A line scan camera has a single line of pixels that image across an object. To build up a two-dimensional image of the object, either the camera or object is moved perpendicular to the line of pixels. This might seem like a complicated way to image an object compared with “frame cameras” that take two-dimensional images (a “frame”). However, when the object is large, continuously moving, or the task needs perfect or high resolution imaging, a line scan camera is often a much better choice than a frame camera.
Suppose a machine vision system is to inspect long rolls of material, such as paper or plastic, for defects. A resolution of 16,000 pixels across the material is needed to detect these defects. The material is unrolled under the camera and the material’s speed is measured by a rotary encoder connected to the unrolling mechanism.
You could use four large-format frame cameras mounted side-by-side across the web of material, but this has a number of disadvantages. First, large-format frame cameras usually have defective pixels that are hidden by filling in a pixel value from neighboring pixels. This can hide the defects you are trying to detect. Second, the frame camera’s exposure time must be short to prevent blurring due to the material’s motion. A short exposure requires high intensity lighting over the entire imaged area to get an acceptable image. The rotary encoder output triggers synchronous image acquisition for all cameras. Last, removing offset and gain variations in pixel responses – necessary to detect low contrast defects – is time consuming with frame cameras.
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