LEDs and LED Reflector Design Challenges, Part 1

Light Emitting Diode (LED) emitters now dominate emergency vehicle lighting, including takedown lights and spotlights. There are many advantages of LEDs. For the power used, they are much brighter than strobe or halogen lights. In these days of high electrical loads, current consumption is far less with LEDs than with conventional lightbars. LEDs are also more reliable, and have a longer service life than strobe or halogen lights.

As LEDs can be programmed into an infinite number of flash patterns, they do not require energy pulling flashers and electric motors to spin the components around as with conventional lightbars. LEDs don’t require a high-voltage power supply.

Being smaller, lighter and thinner, LEDs allow lightbar designs that have less aerodynamic drag, which translates into faster acceleration and less fuel used. Lower electrical demands mean longer alternator and battery life.

The Light Emitting Diode (LED) is a solid-state device derived from the transistor family, also termed a semi-conductor. There is no bulb and they can be manufactured into different colors, thus no color filtering is required.

LED lights are extremely energy-efficient, drawing about 1/5 of the energy of incandescent bulbs. A halogen lightbar will draw up to 75 Amps with everything operating. A strobe-equipped lightbar, with halogen take-down lights operating draws about 30 Amps. In comparison, an LED lightbar only draws about 12 Amps. The low current draw from the LED lights negates the need for heavy-duty switches, relays, and thick cables and prolongs alternator and battery life.

Another safety feature is LEDs "light up" to full brilliance much more quickly than conventional lighting. An incandescent bulb takes about 250 milliseconds to light up, while the LED counterpart delivers full brilliance in only 1.8 milliseconds. This "suddenness" of full illumination adds to the flashing effectiveness.

Being solid-state and having no bulbs or filaments to break, LEDs survive shock, vibration, and other adverse operating conditions far better than incandescent or strobe lights.

Useful life expectancy is far superior to incandescent and strobes; estimated at a 100,000-hour life span compared to 1,000 hours for the typical halogen bulb. Thus, maintenance costs from normal operation are significantly reduced. Currently, most LED lightbars are of modular construction, so if a problem occurs, just replace the defective module and put back in service.

Lightbar Design Challenges

LED light output is highly directional. That means LEDs require sophisticated reflectors, placement angles, and lenses to satisfactorily disperse the light. While the LEDs are supplied by various manufacturers, the housings, reflectors, light diffusers, collimators, and lenses are carefully designed, engineered, and assembled by the lightbar manufacturers. In many cases, these components are proprietary to that

particular manufacturer.

The way LED emitters produce light is drastically different than strobe or halogen lights. Strobe and halogen bulbs produce omni-directional (scattered) light. LEDs produce a direct (focused) light. The design challenge with strobe and halogen lights is to focus that scattered light. The design challenge with LED emitters is to scatter or reflect that focused light beam. So, it is not the LED emitters that make one lightbar different from another. Instead, it is the design of the LED reflector in the light module that makes the various lightbars different.

For the best off-axis warning signal, some lightbar manufacturers angle individual LED modules in that part of the lightbar toward the intersection. In some cases, the entire lightbar has a "V" shaped construction to improve the off-axis signal. Even with angled LED modules and non-linear lightbars, the off-axis warning that comes from the individual LED reflector is still the key to lightbar design.

Design Solutions

This poses some special problems considering the somewhat limited viewing angles at the ends of linear lightbars, and the fact that even clear end-caps tend to distort light-rays. Some manufacturers installed more powerful LED modules at 45-degree angles to improve visibility when the vehicle enters intersections. Others have gone to "U" or "V"-shaped lightbars to ensure 360 degrees of coverage.

The reflector design is a critical component of LED lighting. Reflectors are shiny—usually aluminum or polished stainless steel. Sometimes trick manufacturing processes are used to transform the metal into aluminum particles attached to the housing to reflect the light into slightly different directions for that extra bit of dispersion. Some manufacturers utilize multiple rows of LEDs offset at slightly different angles to obtain a broader range of visibility.

One light-dispersing method is a collimator, a light-modification device that has been around for centuries. The collimator is a clear plastic or glass tube mounted between the LED and outer lens, which can absorb light and then redirect it, ultimately spreading the beam into a wider pattern. Its function is similar to that of a neon light that bends the light into the form of the tube.

Another optical device that successfully adapted the LED to be viable for emergency lighting is the Fresnel lens. Far from being a new invention, the Fresnel lens has been used in lighthouses and other optical devices since 1823. Essentially, the Fresnel lens consists of a series of prisms meticulously ground into a clear surface—glass or plastic—that modifies the light beams, allowing them to be visible over farther distances.

The directional lighting characteristics of LEDs mean that spreading the light out is the major design challenge. In Part Two, we will cover how the major lightbar manufacturers are meeting that challenge.

Published in Law and Order, Nov 2014

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