If you wanted to make an analogy for the history of automotive lighting, you could do worse than to bring up half a trillion years of evolution. As in biology, only a handful of variations existed until very recently and, just like Darwin theorized, time is weeding out the useful advances from the useless. The newest trend in illumination is the use of light-emitting diodes (LEDs) in headlamps, which have the potential to make cars safer and more stylish.
Before grasping what LED headlamps are, it's a good to understand what they're not: namely, bulbs. Both plain-vanilla incandescent lights and blue-cast Xenons use bulbs, and it's precisely that which limits their flexibility to designers. In both cases, the light in each headlamp is generated by a single source, be it a heated tungsten filament in a traditional light bulb or an electric arc in the Xenon capsule. The bulbs' output is bounced onto a single reflector in the back of the headlight, which is contoured and shaped so that it projects usefully onto the road.
LEDs, by contrast, don't have a filament that can burn out and they don't make light through an electric arc. They create light through the movement of electrons across a small "chip" of semiconductor materials - most are only 1-mm square - and varying the materials used produces different colors of light.
With LEDs' small size and high outputs - six can produce the same amount of light as the average car's low beam - headlamps can be packaged in nearly any shape, with each individual LED having a tiny reflector to project its light onto the road. It's this inherent flexibility, the ability to have a designer's sketch translated directly into metal and glass, which has made LED lighting so prevalent in concept cars. Mazda's Ryuga concept, rotating on a turntable at the 2007 Detroit Auto Show, had three narrow, serpentine strakes of LED headlamps transitioning upwards across its fenders, mimicking the equally narrow oblong of its greenhouse. To look at the front of Mercedes-Benz's Ocean Drive concept, with its 144 individual LED lights, is to stare into the sun.
LEDs also have the potential to save energy. A recent study by the Department of Energy estimated that 1.2 billion gallons of gasoline would be saved per year if every bulb on the road were switched to LEDs. There are a lot of hopes pinned on the technology, which is why it's disappointing that the first production LED headlamps to be installed on road cars - you'll pardon our bluntness - suck. "[The] first LED headlamps will be far away from being competitive," says Wolfgang Kuhn, a lighting engineer for Audi. "They will be very expensive, heavy-weighted, and large. And they will have only average light quality."
Average light quality, as it turns out, can be blamed on the white LEDs used for headlamps. There's no known combination of semiconductor materials that produces pure white light, so the best examples actually produce blue. Coating the blue LED is what's known as a phosphor, which has the unique property of glowing after exposure to light. The phosphor coating absorbs most of the blue light, glowing in the process and emitting a large amount of yellow light, along with a smattering of green and red. When combined with what little blue wasn't absorbed, the resulting light is a shade close to what we call daylight. It's an inefficient process, and losses along the way mean that only the most powerful LED emitters available can generate enough usable light to work as headlamps. Those powerful emitters draw a fair bit of power - up to 3 watts each - which bring us to the next obstacle on LEDs' road to a mainstream debut: heat.
For years, LEDs' use in small, modern electronics have given them the reputation of being, quite literally, cool. When they're one sixth the brightness of a car's headlight, LEDs start producing a significant amount of heat, all of which is produced on the 1 mm-by-1 mm surface of LEDs' semiconductor chips. Heat kills LEDs, and those that don't burn out reduce their light output by over one third as they reach temperatures in excess of 250 degrees F. "Heat dissipation has to be very efficient," says Kuhn. "Electrical fans are necessary like in nearly every other electronic device."
LEDs don't fare much better in the cold. At 10 degrees F, many LEDs produce more than 1.5 times their rated light output, so the headlamps blind oncoming drivers until they're warmed up. Since LEDs produce what is known as "cold light,” with no output in the infrared spectrum, there is no radiant heat to melt snow or ice off the headlamps while driving. Exhaust from the cooling fans at the rear of the headlamp must be piped into its front, in order to keep the outer lens clear during a snowstorm.
Like any technology in its infancy, LED lighting will inevitably be refined and improved. If the development timeframe for a new car model is taken into account, overcoming LEDs' current obstacles will determine whether LED headlights are the future of automotive design, or simply an evolutionary dead-end.
