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Colour line scan cameras

Whereas the previous sections described monochrome line scan technology, the following chapters deal with colour line scan technology. For colour segmentation into the three basic colours red, green and blue either single chip cameras with 2/3 lines or 3-chip prism cameras are used.

Prism cameras

In this type of colour line scan camera, the three sensors are arranged around a prism, as with 3-chip area scan cameras. This arrangement can cope with 3D objects without suffering spatial displacement problems, as the R, G and B pixels are coincident for any given object position.

Tri-linear line scan cameras

In this type of colour line scan camera, 3 lines are positioned adjacent to each other, spaced by an even number of pixels. The spatial differences between the lines are compensated for either in the camera, frame grabber or software. This architecture has good sensitivity, but is limited to imaging flat, 2D objects as the diagram demonstrates. A key consideration when comparing different tri-linear cameras is the interline spacing, which can vary between manufacturers. Cameras with closer line spacing are less prone to vibration and web movement errors and the camera's performance will be closer to that of a prism based camera.

Dual-line colour cameras

Colour line scan cameras can also be realised based on colour coding of a dual-line sensor. For each horizontal pixel, real data exists for two out of the three colours. Using onboard preprocessing, the third colour is interpolated from the adjacent pixels, so that image data for red, green and blue can be transmitted. There are a number of formats in which the coloured pixels are positioned. One uses an alternating pattern similar to the area scan Bayer format. Using the other method the first row is covered with green filters, whereas the pixels of the second line use alternated red and blue filter layouts. One advantage of this second format is that we get full resolution in one wavelength. For many applications this is a good compromise in a cost-effective package.

Quad linear line scan cameras

These cameras consist of a sensor with three RGB-lines and an additional line without colour filter, but with a dichroic filter to block either the visible or the IR range. This kind of camera is used in print applications for detection of security features.

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