You have seen them. Those piercing blue-white headlights from oncoming cars seem to bore into your skull like an icy dagger, threatening to blind you.
OK, maybe that’s a slight exaggeration. But nearly 6,000 people were so bothered by the glare from “high-intensity discharge” (HID) headlamps when they first appeared about a decade ago that they complained to the National Highway Traffic Safety Administration, which commissioned research to figure out what was going on.
That started John Bullough down a path that some day soon could yield “smart” headlamps capable of adjusting themselves to shield oncoming drivers from glare while improving road visibility at night.
Bullough, a senior research scientist at Rensselaer Polytechnic Institute’s Lighting Research Center, has studied the elements of headlamp glare, ranging from how high the lights are mounted, to their intrinsic brightness and whether they’re aimed properly. It turns out this familiar technology is kind of complicated.
“Glare from any type of headlight — it doesn’t matter what kind it is – is going to reduce your ability to see,” Bullough says. “If there’s an oncoming car, no matter what kind of headlights those are, you’re going to see worse when those headlights are there than when they’re not there. The light scatters in our eyes, and it’s almost like looking through a foggy window inside our eyes.”
When the tungsten-halogen headlamps now standard on U.S. cars and trucks were introduced a generation ago, they were whiter, brighter and more energy efficient than their predecessors — and they caused a spate of complaints, Bullough says. “Within a few years everyone had these headlamps and what seemed to happen was they no longer were unique, and people stopped complaining about them.”
HID bulbs are brighter still, longer-lived, more energy efficient and noticeably bluer than halogen lamps. They have safety advantages, too.
“The HID headlamps tend to produce more light in the peripheral region below the horizontal,” he says. “They may help you see things that are not necessarily right on the road, but if there are pedestrians or other hazards close to the road, you can actually respond to those more quickly.”
But does a blue-hued HID headlamp actually cause more glare?
“What we found is for the same amount of light at your eye, it didn’t really matter what color the light was,” Bullough says. “It affected your visibility about equally. The other thing we found is when headlamps are aimed properly, you get about the same amount of light in your eyes from a halogen headlight as you do from an HID headlight.”
Average headlamp heights have increased as people have turned to SUVs and pickup trucks, which sit higher off the road. Their headlamps — whether halogen or HID — often point straight into the eyes of other drivers, he says.
Meanwhile, nearly two-thirds of cars have misaimed headlamps. A random survey of 100 cars found 62 percent had at least one headlamp that was either too high (worsening glare for other drivers) or too low (providing insufficient illumination down the road), he says.
European cars with HID lamps are required to have auto-leveling systems, but U.S. regulations are lax, Bullough says. “If our headlamps were aimed more properly, that would go a long way toward reducing a lot of the problems we’re experiencing with headlamp glare,” he says.
Still, he acknowledges, “blue light does create more discomfort for people. If it’s bright enough, you almost feel like a painful sensation in your eyes. It seems that there’s something about that blue color and the blue part of the spectrum of light that makes us more uncomfortable.”
Dr. Martin Mainster, an ophthalmologist who teaches at the University of Kansas medical school, says the physiological basis for aversion to bright lights in the blue end of the spectrum is not well understood, although it is known that daily exposure to blue light is essential for the regulation of the body’s circadian rhythms.
Mainster, who in 2003 published a paper titled, “Why HID headlights bother older drivers,” in the British Journal of Ophthalmology, points out that not all drivers are the same in the face of bright headlamps. As we age, the clear lens that helps to focus light on the retina in the rear of the eye develops imperfections and starts to yellow, Mainster says. That causes more light-scattering inside the eye, softening the contrast needed to see detail clearly.
Mainster wrote the paper when one his patients crashed into a parked car after being blinded by brilliant HID lights. “I was incensed with the B.S. that the manufacturers were coming up with. They said, ‘The lights are a little bluer, so they’re novel, so people stare at them and get dazzled. There’s no problem with them — it’s all an illusion.'”
But if brighter lights tend to disorient older drivers, from a highway safety standpoint, they provide superior visibility.
Bullough’s NHTSA-funded research found that high beams illuminate more of the road and provide much more time to stop than low beams — indeed, he says, people shouldn’t drive faster than 35 or 40 miles an hour with their low beams because of their limited scope. It would be best if we could always use high beams that somehow stay out of the eyes of other drivers, he says.
New technology just might make that possible.
There are already vehicle-based sensors that can track an oncoming vehicle, says Bullough, who belongs to the Transportation Lighting Alliance, a consortium of auto and lighting manufacturers interested in developing adaptive headlamps. A variety of approaches could be used to obscure the few degrees of the light beam that reaches the other driver’s face (one involves a matrix of LED bulbs that could be selectively illuminated or dimmed).
From the oncoming driver’s perspective, “It just looks like fairly dim, low-beam headlights,” he says. “But in actuality, the person who’s driving will have a full high beam in front of them, except in that small area where an oncoming car is located.”
Bullough tested a prototype of this “prime beam” system to see whether drivers liked it and found that reducing light in an angular area of about 3 degrees was sufficient to cut glare without hampering visibility.
“What we came up with was not so much the technology as the recipe,” says Bullough, who will present his NHTSA research this month at the International Symposium on Automotive Lighting at the Technische Universität Darmstadt. “It probably could be implemented within two or three years.”
Bullough likes to imagine a world where smart headlamps have become the norm. For starters, he says, we wouldn’t need as much expensive highway lighting, which means our energy needs would decrease. Moreover, light pollution would be cut because you would only illuminate what you need to.
“This really does have the potential to be a transforming kind of thing,” he says. “It’s not something that’s going to happen overnight. There’s a cost involved in having a system that can detect where an oncoming driver is. However we get there, we should think about it.”
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