After dark, your chances of being involved in a fatal car crash go up sharply. In the United States, for example, more than 20 percent of all fatal crashes occur between midnight and 6 a.m., though less than 3 percent of daily traffic volume is on roadways during these hours.

Of course drinking and fatigue are big factors. But inadequate illumination also ranks high: headlights provide about 50 meters of visibility on a dark road, but it takes nearly 110 meters to come to a full stop from 100 kilometers per hour. At that speed, you may not respond fast enough to an unexpected event, simply because the bright spot provided by your headlights doesn't give you enough time.

Carmakers are trying to do something about that. Mercedes-Benz and BMW, longtime safety technology standard-bearers, made a big splash at the year-end auto shows in 2005 and at the Detroit show in January, announcing that night vision would be on a list of new high-tech safety options that are available in 2006 on new models in their flagship car lines, the S Class and the 7 Series, respectively. Night vision systems use infrared sensors to let drivers see as much as three or four times farther ahead and help them quickly distinguish among objects. Someone walking along the roadside would appear to glow brightly compared with the road surface and the trees in the background, bringing the pedestrian immediately to the driver's attention when the person was all but invisible through the windshield [see photo, "Heads Up"].

Night vision has been in cars since Cadillac introduced it in the 2000 DeVille. But thus far, consumer demand has been anemic—so much so that Cadillac stopped offering it after the 2005 model year. Still, Mercedes and BMW are confident that they can package night vision as part of their renowned suites of safety systems, so that consumers will plunk down an extra US $2000 to $3000. In top-of-the-line BMWs, for example, night vision works in conjunction with headlights that pan in the direction the driver turns the steering wheel, affording better navigation of turns.

Automotive night vision comes in two flavors: near and far infrared. The type of system offered as an option in the Mercedes S Class (and in the Lexus LX470 sport-utility vehicle since the 2004 model year) uses near infrared technology, which detects the portion of the infrared band nearest to visible light. But the near infrared detector needs an assist. Special bulbs mounted next to the standard headlights are aimed straight ahead like a car's high beams, but they don't blind other drivers, because the human eye is insensitive to the infrared light. The infrared reflection of objects is captured and converted to a digital signal by a charge-coupled device (CCD)—similar to those used in digital video cameras—which perches behind the rearview mirror.

The digital signal from the CCD is routed to an image processor that trans-lates it into a format that can be viewed on a black-and-white head-up display beamed onto the windshield—in the Lexus system—or on a liquid-crystal display in the instrument panel behind the steering wheel—in the Mercedes system.

BMW (and Honda, which offers the enhancement on its Legend luxury sedan, sold in Japan) has opted for far infrared technology, which detects energy farther up the infrared band that is emitted by objects as heat. This far infrared night vision is also called passive, because no special light source is required. The special camera these systems use—essentially a phased array of infrared detector elements analogous to the pixels in an ordinary digital camera—creates a temperature pattern called a thermogram, which is refreshed 30 times a second. The heat from a pedestrian or an animal is much greater than the heat coming to the camera from its surroundings.

A signal processor translates the thermogram data to an image suitable for display on a monitor. In the BMW, it shows up on the LCD screen in the center console normally used for its iDrive control system. In Honda's Legend, the night vision image appears on a display that pops up from the dashboard when the system is turned on.

Which technology is better? According to Bert Wolfram, vice president for passenger car information systems at Siemens VDO, which introduced both far and near infrared systems at the International Motor Show in Germany last September, neither has a clear advantage.

Wolfram reports that it would cost a car company more than $1200 per unit to buy the far infrared system in quantity, while the near infrared system costs carmakers just $300 or so. What is more, the camera used in the far infrared, passive system is about the size of a paperback book, while the near infrared's CCD footprint is about the size of a postage stamp. Space in the bumper or behind the front grille has to be created for the bigger camera, because it is sensitive to the artificial temperature environment inside the passenger cabin. A CCD has no such constraints, so it can be mounted on the rearview mirror or near the top of the windshield.

But far infrared systems see farther. Some detect pedestrians more than 400 meters away. In the interest of scaling back the cost to keep the retail price near $2000, BMW uses a system with an imaging range closer to 300 meters. This is still considerably farther than the 150- to 200-meter viewing distance offered by the near infrared systems in the Mercedes S Class and the Lexus LX470.

To be sure, not everyone thinks night vision in cars makes sense, even if the price is appealing and the performance impressive. "It's a terrible idea," says Marc Green, a professor of ophthalmology at West Virginia University, Morgantown, whose research is focused on perception, attention, reaction time, memory, man-machine interfaces, and related areas. He says the biggest problem with night vision is that these systems demand that the driver take his or her focus from the road, which is not a good idea, and he worries drivers will just increase their speed, believing themselves to be less at risk.

Siemens's Wolfram, defending night vision, does agree that "with or without electronic vision support, the ultimate responsibility for recognizing obstacles in the road remains with the driver."