Resampling line-scan camera data
By Ann R. Thryft, Contributing Technical Editor -- Test & Measurement World, 2/1/2009 2:00:00 AM
 The GrablinkExpress frame grabber contains ADR (advanced downweb resampling) technology for line-scan cameras. Courtesy of Euresys. |
Line-scan cameras “build” an image by capturing one line of image data at a time as an object moves past the camera on a production line. To avoid image distortion, the camera's line rate must be synchronized with the changing speeds of items passing underneath. A motion encoder typically performs this synchronization, while an electronic shutter controls exposure inside the camera. But a synchronized camera's continually changing speed can decrease its sensitivity and increase image noise. As demand for faster speeds in electronics inspection grows, image quality is suffering, said Marc Damhaut, senior VP of product management for Euresys. “The faster the camera cycle, the shorter the exposure time and the darker the image, so the need for sensitivity is rising.”
Most line-scan cameras operate best in a particular speed range. If a line-scan camera could run at a constant speed and produce a fixed number of lines per second to build up an image, and if it could maintain continual exposure without a shutter, it would create less image noise and maintain the same brightness level. These combined conditions would result in higher image quality.
A new technique developed by Euresys called ADR (advanced downweb resampling), which is employed in two of the company's frame grabbers, lets a line-scan camera operate at a fixed rate in its optimum speed range without requiring it to adjust to different speeds outside this range, said Damhaut. Instead, ADR electronics inside the frame grabber process the image and resample it at up to twice the camera's maximum speed. The motion encoder's measurements no longer control the camera. An algorithm in the frame grabber uses the encoder's speed information to continually interpolate the data to form an image.
Because the camera is constantly exposed, the image can be built out of more data from the object and the frame grabber can perform its computations in real time, so image accuracy is much greater. Image quality increases as a result of both the continual exposure to lighting and the lack of image noise from the disabled shutter's electrical pulses.
 ADR technology lets line-scan cameras run at a constant speed and maintain continual exposure, which creates less image noise and maintains the same brightness level, resulting in higher image quality. Courtesy of Euresys. |
One of the main trends driving faster camera speeds is the need to take accurate images of large flat-panel displays, said Damhaut. “These require faster speeds because they are so large and they must be inspected so closely, but throughput cannot be compromised.” The combination of size with the needs for close inspection and high throughput poses a challenge to line-scan cameras, which require fairly uniform illumination over the entire field of view. Line-scan cameras also require high amounts of illumination, due to very short integration times, Damhaut said.
Damhaut explained that although no cameras today incorporate the ADR technology, ADR processing could be done by the camera instead of the frame grabber. “You could put a simpler, less expensive sensor inside the camera, such as a fixed-speed CCD or CMOS sensor, implement ADR electronics in the camera, and use it with a standard frame grabber,” he said. The advantages in electronics inspection would be similar to those gained from implementing ADR in the frame grabber.
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