Scan2CAD raster to vector conversion software

EXPLANATION:

Large format scanner accuracy and calibration

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Colour uniformity

Most large format scanners use three or more cameras (CCD scanners) or contact image sensor arrays (CIS scanners) to capture a large format scan. Each camera or array will be slightly different, so no large format scanner can scan absolutely uniformly across the scan width. In addition, temperature gradients inside a scanner caused by the illumination system or other electronics can result in differences in the performance of different components.

To compensate for this, large format scanners are supplied with calibration or maintenance software containing a procedure for "normalisation" or "white balance". This measures any unevenness in a scan caused by internal discrepancies so that it can be compensated for in subsequent scans. It also ensures that when documents that contain white are scanned, the white is scanned as uniform white and is free of any colour cast.

The normalisation procedure takes a few minutes to run and involves scanning a uniform white target or calibration sheet, normally a piece of clean white paper wide enough to cover the scanner's entire scanning width. The calibration sheet supplied with Contex scanners is an "all in one" sheet that includes white and greyscale areas for normalisation, an IT8 target for colour calibration and special markings for automatic camera alignment and for correcting dimensional inaccuracies. This allows all Contex calibration operations to take place in a single pass of the maintenance software.

Because temperature affects the colour characteristics of the scanner components, normalisation should only be carried out once the scanner has warmed up and the light has stabilised. Graphtec and Colortrac CIS scanners do not need warm up time, nor does the Image Access WideTEK - a CCD scanner that uses white LED illumination. However, if the scanner is a CCD scanner that uses fluorescent tubes for illumination it should be switched on for a while before calibration. Colortrac suggest that their CCD scanners should be switched on for an hour before calibration. Contex's literature also suggests an hours warm up time, although Contex now suggest that this is "overkill" and that their scanners can be calibrated after a much shorter time period.

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