Driving at night in falling rain or snow can be treacherous, but not just because the asphalt is slippery – visibility is also greatly reduced, as the driver’s view of the road ahead is obscured by brightly headlight-lit raindrops or snowflakes. In the future, however, that may not be so much of a problem. A team led by Carnegie Mellon University’s Prof. Srinivasa Narasimhan has developed an experimental headlight system that renders most foreground precipitation virtually invisible, while still adequately illuminating the road beyond.

At the heart of the system are a digital light projector (which serves as the actual headlight) and an adjacent video camera. The camera is able to “see” the projector’s exact field of illumination, via a beamsplitter. As a raindrop falls into the top of this field, it is illuminated by the projector and its image is picked up by the camera.

A microprocessor then calculates the drop’s trajectory, and proceeds to selectively deactivate the projector’s light rays along that path. The result is that the raindrop is able to fall through the field of illumination, but with no light rays actually striking it – except for at the very top of the field, as it’s first detected. All of the other rays, or at least those that aren’t lined up with a raindrop falling within three to four meters (10 to 13 feet) in front of the projector, proceed through to light up the road.

The lab setup, consisting of the projector, camera and beamsplitter

This process is carried on for multitudes of drops simultaneously. For each individual raindrop, the amount of time between detection and reaction is approximately 13 milliseconds. Because the light rays are turned off and back on so quickly, there is reportedly no noticeable “flickery” quality to the light.

Needless to say, because numerous rays from the projector are continuously being disabled, it isn’t able to illuminate the road quite as brightly as would be possible otherwise ... although it’s not as pronounced of an effect as one might think. Even in heavy rain, the air volume consists of only about two to three percent raindrops, so the projector’s light output would only drop by about the same amount.

According to a report in Technology Review, the system is able to “hide” 70 percent of raindrops in simulated thunderstorm conditions, at a driving speed of 30 km/h (18.6 mph). That figure drops to 15 to 20 percent when the speed is increased to 100 km/h (62 mph).

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