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LEDs in Emergency Vehicle Lighting

November 5th, 2007

In 1907, Henry Joseph Round observed that when you ran an electrical current through silicon carbide, it emitted a weak, yellow light. This was the first documented case of electroluminescence by which a small electric current passing through a material was converted directly into visible light. Building upon the foundation Round discovered, in the 1960’s, scientists created the first modern light emitting diodes (LEDs). Initially, LEDs were restricted to indicators for the electronics and display industries. However, by the 1990s, the technology had advanced sufficiently that the light output started to equal more traditional lighting sources. In the past 5 years, the use of LEDs on fire trucks has exploded. They have become extremely common as truck clearance lights, turn signal lights, brake lights, warning lights, and even backup or compartment lights. In fact, its rare to find a truck that doesn’t have at least a few LEDs somewhere on the perimeter of the vehicle.

The reason for this explosion has to do with how an LED works. Comparing LEDs to more traditional light sources is like comparing a jet engine to an internal combustion engine. They both burn fuel to create motion, but they approach the task using very different technologies. More traditional lights, such as incandescent lights, rely upon electrical current passing through a generally non-conductive medium (such as a filament or a gas). As the current excites the medium, light is emitted. However, because the medium is not good at conducting electricity, one of two things must also occur (and sometime both). First, there is a lot of wasted energy in the form of heat. Second, the input voltage may have to be bumped up to create an arc (if the light medium is a gas).

LEDs, however, approach the same task on a much more targeted level. As the name suggests, LEDs are diodes, and are designed to allow the unimpeded flow of electricity in one direction. However, as compared to traditional diodes, the base material in the diode has been d”oped” with electroluminescent materials. As Henry Round discovered, the result is a device that “glow” when electricity is applied. Therefore, the voltages may remain very low, and there is very little heat generated.

One could almost think of LEDs as highly efficient light generators. The light is converted directly from electricity at a very high efficiency. As a result, the designer can use the light without having to worry about the heat. In addition, since the LEDs are so much more efficient than more traditional light sources, much less electrical current is required for the same amount of light output. Therefore, on a fire truck, when a traditional light fixture may draw 3 or 4 amps in a given location, an equivalent LED fixture may only draw 2 or 3 tenths of an amp.

However, the difference between LEDs and traditional light sources involves more than just the amount of electricity used. The very nature of the light itself is different. Light from most light sources (such as halogen or strobes) covers a broad range of wavelengths. Just like in a rainbow, each color that the eye can see comes to us in a specific range of wavelengths. When you combine all these ranges together, you have the white light with which we are all familiar. Most traditional light sources generate light across a fairly broad portion of the spectrum. Therefore, if you need to create a red light, for example, using a halogen light, you simply place a red lens in front of the bulb. The lens filters out all but the red color, so that’s all you can see.

LEDs however, emit light in a much smaller range, essentially one color only. Therefore, if you want a red light, you choose LEDs created with a material that emits red light. If you want a blue light, you need a different LED created from a different material. The result is a light that is much more targeted. Lenses will not work to change the color of the light emitted from an LED. In fact, if you use a colored lens with a different colored LED, the result would be the same as putting a cover over the light. However, unlike other lights, the LED light appears much more intense. Because it consists of such a small range of wavelengths, it is much less likely to be washed out by other sources such as sunlight. The result is a light that can be seen more clearly at a greater distance.

In addition, the light itself is more directional from an LED. For halogen and incandescent bulbs, light is emitted in every direction out from the axis along which the electrical current flows. The result is that reflectors may be used to redirect light from behind the bulb in whatever direction the designer wishes it to go. The result is a light that is fairly uniform regardless of from which angle it is observed. LEDs, however, are essentially tiny flat panels in a plastic housing. The flat panel is actually the layer of material that emits the light. The result is that the light travels out directly from the panel in a fairly straight line. When a single LED is viewed from an angle, the intensity of the light quickly fades. Therefore, traditional reflectors won’t work, and the designer has to orientate the LEDs themselves to cover the full range over which the light is to be seen.

There is one more major difference between LEDs and other light sources. In incandescent or strobe bulbs, any damage to the conducting medium will cause the light to fail. Even without specific damage, the higher currents combined with the higher heat will eventually cause the filament or gas to break down. By contrast, an LED is simply two materials bonded to each other with no mechanical connections. The ability to product light is inherent in the base material. Therefore, LEDs don’t “burn-out”, and they have a much longer life span than traditional bulbs.

To be sure, designers of LED lights have to account for many things that dont come up with other light sources. Its only been within the last 10 years that technology allowed the brightness of LED lights to equal other sources. In fact, LED light fixtures arent a single light source, but rather a colony of individual LEDs. Also, since reflectors work differently inside LED fixtures, the actual LEDs must be positioned so that they are visible across the desired arc-of-visibility. And the limited color output means that frequently designers mix-and-match different colored LEDs to get the desired color effect. White LEDs, for example, are actually often a mixture of bright blue with a small smattering of other colors such as amber. However, despite these challenges, the low electrical use makes LEDs a popular alternative in many applications. With so many other things competing for electricity on a fire truck, one can see why LEDs are only going to get more popular.

1 A Brief History of the Light Emitting Diode, Wavicle LED Lighting Technology, http://www.wavicle.biz/led_history.html, 2006

This entry was posted on Monday, November 5th, 2007 at 12:00 am and is filed under commentary articles. There are One Response to “LEDs in Emergency Vehicle Lighting” :
Donald Singley Says:

With the development of high efficiency and high power LEDs it has become possible to incorporate LEDs in lighting and illumination. Replacement light bulbs have been made as well as dedicated fixtures and LED lamps. LEDs are used as street lights and in other architectural lighting where color changing is used. The mechanical robustness and long lifetime is used in automotive lighting on cars, motorcycles and on bicycle lights.

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