of electricity, compared to 26 lumens per watt for tungsten halogen bulbs). This is partly because the incandescent bulb expends 80% of its energy creating heat (infrared wavelengths), whereas HMIs convert that same percentage of its energy into usable illumination. As a result, HMIs operate somewhat cooler than their tungsten equivalents. However, in order to match the color temperature of an HMI (which is daylight) with the tungsten lamp, we will have to add a blue filter in front of the HMI light that will reduce about 50% of the light output so in actuality the HMI lamp puts out about eight times as much light as a tungsten light.
Common film lighting HMI fixtures: Fresnels, PAR, open face, soft-lights
HMI FRESNEL
Small Fresnels are commonly used as interior direct light when daylight color balance is needed. Large HMI Fresnels are used for fill on a day exterior shots or to cover wide exterior night shots.
Another common application is making sunlight effects through window, both on location and inside a studio. They may be put through diffusion or bounced to create a large, bright soft source.
HMI PAR
PAR lights are more efficient than Fresnels, and they deliver a strong punch of light with a longer throw. They come with a case of three or four spreader lenses.
Today’s HMI PAR lights use a single-ended HMI globe. It is axially mounted in the fixture in front of a highly efficient parabolic reflector. UV protective glass covers the front of the unit, and the spreader lens slides into slots in the ears. The spreader lenses affect both the shape and angle of the beam.
HMI OPEN FACE
There are HMI open-face lights in the sense that they have only clear UV protective glass in front of the bulb and reflector, no lens. While their beam is broad and unfocused, these fixtures have the unique ability to be oriented in any direction.
They can be used pointing straight down as an overhead soft light (through heavy diffusion) or straight up for architectural lighting. They cast a hard shadow and would be a good choice if a hard silhouette were needed.
HMI SOFT LIGHT
As with incandescent soft lights, the lamp of an HMI soft light is aimed into a white concave reflector. The bounced light and large aperture create a soft, even field of daylight-balanced light.
The HMI soft light, of course, produces much greater quantities of light than an incandescent.
