Image Sensors: Vision Safety Warms Up

Wednesday, September 22, 2010 @ 09:09 PM gHale


Image sensors, used as electronic parking aids in cars or for quality control in production systems, have to be able to withstand very high temperatures that prevail in these environments.
However, as hot as they can now stand, researchers just turned it up a notch as they made a CMOS chip which functions at 115 degrees Celsius.
More and more car manufacturers are equipping their vehicles with image sensors so drivers can see pedestrians or vehicles in the blind spot or to detect obstacles when parking.
The sensors must be able to function in extremely high temperatures and in blazing sunlight. If installed behind the rear view mirror or on the instrument panel, they can get very hot. The Fraunhofer Institute for Microelectronic Circuits and Systems IMS in Duisburg, Germany, created a CMOS (complementary metal oxide semiconductor) image sensor for an industrial customer that can withstand temperatures ranging from -40 to +115 degrees Celsius.
The CCD (charged coupled device) image sensors now out in the market fails when the temperature goes beyond 60 degrees. “Our chip is not only heat-resistant, it even functions at arctic temperatures,” said Werner Brockherde, head of department at the IMS.
The research scientists developed pixels that exhibit an extremely low dark current. This reduction of residual current, which flows in complete darkness, makes it possible to capture very high-quality images even in extreme heat.
“It was not easy to achieve a low dark current,” Brockherde said. “An increase in temperature of just eight degrees doubles the dark current, resulting in image noise and reduced dynamics. Ghosting occurs in the form of artifacts or fuzziness and degrades the image.”
One other feature is its image size of 2.5 x 2.5 centimeters. This offers the advantage for special applications with weak illumination or for capturing images in the infrared or UV range the sensor can connect directly to an electronic image intensifier.
The sensor has a resolution of 256 x 256 pixels. Its high dynamic range or exposure latitude of 90 decibels provides increased contrast and optimized detail accuracy in shadow as well as in very bright areas, Brockherde said.
The sensor can precisely reproduce nuances of light. Because of efficient light absorption, the image sensor reacts with high sensitivity even in weak light conditions, he said. Another application could be for night vision equipment. The chip supports cameras with synchronous as well as asynchronous shutters. The synchronous shutter prevents motion artifacts when recording rapid movements, reducing movement fuzziness. The rolling shutter permits a higher image frame rate and continuous image recording. The effect of this is to minimize image noise.
“We produced the sensor in a standard process using 0.5 micrometer CMOS technology in our own semiconductor factory,” Brockherde said. “We also produce special components here for industrial customers.”
In addition to the automotive sector Brockherde can see other potential markets. “Our chip is suitable for deployment in chemical and steel production facilities, where it can be used for process and quality control,” he said. “Very high temperatures prevail, for example, in rolling mills where sheet metal is produced.”



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