Tech Briefs

Global Shutter Imaging

Recent improvements in CMOS sensors enable advanced high-speed machine vision.

In order to deliver crisp images in high-speed machine vision applications, image sensors require high-speed shuttering ability. The industry has traditionally relied on charge-coupled device (CCD) image sensors using interline transfer architectures to deliver this functionality. However, recent improvements in CMOS sensor design have enabled CMOS technology to achieve the image quality and true global shuttering performance necessary to meet high-speed machine vision demands. And with parallel outputs, windowing, and on-chip integration, well-designed CMOS imagers can offer compelling advantages in speed and system throughput.

True Global Shutter Imaging delivers high-speed, smear-free still image capture." class="caption" align="right">Machine vision imaging often involves inspecting targets such as printed circuit boards, silicon wafers, glass containers, or mechanical components for defects at very high speeds. As various industries move towards 100% inspection at multiple points in their production lines to improve yield and quality control, inspection systems are constantly pushed to improve system throughput and reduce cycle times.

Image sensors, as fundamental limiters of a machine vision system’s performance, must be carefully designed to excel in high-speed applications. The traditional electronic imagers of choice have been interline-transfer (ILT) CCDs due to their ability to provide high-quality images of fast-moving objects using electronic shutters to avoid motion blur or smear. ILT CCDs are not the only options.

Applications such as traffic management, metrology, and robotics inspection all need to image fast-moving objects without smear or distortion. To meet this need, cameras with true global shutter CMOS and interline transfer CCDs are available. With both technologies, the sensor has the ability to electronically shutter itself and “stop the action.”

To stop the action, the camera starts and stops exposure of all pixels in an array simultaneously, under electronic control. This functionality requires specific design features, and not all sensors have them. Full-frame CCDs require mechanical shutters or strobing to achieve this result, introducing more cost and complexity into the vision system. Frame-transfer CCDs may not provide the speed required, or may be difficult to cost-justify due to the extra silicon required for the storage area. Standard rolling-shutter CMOS sensors distort images of moving objects because they integrate different lines in the sensor at a time — by the time they read the next line, the object has moved. This distortion is a major obstacle for machine vision applications such as automated sorting or defect inspection. Objects that should be a “pass” are judged as “fail” because the image fed into the pattern- matching algorithms is distorted. For example, instead of valid round holes, the system would see unacceptable ellipses.

But interline CCDs and true global shutter CMOS sensors are ideally suited to imaging fast-moving objects. They capture crisp images without smear or distortion by electronically shuttering all pixels simultaneously even at short exposure times.

This work was done by Eric Fox, Technical Director CMOS ICs, Teledyne DALSA. For more information, visit http://info.hotims.com/45605-122.