Advanced Imaging


Advanced Imaging Magazine

Updated: July 8th, 2008 05:26 PM CDT

How to... Improve the Quality of Images in Line-scan Acquisition Systems


Common line-scan applications represent a growing part of the machine vision market. The traditional inspection of non finite objects (such as paper, textile, plastic or steel) is called web inspection. But a growing type of line-scan applications uses line-scan cameras because of their ability to provide a very high-resolution image of a finite object to be inspected. This is the case for PCB inspection.

These new applications require the acquisition of a very precise image, with a controlled pixel aspect ratio. However, users often face pitfalls such as distortion in the image, or non-constant contrast or brightness.

The Advanced Downweb Resampling (patent pending) device, available in the latest Euresys Grablink frame grabber, is the ultimate method for digital acquisition from line-scan cameras at a variable web speed. Compared to traditional methods, ADR achieves high-quality images from a simpler and more reliable line-scan acquisition system.

Traditionally, line-scan cameras are used to acquire the image of objects moving in front of the camera (scanner). A motion encoder is used to measure the speed of the object and this information is used by the frame grabber to synchronize the camera line rate with the movement of the object, ensuring that the image is acquired at a constant pixel aspect ratio and does not exhibit distortion. When synchronizing the camera with the motion encoder, the camera line rate is not constant and the electronic shutter of the camera has to be used to insure a constant light sensitivity (even contrast and brightness) of the image.

When using ADR, the line-scan camera operates at a constant line rate, without using its electronic shutter (if any). A motion encoder is also used to measure the speed of the object but, instead of using the speed information to control the camera line rate, a high-accuracy real-time digital processing is performed by the ADR FPGA of the Grablink Avenue to deliver images reproducing exactly the observed objects, without distortion. By design, ADR faithfully preserves the original shape and color scale features.

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