Windshield-Embedded System Senses Fog, Darkness

An optical sensor designed for use inside windshields can tell the difference between fog and darkness, providing driver-assistance systems with information they need to help prevent accidents. Quite simply, the more a car knows about its surroundings, the more intelligently it can respond to them.

A multifunctional system consisting of an infrared LED, two sensors (right and left in the front) and a camera distinguishes between darkness and fog. (Image: Fraunhofer IZM)

To monitor the surroundings during a journey, complex systems equipped with cameras and other sensors are used. These systems can register difficult-visibility areas near the vehicle – such as during parking – and automatically analyze the pictures generated. These sensors are mounted between the windshield and the rear-view mirror. In addition to imaging data, they also deliver information about ambient light conditions; for instance, they can distinguish between darkness and fog. The sensors interpret the optical data and analyze weather conditions.

Thus far, such high-tech systems have been too expensive for standard-size and small automobiles because, 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 have been able to offset these effects.

Now, however, researchers at the Fraunhofer Institute for Reliability and Microintegration (Fraunhofer IZM), working with Centro Ricerche Fiat of Orbassano, Italy, and the chip manufacturer STMicroelectronics of Geneva have developed a sensor system that can be inexpensively produced for medium-size and small cars as well.

"Our multifunctional system consists of an entire camera, two sensors equipped with Fresnel lenses to detect light signals, and an infrared LED,” said Fraunhofer IZM group manager 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 lightwaves 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, because the middle of the chip is reserved for recording the camera image,” he added.

To make this possible, the researcher and his team developed light pipes in a hot stamping procedure. Until now, optical fibers have been used to transmit these signals, but these snap at even low bending radii, are expensive and must be painstakingly mounted 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,” Schroeder said. The hot stamping method involves several optical channels being produced in a single pass, simplifying assembly considerably.

The Fraunhofer 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, which was carried out via rapid prototyping. A prototype of the sensor module is already in hand. Centro Ricerche Fiat is putting it through initial field tests.

For more information, visit: www.fraunhofer.de/en