HID Scene Lighting

November 17th, 2007

For most of the years since Thomas Edison successfully patented the carbon-filament light bulb in 1889, lighting technology has changed very little. Of course, there have been improvements of the art, but at the heart, most lighting technology has relied on one key: an electrical current passing through a thin filament creating the light. This has been particularly true for automotive lighting such as those found on emergency vehicles. With the exception of strobe lighting, the state-of-the-art in vehicle lighting for most of those years relied on the same basic concept demonstrated by Edison’s patent.

Not anymore.

The last two decades have seen an explosion of new technologies that have changed the very basics as to how light is generated.

The best example is the explosion of LED lighting used in vehicles over the past few years. However, there is another example as well, and it is still at the beginning of its march into the marketplace. It’s called High Intensity Discharge lighting, or HID for short.
You can recognize HID lighting by two characteristics. First, each HID light will have a ballast used to step up the voltage on the light in order to create the arc across the gas. Second, when first turned on, the light will take about 15-20 seconds to come up to full intensity.

HID lighting differs from Edison’s incandescent lighting because it replaces the filament with a capsule of gas.

When sufficient voltage is applied across the gas, the resulting current allows free electrons to collide with the atoms inside the gas. The result of the collision is that some of the orbital electrons in the gas atoms are pushed into higher orbits around the nucleus. When the electrons return to their normal state, they release energy in the form of a photon. Photons are the basis for all electromagnetic radiation, including visible light.

To be sure, the concept of HID lighting is not new by itself. The concept has been around for years, finding its best known commercial application in area lighting (such as in stadiums or large warehouses). You may know these lights by another name, mercury vapor lamps. Everyone has seen those, and they can be readily identified by the amount of time it takes for the lights to come up to full power after they are turned on.
Recently, however, new HID bulbs have been developed that rely on different combinations of metallic halide gases within the bulbs. One example relatively new to widespread commercial use is the xenon headlamp, which is also a form of HID technology. However, for the emergency worker, there is another application more near and dear to the heart: scene lighting.

Scene lighting is crucial to the success of nighttime emergency operations.

You can never have too much light. However, light fixtures are frequently large and require lots of power at the scene just to reach the minimum acceptable level of area lighting. The result is large, often noisy generators just for the lights, and that frequently translates into more vehicles and more confusion at the scene.

That’s because until recently, vehicle mounted scene lighting relied on halogen bulb technology. Halogen bulbs are a form of incandescent light that rely on a halogen gas (such as iodine or bromine) around a tungsten filament. It’s sound technology, but as anyone who has used them knows, they take up a lot of power, the light often has a yellowish tint, and sometimes the bulbs will not last as long as you might like.
This is where HID lighting is so promising. To begin with, modern HID lighting is more efficient than halogen lighting. For the same electrical power, an HID light will give off more light and less heat. For example, a 150 watt HID bulb can produce more light than a 500 watt halogen bulb. Or to look at it as a function of light output, you can get 11,250 lumens of lighting from a 150 watt HID fixture compared with 10,500 lumens from a 500 watt halogen filament fixture.

Because of the increased efficiency, HID bulbs have lower power ratings.

Because they have lower power ratings, they can be used with smaller power sources. The result? Emergency vehicles today can have the same area of light using the 12 volt system that vehicles 10 years ago had using AC generators with the added benefit of less noise on the scene. It is certainly true that smaller vehicles can now carry lighting that previously was restricted to the big units.
A second advantage of HID lighting is an improved bulb life.
Typically, HID bulbs will last 3-5 times as long as equivalent halogen bulbs.
This is because a halogen bulb is dependent on the life of the filament. In addition to being susceptible to damage from mechanical shock, the filament itself will degrade under normal operation. When the current is passed through the filament, the tungsten will actually evaporate and deposit on other places inside the bulb (such as on the inside of the glass). However, HID bulbs do not have a filament, and the entire volume of gas is used to generate light. When they fail, they do not usually fail suddenly. Instead, over time, as the gas heats up, it requires more voltage, resulting in a loss of light intensity. When the bulb cools, it appears to work again.
A third advantage of the HID light is the quality of the light produced.
The bulbs used for HID scene lighting produce an output that is much “whiter” in the spectrum.
Of course, this makes it easier on the eyes at the scene (as long as you are not looking directly at the source). Some have even gone so far as to describe the light as being closer to daylight. While this is not true in the purely technical sense, it indicates a perception on the part of the user. And after all, the ideal scene lighting is the sun itself.
HID lighting is the next step in the evolution of how light is produced. It holds the promise of cleaner light using less power. And it has the advantage of improving bulb life. For the emergency worker, as you are considering how to outfit your next vehicle, it’s certainly worth taking a second look.

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