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The more a car knows about its surroundings, the more intelligently it can respond to them. That is why an optical sensor is under development for a windshield that can tell the difference between fog and darkness.

As studies show, the amount of fatalities on the road is in a decline. That is because of the numerous new driver-assistance systems that react more quickly than a human being can. In Germany alone the number of traffic fatalities has steadily fallen.

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These new systems identify risks, warn of hazards and assist the driver in critical situations. Radar sensors, for instance, scan surrounding traffic conditions, monitoring the vehicle’s blind spot or maintaining a safe distance to the vehicle in front; infrared detectors improve night vision, and fatigue sensors sound an alarm if there is a risk of momentary driver drowsiness.

To monitor the surroundings during a journey, complex systems equipped not only with a camera but also with sensors are now in use.

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A windshield sensor system can now be inexpensively produced for medium-sized and small cars.

A windshield sensor system can now be inexpensively produced for medium-sized and small cars.

These systems can register difficult-visibility areas near the vehicle and automatically analyze the camera pictures generated. These sensors are between the windshield and the rear-view mirror. In addition to imaging data, they also deliver information about ambient light conditions. They can distinguish between darkness and fog. The sensors interpret the optical data and analyze weather conditions. Still, these high-tech systems have found their way only onto high-priced vehicles. They have been too expensive for standard-size and small models. The reason for that is with conventional components, constant use results in imprecise measurements – the integrated LEDs become less powerful over time, and the needed light detectors lose some of their sensitivity. To date, only expensive components are able to offset these effects.

A sensor system that can now be inexpensively produced for medium-sized and small cars, said researchers at Centro Ricerche Fiat and the chip manufacturer STMicroelectronics and researchers at the Fraunhofer Institute for Reliability and Microintegration IZM in Berlin.

“Our multifunctional system consists of an entire camera, two sensors equipped with Fresnel lenses to detect light signals, and an infrared LED,” said IZM group manager Dr.-Ing. Henning Schroeder. “Because fog and darkness can exhibit optically identical spectra, it is difficult to distinguish between these two light phenomena. That’s why the infrared LED emits light waves that are scattered back in fog but not in conditions of darkness.”

“It’s particularly difficult to capture the light signal from a broad aperture angle, to bundle the signal and pass it along the circuit board to the four corners of the camera chip,” Schroeder said. “Because the middle of the chip is reserved for recording the camera image.”

To make this possible, Schroeder’s team developed light pipes in a hot stamping procedure. These are hollow, mirrored tubes that can deflect a light signal by as much as 90 degrees. Up until now, optical fibers transmitted these signals. But these snap at even low bending radii, are expensive and must mount in place manually. “With the light pipes, we have succeeded in making the optical signal transmission more efficient, making the entire system smaller and reducing costs as a result,” he said. The hot stamping method involves several optical channels produced in a single pass, simplifying assembly considerably. The trick: the IZM scientists’ system is scalable and can expand through the addition of additional light pipes – to record things like solar radiation.

The IZM researchers developed not only the light pipes but also the Fresnel lenses for these sensors. They are also responsible for the design of the sensor module, carried out via Rapid Prototyping. A prototype of the sensor module is already in hand. Centro Ricerche Fiat is currently testing it in the field.

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